Introduce core.trustPackedRefsStat config

Currently, we always read packed-refs file when 'trustFolderStat'
is false. Introduce a new config 'trustPackedRefsStat' which takes
precedence over 'trustFolderStat' when reading packed refs. Possible
values for this new config are:

* always: Trust packed-refs file attributes
* after_open: Same as 'always', but refresh the file attributes of
              packed-refs before trusting it
* never: Always read the packed-refs file
* unset: Fallback to 'trustFolderStat' to determine if the file
  attributes of packed-refs can be trusted

Folks whose repositories are on NFS and have traditionally been
setting 'trustFolderStat=false' can now get some performance improvement
with 'trustPackedRefsStat=after_open' as it refreshes the file
attributes of packed-refs (at least on some NFS clients) before
considering it.

For example, consider a repository on NFS with ~500k packed-refs. Here
are some stats which illustrate the improvement with this new config
when reading packed refs on NFS:

trustFolderStat=true trustPackedRefsStat=unset: 0.2ms
trustFolderStat=false trustPackedRefsStat=unset: 155ms
trustFolderStat=false trustPackedRefsStat=after_open: 1.5ms

Change-Id: I00da88e4cceebbcf3475be0fc0011ff65767c111
Signed-off-by: Kaushik Lingarkar <quic_kaushikl@quicinc.com>

4 files changed, 72 insertions, 2 deletions

diff --git a/org.eclipse.jgit/.settings/.api_filters b/org.eclipse.jgit/.settings/.api_filters
index a6f18310d6..0f84330082 100644
--- a/org.eclipse.jgit/.settings/.api_filters
+++ b/org.eclipse.jgit/.settings/.api_filters
@@ -15,6 +15,20 @@
                 <message_argument value="SHA1_IMPLEMENTATION"/>
             </message_arguments>
         </filter>
+        <filter id="1142947843">
+            <message_arguments>
+                <message_argument value="6.1.1"/>
+                <message_argument value="CONFIG_KEY_TRUST_PACKED_REFS_STAT"/>
+            </message_arguments>
+        </filter>
+    </resource>
+    <resource path="src/org/eclipse/jgit/lib/CoreConfig.java" type="org.eclipse.jgit.lib.CoreConfig$TrustPackedRefsStat">
+        <filter id="1142947843">
+            <message_arguments>
+                <message_argument value="6.1.1"/>
+                <message_argument value="TrustPackedRefsStat"/>
+            </message_arguments>
+        </filter>
     </resource>
     <resource path="src/org/eclipse/jgit/lib/ObjectDatabase.java" type="org.eclipse.jgit.lib.ObjectDatabase">
         <filter id="336695337">
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/RefDirectory.java b/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/RefDirectory.java
index b46ffe3670..43ebce3918 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/RefDirectory.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/RefDirectory.java
@@ -30,6 +30,7 @@ import java.io.File;
 import java.io.FileInputStream;
 import java.io.FileNotFoundException;
 import java.io.IOException;
+import java.io.InputStream;
 import java.io.InputStreamReader;
 import java.io.InterruptedIOException;
 import java.nio.file.DirectoryNotEmptyException;
@@ -60,6 +61,7 @@ import org.eclipse.jgit.events.RefsChangedEvent;
 import org.eclipse.jgit.internal.JGitText;
 import org.eclipse.jgit.lib.ConfigConstants;
 import org.eclipse.jgit.lib.Constants;
+import org.eclipse.jgit.lib.CoreConfig.TrustPackedRefsStat;
 import org.eclipse.jgit.lib.ObjectId;
 import org.eclipse.jgit.lib.ObjectIdRef;
 import org.eclipse.jgit.lib.Ref;
@@ -892,10 +894,37 @@ public class RefDirectory extends RefDatabase {
 		boolean trustFolderStat = getRepository().getConfig().getBoolean(
 				ConfigConstants.CONFIG_CORE_SECTION,
 				ConfigConstants.CONFIG_KEY_TRUSTFOLDERSTAT, true);
+		TrustPackedRefsStat trustPackedRefsStat =
+				getRepository().getConfig().getEnum(
+						ConfigConstants.CONFIG_CORE_SECTION,
+						null,
+						ConfigConstants.CONFIG_KEY_TRUST_PACKED_REFS_STAT,
+						TrustPackedRefsStat.UNSET);
 
 		final PackedRefList curList = packedRefs.get();
-		if (trustFolderStat && !curList.snapshot.isModified(packedRefsFile)) {
-			return curList;
+
+		switch (trustPackedRefsStat) {
+		case NEVER:
+			break;
+		case AFTER_OPEN:
+			try (InputStream stream = Files
+					.newInputStream(packedRefsFile.toPath())) {
+				// open the file to refresh attributes (on some NFS clients)
+			} catch (FileNotFoundException e) {
+				// Ignore as packed-refs may not exist
+			}
+			//$FALL-THROUGH$
+		case ALWAYS:
+			if (!curList.snapshot.isModified(packedRefsFile)) {
+				return curList;
+			}
+			break;
+		case UNSET:
+			if (trustFolderStat
+					&& !curList.snapshot.isModified(packedRefsFile)) {
+				return curList;
+			}
+			break;
 		}
 
 		final PackedRefList newList = readPackedRefs();
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/lib/ConfigConstants.java b/org.eclipse.jgit/src/org/eclipse/jgit/lib/ConfigConstants.java
index 649bcde25a..9398419af9 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/lib/ConfigConstants.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/lib/ConfigConstants.java
@@ -850,4 +850,10 @@ public final class ConfigConstants {
 	 */
 	public static final String CONFIG_KEY_ABBREV = "abbrev";
 
+	/**
+	 * The "trustPackedRefsStat" key
+	 *
+	 * @since 6.1.1
+	 */
+	public static final String CONFIG_KEY_TRUST_PACKED_REFS_STAT = "trustPackedRefsStat";
 }
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/lib/CoreConfig.java b/org.eclipse.jgit/src/org/eclipse/jgit/lib/CoreConfig.java
index f23c6e08d1..fc82a5fead 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/lib/CoreConfig.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/lib/CoreConfig.java
@@ -116,6 +116,27 @@ public class CoreConfig {
 		ALWAYS
 	}
 
+	/**
+	 * Permissible values for {@code core.trustPackedRefsStat}.
+	 *
+	 * @since 6.1.1
+	 */
+	public enum TrustPackedRefsStat {
+		/** Do not trust file attributes of the packed-refs file. */
+		NEVER,
+
+		/** Trust file attributes of the packed-refs file. */
+		ALWAYS,
+
+		/** Open and close the packed-refs file to refresh its file attributes
+		 * and then trust it. */
+		AFTER_OPEN,
+
+		/** {@code core.trustPackedRefsStat} defaults to this when it is
+		 * not set */
+		UNSET
+	}
+
 	private final int compression;
 
 	private final int packIndexVersion;