Support agent= capability in wire protocol

Since git-core ff5effd (v1.7.12.1) the native wire protocol transmits
the server and client implementation and version strings using
capability "agent=git/1.7.12.1" or similar.

Support this in JGit and hang the implementation data off UploadPack
and ReceivePack.  On HTTP transports default to the User-Agent HTTP
header until the client overrides this with the optional capability
string in the first line.

Extract the user agent string into a UserAgent class under transport
where it can be specified to a different value if the application's
build process has broken the Implementation-Version header in the
JGit package.

Change-Id: Icfc6524d84a787386d1786310b421b2f92ae9e65

org.eclipse.jgit.http.server/src/org/eclipse/jgit/http/server/ReceivePackServlet.java

@@ -76,6 +76,7 @@ import javax.servlet.http.HttpServletResponse;
 
 import org.eclipse.jgit.errors.UnpackException;
 import org.eclipse.jgit.lib.Repository;
+import org.eclipse.jgit.transport.InternalHttpServerGlue;
 import org.eclipse.jgit.transport.ReceivePack;
 import org.eclipse.jgit.transport.RefAdvertiser.PacketLineOutRefAdvertiser;
 import org.eclipse.jgit.transport.resolver.ReceivePackFactory;
@@ -100,6 +101,9 @@ class ReceivePackServlet extends HttpServlet {
 				throws IOException, ServiceNotEnabledException,
 				ServiceNotAuthorizedException {
 			ReceivePack rp = receivePackFactory.create(req, db);
+			InternalHttpServerGlue.setPeerUserAgent(
+					rp,
+					req.getHeader(HDR_USER_AGENT));
 			req.setAttribute(ATTRIBUTE_HANDLER, rp);
 		}
 

org.eclipse.jgit.http.server/src/org/eclipse/jgit/http/server/UploadPackServlet.java

@@ -74,6 +74,7 @@ import javax.servlet.http.HttpServletRequest;
 import javax.servlet.http.HttpServletResponse;
 
 import org.eclipse.jgit.lib.Repository;
+import org.eclipse.jgit.transport.InternalHttpServerGlue;
 import org.eclipse.jgit.transport.RefAdvertiser.PacketLineOutRefAdvertiser;
 import org.eclipse.jgit.transport.UploadPack;
 import org.eclipse.jgit.transport.UploadPackInternalServerErrorException;
@@ -100,6 +101,9 @@ class UploadPackServlet extends HttpServlet {
 				throws IOException, ServiceNotEnabledException,
 				ServiceNotAuthorizedException {
 			UploadPack up = uploadPackFactory.create(req, db);
+			InternalHttpServerGlue.setPeerUserAgent(
+					up,
+					req.getHeader(HDR_USER_AGENT));
 			req.setAttribute(ATTRIBUTE_HANDLER, up);
 		}
 

org.eclipse.jgit/src/org/eclipse/jgit/transport/BaseConnection.java

@@ -65,6 +65,8 @@ import org.eclipse.jgit.lib.Ref;
 public abstract class BaseConnection implements Connection {
 	private Map<String, Ref> advertisedRefs = Collections.emptyMap();
 
+	private String peerUserAgent;
+
 	private boolean startedOperation;
 
 	private Writer messageWriter;
@@ -85,6 +87,28 @@ public abstract class BaseConnection implements Connection {
 		return messageWriter != null ? messageWriter.toString() : ""; //$NON-NLS-1$
 	}
 
+	/**
+	 * User agent advertised by the remote server.
+	 *
+	 * @return agent (version of Git) running on the remote server. Null if the
+	 *         server does not advertise this version.
+	 * @since 4.0
+	 */
+	public String getPeerUserAgent() {
+		return peerUserAgent;
+	}
+
+	/**
+	 * Remember the remote peer's agent.
+	 *
+	 * @param agent
+	 *            remote peer agent string.
+	 * @since 4.0
+	 */
+	protected void setPeerUserAgent(String agent) {
+		peerUserAgent = agent;
+	}
+
 	public abstract void close();
 
 	/**

org.eclipse.jgit/src/org/eclipse/jgit/transport/BasePackConnection.java

@@ -46,6 +46,8 @@
 
 package org.eclipse.jgit.transport;
 
+import static org.eclipse.jgit.transport.GitProtocolConstants.OPTION_AGENT;
+
 import java.io.EOFException;
 import java.io.IOException;
 import java.io.InputStream;
@@ -275,6 +277,18 @@ abstract class BasePackConnection extends BaseConnection {
 		return true;
 	}
 
+	protected void addUserAgentCapability(StringBuilder b) {
+		String a = UserAgent.get();
+		if (a != null && UserAgent.hasAgent(remoteCapablities)) {
+			b.append(' ').append(OPTION_AGENT).append('=').append(a);
+		}
+	}
+
+	@Override
+	public String getPeerUserAgent() {
+		return UserAgent.getAgent(remoteCapablities, super.getPeerUserAgent());
+	}
+
 	private PackProtocolException duplicateAdvertisement(final String name) {
 		return new PackProtocolException(uri, MessageFormat.format(JGitText.get().duplicateAdvertisementsOf, name));
 	}

org.eclipse.jgit/src/org/eclipse/jgit/transport/BasePackFetchConnection.java

@@ -521,6 +521,7 @@ public abstract class BasePackFetchConnection extends BasePackConnection
 					OPTION_MULTI_ACK_DETAILED));
 		}
 
+		addUserAgentCapability(line);
 		return line.toString();
 	}
 

org.eclipse.jgit/src/org/eclipse/jgit/transport/BasePackPushConnection.java

@@ -268,6 +268,7 @@ public abstract class BasePackPushConnection extends BasePackConnection implemen
 					outputStream);
 			pckIn = new PacketLineIn(in);
 		}
+		addUserAgentCapability(line);
 
 		if (line.length() > 0)
 			line.setCharAt(0, '\0');

org.eclipse.jgit/src/org/eclipse/jgit/transport/BaseReceivePack.java

@@ -48,6 +48,7 @@ import static org.eclipse.jgit.transport.GitProtocolConstants.CAPABILITY_DELETE_
 import static org.eclipse.jgit.transport.GitProtocolConstants.CAPABILITY_OFS_DELTA;
 import static org.eclipse.jgit.transport.GitProtocolConstants.CAPABILITY_REPORT_STATUS;
 import static org.eclipse.jgit.transport.GitProtocolConstants.CAPABILITY_SIDE_BAND_64K;
+import static org.eclipse.jgit.transport.GitProtocolConstants.OPTION_AGENT;
 import static org.eclipse.jgit.transport.SideBandOutputStream.CH_DATA;
 import static org.eclipse.jgit.transport.SideBandOutputStream.CH_PROGRESS;
 import static org.eclipse.jgit.transport.SideBandOutputStream.MAX_BUF;
@@ -224,6 +225,7 @@ public abstract class BaseReceivePack {
 
 	/** Capabilities requested by the client. */
 	private Set<String> enabledCapabilities;
+	String userAgent;
 	private Set<ObjectId> clientShallowCommits;
 	private List<ReceiveCommand> commands;
 
@@ -738,6 +740,25 @@ public abstract class BaseReceivePack {
 		return enabledCapabilities.contains(CAPABILITY_SIDE_BAND_64K);
 	}
 
+	/**
+	 * Get the user agent of the client.
+	 * <p>
+	 * If the client is new enough to use {@code agent=} capability that value
+	 * will be returned. Older HTTP clients may also supply their version using
+	 * the HTTP {@code User-Agent} header. The capability overrides the HTTP
+	 * header if both are available.
+	 * <p>
+	 * When an HTTP request has been received this method returns the HTTP
+	 * {@code User-Agent} header value until capabilities have been parsed.
+	 *
+	 * @return user agent supplied by the client. Available only if the client
+	 *         is new enough to advertise its user agent.
+	 * @since 4.0
+	 */
+	public String getPeerUserAgent() {
+		return UserAgent.getAgent(enabledCapabilities, userAgent);
+	}
+
 	/** @return all of the command received by the current request. */
 	public List<ReceiveCommand> getAllCommands() {
 		return Collections.unmodifiableList(commands);
@@ -955,6 +976,7 @@ public abstract class BaseReceivePack {
 			adv.advertiseCapability(CAPABILITY_ATOMIC);
 		if (allowOfsDelta)
 			adv.advertiseCapability(CAPABILITY_OFS_DELTA);
+		adv.advertiseCapability(OPTION_AGENT, UserAgent.get());
 		adv.send(getAdvertisedOrDefaultRefs());
 		for (ObjectId obj : advertisedHaves)
 			adv.advertiseHave(obj);

org.eclipse.jgit/src/org/eclipse/jgit/transport/Connection.java

@@ -127,4 +127,13 @@ public interface Connection {
 	 *         remote produced no additional messages.
 	 */
 	public String getMessages();
+
+	/**
+	 * User agent advertised by the remote server.
+	 *
+	 * @return agent (version of Git) running on the remote server. Null if the
+	 *         server does not advertise this version.
+	 * @since 4.0
+	 */
+	public String getPeerUserAgent();
 }

org.eclipse.jgit/src/org/eclipse/jgit/transport/FetchProcess.java

@@ -136,6 +136,7 @@ class FetchProcess {
 		conn = transport.openFetch();
 		try {
 			result.setAdvertisedRefs(transport.getURI(), conn.getRefsMap());
+			result.peerUserAgent = conn.getPeerUserAgent();
 			final Set<Ref> matched = new HashSet<Ref>();
 			for (final RefSpec spec : toFetch) {
 				if (spec.getSource() == null)

org.eclipse.jgit/src/org/eclipse/jgit/transport/GitProtocolConstants.java

@@ -186,6 +186,13 @@ public class GitProtocolConstants {
 	 */
 	public static final String CAPABILITY_PUSH_CERT = "push-cert"; //$NON-NLS-1$
 
+	/**
+	 * Implementation name and version of the client or server.
+	 *
+	 * @since 4.0
+	 */
+	public static final String OPTION_AGENT = "agent"; //$NON-NLS-1$
+
 	static enum MultiAck {
 		OFF, CONTINUE, DETAILED;
 	}

org.eclipse.jgit/src/org/eclipse/jgit/transport/InternalHttpServerGlue.java

@@ -0,0 +1,80 @@
+/*
+ * Copyright (C) 2015, Google Inc.
+ * and other copyright owners as documented in the project's IP log.
+ *
+ * This program and the accompanying materials are made available
+ * under the terms of the Eclipse Distribution License v1.0 which
+ * accompanies this distribution, is reproduced below, and is
+ * available at http://www.eclipse.org/org/documents/edl-v10.php
+ *
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or
+ * without modification, are permitted provided that the following
+ * conditions are met:
+ *
+ * - Redistributions of source code must retain the above copyright
+ *   notice, this list of conditions and the following disclaimer.
+ *
+ * - Redistributions in binary form must reproduce the above
+ *   copyright notice, this list of conditions and the following
+ *   disclaimer in the documentation and/or other materials provided
+ *   with the distribution.
+ *
+ * - Neither the name of the Eclipse Foundation, Inc. nor the
+ *   names of its contributors may be used to endorse or promote
+ *   products derived from this software without specific prior
+ *   written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND
+ * CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES,
+ * INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
+ * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR
+ * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
+ * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
+ * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
+ * STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
+ * ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+package org.eclipse.jgit.transport;
+
+/**
+ * Internal API to to assist {@code org.eclipse.jgit.http.server}.
+ * <p>
+ * <b>Do not call.</b>
+ *
+ * @since 4.0
+ */
+public class InternalHttpServerGlue {
+	/**
+	 * Apply a default user agent for a request.
+	 *
+	 * @param up
+	 *            current UploadPack instance.
+	 * @param agent
+	 *            user agent string from the HTTP headers.
+	 */
+	public static void setPeerUserAgent(UploadPack up, String agent) {
+		up.userAgent = agent;
+	}
+
+	/**
+	 * Apply a default user agent for a request.
+	 *
+	 * @param rp
+	 *            current ReceivePack instance.
+	 * @param agent
+	 *            user agent string from the HTTP headers.
+	 */
+	public static void setPeerUserAgent(ReceivePack rp, String agent) {
+		rp.userAgent = agent;
+	}
+
+	private InternalHttpServerGlue() {
+	}
+}

org.eclipse.jgit/src/org/eclipse/jgit/transport/OperationResult.java

@@ -68,6 +68,8 @@ public abstract class OperationResult {
 
 	StringBuilder messageBuffer;
 
+	String peerUserAgent;
+
 	/**
 	 * Get the URI this result came from.
 	 * <p>
@@ -165,4 +167,15 @@ public abstract class OperationResult {
 				messageBuffer.append('\n');
 		}
 	}
+
+	/**
+	 * Get the user agent advertised by the peer server, if available.
+	 *
+	 * @return advertised user agent, e.g. {@code "JGit/4.0"}. Null if the peer
+	 *         did not advertise version information.
+	 * @since 4.0
+	 */
+	public String getPeerUserAgent() {
+		return peerUserAgent;
+	}
 }

org.eclipse.jgit/src/org/eclipse/jgit/transport/PushProcess.java

@@ -155,6 +155,7 @@ class PushProcess {
 			try {
 				res.setAdvertisedRefs(transport.getURI(), connection
 						.getRefsMap());
+				res.peerUserAgent = connection.getPeerUserAgent();
 				res.setRemoteUpdates(toPush);
 				monitor.endTask();
 

org.eclipse.jgit/src/org/eclipse/jgit/transport/RefAdvertiser.java

@@ -147,6 +147,21 @@ public abstract class RefAdvertiser {
 		capablities.add(name);
 	}
 
+	/**
+	 * Add one protocol capability with a value ({@code "name=value"}).
+	 *
+	 * @param name
+	 *            name of the capability.
+	 * @param value
+	 *            value. If null the capability will not be added.
+	 * @since 4.0
+	 */
+	public void advertiseCapability(String name, String value) {
+		if (value != null) {
+			capablities.add(name + '=' + value);
+		}
+	}
+
 	/**
 	 * Add a symbolic ref to capabilities.
 	 * <p>
@@ -164,8 +179,7 @@ public abstract class RefAdvertiser {
 	 * @since 3.6
 	 */
 	public void addSymref(String from, String to) {
-		String symref = String.format("%s=%s:%s", OPTION_SYMREF, from, to); //$NON-NLS-1$
-		advertiseCapability(symref);
+		advertiseCapability(OPTION_SYMREF, from + ':' + to);
 	}
 
 	/**

org.eclipse.jgit/src/org/eclipse/jgit/transport/TransportHttp.java

@@ -134,8 +134,6 @@ public class TransportHttp extends HttpTransport implements WalkTransport,
 
 	private static final String SVC_RECEIVE_PACK = "git-receive-pack"; //$NON-NLS-1$
 
-	private static final String userAgent = computeUserAgent();
-
 	static final TransportProtocol PROTO_HTTP = new TransportProtocol() {
 		private final String[] schemeNames = { "http", "https" }; //$NON-NLS-1$ //$NON-NLS-2$
 
@@ -204,17 +202,6 @@ public class TransportHttp extends HttpTransport implements WalkTransport,
 		}
 	};
 
-	private static String computeUserAgent() {
-		String version;
-		final Package pkg = TransportHttp.class.getPackage();
-		if (pkg != null && pkg.getImplementationVersion() != null) {
-			version = pkg.getImplementationVersion();
-		} else {
-			version = "unknown"; //$NON-NLS-1$
-		}
-		return "JGit/" + version; //$NON-NLS-1$
-	}
-
 	private static final Config.SectionParser<HttpConfig> HTTP_KEY = new SectionParser<HttpConfig>() {
 		public HttpConfig parse(final Config cfg) {
 			return new HttpConfig(cfg);
@@ -309,16 +296,17 @@ public class TransportHttp extends HttpTransport implements WalkTransport,
 			final HttpConnection c = connect(service);
 			final InputStream in = openInputStream(c);
 			try {
+				BaseConnection f;
 				if (isSmartHttp(c, service)) {
 					readSmartHeaders(in, service);
-					return new SmartHttpFetchConnection(in);
-
+					f = new SmartHttpFetchConnection(in);
 				} else {
 					// Assume this server doesn't support smart HTTP fetch
 					// and fall back on dumb object walking.
-					//
-					return newDumbConnection(in);
+					f = newDumbConnection(in);
 				}
+				f.setPeerUserAgent(c.getHeaderField(HttpSupport.HDR_SERVER));
+				return (FetchConnection) f;
 			} finally {
 				in.close();
 			}
@@ -331,7 +319,7 @@ public class TransportHttp extends HttpTransport implements WalkTransport,
 		}
 	}
 
-	private FetchConnection newDumbConnection(InputStream in)
+	private WalkFetchConnection newDumbConnection(InputStream in)
 			throws IOException, PackProtocolException {
 		HttpObjectDB d = new HttpObjectDB(objectsUrl);
 		BufferedReader br = toBufferedReader(in);
@@ -400,9 +388,7 @@ public class TransportHttp extends HttpTransport implements WalkTransport,
 			final InputStream in = openInputStream(c);
 			try {
 				if (isSmartHttp(c, service)) {
-					readSmartHeaders(in, service);
-					return new SmartHttpPushConnection(in);
-
+					return smartPush(service, c, in);
 				} else if (!useSmartHttp) {
 					final String msg = JGitText.get().smartHTTPPushDisabled;
 					throw new NotSupportedException(msg);
@@ -423,6 +409,14 @@ 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);
+		p.setPeerUserAgent(c.getHeaderField(HttpSupport.HDR_SERVER));
+		return p;
+	}
+
 	@Override
 	public void close() {
 		// No explicit connections are maintained.
@@ -551,7 +545,9 @@ public class TransportHttp extends HttpTransport implements WalkTransport,
 		conn.setUseCaches(false);
 		conn.setRequestProperty(HDR_ACCEPT_ENCODING, ENCODING_GZIP);
 		conn.setRequestProperty(HDR_PRAGMA, "no-cache"); //$NON-NLS-1$
-		conn.setRequestProperty(HDR_USER_AGENT, userAgent);
+		if (UserAgent.get() != null) {
+			conn.setRequestProperty(HDR_USER_AGENT, UserAgent.get());
+		}
 		int timeOut = getTimeout();
 		if (timeOut != -1) {
 			int effTimeOut = timeOut * 1000;

org.eclipse.jgit/src/org/eclipse/jgit/transport/UploadPack.java

@@ -44,6 +44,7 @@
 package org.eclipse.jgit.transport;
 
 import static org.eclipse.jgit.lib.RefDatabase.ALL;
+import static org.eclipse.jgit.transport.GitProtocolConstants.OPTION_AGENT;
 import static org.eclipse.jgit.transport.GitProtocolConstants.OPTION_ALLOW_TIP_SHA1_IN_WANT;
 import static org.eclipse.jgit.transport.GitProtocolConstants.OPTION_INCLUDE_TAG;
 import static org.eclipse.jgit.transport.GitProtocolConstants.OPTION_MULTI_ACK;
@@ -253,6 +254,7 @@ public class UploadPack {
 
 	/** Capabilities requested by the client. */
 	private Set<String> options;
+	String userAgent;
 
 	/** Raw ObjectIds the client has asked for, before validating them. */
 	private final Set<ObjectId> wantIds = new HashSet<ObjectId>();
@@ -806,6 +808,7 @@ public class UploadPack {
 				|| policy == RequestPolicy.REACHABLE_COMMIT_TIP
 				|| policy == null)
 			adv.advertiseCapability(OPTION_ALLOW_TIP_SHA1_IN_WANT);
+		adv.advertiseCapability(OPTION_AGENT, UserAgent.get());
 		adv.setDerefTags(true);
 		Map<String, Ref> refs = getAdvertisedOrDefaultRefs();
 		findSymrefs(adv, refs);
@@ -891,12 +894,30 @@ public class UploadPack {
 	 * @since 4.0
 	 */
 	public int getDepth() {
-		if (options == null) {
-			throw new IllegalStateException("do not call getDepth() before recvWants()");
-		}
+		if (options == null)
+			throw new RequestNotYetReadException();
 		return depth;
 	}
 
+	/**
+	 * Get the user agent of the client.
+	 * <p>
+	 * If the client is new enough to use {@code agent=} capability that value
+	 * will be returned. Older HTTP clients may also supply their version using
+	 * the HTTP {@code User-Agent} header. The capability overrides the HTTP
+	 * header if both are available.
+	 * <p>
+	 * When an HTTP request has been received this method returns the HTTP
+	 * {@code User-Agent} header value until capabilities have been parsed.
+	 *
+	 * @return user agent supplied by the client. Available only if the client
+	 *         is new enough to advertise its user agent.
+	 * @since 4.0
+	 */
+	public String getPeerUserAgent() {
+		return UserAgent.getAgent(options, userAgent);
+	}
+
 	private boolean negotiate() throws IOException {
 		okToGiveUp = Boolean.FALSE;
 

org.eclipse.jgit/src/org/eclipse/jgit/transport/UserAgent.java

@@ -0,0 +1,147 @@
+/*
+ * Copyright (C) 2015, Google Inc.
+ * and other copyright owners as documented in the project's IP log.
+ *
+ * This program and the accompanying materials are made available
+ * under the terms of the Eclipse Distribution License v1.0 which
+ * accompanies this distribution, is reproduced below, and is
+ * available at http://www.eclipse.org/org/documents/edl-v10.php
+ *
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or
+ * without modification, are permitted provided that the following
+ * conditions are met:
+ *
+ * - Redistributions of source code must retain the above copyright
+ *   notice, this list of conditions and the following disclaimer.
+ *
+ * - Redistributions in binary form must reproduce the above
+ *   copyright notice, this list of conditions and the following
+ *   disclaimer in the documentation and/or other materials provided
+ *   with the distribution.
+ *
+ * - Neither the name of the Eclipse Foundation, Inc. nor the
+ *   names of its contributors may be used to endorse or promote
+ *   products derived from this software without specific prior
+ *   written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND
+ * CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES,
+ * INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
+ * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR
+ * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
+ * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
+ * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
+ * STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
+ * ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+package org.eclipse.jgit.transport;
+
+import static org.eclipse.jgit.transport.GitProtocolConstants.OPTION_AGENT;
+
+import java.util.Set;
+
+import org.eclipse.jgit.util.StringUtils;
+
+/**
+ * User agent to be reported by this JGit client and server on the network.
+ * <p>
+ * On HTTP transports this user agent string is always supplied by the JGit
+ * client in the {@code User-Agent} HTTP header.
+ * <p>
+ * On native transports this user agent string is always sent when JGit is a
+ * server. When JGit is a client the user agent string will be supplied to the
+ * remote server only if the remote server advertises its own agent identity.
+ *
+ * @since 4.0
+ */
+public class UserAgent {
+	private static volatile String userAgent = computeUserAgent();
+
+	private static String computeUserAgent() {
+		return clean("JGit/" + computeVersion()); //$NON-NLS-1$
+	}
+
+	private static String computeVersion() {
+		Package pkg = UserAgent.class.getPackage();
+		if (pkg != null) {
+			String ver = pkg.getImplementationVersion();
+			if (!StringUtils.isEmptyOrNull(ver)) {
+				return ver;
+			}
+		}
+		return "unknown"; //$NON-NLS-1$
+	}
+
+	private static String clean(String s) {
+		s = s.trim();
+		StringBuilder b = new StringBuilder(s.length());
+		for (int i = 0; i < s.length(); i++) {
+			char c = s.charAt(i);
+			if (c <= 32 || c >= 127) {
+				if (b.length() > 0 && b.charAt(b.length() - 1) == '.')
+					continue;
+				c = '.';
+			}
+			b.append(c);
+		}
+		return b.length() > 0 ? b.toString() : null;
+	}
+
+	/**
+	 * Get the user agent string advertised by JGit.
+	 *
+	 * @return a string similar to {@code "JGit/4.0"}; null if the agent has
+	 *         been cleared and should not be shared with a peer.
+	 */
+	public static String get() {
+		return userAgent;
+	}
+
+	/**
+	 * Change the user agent string advertised by JGit.
+	 * <p>
+	 * The new string should start with {@code "JGit/"} (for example
+	 * {@code "JGit/4.0"}) to advertise the implementation as JGit based.
+	 * <p>
+	 * Spaces and other whitespace should be avoided as these will be
+	 * automatically converted to {@code "."}.
+	 * <p>
+	 * User agent strings are restricted to printable ASCII.
+	 *
+	 * @param agent
+	 *            new user agent string for this running JGit library. Setting
+	 *            to null or empty string will avoid sending any identification
+	 *            to the peer.
+	 */
+	public static void set(String agent) {
+		userAgent = StringUtils.isEmptyOrNull(agent) ? null : clean(agent);
+	}
+
+	static String getAgent(Set<String> options, String transportAgent) {
+		if (options == null || options.isEmpty()) {
+			return transportAgent;
+		}
+		for (String o : options) {
+			if (o.startsWith(OPTION_AGENT)
+					&& o.length() > OPTION_AGENT.length()
+					&& o.charAt(OPTION_AGENT.length()) == '=') {
+				return o.substring(OPTION_AGENT.length() + 1);
+			}
+		}
+		return transportAgent;
+	}
+
+	static boolean hasAgent(Set<String> options) {
+		return getAgent(options, null) != null;
+	}
+
+	private UserAgent() {
+	}
+}

org.eclipse.jgit/src/org/eclipse/jgit/util/HttpSupport.java

@@ -74,6 +74,12 @@ public class HttpSupport {
 	/** The {@code User-Agent} header. */
 	public static final String HDR_USER_AGENT = "User-Agent"; //$NON-NLS-1$
 
+	/**
+	 * The {@code Server} header.
+	 * @since 4.0
+	 */
+	public static final String HDR_SERVER = "Server"; //$NON-NLS-1$
+
 	/** The {@code Date} header. */
 	public static final String HDR_DATE = "Date"; //$NON-NLS-1$