Shortcut during git fetch for avoiding looping through all local refs

The FetchProcess needs to verify that all the refs received point
to objects that are reachable from the local refs, which could be
very expensive but is needed to avoid missing objects exceptions
because of broken chains.

When the local repository has a lot of refs (e.g. millions) and the
client is fetching a non-commit object (e.g. refs/sequences/changes in
Gerrit) the reachability check on all local refs can be very expensive
compared to the time to fetch the remote ref.

Example for a 2M refs repository:
- fetching a single non-commit object: 50ms
- checking the reachability of local refs: 30s

A ref pointing to a non-commit object doesn't have any parent or
successor objects, hence would never need to have a reachability check
done. Skipping the askForIsComplete() altogether would save the 30s
time spent in an unnecessary phase.

Signed-off-by: Luca Milanesio <luca.milanesio@gmail.com>
Change-Id: I09ac66ded45cede199ba30f9e71cc1055f00941b

1 files changed, 14 insertions, 5 deletions

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 2cedd4b07e..507795af52 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/transport/FetchProcess.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/transport/FetchProcess.java
@@ -47,6 +47,7 @@ import org.eclipse.jgit.lib.ProgressMonitor;
 import org.eclipse.jgit.lib.Ref;
 import org.eclipse.jgit.lib.RefDatabase;
 import org.eclipse.jgit.revwalk.ObjectWalk;
+import org.eclipse.jgit.revwalk.RevObject;
 import org.eclipse.jgit.revwalk.RevWalk;
 import org.eclipse.jgit.util.StringUtils;
 
@@ -378,11 +379,19 @@ class FetchProcess {
 	private boolean askForIsComplete() throws TransportException {
 		try {
 			try (ObjectWalk ow = new ObjectWalk(transport.local)) {
-				for (ObjectId want : askFor.keySet())
-					ow.markStart(ow.parseAny(want));
-				for (Ref ref : localRefs().values())
-					ow.markUninteresting(ow.parseAny(ref.getObjectId()));
-				ow.checkConnectivity();
+				boolean hasCommitObject = false;
+				for (ObjectId want : askFor.keySet()) {
+					RevObject obj = ow.parseAny(want);
+					ow.markStart(obj);
+					hasCommitObject |= obj.getType() == Constants.OBJ_COMMIT;
+				}
+				// Checking connectivity makes sense on commits only
+				if (hasCommitObject) {
+					for (Ref ref : localRefs().values()) {
+						ow.markUninteresting(ow.parseAny(ref.getObjectId()));
+					}
+					ow.checkConnectivity();
+				}
 			}
 			return true;
 		} catch (MissingObjectException e) {