Do not create reflog for remote tracking branches during clone

When using JGit on a non-bare repository, the CloneCommand
it previously created local reflogs for all branches including remote
tracking ones, causing the generation of a potentially large
number of files on the local filesystem.

The creation of the remote-tracking branches (refs/remotes/*) during
clone is not an issue for the local filesystem because all of them are
stored in a single packed-refs file. However, the creation of a large
number of ref logs on a local filesystem IS an issue because it
may not be tuned or initialised in term of inodes to contain a very
large number of files.

When a user (or a CI system) performs the CloneCommand against
a potentially large repository (e.g., millions of branches), it is
interested in working or validating a single branch or tag and is
unlikely to work with all the remote-tracking branches.
The eager creation of a reflogs for all the remote-tracking branches is
not just a performance issue but may also compromise the ability to
use JGit for cloning a large repository.

The behaviour implemented in this change is also consistent with the
optimisation done in the C code-base [1].

We differentiate between clone and fetch commands using --branch
<initialBranch> option, that is only available in clone command,
and is set as HEAD per default.

[1] 58f233ce1e

Bug: 579805
Change-Id: I58d0d36a8a4ce42e0f59b8bf063747c4b81bd859
Signed-off-by: Luca Milanesio <luca.milanesio@gmail.com>

org.eclipse.jgit.test/tst/org/eclipse/jgit/api/CloneCommandTest.java

@@ -22,6 +22,7 @@
 import java.util.Collections;
 import java.util.List;
 import java.util.Map;
+import java.util.stream.Stream;
 
 import org.eclipse.jgit.api.ListBranchCommand.ListMode;
 import org.eclipse.jgit.api.errors.GitAPIException;
@@ -114,6 +115,49 @@ public void testCloneRepository() throws IOException,
 		assertTagOption(git2.getRepository(), TagOpt.AUTO_FOLLOW);
 	}
 
+	@Test
+	public void testCloneRepository_refLogForLocalRefs()
+			throws IOException, JGitInternalException, GitAPIException {
+		File directory = createTempDirectory("testCloneRepository");
+		CloneCommand command = Git.cloneRepository();
+		command.setDirectory(directory);
+		command.setURI(fileUri());
+		Git git2 = command.call();
+		Repository clonedRepo = git2.getRepository();
+		addRepoToClose(clonedRepo);
+
+		List<Ref> clonedRefs = clonedRepo.getRefDatabase().getRefs();
+		Stream<Ref> remoteRefs = clonedRefs.stream()
+				.filter(CloneCommandTest::isRemote);
+		Stream<Ref> localHeadsRefs = clonedRefs.stream()
+				.filter(CloneCommandTest::isLocalHead);
+
+		remoteRefs.forEach(ref -> assertFalse(
+				"Ref " + ref.getName()
+						+ " is remote and should not have a reflog",
+				hasRefLog(clonedRepo, ref)));
+		localHeadsRefs.forEach(ref -> assertTrue(
+				"Ref " + ref.getName()
+						+ " is local head and should have a reflog",
+				hasRefLog(clonedRepo, ref)));
+	}
+
+	private static boolean isRemote(Ref ref) {
+		return ref.getName().startsWith(Constants.R_REMOTES);
+	}
+
+	private static boolean isLocalHead(Ref ref) {
+		return !isRemote(ref) && ref.getName().startsWith(Constants.R_HEADS);
+	}
+
+	private static boolean hasRefLog(Repository repo, Ref ref) {
+		try {
+			return repo.getReflogReader(ref.getName()).getLastEntry() != null;
+		} catch (IOException ioe) {
+			throw new IllegalStateException(ioe);
+		}
+	}
+
 	@Test
 	public void testCloneRepositoryExplicitGitDir() throws IOException,
 			JGitInternalException, GitAPIException {

org.eclipse.jgit.test/tst/org/eclipse/jgit/api/FetchCommandTest.java

@@ -76,6 +76,26 @@ public void testFetch() throws Exception {
 				db.resolve(tagRef.getObjectId().getName()));
 	}
 
+	@Test
+	public void testFetchHasRefLogForRemoteRef() throws Exception {
+		// create an initial commit SHA1 for the default branch
+		ObjectId defaultBranchSha1 = remoteGit.commit()
+				.setMessage("initial commit").call().getId();
+
+		git.fetch().setRemote("test")
+				.setRefSpecs("refs/heads/*:refs/remotes/origin/*").call();
+
+		List<Ref> allFetchedRefs = git.getRepository().getRefDatabase()
+				.getRefs();
+		assertEquals(allFetchedRefs.size(), 1);
+		Ref remoteRef = allFetchedRefs.get(0);
+
+		assertTrue(remoteRef.getName().startsWith(Constants.R_REMOTES));
+		assertEquals(defaultBranchSha1, remoteRef.getObjectId());
+		assertNotNull(git.getRepository().getReflogReader(remoteRef.getName())
+				.getLastEntry());
+	}
+
 	@Test
 	public void testForcedFetch() throws Exception {
 		remoteGit.commit().setMessage("commit").call();

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

@@ -205,8 +205,13 @@ else if (tagopt == TagOpt.FETCH_TAGS)
 
 		BatchRefUpdate batch = transport.local.getRefDatabase()
 				.newBatchUpdate()
-				.setAllowNonFastForwards(true)
-				.setRefLogMessage("fetch", true); //$NON-NLS-1$
+				.setAllowNonFastForwards(true);
+
+		// Generate reflog only when fetching updates and not at the first clone
+		if (initialBranch == null) {
+			batch.setRefLogMessage("fetch", true); //$NON-NLS-1$
+		}
+
 		try (RevWalk walk = new RevWalk(transport.local)) {
 			walk.setRetainBody(false);
 			if (monitor instanceof BatchingProgressMonitor) {