Client-side protocol V2 support for fetching

Make all transports request protocol V2 when fetching. Depending on
the transport, set the GIT_PROTOCOL environment variable (file and
ssh), pass the Git-Protocol header (http), or set the hidden
"\0version=2\0" (git anon). We'll fall back to V0 if the server
doesn't reply with a version 2 answer.

A user can control which protocol the client requests via the git
config protocol.version; if not set, JGit requests protocol V2 for
fetching. Pushing always uses protocol V0 still.

In the API, there is only a new Transport.openFetch() version that
takes a collection of RefSpecs plus additional patterns to construct
the Ref prefixes for the "ls-refs" command in protocol V2. If none
are given, the server will still advertise all refs, even in protocol
V2.

BasePackConnection.readAdvertisedRefs() handles falling back to
protocol V0. It newly returns true if V0 was used and the advertised
refs were read, and false if V2 is used and an explicit "ls-refs" is
needed. (This can't be done transparently inside readAdvertisedRefs()
because a "stateless RPC" transport like TransportHttp may need to
open a new connection for writing.)

BasePackFetchConnection implements the changes needed for the protocol
V2 "fetch" command (stateless protocol, simplified ACK handling,
delimiters, section headers).

In TransportHttp, change readSmartHeaders() to also recognize the
"version 2" packet line as a valid smart server indication.

Adapt tests, and run all the HTTP tests not only with both HTTP
connection factories (JDK and Apache HttpClient) but also with both
protocol V0 and V2. The SSH tests are much slower and much more
focused on the SSH protocol and SSH key handling. Factor out two
very simple cloning and pulling tests and make those run with
protocol V2.

Bug: 553083
Change-Id: I357c7f5daa7efb2872f1c64ee6f6d54229031ae1
Signed-off-by: Thomas Wolf <thomas.wolf@paranor.ch>

17 files changed, 1070 insertions, 182 deletions

diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/api/LsRemoteCommand.java b/org.eclipse.jgit/src/org/eclipse/jgit/api/LsRemoteCommand.java
index a4ca309095..0c691062f9 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/api/LsRemoteCommand.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/api/LsRemoteCommand.java
@@ -1,5 +1,5 @@
 /*
- * Copyright (C) 2011, Christoph Brill <egore911@egore911.de> and others
+ * Copyright (C) 2011, 2020 Christoph Brill <egore911@egore911.de> and others
  *
  * This program and the accompanying materials are made available under the
  * terms of the Eclipse Distribution License v. 1.0 which is available at
@@ -164,7 +164,7 @@ public class LsRemoteCommand extends
 				refSpecs.add(new RefSpec("refs/heads/*:refs/remotes/origin/*")); //$NON-NLS-1$
 			Collection<Ref> refs;
 			Map<String, Ref> refmap = new HashMap<>();
-			try (FetchConnection fc = transport.openFetch()) {
+			try (FetchConnection fc = transport.openFetch(refSpecs)) {
 				refs = fc.getRefs();
 				if (refSpecs.isEmpty())
 					for (Ref r : refs)
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/internal/JGitText.java b/org.eclipse.jgit/src/org/eclipse/jgit/internal/JGitText.java
index 154f32c25f..09ec529fc9 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/internal/JGitText.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/internal/JGitText.java
@@ -261,6 +261,7 @@ public class JGitText extends TranslationBundle {
 	/***/ public String downloadCancelledDuringIndexing;
 	/***/ public String duplicateAdvertisementsOf;
 	/***/ public String duplicateRef;
+	/***/ public String duplicateRefAttribute;
 	/***/ public String duplicateRemoteRefUpdateIsIllegal;
 	/***/ public String duplicateStagesNotAllowed;
 	/***/ public String eitherGitDirOrWorkTreeRequired;
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/transport/BasePackConnection.java b/org.eclipse.jgit/src/org/eclipse/jgit/transport/BasePackConnection.java
index 3a36398629..3826bf7401 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/transport/BasePackConnection.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/transport/BasePackConnection.java
@@ -1,8 +1,8 @@
 /*
- * Copyright (C) 2008-2010, Google Inc.
+ * Copyright (C) 2008, 2010 Google Inc.
  * Copyright (C) 2008, Marek Zawirski <marek.zawirski@gmail.com>
  * Copyright (C) 2008, Robin Rosenberg <robin.rosenberg@dewire.com>
- * Copyright (C) 2008, Shawn O. Pearce <spearce@spearce.org> and others
+ * Copyright (C) 2008, 2020 Shawn O. Pearce <spearce@spearce.org> and others
  *
  * This program and the accompanying materials are made available under the
  * terms of the Eclipse Distribution License v. 1.0 which is available at
@@ -13,7 +13,12 @@
 
 package org.eclipse.jgit.transport;
 
+import static org.eclipse.jgit.transport.GitProtocolConstants.COMMAND_LS_REFS;
 import static org.eclipse.jgit.transport.GitProtocolConstants.OPTION_AGENT;
+import static org.eclipse.jgit.transport.GitProtocolConstants.REF_ATTR_PEELED;
+import static org.eclipse.jgit.transport.GitProtocolConstants.REF_ATTR_SYMREF_TARGET;
+import static org.eclipse.jgit.transport.GitProtocolConstants.VERSION_1;
+import static org.eclipse.jgit.transport.GitProtocolConstants.VERSION_2;
 
 import java.io.EOFException;
 import java.io.IOException;
@@ -22,23 +27,29 @@ import java.io.OutputStream;
 import java.text.MessageFormat;
 import java.util.Arrays;
 import java.util.Collection;
+import java.util.Collections;
+import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Iterator;
 import java.util.LinkedHashMap;
 import java.util.Map;
+import java.util.Objects;
 import java.util.Set;
 
+import org.eclipse.jgit.annotations.NonNull;
 import org.eclipse.jgit.errors.InvalidObjectIdException;
 import org.eclipse.jgit.errors.NoRemoteRepositoryException;
 import org.eclipse.jgit.errors.PackProtocolException;
 import org.eclipse.jgit.errors.RemoteRepositoryException;
 import org.eclipse.jgit.errors.TransportException;
 import org.eclipse.jgit.internal.JGitText;
+import org.eclipse.jgit.lib.Constants;
 import org.eclipse.jgit.lib.ObjectId;
 import org.eclipse.jgit.lib.ObjectIdRef;
 import org.eclipse.jgit.lib.Ref;
 import org.eclipse.jgit.lib.Repository;
 import org.eclipse.jgit.lib.SymbolicRef;
+import org.eclipse.jgit.util.StringUtils;
 import org.eclipse.jgit.util.io.InterruptTimer;
 import org.eclipse.jgit.util.io.TimeoutInputStream;
 import org.eclipse.jgit.util.io.TimeoutOutputStream;
@@ -92,17 +103,27 @@ abstract class BasePackConnection extends BaseConnection {
 	protected boolean statelessRPC;
 
 	/** Capability tokens advertised by the remote side. */
-	private final Set<String> remoteCapablities = new HashSet<>();
+	private final Map<String, String> remoteCapabilities = new HashMap<>();
 
 	/** Extra objects the remote has, but which aren't offered as refs. */
 	protected final Set<ObjectId> additionalHaves = new HashSet<>();
 
+	private TransferConfig.ProtocolVersion protocol = TransferConfig.ProtocolVersion.V0;
+
 	BasePackConnection(PackTransport packTransport) {
 		transport = (Transport) packTransport;
 		local = transport.local;
 		uri = transport.uri;
 	}
 
+	TransferConfig.ProtocolVersion getProtocolVersion() {
+		return protocol;
+	}
+
+	void setProtocolVersion(@NonNull TransferConfig.ProtocolVersion protocol) {
+		this.protocol = protocol;
+	}
+
 	/**
 	 * Configure this connection with the directional pipes.
 	 *
@@ -147,12 +168,15 @@ abstract class BasePackConnection extends BaseConnection {
 	 * {@link #close()} and the exception is wrapped (if necessary) and thrown
 	 * as a {@link org.eclipse.jgit.errors.TransportException}.
 	 *
+	 * @return {@code true} if the refs were read; {@code false} otherwise
+	 *         indicating that {@link #lsRefs} must be called
+	 *
 	 * @throws org.eclipse.jgit.errors.TransportException
 	 *             the reference list could not be scanned.
 	 */
-	protected void readAdvertisedRefs() throws TransportException {
+	protected boolean readAdvertisedRefs() throws TransportException {
 		try {
-			readAdvertisedRefsImpl();
+			return readAdvertisedRefsImpl();
 		} catch (TransportException err) {
 			close();
 			throw err;
@@ -162,35 +186,79 @@ abstract class BasePackConnection extends BaseConnection {
 		}
 	}
 
-	private void readAdvertisedRefsImpl() throws IOException {
-		final LinkedHashMap<String, Ref> avail = new LinkedHashMap<>();
-		for (;;) {
-			String line;
-
-			try {
-				line = pckIn.readString();
-			} catch (EOFException eof) {
-				if (avail.isEmpty())
-					throw noRepository();
-				throw eof;
-			}
-			if (PacketLineIn.isEnd(line))
-				break;
+	private String readLine() throws IOException {
+		String line = pckIn.readString();
+		if (PacketLineIn.isEnd(line)) {
+			return null;
+		}
+		if (line.startsWith("ERR ")) { //$NON-NLS-1$
+			// This is a customized remote service error.
+			// Users should be informed about it.
+			throw new RemoteRepositoryException(uri, line.substring(4));
+		}
+		return line;
+	}
 
-			if (line.startsWith("ERR ")) { //$NON-NLS-1$
-				// This is a customized remote service error.
-				// Users should be informed about it.
-				throw new RemoteRepositoryException(uri, line.substring(4));
-			}
+	private boolean readAdvertisedRefsImpl() throws IOException {
+		final Map<String, Ref> avail = new LinkedHashMap<>();
+		final Map<String, String> symRefs = new LinkedHashMap<>();
+		for (boolean first = true;; first = false) {
+			String line;
 
-			if (avail.isEmpty()) {
+			if (first) {
+				boolean isV1 = false;
+				try {
+					line = readLine();
+				} catch (EOFException e) {
+					TransportException noRepo = noRepository();
+					noRepo.initCause(e);
+					throw noRepo;
+				}
+				if (line != null && VERSION_1.equals(line)) {
+					// Same as V0, except for this extra line. We shouldn't get
+					// it since we never request V1.
+					setProtocolVersion(TransferConfig.ProtocolVersion.V0);
+					isV1 = true;
+					line = readLine();
+				}
+				if (line == null) {
+					break;
+				}
 				final int nul = line.indexOf('\0');
 				if (nul >= 0) {
-					// The first line (if any) may contain "hidden"
-					// capability values after a NUL byte.
-					remoteCapablities.addAll(
-							Arrays.asList(line.substring(nul + 1).split(" "))); //$NON-NLS-1$
+					// Protocol V0: The first line (if any) may contain
+					// "hidden" capability values after a NUL byte.
+					for (String capability : line.substring(nul + 1)
+							.split(" ")) { //$NON-NLS-1$
+						if (capability.startsWith(CAPABILITY_SYMREF_PREFIX)) {
+							String[] parts = capability
+									.substring(
+											CAPABILITY_SYMREF_PREFIX.length())
+									.split(":", 2); //$NON-NLS-1$
+							if (parts.length == 2) {
+								symRefs.put(parts[0], parts[1]);
+							}
+						} else {
+							addCapability(capability);
+						}
+					}
 					line = line.substring(0, nul);
+					setProtocolVersion(TransferConfig.ProtocolVersion.V0);
+				} else if (!isV1 && VERSION_2.equals(line)) {
+					// Protocol V2: remaining lines are capabilities as
+					// key=value pairs
+					setProtocolVersion(TransferConfig.ProtocolVersion.V2);
+					readCapabilitiesV2();
+					// Break out here so that stateless RPC transports get a
+					// chance to set up the output stream.
+					return false;
+				} else {
+					setProtocolVersion(TransferConfig.ProtocolVersion.V0);
+				}
+			} else {
+				line = readLine();
+				if (line == null) {
+					break;
 				}
 			}
 
@@ -199,73 +267,214 @@ abstract class BasePackConnection extends BaseConnection {
 				throw invalidRefAdvertisementLine(line);
 			}
 			String name = line.substring(41, line.length());
-			if (avail.isEmpty() && name.equals("capabilities^{}")) { //$NON-NLS-1$
-				// special line from git-receive-pack to show
+			if (first && name.equals("capabilities^{}")) { //$NON-NLS-1$
+				// special line from git-receive-pack (protocol V0) to show
 				// capabilities when there are no refs to advertise
 				continue;
 			}
 
-			final ObjectId id;
-			try {
-				id  = ObjectId.fromString(line.substring(0, 40));
-			} catch (InvalidObjectIdException e) {
-				PackProtocolException ppe = invalidRefAdvertisementLine(line);
-				ppe.initCause(e);
-				throw ppe;
-			}
+			final ObjectId id = toId(line, line.substring(0, 40));
 			if (name.equals(".have")) { //$NON-NLS-1$
 				additionalHaves.add(id);
-			} else if (name.endsWith("^{}")) { //$NON-NLS-1$
-				name = name.substring(0, name.length() - 3);
-				final Ref prior = avail.get(name);
-				if (prior == null)
-					throw new PackProtocolException(uri, MessageFormat.format(
-							JGitText.get().advertisementCameBefore, name, name));
-
-				if (prior.getPeeledObjectId() != null)
-					throw duplicateAdvertisement(name + "^{}"); //$NON-NLS-1$
-
-				avail.put(name, new ObjectIdRef.PeeledTag(
-						Ref.Storage.NETWORK, name, prior.getObjectId(), id));
 			} else {
-				final Ref prior = avail.put(name, new ObjectIdRef.PeeledNonTag(
-						Ref.Storage.NETWORK, name, id));
-				if (prior != null)
-					throw duplicateAdvertisement(name);
+				processLineV1(name, id, avail);
 			}
 		}
-		updateWithSymRefs(avail, extractSymRefsFromCapabilities(remoteCapablities));
+		updateWithSymRefs(avail, symRefs);
 		available(avail);
+		return true;
 	}
 
 	/**
-	 * Finds values in the given capabilities of the form:
-	 *
-	 * <pre>
-	 * symref=<em>source</em>:<em>target</em>
-	 * </pre>
+	 * Issue a protocol V2 ls-refs command and read its response.
 	 *
-	 * And returns a Map of source->target entries.
-	 *
-	 * @param capabilities
-	 *            the capabilities lines
-	 * @return a Map of the symref entries from capabilities
-	 * @throws NullPointerException
-	 *             if capabilities, or any entry in it, is null
+	 * @param refSpecs
+	 *            to produce ref prefixes from if the server supports git
+	 *            protocol V2
+	 * @param additionalPatterns
+	 *            to use for ref prefixes if the server supports git protocol V2
+	 * @throws TransportException
+	 *             if the command could not be run or its output not be read
 	 */
-	static Map<String, String> extractSymRefsFromCapabilities(Collection<String> capabilities) {
+	protected void lsRefs(Collection<RefSpec> refSpecs,
+			String... additionalPatterns) throws TransportException {
+		try {
+			lsRefsImpl(refSpecs, additionalPatterns);
+		} catch (TransportException err) {
+			close();
+			throw err;
+		} catch (IOException | RuntimeException err) {
+			close();
+			throw new TransportException(err.getMessage(), err);
+		}
+	}
+
+	private void lsRefsImpl(Collection<RefSpec> refSpecs,
+			String... additionalPatterns) throws IOException {
+		pckOut.writeString("command=" + COMMAND_LS_REFS); //$NON-NLS-1$
+		// Add the user-agent
+		String agent = UserAgent.get();
+		if (agent != null && isCapableOf(OPTION_AGENT)) {
+			pckOut.writeString(OPTION_AGENT + '=' + agent);
+		}
+		pckOut.writeDelim();
+		pckOut.writeString("peel"); //$NON-NLS-1$
+		pckOut.writeString("symrefs"); //$NON-NLS-1$
+		for (String refPrefix : getRefPrefixes(refSpecs, additionalPatterns)) {
+			pckOut.writeString("ref-prefix " + refPrefix); //$NON-NLS-1$
+		}
+		pckOut.end();
+		final Map<String, Ref> avail = new LinkedHashMap<>();
 		final Map<String, String> symRefs = new LinkedHashMap<>();
-		for (String option : capabilities) {
-			if (option.startsWith(CAPABILITY_SYMREF_PREFIX)) {
-				String[] symRef = option
-						.substring(CAPABILITY_SYMREF_PREFIX.length())
-						.split(":", 2); //$NON-NLS-1$
-				if (symRef.length == 2) {
-					symRefs.put(symRef[0], symRef[1]);
+		for (;;) {
+			String line = readLine();
+			if (line == null) {
+				break;
+			}
+			// Expecting to get a line in the form "sha1 refname"
+			if (line.length() < 41 || line.charAt(40) != ' ') {
+				throw invalidRefAdvertisementLine(line);
+			}
+			String name = line.substring(41, line.length());
+			final ObjectId id = toId(line, line.substring(0, 40));
+			if (name.equals(".have")) { //$NON-NLS-1$
+				additionalHaves.add(id);
+			} else {
+				processLineV2(line, id, name, avail, symRefs);
+			}
+		}
+		updateWithSymRefs(avail, symRefs);
+		available(avail);
+	}
+
+	private Collection<String> getRefPrefixes(Collection<RefSpec> refSpecs,
+			String... additionalPatterns) {
+		if (refSpecs.isEmpty() && (additionalPatterns == null
+				|| additionalPatterns.length == 0)) {
+			return Collections.emptyList();
+		}
+		Set<String> patterns = new HashSet<>();
+		if (additionalPatterns != null) {
+			Arrays.stream(additionalPatterns).filter(Objects::nonNull)
+					.forEach(patterns::add);
+		}
+		for (RefSpec spec : refSpecs) {
+			// TODO: for now we only do protocol V2 for fetch. For push
+			// RefSpecs, the logic would need to be different. (At the
+			// minimum, take spec.getDestination().)
+			String src = spec.getSource();
+			if (ObjectId.isId(src)) {
+				continue;
+			}
+			if (spec.isWildcard()) {
+				patterns.add(src.substring(0, src.indexOf('*')));
+			} else {
+				patterns.add(src);
+				patterns.add(Constants.R_REFS + src);
+				patterns.add(Constants.R_HEADS + src);
+				patterns.add(Constants.R_TAGS + src);
+			}
+		}
+		return patterns;
+	}
+
+	private void readCapabilitiesV2() throws IOException {
+		// In git protocol V2, capabilities are different. If it's a key-value
+		// pair, the key may be a command name, and the value a space-separated
+		// list of capabilities for that command. We still store it in the same
+		// map as for protocol v0/v1. Protocol v2 code has to account for this.
+		for (;;) {
+			String line = readLine();
+			if (line == null) {
+				break;
+			}
+			addCapability(line);
+		}
+	}
+
+	private void addCapability(String capability) {
+		String parts[] = capability.split("=", 2); //$NON-NLS-1$
+		if (parts.length == 2) {
+			remoteCapabilities.put(parts[0], parts[1]);
+		}
+		remoteCapabilities.put(capability, null);
+	}
+
+	private ObjectId toId(String line, String value)
+			throws PackProtocolException {
+		try {
+			return ObjectId.fromString(value);
+		} catch (InvalidObjectIdException e) {
+			PackProtocolException ppe = invalidRefAdvertisementLine(line);
+			ppe.initCause(e);
+			throw ppe;
+		}
+	}
+
+	private void processLineV1(String name, ObjectId id, Map<String, Ref> avail)
+			throws IOException {
+		if (name.endsWith("^{}")) { //$NON-NLS-1$
+			name = name.substring(0, name.length() - 3);
+			final Ref prior = avail.get(name);
+			if (prior == null) {
+				throw new PackProtocolException(uri, MessageFormat.format(
+						JGitText.get().advertisementCameBefore, name, name));
+			}
+			if (prior.getPeeledObjectId() != null) {
+				throw duplicateAdvertisement(name + "^{}"); //$NON-NLS-1$
+			}
+			avail.put(name, new ObjectIdRef.PeeledTag(Ref.Storage.NETWORK, name,
+					prior.getObjectId(), id));
+		} else {
+			final Ref prior = avail.put(name, new ObjectIdRef.PeeledNonTag(
+					Ref.Storage.NETWORK, name, id));
+			if (prior != null) {
+				throw duplicateAdvertisement(name);
+			}
+		}
+	}
+
+	private void processLineV2(String line, ObjectId id, String rest,
+			Map<String, Ref> avail, Map<String, String> symRefs)
+			throws IOException {
+		String[] parts = rest.split(" "); //$NON-NLS-1$
+		String name = parts[0];
+		// Two attributes possible, symref-target or peeled
+		String symRefTarget = null;
+		String peeled = null;
+		for (int i = 1; i < parts.length; i++) {
+			if (parts[i].startsWith(REF_ATTR_SYMREF_TARGET)) {
+				if (symRefTarget != null) {
+					throw new PackProtocolException(uri, MessageFormat.format(
+							JGitText.get().duplicateRefAttribute, line));
+				}
+				symRefTarget = parts[i]
+						.substring(REF_ATTR_SYMREF_TARGET.length());
+			} else if (parts[i].startsWith(REF_ATTR_PEELED)) {
+				if (peeled != null) {
+					throw new PackProtocolException(uri, MessageFormat.format(
+							JGitText.get().duplicateRefAttribute, line));
 				}
+				peeled = parts[i].substring(REF_ATTR_PEELED.length());
 			}
+			if (peeled != null && symRefTarget != null) {
+				break;
+			}
+		}
+		Ref idRef;
+		if (peeled != null) {
+			idRef = new ObjectIdRef.PeeledTag(Ref.Storage.NETWORK, name, id,
+					toId(line, peeled));
+		} else {
+			idRef = new ObjectIdRef.PeeledNonTag(Ref.Storage.NETWORK, name, id);
+		}
+		Ref prior = avail.put(name, idRef);
+		if (prior != null) {
+			throw duplicateAdvertisement(name);
+		}
+		if (!StringUtils.isEmptyOrNull(symRefTarget)) {
+			symRefs.put(name, symRefTarget);
 		}
-		return symRefs;
 	}
 
 	/**
@@ -334,6 +543,22 @@ abstract class BasePackConnection extends BaseConnection {
 				}
 			}
 		}
+		// If HEAD is still in the symRefs map here, the real ref was not
+		// reported, but we know it must point to the object reported for HEAD.
+		// So fill it in in the refMap.
+		String headRefName = symRefs.get(Constants.HEAD);
+		if (headRefName != null && !refMap.containsKey(headRefName)) {
+			Ref headRef = refMap.get(Constants.HEAD);
+			if (headRef != null) {
+				ObjectId headObj = headRef.getObjectId();
+				headRef = new ObjectIdRef.PeeledNonTag(Ref.Storage.NETWORK,
+						headRefName, headObj);
+				refMap.put(headRefName, headRef);
+				headRef = new SymbolicRef(Constants.HEAD, headRef);
+				refMap.put(Constants.HEAD, headRef);
+				symRefs.remove(Constants.HEAD);
+			}
+		}
 	}
 
 	/**
@@ -357,7 +582,7 @@ abstract class BasePackConnection extends BaseConnection {
 	 * @return whether this option is supported
 	 */
 	protected boolean isCapableOf(String option) {
-		return remoteCapablities.contains(option);
+		return remoteCapabilities.containsKey(option);
 	}
 
 	/**
@@ -378,6 +603,17 @@ abstract class BasePackConnection extends BaseConnection {
 	}
 
 	/**
+	 * Return a capability value.
+	 *
+	 * @param option
+	 *            to get
+	 * @return the value stored, if any.
+	 */
+	protected String getCapability(String option) {
+		return remoteCapabilities.get(option);
+	}
+
+	/**
 	 * Add user agent capability
 	 *
 	 * @param b
@@ -385,7 +621,7 @@ abstract class BasePackConnection extends BaseConnection {
 	 */
 	protected void addUserAgentCapability(StringBuilder b) {
 		String a = UserAgent.get();
-		if (a != null && UserAgent.hasAgent(remoteCapablities)) {
+		if (a != null && remoteCapabilities.get(OPTION_AGENT) != null) {
 			b.append(' ').append(OPTION_AGENT).append('=').append(a);
 		}
 	}
@@ -393,7 +629,8 @@ abstract class BasePackConnection extends BaseConnection {
 	/** {@inheritDoc} */
 	@Override
 	public String getPeerUserAgent() {
-		return UserAgent.getAgent(remoteCapablities, super.getPeerUserAgent());
+		String agent = remoteCapabilities.get(OPTION_AGENT);
+		return agent != null ? agent : super.getPeerUserAgent();
 	}
 
 	private PackProtocolException duplicateAdvertisement(String name) {
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/transport/BasePackFetchConnection.java b/org.eclipse.jgit/src/org/eclipse/jgit/transport/BasePackFetchConnection.java
index a2fb51f46d..fa0c0c6670 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/transport/BasePackFetchConnection.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/transport/BasePackFetchConnection.java
@@ -1,7 +1,7 @@
 /*
- * Copyright (C) 2008-2010, Google Inc.
+ * Copyright (C) 2008, 2010 Google Inc.
  * Copyright (C) 2008, Robin Rosenberg <robin.rosenberg@dewire.com>
- * Copyright (C) 2008, Shawn O. Pearce <spearce@spearce.org> and others
+ * Copyright (C) 2008, 2020 Shawn O. Pearce <spearce@spearce.org> and others
  *
  * This program and the accompanying materials are made available under the
  * terms of the Eclipse Distribution License v. 1.0 which is available at
@@ -16,18 +16,21 @@ import java.io.IOException;
 import java.io.InputStream;
 import java.io.OutputStream;
 import java.text.MessageFormat;
+import java.util.Arrays;
 import java.util.Collection;
 import java.util.Collections;
 import java.util.Date;
+import java.util.HashSet;
+import java.util.LinkedHashSet;
 import java.util.Set;
 
 import org.eclipse.jgit.errors.PackProtocolException;
+import org.eclipse.jgit.errors.RemoteRepositoryException;
 import org.eclipse.jgit.errors.TransportException;
 import org.eclipse.jgit.internal.JGitText;
 import org.eclipse.jgit.internal.storage.file.PackLock;
 import org.eclipse.jgit.lib.AnyObjectId;
 import org.eclipse.jgit.lib.Config;
-import org.eclipse.jgit.lib.Constants;
 import org.eclipse.jgit.lib.MutableObjectId;
 import org.eclipse.jgit.lib.NullProgressMonitor;
 import org.eclipse.jgit.lib.ObjectId;
@@ -44,6 +47,7 @@ import org.eclipse.jgit.revwalk.filter.CommitTimeRevFilter;
 import org.eclipse.jgit.revwalk.filter.RevFilter;
 import org.eclipse.jgit.transport.GitProtocolConstants.MultiAck;
 import org.eclipse.jgit.transport.PacketLineIn.AckNackResult;
+import org.eclipse.jgit.util.StringUtils;
 import org.eclipse.jgit.util.TemporaryBuffer;
 
 /**
@@ -207,7 +211,10 @@ public abstract class BasePackFetchConnection extends BasePackConnection
 
 	private int maxHaves;
 
-	/** RPC state, if {@link BasePackConnection#statelessRPC} is true. */
+	/**
+	 * RPC state, if {@link BasePackConnection#statelessRPC} is true or protocol
+	 * V2 is used.
+	 */
 	private TemporaryBuffer.Heap state;
 
 	private PacketLineOut pckState;
@@ -321,6 +328,13 @@ public abstract class BasePackFetchConnection extends BasePackConnection
 		return Collections.<PackLock> emptyList();
 	}
 
+	private void clearState() {
+		walk.dispose();
+		reachableCommits = null;
+		state = null;
+		pckState = null;
+	}
+
 	/**
 	 * Execute common ancestor negotiation and fetch the objects.
 	 *
@@ -349,18 +363,34 @@ public abstract class BasePackFetchConnection extends BasePackConnection
 			markRefsAdvertised();
 			markReachable(have, maxTimeWanted(want));
 
+			if (TransferConfig.ProtocolVersion.V2
+					.equals(getProtocolVersion())) {
+				// Protocol V2 always is a "stateless" protocol, even over a
+				// bidirectional pipe: the server serves one "fetch" request and
+				// then forgets anything it has learned, so the next fetch
+				// request has to re-send all wants and previously determined
+				// common objects as "have"s again.
+				state = new TemporaryBuffer.Heap(Integer.MAX_VALUE);
+				pckState = new PacketLineOut(state);
+				try {
+					doFetchV2(monitor, want, outputStream);
+				} finally {
+					clearState();
+				}
+				return;
+			}
+			// Protocol V0/1
 			if (statelessRPC) {
 				state = new TemporaryBuffer.Heap(Integer.MAX_VALUE);
 				pckState = new PacketLineOut(state);
 			}
-
-			if (sendWants(want)) {
+			PacketLineOut output = statelessRPC ? pckState : pckOut;
+			if (sendWants(want, output)) {
+				output.end();
+				outNeedsEnd = false;
 				negotiate(monitor);
 
-				walk.dispose();
-				reachableCommits = null;
-				state = null;
-				pckState = null;
+				clearState();
 
 				receivePack(monitor, outputStream);
 			}
@@ -373,6 +403,180 @@ public abstract class BasePackFetchConnection extends BasePackConnection
 		}
 	}
 
+	private void doFetchV2(ProgressMonitor monitor, Collection<Ref> want,
+			OutputStream outputStream) throws IOException, CancelledException {
+		sideband = true;
+		negotiateBegin();
+
+		pckState.writeString("command=" + GitProtocolConstants.COMMAND_FETCH); //$NON-NLS-1$
+		// Capabilities are sent as command arguments in protocol V2
+		String agent = UserAgent.get();
+		if (agent != null && isCapableOf(GitProtocolConstants.OPTION_AGENT)) {
+			pckState.writeString(
+					GitProtocolConstants.OPTION_AGENT + '=' + agent);
+		}
+		Set<String> capabilities = new HashSet<>();
+		String advertised = getCapability(GitProtocolConstants.COMMAND_FETCH);
+		if (!StringUtils.isEmptyOrNull(advertised)) {
+			capabilities.addAll(Arrays.asList(advertised.split("\\s+"))); //$NON-NLS-1$
+		}
+		// Arguments
+		pckState.writeDelim();
+		for (String capability : getCapabilitiesV2(capabilities)) {
+			pckState.writeString(capability);
+		}
+		if (!sendWants(want, pckState)) {
+			// We already have everything we wanted.
+			return;
+		}
+		// If we send something, we always close it properly ourselves.
+		outNeedsEnd = false;
+
+		FetchStateV2 fetchState = new FetchStateV2();
+		boolean sentDone = false;
+		for (;;) {
+			// The "state" buffer contains the full fetch request with all
+			// common objects found so far.
+			state.writeTo(out, monitor);
+			sentDone = sendNextHaveBatch(fetchState, pckOut, monitor);
+			if (sentDone) {
+				break;
+			}
+			if (readAcknowledgments(fetchState, pckIn, monitor)) {
+				// We got a "ready": next should be a patch file.
+				break;
+			}
+			// Note: C git reads and requires here (and after a packfile) a
+			// "0002" packet in stateless RPC transports (https). This "response
+			// end" packet is even mentioned in the protocol V2 technical
+			// documentation. However, it is not actually part of the public
+			// protocol; it occurs only in an internal protocol wrapper in the C
+			// git implementation.
+		}
+		clearState();
+		String line = pckIn.readString();
+		// If we sent a done, we may have an error reply here.
+		if (sentDone && line.startsWith("ERR ")) { //$NON-NLS-1$
+			throw new RemoteRepositoryException(uri, line.substring(4));
+		}
+		// "shallow-info", "wanted-refs", and "packfile-uris" would have to be
+		// handled here in that order.
+		if (!GitProtocolConstants.SECTION_PACKFILE.equals(line)) {
+			throw new PackProtocolException(
+					MessageFormat.format(JGitText.get().expectedGot,
+							GitProtocolConstants.SECTION_PACKFILE, line));
+		}
+		receivePack(monitor, outputStream);
+	}
+
+	/**
+	 * Sends the next batch of "have"s and terminates the {@code output}.
+	 *
+	 * @param fetchState
+	 *            is updated with information about the number of items written,
+	 *            and whether to expect a packfile next
+	 * @param output
+	 *            to write to
+	 * @param monitor
+	 *            for progress reporting and cancellation
+	 * @return {@code true} if a "done" was written and we should thus expect a
+	 *         packfile next
+	 * @throws IOException
+	 *             on errors
+	 * @throws CancelledException
+	 *             on cancellation
+	 */
+	private boolean sendNextHaveBatch(FetchStateV2 fetchState,
+			PacketLineOut output, ProgressMonitor monitor)
+			throws IOException, CancelledException {
+		long n = 0;
+		while (n < fetchState.havesToSend) {
+			final RevCommit c = walk.next();
+			if (c == null) {
+				break;
+			}
+			output.writeString("have " + c.getId().name() + '\n'); //$NON-NLS-1$
+			n++;
+			if (n % 10 == 0 && monitor.isCancelled()) {
+				throw new CancelledException();
+			}
+		}
+		fetchState.havesTotal += n;
+		if (n == 0
+				|| fetchState.havesWithoutAck > MAX_HAVES
+				|| fetchState.havesTotal > maxHaves) {
+			output.writeString("done\n"); //$NON-NLS-1$
+			output.end();
+			return true;
+		}
+		fetchState.havesWithoutAck += n;
+		output.end();
+		fetchState.incHavesToSend(statelessRPC);
+		return false;
+	}
+
+	/**
+	 * Reads and processes acknowledgments, adding ACKed objects as "have"s to
+	 * the global state {@link TemporaryBuffer}.
+	 *
+	 * @param fetchState
+	 *            to update
+	 * @param input
+	 *            to read from
+	 * @param monitor
+	 *            for progress reporting and cancellation
+	 * @return {@code true} if a "ready" was received and a packfile is expected
+	 *         next
+	 * @throws IOException
+	 *             on errors
+	 * @throws CancelledException
+	 *             on cancellation
+	 */
+	private boolean readAcknowledgments(FetchStateV2 fetchState,
+			PacketLineIn input, ProgressMonitor monitor)
+			throws IOException, CancelledException {
+		String line = input.readString();
+		if (!GitProtocolConstants.SECTION_ACKNOWLEDGMENTS.equals(line)) {
+			throw new PackProtocolException(MessageFormat.format(
+					JGitText.get().expectedGot,
+					GitProtocolConstants.SECTION_ACKNOWLEDGMENTS, line));
+		}
+		MutableObjectId returnedId = new MutableObjectId();
+		line = input.readString();
+		boolean gotReady = false;
+		long n = 0;
+		while (!PacketLineIn.isEnd(line) && !PacketLineIn.isDelimiter(line)) {
+			AckNackResult ack = PacketLineIn.parseACKv2(line, returnedId);
+			// If we got a "ready", we just skip the remaining lines after
+			// having checked them for being valid. (Normally, the "ready"
+			// should be the last line anyway.)
+			if (!gotReady) {
+				if (ack == AckNackResult.ACK_COMMON) {
+					// markCommon appends the object to the "state"
+					markCommon(walk.parseAny(returnedId), ack, true);
+					fetchState.havesWithoutAck = 0;
+				} else if (ack == AckNackResult.ACK_READY) {
+					gotReady = true;
+				}
+			}
+			n++;
+			if (n % 10 == 0 && monitor.isCancelled()) {
+				throw new CancelledException();
+			}
+			line = input.readString();
+		}
+		if (gotReady) {
+			if (!PacketLineIn.isDelimiter(line)) {
+				throw new PackProtocolException(MessageFormat
+						.format(JGitText.get().expectedGot, "0001", line)); //$NON-NLS-1$
+			}
+		} else if (!PacketLineIn.isEnd(line)) {
+			throw new PackProtocolException(MessageFormat
+					.format(JGitText.get().expectedGot, "0000", line)); //$NON-NLS-1$
+		}
+		return gotReady;
+	}
+
 	/** {@inheritDoc} */
 	@Override
 	public void close() {
@@ -456,8 +660,8 @@ public abstract class BasePackFetchConnection extends BasePackConnection
 		}
 	}
 
-	private boolean sendWants(Collection<Ref> want) throws IOException {
-		final PacketLineOut p = statelessRPC ? pckState : pckOut;
+	private boolean sendWants(Collection<Ref> want, PacketLineOut p)
+			throws IOException {
 		boolean first = true;
 		for (Ref r : want) {
 			ObjectId objectId = r.getObjectId();
@@ -479,10 +683,11 @@ public abstract class BasePackFetchConnection extends BasePackConnection
 			final StringBuilder line = new StringBuilder(46);
 			line.append("want "); //$NON-NLS-1$
 			line.append(objectId.name());
-			if (first) {
+			if (first && TransferConfig.ProtocolVersion.V0
+					.equals(getProtocolVersion())) {
 				line.append(enableCapabilities());
-				first = false;
 			}
+			first = false;
 			line.append('\n');
 			p.writeString(line.toString());
 		}
@@ -492,11 +697,34 @@ public abstract class BasePackFetchConnection extends BasePackConnection
 		if (!filterSpec.isNoOp()) {
 			p.writeString(filterSpec.filterLine());
 		}
-		p.end();
-		outNeedsEnd = false;
 		return true;
 	}
 
+	private Set<String> getCapabilitiesV2(Set<String> advertisedCapabilities)
+			throws TransportException {
+		Set<String> capabilities = new LinkedHashSet<>();
+		// Protocol V2 is implicitly capable of all these.
+		if (noProgress) {
+			capabilities.add(OPTION_NO_PROGRESS);
+		}
+		if (includeTags) {
+			capabilities.add(OPTION_INCLUDE_TAG);
+		}
+		if (allowOfsDelta) {
+			capabilities.add(OPTION_OFS_DELTA);
+		}
+		if (thinPack) {
+			capabilities.add(OPTION_THIN_PACK);
+		}
+		if (!filterSpec.isNoOp()
+				&& !advertisedCapabilities.contains(OPTION_FILTER)) {
+			throw new PackProtocolException(uri,
+					JGitText.get().filterRequiresCapability);
+		}
+		// The FilterSpec will be added later in sendWants().
+		return capabilities;
+	}
+
 	private String enableCapabilities() throws TransportException {
 		final StringBuilder line = new StringBuilder();
 		if (noProgress)
@@ -622,7 +850,7 @@ public abstract class BasePackFetchConnection extends BasePackConnection
 					// we need to continue to talk about other parts of
 					// our local history.
 					//
-					markCommon(walk.parseAny(ackId), anr);
+					markCommon(walk.parseAny(ackId), anr, statelessRPC);
 					receivedAck = true;
 					receivedContinue = true;
 					havesSinceLastContinue = 0;
@@ -757,16 +985,10 @@ public abstract class BasePackFetchConnection extends BasePackConnection
 		}
 	}
 
-	private void markCommon(RevObject obj, AckNackResult anr)
+	private void markCommon(RevObject obj, AckNackResult anr, boolean useState)
 			throws IOException {
-		if (statelessRPC && anr == AckNackResult.ACK_COMMON && !obj.has(STATE)) {
-			StringBuilder s;
-
-			s = new StringBuilder(6 + Constants.OBJECT_ID_STRING_LENGTH);
-			s.append("have "); //$NON-NLS-1$
-			s.append(obj.name());
-			s.append('\n');
-			pckState.writeString(s.toString());
+		if (useState && anr == AckNackResult.ACK_COMMON && !obj.has(STATE)) {
+			pckState.writeString("have " + obj.name() + '\n'); //$NON-NLS-1$
 			obj.add(STATE);
 		}
 		obj.add(COMMON);
@@ -806,4 +1028,26 @@ public abstract class BasePackFetchConnection extends BasePackConnection
 	private static class CancelledException extends Exception {
 		private static final long serialVersionUID = 1L;
 	}
+
+	private static class FetchStateV2 {
+
+		long havesToSend = 32;
+
+		long havesTotal;
+
+		long havesWithoutAck;
+
+		void incHavesToSend(boolean statelessRPC) {
+			if (statelessRPC) {
+				// Increase this quicker since connection setup costs accumulate
+				if (havesToSend < 16384) {
+					havesToSend *= 2;
+				} else {
+					havesToSend = havesToSend * 11 / 10;
+				}
+			} else {
+				havesToSend += 32;
+			}
+		}
+	}
 }
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/transport/FetchProcess.java b/org.eclipse.jgit/src/org/eclipse/jgit/transport/FetchProcess.java
index 00b726e063..bdebfa607b 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/transport/FetchProcess.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/transport/FetchProcess.java
@@ -1,6 +1,6 @@
 /*
  * Copyright (C) 2008, Robin Rosenberg <robin.rosenberg@dewire.com>
- * Copyright (C) 2008, Shawn O. Pearce <spearce@spearce.org> and others
+ * Copyright (C) 2008, 2020 Shawn O. Pearce <spearce@spearce.org> and others
  *
  * This program and the accompanying materials are made available under the
  * terms of the Eclipse Distribution License v. 1.0 which is available at
@@ -110,7 +110,21 @@ class FetchProcess {
 	private void executeImp(final ProgressMonitor monitor,
 			final FetchResult result) throws NotSupportedException,
 			TransportException {
-		conn = transport.openFetch();
+		final TagOpt tagopt = transport.getTagOpt();
+		String getTags = (tagopt == TagOpt.NO_TAGS) ? null : Constants.R_TAGS;
+		String getHead = null;
+		try {
+			// If we don't have a HEAD yet, we're cloning and need to get the
+			// upstream HEAD, too.
+			Ref head = transport.local.exactRef(Constants.HEAD);
+			ObjectId id = head != null ? head.getObjectId() : null;
+			if (id == null || id.equals(ObjectId.zeroId())) {
+				getHead = Constants.HEAD;
+			}
+		} catch (IOException e) {
+			// Ignore
+		}
+		conn = transport.openFetch(toFetch, getTags, getHead);
 		try {
 			result.setAdvertisedRefs(transport.getURI(), conn.getRefsMap());
 			result.peerUserAgent = conn.getPeerUserAgent();
@@ -127,7 +141,6 @@ class FetchProcess {
 			}
 
 			Collection<Ref> additionalTags = Collections.<Ref> emptyList();
-			final TagOpt tagopt = transport.getTagOpt();
 			if (tagopt == TagOpt.AUTO_FOLLOW)
 				additionalTags = expandAutoFollowTags();
 			else if (tagopt == TagOpt.FETCH_TAGS)
@@ -261,7 +274,17 @@ class FetchProcess {
 		if (conn != null)
 			return;
 
-		conn = transport.openFetch();
+		// Build prefixes
+		Set<String> prefixes = new HashSet<>();
+		for (Ref toGet : askFor.values()) {
+			String src = toGet.getName();
+			prefixes.add(src);
+			prefixes.add(Constants.R_REFS + src);
+			prefixes.add(Constants.R_HEADS + src);
+			prefixes.add(Constants.R_TAGS + src);
+		}
+		conn = transport.openFetch(Collections.emptyList(),
+				prefixes.toArray(new String[0]));
 
 		// Since we opened a new connection we cannot be certain
 		// that the system we connected to has the same exact set
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/transport/GitProtocolConstants.java b/org.eclipse.jgit/src/org/eclipse/jgit/transport/GitProtocolConstants.java
index 35e2978bc4..36fce7a3f5 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/transport/GitProtocolConstants.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/transport/GitProtocolConstants.java
@@ -1,7 +1,7 @@
 /*
- * Copyright (C) 2008-2013, Google Inc.
+ * Copyright (C) 2008, 2013 Google Inc.
  * Copyright (C) 2008, Robin Rosenberg <robin.rosenberg@dewire.com>
- * Copyright (C) 2008, Shawn O. Pearce <spearce@spearce.org> and others
+ * Copyright (C) 2008, 2020 Shawn O. Pearce <spearce@spearce.org> and others
  *
  * This program and the accompanying materials are made available under the
  * terms of the Eclipse Distribution License v. 1.0 which is available at
@@ -247,6 +247,74 @@ public final class GitProtocolConstants {
 	 */
 	public static final String COMMAND_FETCH = "fetch"; //$NON-NLS-1$
 
+	/**
+	 * HTTP header to set by clients to request a specific git protocol version
+	 * in the HTTP transport.
+	 *
+	 * @since 5.11
+	 */
+	public static final String PROTOCOL_HEADER = "Git-Protocol"; //$NON-NLS-1$
+
+	/**
+	 * Environment variable to set by clients to request a specific git protocol
+	 * in the file:// and ssh:// transports.
+	 *
+	 * @since 5.11
+	 */
+	public static final String PROTOCOL_ENVIRONMENT_VARIABLE = "GIT_PROTOCOL"; //$NON-NLS-1$
+
+	/**
+	 * Protocol V2 ref advertisement attribute containing the peeled object id
+	 * for annotated tags.
+	 *
+	 * @since 5.11
+	 */
+	public static final String REF_ATTR_PEELED = "peeled:"; //$NON-NLS-1$
+
+	/**
+	 * Protocol V2 ref advertisement attribute containing the name of the ref
+	 * for symbolic refs.
+	 *
+	 * @since 5.11
+	 */
+	public static final String REF_ATTR_SYMREF_TARGET = "symref-target:"; //$NON-NLS-1$
+
+	/**
+	 * Protocol V2 acknowledgments section header.
+	 *
+	 * @since 5.11
+	 */
+	public static final String SECTION_ACKNOWLEDGMENTS = "acknowledgments"; //$NON-NLS-1$
+
+	/**
+	 * Protocol V2 packfile section header.
+	 *
+	 * @since 5.11
+	 */
+	public static final String SECTION_PACKFILE = "packfile"; //$NON-NLS-1$
+
+	/**
+	 * Protocol announcement for protocol version 1. This is the same as V0,
+	 * except for this initial line.
+	 *
+	 * @since 5.11
+	 */
+	public static final String VERSION_1 = "version 1"; //$NON-NLS-1$
+
+	/**
+	 * Protocol announcement for protocol version 2.
+	 *
+	 * @since 5.11
+	 */
+	public static final String VERSION_2 = "version 2"; //$NON-NLS-1$
+
+	/**
+	 * Protocol request for protocol version 2.
+	 *
+	 * @since 5.11
+	 */
+	public static final String VERSION_2_REQUEST = "version=2"; //$NON-NLS-1$
+
 	enum MultiAck {
 		OFF, CONTINUE, DETAILED;
 	}
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/transport/PacketLineIn.java b/org.eclipse.jgit/src/org/eclipse/jgit/transport/PacketLineIn.java
index 350311ecc8..68c5b348ad 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/transport/PacketLineIn.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/transport/PacketLineIn.java
@@ -103,6 +103,38 @@ public class PacketLineIn {
 		this.limit = limit;
 	}
 
+	/**
+	 * Parses a ACK/NAK line in protocol V2.
+	 *
+	 * @param line
+	 *            to parse
+	 * @param returnedId
+	 *            in case of {@link AckNackResult#ACK_COMMON ACK_COMMON}
+	 * @return one of {@link AckNackResult#NAK NAK},
+	 *         {@link AckNackResult#ACK_COMMON ACK_COMMON}, or
+	 *         {@link AckNackResult#ACK_READY ACK_READY}
+	 * @throws IOException
+	 *             on protocol or transport errors
+	 */
+	static AckNackResult parseACKv2(String line, MutableObjectId returnedId)
+			throws IOException {
+		if ("NAK".equals(line)) { //$NON-NLS-1$
+			return AckNackResult.NAK;
+		}
+		if (line.startsWith("ACK ") && line.length() == 44) { //$NON-NLS-1$
+			returnedId.fromString(line.substring(4, 44));
+			return AckNackResult.ACK_COMMON;
+		}
+		if ("ready".equals(line)) { //$NON-NLS-1$
+			return AckNackResult.ACK_READY;
+		}
+		if (line.startsWith("ERR ")) { //$NON-NLS-1$
+			throw new PackProtocolException(line.substring(4));
+		}
+		throw new PackProtocolException(
+				MessageFormat.format(JGitText.get().expectedACKNAKGot, line));
+	}
+
 	AckNackResult readACK(MutableObjectId returnedId) throws IOException {
 		final String line = readString();
 		if (line.length() == 0)
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/transport/PacketLineOut.java b/org.eclipse.jgit/src/org/eclipse/jgit/transport/PacketLineOut.java
index 6fc2042e1f..9fe3f0defc 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/transport/PacketLineOut.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/transport/PacketLineOut.java
@@ -1,7 +1,7 @@
 /*
- * Copyright (C) 2008-2010, Google Inc.
- * Copyright (C) 2008-2009, Robin Rosenberg <robin.rosenberg@dewire.com>
- * Copyright (C) 2008, Shawn O. Pearce <spearce@spearce.org> and others
+ * Copyright (C) 2008, 2010 Google Inc.
+ * Copyright (C) 2008, 2009 Robin Rosenberg <robin.rosenberg@dewire.com>
+ * Copyright (C) 2008, 2020 Shawn O. Pearce <spearce@spearce.org> and others
  *
  * This program and the accompanying materials are made available under the
  * terms of the Eclipse Distribution License v. 1.0 which is available at
@@ -140,8 +140,14 @@ public class PacketLineOut {
 		}
 		out.write(buf, pos, len);
 		if (log.isDebugEnabled()) {
-			String s = RawParseUtils.decode(UTF_8, buf, pos, len);
-			log.debug("git> " + s); //$NON-NLS-1$
+			// Escape a trailing \n to avoid empty lines in the log.
+			if (len > 0 && buf[pos + len - 1] == '\n') {
+				log.debug(
+						"git> " + RawParseUtils.decode(UTF_8, buf, pos, len - 1) //$NON-NLS-1$
+								+ "\\n"); //$NON-NLS-1$
+			} else {
+				log.debug("git> " + RawParseUtils.decode(UTF_8, buf, pos, len)); //$NON-NLS-1$
+			}
 		}
 	}
 
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/transport/RefAdvertiser.java b/org.eclipse.jgit/src/org/eclipse/jgit/transport/RefAdvertiser.java
index 3adebba03c..c525e66848 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/transport/RefAdvertiser.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/transport/RefAdvertiser.java
@@ -1,5 +1,5 @@
 /*
- * Copyright (C) 2008-2010, Google Inc. and others
+ * Copyright (C) 2008, 2020 Google Inc. and others
  *
  * This program and the accompanying materials are made available under the
  * terms of the Eclipse Distribution License v. 1.0 which is available at
@@ -13,6 +13,8 @@ package org.eclipse.jgit.transport;
 import static java.nio.charset.StandardCharsets.UTF_8;
 import static org.eclipse.jgit.lib.Constants.OBJECT_ID_STRING_LENGTH;
 import static org.eclipse.jgit.transport.GitProtocolConstants.OPTION_SYMREF;
+import static org.eclipse.jgit.transport.GitProtocolConstants.REF_ATTR_PEELED;
+import static org.eclipse.jgit.transport.GitProtocolConstants.REF_ATTR_SYMREF_TARGET;
 
 import java.io.IOException;
 import java.nio.ByteBuffer;
@@ -287,7 +289,8 @@ public abstract class RefAdvertiser {
 
 			if (useProtocolV2) {
 				String symrefPart = symrefs.containsKey(ref.getName())
-						? (" symref-target:" + symrefs.get(ref.getName())) //$NON-NLS-1$
+						? (' ' + REF_ATTR_SYMREF_TARGET
+								+ symrefs.get(ref.getName()))
 						: ""; //$NON-NLS-1$
 				String peelPart = ""; //$NON-NLS-1$
 				if (derefTags) {
@@ -296,7 +299,8 @@ public abstract class RefAdvertiser {
 					}
 					ObjectId peeledObjectId = ref.getPeeledObjectId();
 					if (peeledObjectId != null) {
-						peelPart = " peeled:" + peeledObjectId.getName(); //$NON-NLS-1$
+						peelPart = ' ' + REF_ATTR_PEELED
+								+ peeledObjectId.getName();
 					}
 				}
 				writeOne(objectId.getName() + " " + ref.getName() + symrefPart //$NON-NLS-1$
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/transport/RemoteSession2.java b/org.eclipse.jgit/src/org/eclipse/jgit/transport/RemoteSession2.java
new file mode 100644
index 0000000000..23f670ae25
--- /dev/null
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/transport/RemoteSession2.java
@@ -0,0 +1,45 @@
+/*
+ * Copyright (C) 2020, Thomas Wolf <thomas.wolf@paranor.ch> and others
+ *
+ * This program and the accompanying materials are made available under the
+ * terms of the Eclipse Distribution License v. 1.0 which is available at
+ * https://www.eclipse.org/org/documents/edl-v10.php.
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ */
+
+package org.eclipse.jgit.transport;
+
+import java.io.IOException;
+import java.util.Map;
+
+/**
+ * A {@link RemoteSession} that supports passing environment variables to
+ * commands.
+ *
+ * @since 5.11
+ */
+public interface RemoteSession2 extends RemoteSession {
+
+	/**
+	 * Creates a new remote {@link Process} to execute the given command. The
+	 * returned process's streams exist and are connected, and execution of the
+	 * process is already started.
+	 *
+	 * @param commandName
+	 *            command to execute
+	 * @param environment
+	 *            environment variables to pass on
+	 * @param timeout
+	 *            timeout value, in seconds, for creating the remote process
+	 * @return a new remote process, already started
+	 * @throws java.io.IOException
+	 *             may be thrown in several cases. For example, on problems
+	 *             opening input or output streams or on problems connecting or
+	 *             communicating with the remote host. For the latter two cases,
+	 *             a TransportException may be thrown (a subclass of
+	 *             java.io.IOException).
+	 */
+	Process exec(String commandName, Map<String, String> environment,
+			int timeout) throws IOException;
+}
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/transport/TransferConfig.java b/org.eclipse.jgit/src/org/eclipse/jgit/transport/TransferConfig.java
index 0b38159c09..83ffd4123a 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/transport/TransferConfig.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/transport/TransferConfig.java
@@ -1,5 +1,5 @@
 /*
- * Copyright (C) 2008-2009, Google Inc. and others
+ * Copyright (C) 2008, 2020 Google Inc. and others
  *
  * This program and the accompanying materials are made available under the
  * terms of the Eclipse Distribution License v. 1.0 which is available at
@@ -101,6 +101,9 @@ public class TransferConfig {
 					return v;
 				}
 			}
+			if ("1".equals(name)) { //$NON-NLS-1$
+				return V0;
+			}
 			return null;
 		}
 	}
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/transport/Transport.java b/org.eclipse.jgit/src/org/eclipse/jgit/transport/Transport.java
index 2ddd0a6128..1c998f4e8c 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/transport/Transport.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/transport/Transport.java
@@ -1,8 +1,8 @@
 /*
- * Copyright (C) 2008-2009, Google Inc.
+ * Copyright (C) 2008, 2009 Google Inc.
  * Copyright (C) 2008, Marek Zawirski <marek.zawirski@gmail.com>
  * Copyright (C) 2008, Robin Rosenberg <robin.rosenberg@dewire.com>
- * Copyright (C) 2008, Shawn O. Pearce <spearce@spearce.org> and others
+ * Copyright (C) 2008, 2020 Shawn O. Pearce <spearce@spearce.org> and others
  *
  * This program and the accompanying materials are made available under the
  * terms of the Eclipse Distribution License v. 1.0 which is available at
@@ -39,6 +39,7 @@ import java.util.Vector;
 import java.util.concurrent.CopyOnWriteArrayList;
 
 import org.eclipse.jgit.annotations.NonNull;
+import org.eclipse.jgit.annotations.Nullable;
 import org.eclipse.jgit.api.errors.AbortedByHookException;
 import org.eclipse.jgit.errors.NotSupportedException;
 import org.eclipse.jgit.errors.TransportException;
@@ -774,6 +775,10 @@ public abstract class Transport implements AutoCloseable {
 	private PrintStream hookOutRedirect;
 
 	private PrePushHook prePush;
+
+	@Nullable
+	TransferConfig.ProtocolVersion protocol;
+
 	/**
 	 * Create a new transport instance.
 	 *
@@ -789,6 +794,7 @@ public abstract class Transport implements AutoCloseable {
 		final TransferConfig tc = local.getConfig().get(TransferConfig.KEY);
 		this.local = local;
 		this.uri = uri;
+		this.protocol = tc.protocolVersion;
 		this.objectChecker = tc.newObjectChecker();
 		this.credentialsProvider = CredentialsProvider.getDefault();
 		prePush = Hooks.prePush(local, hookOutRedirect);
@@ -1453,6 +1459,43 @@ public abstract class Transport implements AutoCloseable {
 			TransportException;
 
 	/**
+	 * Begins a new connection for fetching from the remote repository.
+	 * <p>
+	 * If the transport has no local repository, the fetch connection can only
+	 * be used for reading remote refs.
+	 * </p>
+	 * <p>
+	 * If the server supports git protocol V2, the {@link RefSpec}s and the
+	 * additional patterns, if any, are used to restrict the server's ref
+	 * advertisement to matching refs only.
+	 * </p>
+	 * <p>
+	 * Transports that want to support git protocol V2 <em>must</em> override
+	 * this; the default implementation ignores its arguments and calls
+	 * {@link #openFetch()}.
+	 * </p>
+	 *
+	 * @param refSpecs
+	 *            that will be fetched via
+	 *            {@link FetchConnection#fetch(ProgressMonitor, Collection, java.util.Set, OutputStream)} later
+	 * @param additionalPatterns
+	 *            that will be set as ref prefixes if the server supports git
+	 *            protocol V2; {@code null} values are ignored
+	 *
+	 * @return a fresh connection to fetch from the remote repository.
+	 * @throws org.eclipse.jgit.errors.NotSupportedException
+	 *             the implementation does not support fetching.
+	 * @throws org.eclipse.jgit.errors.TransportException
+	 *             the remote connection could not be established.
+	 * @since 5.11
+	 */
+	public FetchConnection openFetch(Collection<RefSpec> refSpecs,
+			String... additionalPatterns)
+			throws NotSupportedException, TransportException {
+		return openFetch();
+	}
+
+	/**
 	 * Begins a new connection for pushing into the remote repository.
 	 *
 	 * @return a fresh connection to push into the remote repository.
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/transport/TransportGitAnon.java b/org.eclipse.jgit/src/org/eclipse/jgit/transport/TransportGitAnon.java
index 820ec1a67a..fa4392d5aa 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/transport/TransportGitAnon.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/transport/TransportGitAnon.java
@@ -1,7 +1,7 @@
 /*
  * Copyright (C) 2008, Marek Zawirski <marek.zawirski@gmail.com>
  * Copyright (C) 2008, Robin Rosenberg <robin.rosenberg@dewire.com>
- * Copyright (C) 2008, Shawn O. Pearce <spearce@spearce.org> and others
+ * Copyright (C) 2008, 2020 Shawn O. Pearce <spearce@spearce.org> and others
  *
  * This program and the accompanying materials are made available under the
  * terms of the Eclipse Distribution License v. 1.0 which is available at
@@ -22,6 +22,7 @@ import java.net.InetAddress;
 import java.net.InetSocketAddress;
 import java.net.Socket;
 import java.net.UnknownHostException;
+import java.util.Collection;
 import java.util.Collections;
 import java.util.EnumSet;
 import java.util.Set;
@@ -94,6 +95,13 @@ class TransportGitAnon extends TcpTransport implements PackTransport {
 		return new TcpFetchConnection();
 	}
 
+	@Override
+	public FetchConnection openFetch(Collection<RefSpec> refSpecs,
+			String... additionalPatterns)
+			throws NotSupportedException, TransportException {
+		return new TcpFetchConnection(refSpecs, additionalPatterns);
+	}
+
 	/** {@inheritDoc} */
 	@Override
 	public PushConnection openPush() throws TransportException {
@@ -130,7 +138,8 @@ class TransportGitAnon extends TcpTransport implements PackTransport {
 		return s;
 	}
 
-	void service(String name, PacketLineOut pckOut)
+	void service(String name, PacketLineOut pckOut,
+			TransferConfig.ProtocolVersion gitProtocol)
 			throws IOException {
 		final StringBuilder cmd = new StringBuilder();
 		cmd.append(name);
@@ -144,6 +153,11 @@ class TransportGitAnon extends TcpTransport implements PackTransport {
 			cmd.append(uri.getPort());
 		}
 		cmd.append('\0');
+		if (TransferConfig.ProtocolVersion.V2.equals(gitProtocol)) {
+			cmd.append('\0');
+			cmd.append(GitProtocolConstants.VERSION_2_REQUEST);
+			cmd.append('\0');
+		}
 		pckOut.writeString(cmd.toString());
 		pckOut.flush();
 	}
@@ -152,6 +166,11 @@ class TransportGitAnon extends TcpTransport implements PackTransport {
 		private Socket sock;
 
 		TcpFetchConnection() throws TransportException {
+			this(Collections.emptyList());
+		}
+
+		TcpFetchConnection(Collection<RefSpec> refSpecs,
+				String... additionalPatterns) throws TransportException {
 			super(TransportGitAnon.this);
 			sock = openConnection();
 			try {
@@ -162,13 +181,19 @@ class TransportGitAnon extends TcpTransport implements PackTransport {
 				sOut = new BufferedOutputStream(sOut);
 
 				init(sIn, sOut);
-				service("git-upload-pack", pckOut); //$NON-NLS-1$
+				TransferConfig.ProtocolVersion gitProtocol = protocol;
+				if (gitProtocol == null) {
+					gitProtocol = TransferConfig.ProtocolVersion.V2;
+				}
+				service("git-upload-pack", pckOut, gitProtocol); //$NON-NLS-1$
 			} catch (IOException err) {
 				close();
 				throw new TransportException(uri,
 						JGitText.get().remoteHungUpUnexpectedly, err);
 			}
-			readAdvertisedRefs();
+			if (!readAdvertisedRefs()) {
+				lsRefs(refSpecs, additionalPatterns);
+			}
 		}
 
 		@Override
@@ -201,7 +226,7 @@ class TransportGitAnon extends TcpTransport implements PackTransport {
 				sOut = new BufferedOutputStream(sOut);
 
 				init(sIn, sOut);
-				service("git-receive-pack", pckOut); //$NON-NLS-1$
+				service("git-receive-pack", pckOut, null); //$NON-NLS-1$
 			} catch (IOException err) {
 				close();
 				throw new TransportException(uri,
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/transport/TransportGitSsh.java b/org.eclipse.jgit/src/org/eclipse/jgit/transport/TransportGitSsh.java
index b9cb2484d8..19ed4fbcc1 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/transport/TransportGitSsh.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/transport/TransportGitSsh.java
@@ -1,8 +1,8 @@
 /*
- * Copyright (C) 2008-2010, Google Inc.
+ * Copyright (C) 2008, 2010 Google Inc.
  * Copyright (C) 2008, Marek Zawirski <marek.zawirski@gmail.com>
  * Copyright (C) 2008, Robin Rosenberg <robin.rosenberg@dewire.com>
- * Copyright (C) 2008, Shawn O. Pearce <spearce@spearce.org> and others
+ * Copyright (C) 2008, 2020 Shawn O. Pearce <spearce@spearce.org> and others
  *
  * This program and the accompanying materials are made available under the
  * terms of the Eclipse Distribution License v. 1.0 which is available at
@@ -19,11 +19,13 @@ import java.io.InputStream;
 import java.text.MessageFormat;
 import java.util.ArrayList;
 import java.util.Arrays;
+import java.util.Collection;
 import java.util.Collections;
 import java.util.EnumSet;
 import java.util.LinkedHashSet;
 import java.util.List;
 import java.util.Locale;
+import java.util.Map;
 import java.util.Set;
 
 import org.eclipse.jgit.errors.NoRemoteRepositoryException;
@@ -144,6 +146,13 @@ public class TransportGitSsh extends SshTransport implements PackTransport {
 		return new SshFetchConnection();
 	}
 
+	@Override
+	public FetchConnection openFetch(Collection<RefSpec> refSpecs,
+			String... additionalPatterns)
+			throws NotSupportedException, TransportException {
+		return new SshFetchConnection(refSpecs, additionalPatterns);
+	}
+
 	/** {@inheritDoc} */
 	@Override
 	public PushConnection openPush() throws TransportException {
@@ -196,29 +205,38 @@ public class TransportGitSsh extends SshTransport implements PackTransport {
 		return SystemReader.getInstance().getenv("GIT_SSH") != null; //$NON-NLS-1$
 	}
 
-	private class ExtSession implements RemoteSession {
+	private class ExtSession implements RemoteSession2 {
+
 		@Override
 		public Process exec(String command, int timeout)
 				throws TransportException {
+			return exec(command, null, timeout);
+		}
+
+		@Override
+		public Process exec(String command, Map<String, String> environment,
+				int timeout) throws TransportException {
 			String ssh = SystemReader.getInstance().getenv("GIT_SSH"); //$NON-NLS-1$
 			boolean putty = ssh.toLowerCase(Locale.ROOT).contains("plink"); //$NON-NLS-1$
 
 			List<String> args = new ArrayList<>();
 			args.add(ssh);
-			if (putty
-					&& !ssh.toLowerCase(Locale.ROOT).contains("tortoiseplink")) //$NON-NLS-1$
+			if (putty && !ssh.toLowerCase(Locale.ROOT)
+					.contains("tortoiseplink")) {//$NON-NLS-1$
 				args.add("-batch"); //$NON-NLS-1$
+			}
 			if (0 < getURI().getPort()) {
 				args.add(putty ? "-P" : "-p"); //$NON-NLS-1$ //$NON-NLS-2$
 				args.add(String.valueOf(getURI().getPort()));
 			}
-			if (getURI().getUser() != null)
+			if (getURI().getUser() != null) {
 				args.add(getURI().getUser() + "@" + getURI().getHost()); //$NON-NLS-1$
-			else
+			} else {
 				args.add(getURI().getHost());
+			}
 			args.add(command);
 
-			ProcessBuilder pb = createProcess(args);
+			ProcessBuilder pb = createProcess(args, environment);
 			try {
 				return pb.start();
 			} catch (IOException err) {
@@ -226,9 +244,13 @@ public class TransportGitSsh extends SshTransport implements PackTransport {
 			}
 		}
 
-		private ProcessBuilder createProcess(List<String> args) {
+		private ProcessBuilder createProcess(List<String> args,
+				Map<String, String> environment) {
 			ProcessBuilder pb = new ProcessBuilder();
 			pb.command(args);
+			if (environment != null) {
+				pb.environment().putAll(environment);
+			}
 			File directory = local != null ? local.getDirectory() : null;
 			if (directory != null) {
 				pb.environment().put(Constants.GIT_DIR_KEY,
@@ -249,10 +271,31 @@ public class TransportGitSsh extends SshTransport implements PackTransport {
 		private StreamCopyThread errorThread;
 
 		SshFetchConnection() throws TransportException {
+			this(Collections.emptyList());
+		}
+
+		SshFetchConnection(Collection<RefSpec> refSpecs,
+				String... additionalPatterns) throws TransportException {
 			super(TransportGitSsh.this);
 			try {
-				process = getSession().exec(commandFor(getOptionUploadPack()),
-						getTimeout());
+				RemoteSession session = getSession();
+				TransferConfig.ProtocolVersion gitProtocol = protocol;
+				if (gitProtocol == null) {
+					gitProtocol = TransferConfig.ProtocolVersion.V2;
+				}
+				if (session instanceof RemoteSession2
+						&& TransferConfig.ProtocolVersion.V2
+								.equals(gitProtocol)) {
+					process = ((RemoteSession2) session).exec(
+							commandFor(getOptionUploadPack()), Collections
+									.singletonMap(
+											GitProtocolConstants.PROTOCOL_ENVIRONMENT_VARIABLE,
+											GitProtocolConstants.VERSION_2_REQUEST),
+							getTimeout());
+				} else {
+					process = session.exec(commandFor(getOptionUploadPack()),
+							getTimeout());
+				}
 				final MessageWriter msg = new MessageWriter();
 				setMessageWriter(msg);
 
@@ -272,7 +315,9 @@ public class TransportGitSsh extends SshTransport implements PackTransport {
 			}
 
 			try {
-				readAdvertisedRefs();
+				if (!readAdvertisedRefs()) {
+					lsRefs(refSpecs, additionalPatterns);
+				}
 			} catch (NoRemoteRepositoryException notFound) {
 				final String msgs = getMessages();
 				checkExecFailure(process.exitValue(), getOptionUploadPack(),
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/transport/TransportHttp.java b/org.eclipse.jgit/src/org/eclipse/jgit/transport/TransportHttp.java
index 6768387e65..9d40f02466 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/transport/TransportHttp.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/transport/TransportHttp.java
@@ -1,8 +1,8 @@
 /*
- * Copyright (C) 2008-2010, Google Inc.
+ * Copyright (C) 2008, 2010 Google Inc.
  * Copyright (C) 2008, Shawn O. Pearce <spearce@spearce.org>
  * Copyright (C) 2013, Matthias Sohn <matthias.sohn@sap.com>
- * Copyright (C) 2017, Thomas Wolf <thomas.wolf@paranor.ch> and others
+ * Copyright (C) 2017, 2020 Thomas Wolf <thomas.wolf@paranor.ch> and others
  *
  * This program and the accompanying materials are made available under the
  * terms of the Eclipse Distribution License v. 1.0 which is available at
@@ -33,8 +33,8 @@ import static org.eclipse.jgit.util.HttpSupport.HDR_WWW_AUTHENTICATE;
 import static org.eclipse.jgit.util.HttpSupport.METHOD_GET;
 import static org.eclipse.jgit.util.HttpSupport.METHOD_POST;
 
+import java.io.BufferedInputStream;
 import java.io.BufferedReader;
-import java.io.ByteArrayInputStream;
 import java.io.FileNotFoundException;
 import java.io.IOException;
 import java.io.InputStream;
@@ -132,6 +132,9 @@ public class TransportHttp extends HttpTransport implements WalkTransport,
 
 	private static final String SVC_RECEIVE_PACK = "git-receive-pack"; //$NON-NLS-1$
 
+	private static final byte[] VERSION = "version" //$NON-NLS-1$
+			.getBytes(StandardCharsets.US_ASCII);
+
 	/**
 	 * Accept-Encoding header in the HTTP request
 	 * (https://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html).
@@ -339,11 +342,15 @@ public class TransportHttp extends HttpTransport implements WalkTransport,
 
 	@SuppressWarnings("resource") // Closed by caller
 	private FetchConnection getConnection(HttpConnection c, InputStream in,
-			String service) throws IOException {
+			String service, Collection<RefSpec> refSpecs,
+			String... additionalPatterns) throws IOException {
 		BaseConnection f;
 		if (isSmartHttp(c, service)) {
-			readSmartHeaders(in, service);
-			f = new SmartHttpFetchConnection(in);
+			InputStream withMark = in.markSupported() ? in
+					: new BufferedInputStream(in);
+			readSmartHeaders(withMark, service);
+			f = new SmartHttpFetchConnection(withMark, refSpecs,
+					additionalPatterns);
 		} else {
 			// Assume this server doesn't support smart HTTP fetch
 			// and fall back on dumb object walking.
@@ -357,11 +364,23 @@ public class TransportHttp extends HttpTransport implements WalkTransport,
 	@Override
 	public FetchConnection openFetch() throws TransportException,
 			NotSupportedException {
+		return openFetch(Collections.emptyList());
+	}
+
+	@Override
+	public FetchConnection openFetch(Collection<RefSpec> refSpecs,
+			String... additionalPatterns)
+			throws NotSupportedException, TransportException {
 		final String service = SVC_UPLOAD_PACK;
 		try {
-			final HttpConnection c = connect(service);
+			TransferConfig.ProtocolVersion gitProtocol = protocol;
+			if (gitProtocol == null) {
+				gitProtocol = TransferConfig.ProtocolVersion.V2;
+			}
+			HttpConnection c = connect(service, gitProtocol);
 			try (InputStream in = openInputStream(c)) {
-				return getConnection(c, in, service);
+				return getConnection(c, in, service, refSpecs,
+						additionalPatterns);
 			}
 		} catch (NotSupportedException | TransportException err) {
 			throw err;
@@ -456,8 +475,9 @@ public class TransportHttp extends HttpTransport implements WalkTransport,
 
 	private PushConnection smartPush(String service, HttpConnection c,
 			InputStream in) throws IOException, TransportException {
-		readSmartHeaders(in, service);
-		SmartHttpPushConnection p = new SmartHttpPushConnection(in);
+		BufferedInputStream inBuf = new BufferedInputStream(in);
+		readSmartHeaders(inBuf, service);
+		SmartHttpPushConnection p = new SmartHttpPushConnection(inBuf);
 		p.setPeerUserAgent(c.getHeaderField(HttpSupport.HDR_SERVER));
 		return p;
 	}
@@ -494,6 +514,12 @@ public class TransportHttp extends HttpTransport implements WalkTransport,
 
 	private HttpConnection connect(String service)
 			throws TransportException, NotSupportedException {
+		return connect(service, null);
+	}
+
+	private HttpConnection connect(String service,
+			TransferConfig.ProtocolVersion protocolVersion)
+			throws TransportException, NotSupportedException {
 		URL u = getServiceURL(service);
 		int authAttempts = 1;
 		int redirects = 0;
@@ -507,6 +533,11 @@ public class TransportHttp extends HttpTransport implements WalkTransport,
 				} else {
 					conn.setRequestProperty(HDR_ACCEPT, "*/*"); //$NON-NLS-1$
 				}
+				if (TransferConfig.ProtocolVersion.V2.equals(protocolVersion)) {
+					conn.setRequestProperty(
+							GitProtocolConstants.PROTOCOL_HEADER,
+							GitProtocolConstants.VERSION_2_REQUEST);
+				}
 				final int status = HttpSupport.response(conn);
 				processResponseCookies(conn);
 				switch (status) {
@@ -1148,20 +1179,37 @@ public class TransportHttp extends HttpTransport implements WalkTransport,
 
 	private void readSmartHeaders(InputStream in, String service)
 			throws IOException {
-		// A smart reply will have a '#' after the first 4 bytes, but
-		// a dumb reply cannot contain a '#' until after byte 41. Do a
+		// A smart protocol V0 reply will have a '#' after the first 4 bytes,
+		// but a dumb reply cannot contain a '#' until after byte 41. Do a
 		// quick check to make sure its a smart reply before we parse
 		// as a pkt-line stream.
 		//
-		final byte[] magic = new byte[5];
+		// There appears to be a confusion about this in protocol V2. Github
+		// sends the # service line as a git (not http) header also when
+		// protocol V2 is used. Gitlab also does so. JGit's UploadPack doesn't,
+		// and thus Gerrit also does not.
+		final byte[] magic = new byte[14];
+		if (!in.markSupported()) {
+			throw new TransportException(uri,
+					JGitText.get().inputStreamMustSupportMark);
+		}
+		in.mark(14);
 		IO.readFully(in, magic, 0, magic.length);
+		// Did we get 000dversion 2 or similar? (Canonical is 000eversion 2\n,
+		// but JGit and thus Gerrit omits the \n.)
+		if (Arrays.equals(Arrays.copyOfRange(magic, 4, 11), VERSION)
+				&& magic[12] >= '1' && magic[12] <= '9') {
+			// It's a smart server doing version 1 or greater, but not sending
+			// the # service line header. Don't consume the version line.
+			in.reset();
+			return;
+		}
 		if (magic[4] != '#') {
 			throw new TransportException(uri, MessageFormat.format(
 					JGitText.get().expectedPktLineWithService, RawParseUtils.decode(magic)));
 		}
-
-		final PacketLineIn pckIn = new PacketLineIn(new UnionInputStream(
-				new ByteArrayInputStream(magic), in));
+		in.reset();
+		final PacketLineIn pckIn = new PacketLineIn(in);
 		final String exp = "# service=" + service; //$NON-NLS-1$
 		final String act = pckIn.readString();
 		if (!exp.equals(act)) {
@@ -1327,12 +1375,24 @@ public class TransportHttp extends HttpTransport implements WalkTransport,
 
 		SmartHttpFetchConnection(InputStream advertisement)
 				throws TransportException {
+			this(advertisement, Collections.emptyList());
+		}
+
+		SmartHttpFetchConnection(InputStream advertisement,
+				Collection<RefSpec> refSpecs, String... additionalPatterns)
+				throws TransportException {
 			super(TransportHttp.this);
 			statelessRPC = true;
 
 			init(advertisement, DisabledOutputStream.INSTANCE);
 			outNeedsEnd = false;
-			readAdvertisedRefs();
+			if (!readAdvertisedRefs()) {
+				// Must be protocol V2
+				LongPollService service = new LongPollService(SVC_UPLOAD_PACK,
+						getProtocolVersion());
+				init(service.getInputStream(), service.getOutputStream());
+				lsRefs(refSpecs, additionalPatterns);
+			}
 		}
 
 		@Override
@@ -1340,7 +1400,8 @@ public class TransportHttp extends HttpTransport implements WalkTransport,
 				final Collection<Ref> want, final Set<ObjectId> have,
 				final OutputStream outputStream) throws TransportException {
 			try {
-				svc = new MultiRequestService(SVC_UPLOAD_PACK);
+				svc = new MultiRequestService(SVC_UPLOAD_PACK,
+						getProtocolVersion());
 				init(svc.getInputStream(), svc.getOutputStream());
 				super.doFetch(monitor, want, have, outputStream);
 			} finally {
@@ -1369,7 +1430,8 @@ public class TransportHttp extends HttpTransport implements WalkTransport,
 		protected void doPush(final ProgressMonitor monitor,
 				final Map<String, RemoteRefUpdate> refUpdates,
 				OutputStream outputStream) throws TransportException {
-			final Service svc = new MultiRequestService(SVC_RECEIVE_PACK);
+			final Service svc = new MultiRequestService(SVC_RECEIVE_PACK,
+					getProtocolVersion());
 			init(svc.getInputStream(), svc.getOutputStream());
 			super.doPush(monitor, refUpdates, outputStream);
 		}
@@ -1389,10 +1451,14 @@ public class TransportHttp extends HttpTransport implements WalkTransport,
 
 		protected final HttpExecuteStream execute;
 
+		protected final TransferConfig.ProtocolVersion protocolVersion;
+
 		final UnionInputStream in;
 
-		Service(String serviceName) {
+		Service(String serviceName,
+				TransferConfig.ProtocolVersion protocolVersion) {
 			this.serviceName = serviceName;
+			this.protocolVersion = protocolVersion;
 			this.requestType = "application/x-" + serviceName + "-request"; //$NON-NLS-1$ //$NON-NLS-2$
 			this.responseType = "application/x-" + serviceName + "-result"; //$NON-NLS-1$ //$NON-NLS-2$
 
@@ -1408,6 +1474,10 @@ public class TransportHttp extends HttpTransport implements WalkTransport,
 			conn.setDoOutput(true);
 			conn.setRequestProperty(HDR_CONTENT_TYPE, requestType);
 			conn.setRequestProperty(HDR_ACCEPT, responseType);
+			if (TransferConfig.ProtocolVersion.V2.equals(protocolVersion)) {
+				conn.setRequestProperty(GitProtocolConstants.PROTOCOL_HEADER,
+						GitProtocolConstants.VERSION_2_REQUEST);
+			}
 		}
 
 		void sendRequest() throws IOException {
@@ -1663,8 +1733,9 @@ public class TransportHttp extends HttpTransport implements WalkTransport,
 	class MultiRequestService extends Service {
 		boolean finalRequest;
 
-		MultiRequestService(String serviceName) {
-			super(serviceName);
+		MultiRequestService(String serviceName,
+				TransferConfig.ProtocolVersion protocolVersion) {
+			super(serviceName, protocolVersion);
 		}
 
 		/** Keep opening send-receive pairs to the given URI. */
@@ -1701,11 +1772,10 @@ public class TransportHttp extends HttpTransport implements WalkTransport,
 
 	/** Service for maintaining a single long-poll connection. */
 	class LongPollService extends Service {
-		/**
-		 * @param serviceName
-		 */
-		LongPollService(String serviceName) {
-			super(serviceName);
+
+		LongPollService(String serviceName,
+				TransferConfig.ProtocolVersion protocolVersion) {
+			super(serviceName, protocolVersion);
 		}
 
 		/** Only open one send-receive request. */
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/transport/TransportLocal.java b/org.eclipse.jgit/src/org/eclipse/jgit/transport/TransportLocal.java
index 403f98d869..77d1419ea2 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/transport/TransportLocal.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/transport/TransportLocal.java
@@ -1,9 +1,9 @@
 /*
  * Copyright (C) 2007, Dave Watson <dwatson@mimvista.com>
- * Copyright (C) 2008-2010, Google Inc.
+ * Copyright (C) 2008, 2010 Google Inc.
  * Copyright (C) 2008, Marek Zawirski <marek.zawirski@gmail.com>
  * Copyright (C) 2008, Robin Rosenberg <robin.rosenberg@dewire.com>
- * Copyright (C) 2008, Shawn O. Pearce <spearce@spearce.org> and others
+ * Copyright (C) 2008, 2020 Shawn O. Pearce <spearce@spearce.org> and others
  *
  * This program and the accompanying materials are made available under the
  * terms of the Eclipse Distribution License v. 1.0 which is available at
@@ -20,6 +20,7 @@ import java.io.File;
 import java.io.IOException;
 import java.io.InputStream;
 import java.io.OutputStream;
+import java.util.Collection;
 import java.util.Collections;
 import java.util.Map;
 import java.util.Set;
@@ -153,11 +154,17 @@ class TransportLocal extends Transport implements PackTransport {
 	/** {@inheritDoc} */
 	@Override
 	public FetchConnection openFetch() throws TransportException {
+		return openFetch(Collections.emptyList());
+	}
+
+	@Override
+	public FetchConnection openFetch(Collection<RefSpec> refSpecs,
+			String... additionalPatterns) throws TransportException {
 		final String up = getOptionUploadPack();
 		if (!"git-upload-pack".equals(up) //$NON-NLS-1$
-				&& !"git upload-pack".equals(up)) //$NON-NLS-1$
-			return new ForkLocalFetchConnection();
-
+				&& !"git upload-pack".equals(up)) {//$NON-NLS-1$
+			return new ForkLocalFetchConnection(refSpecs, additionalPatterns);
+		}
 		UploadPackFactory<Void> upf = (Void req,
 				Repository db) -> createUploadPack(db);
 		return new InternalFetchConnection<>(this, upf, null, openRepo());
@@ -193,6 +200,23 @@ class TransportLocal extends Transport implements PackTransport {
 	 */
 	protected Process spawn(String cmd)
 			throws TransportException {
+		return spawn(cmd, null);
+	}
+
+	/**
+	 * Spawn process
+	 *
+	 * @param cmd
+	 *            command
+	 * @param protocolVersion
+	 *            to use
+	 * @return a {@link java.lang.Process} object.
+	 * @throws org.eclipse.jgit.errors.TransportException
+	 *             if any.
+	 */
+	private Process spawn(String cmd,
+			TransferConfig.ProtocolVersion protocolVersion)
+			throws TransportException {
 		try {
 			String[] args = { "." }; //$NON-NLS-1$
 			ProcessBuilder proc = local.getFS().runInShell(cmd, args);
@@ -208,7 +232,10 @@ class TransportLocal extends Transport implements PackTransport {
 			env.remove("GIT_GRAFT_FILE"); //$NON-NLS-1$
 			env.remove("GIT_INDEX_FILE"); //$NON-NLS-1$
 			env.remove("GIT_NO_REPLACE_OBJECTS"); //$NON-NLS-1$
-
+			if (TransferConfig.ProtocolVersion.V2.equals(protocolVersion)) {
+				env.put(GitProtocolConstants.PROTOCOL_ENVIRONMENT_VARIABLE,
+						GitProtocolConstants.VERSION_2_REQUEST);
+			}
 			return proc.start();
 		} catch (IOException err) {
 			throw new TransportException(uri, err.getMessage(), err);
@@ -221,12 +248,21 @@ class TransportLocal extends Transport implements PackTransport {
 		private Thread errorReaderThread;
 
 		ForkLocalFetchConnection() throws TransportException {
+			this(Collections.emptyList());
+		}
+
+		ForkLocalFetchConnection(Collection<RefSpec> refSpecs,
+				String... additionalPatterns) throws TransportException {
 			super(TransportLocal.this);
 
 			final MessageWriter msg = new MessageWriter();
 			setMessageWriter(msg);
 
-			uploadPack = spawn(getOptionUploadPack());
+			TransferConfig.ProtocolVersion gitProtocol = protocol;
+			if (gitProtocol == null) {
+				gitProtocol = TransferConfig.ProtocolVersion.V2;
+			}
+			uploadPack = spawn(getOptionUploadPack(), gitProtocol);
 
 			final InputStream upErr = uploadPack.getErrorStream();
 			errorReaderThread = new StreamCopyThread(upErr, msg.getRawStream());
@@ -239,7 +275,9 @@ class TransportLocal extends Transport implements PackTransport {
 			upOut = new BufferedOutputStream(upOut);
 
 			init(upIn, upOut);
-			readAdvertisedRefs();
+			if (!readAdvertisedRefs()) {
+				lsRefs(refSpecs, additionalPatterns);
+			}
 		}
 
 		@Override
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/transport/UploadPack.java b/org.eclipse.jgit/src/org/eclipse/jgit/transport/UploadPack.java
index 1242ef1b4a..afe4d3fd9f 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/transport/UploadPack.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/transport/UploadPack.java
@@ -1,5 +1,5 @@
 /*
- * Copyright (C) 2008-2010, Google Inc. and others
+ * Copyright (C) 2008, 2020 Google Inc. and others
  *
  * This program and the accompanying materials are made available under the
  * terms of the Eclipse Distribution License v. 1.0 which is available at
@@ -33,6 +33,7 @@ import static org.eclipse.jgit.transport.GitProtocolConstants.OPTION_SIDEBAND_AL
 import static org.eclipse.jgit.transport.GitProtocolConstants.OPTION_SIDE_BAND;
 import static org.eclipse.jgit.transport.GitProtocolConstants.OPTION_SIDE_BAND_64K;
 import static org.eclipse.jgit.transport.GitProtocolConstants.OPTION_THIN_PACK;
+import static org.eclipse.jgit.transport.GitProtocolConstants.VERSION_2_REQUEST;
 import static org.eclipse.jgit.util.RefMap.toRefMap;
 
 import java.io.ByteArrayOutputStream;
@@ -709,7 +710,7 @@ public class UploadPack {
 	 * @since 5.0
 	 */
 	public void setExtraParameters(Collection<String> params) {
-		this.clientRequestedV2 = params.contains("version=2"); //$NON-NLS-1$
+		this.clientRequestedV2 = params.contains(VERSION_2_REQUEST);
 	}
 
 	/**
@@ -1194,7 +1195,8 @@ public class UploadPack {
 					new PacketLineOut(NullOutputStream.INSTANCE),
 					accumulator);
 		} else {
-			pckOut.writeString("acknowledgments\n"); //$NON-NLS-1$
+			pckOut.writeString(
+					GitProtocolConstants.SECTION_ACKNOWLEDGMENTS + '\n');
 			for (ObjectId id : req.getPeerHas()) {
 				if (walk.getObjectReader().has(id)) {
 					pckOut.writeString("ACK " + id.getName() + "\n"); //$NON-NLS-1$ //$NON-NLS-2$
@@ -1243,7 +1245,8 @@ public class UploadPack {
 			if (!pckOut.isUsingSideband()) {
 				// sendPack will write "packfile\n" for us if sideband-all is used.
 				// But sideband-all is not used, so we have to write it ourselves.
-				pckOut.writeString("packfile\n"); //$NON-NLS-1$
+				pckOut.writeString(
+						GitProtocolConstants.SECTION_PACKFILE + '\n');
 			}
 
 			accumulator.timeNegotiating = Duration
@@ -2327,7 +2330,8 @@ public class UploadPack {
 					// for us if provided a PackfileUriConfig. In this case, we
 					// are not providing a PackfileUriConfig, so we have to
 					// write this line ourselves.
-					pckOut.writeString("packfile\n"); //$NON-NLS-1$
+					pckOut.writeString(
+							GitProtocolConstants.SECTION_PACKFILE + '\n');
 				}
 			}
 			pw.writePack(pm, NullProgressMonitor.INSTANCE, packOut);