Restore preserved packs during missing object seeks

Provide a recovery path for objects being referenced during the pack
pruning race. Due to the pack pruning race, it is possible for objects
to become referenced after a pack has been deemed safe to prune, but
before it actually gets pruned. If this happened previously, the newly
referenced objects would be missing and potentially result in a
corrupted ref.

Add the ability to recover from this situation when an object is missing
but happens to still be available in a pack in the "preserved"
directory. This is likely only useful when used in conjunction with the
--preserve-old-packs GC option, which prunes packs by hard-linking to
the preserved directory. If an object is missing and found in a pack in
the preserved directory, immediately recover that pack and its
associated files (idx, bitmaps...) by moving them back to the original
pack directory, and then retry the operation that would have failed due
to the missing object. This retry can now succeed and the repository
may avoid corruption. This approach should drastically reduce the
chance of a corrupt repository during pack pruning at very little extra
cost. This extra cost should only be incurred when objects are missing
and a failure would normally occur.

Change-Id: I2a704e3276b88cc892159d9bfe2455c6eec64252
Signed-off-by: Martin Fick <quic_mfick@quicinc.com>
Signed-off-by: Nasser Grainawi <quic_nasserg@quicinc.com>

1 files changed, 51 insertions, 0 deletions

diff --git a/org.eclipse.jgit.test/tst/org/eclipse/jgit/internal/storage/file/GcBasicPackingTest.java b/org.eclipse.jgit.test/tst/org/eclipse/jgit/internal/storage/file/GcBasicPackingTest.java
index 61538c9c24..8dc1ddb9f6 100644
--- a/org.eclipse.jgit.test/tst/org/eclipse/jgit/internal/storage/file/GcBasicPackingTest.java
+++ b/org.eclipse.jgit.test/tst/org/eclipse/jgit/internal/storage/file/GcBasicPackingTest.java
@@ -23,6 +23,7 @@ import java.util.List;
 
 import org.eclipse.jgit.junit.TestRepository.BranchBuilder;
 import org.eclipse.jgit.lib.ConfigConstants;
+import org.eclipse.jgit.lib.ObjectId;
 import org.eclipse.jgit.lib.RefUpdate;
 import org.eclipse.jgit.revwalk.RevCommit;
 import org.eclipse.jgit.storage.file.FileBasedConfig;
@@ -314,6 +315,56 @@ public class GcBasicPackingTest extends GcTestCase {
 		assertTrue(preservedPackFile.exists());
 	}
 
+	@Test
+	public void testPruneAndRestoreOldPacks() throws Exception {
+		String tempRef = "refs/heads/soon-to-be-unreferenced";
+		BranchBuilder bb = tr.branch(tempRef);
+		bb.commit().add("A", "A").add("B", "B").create();
+
+		// Verify setup conditions
+		stats = gc.getStatistics();
+		assertEquals(4, stats.numberOfLooseObjects);
+		assertEquals(0, stats.numberOfPackedObjects);
+
+		// Force all referenced objects into packs (to avoid having loose objects)
+		configureGc(gc, false);
+		gc.setExpireAgeMillis(0);
+		gc.setPackExpireAgeMillis(0);
+		gc.gc();
+		stats = gc.getStatistics();
+		assertEquals(0, stats.numberOfLooseObjects);
+		assertEquals(4, stats.numberOfPackedObjects);
+		assertEquals(1, stats.numberOfPackFiles);
+
+		// Delete the temp ref, orphaning its commit
+		RefUpdate update = tr.getRepository().getRefDatabase().newUpdate(tempRef, false);
+		update.setForceUpdate(true);
+		ObjectId objectId = update.getOldObjectId(); // remember it so we can restore it!
+		RefUpdate.Result result = update.delete();
+		assertEquals(RefUpdate.Result.FORCED, result);
+
+		fsTick();
+
+		// Repack with only orphaned commit, so packfile will be pruned
+		configureGc(gc, false).setPreserveOldPacks(true);
+		gc.gc();
+		stats = gc.getStatistics();
+		assertEquals(0, stats.numberOfLooseObjects);
+		assertEquals(0, stats.numberOfPackedObjects);
+		assertEquals(0, stats.numberOfPackFiles);
+
+		// Restore the temp ref to the deleted commit, should restore old-packs!
+		update = tr.getRepository().getRefDatabase().newUpdate(tempRef, false);
+		update.setNewObjectId(objectId);
+		update.setExpectedOldObjectId(null);
+		result = update.update();
+		assertEquals(RefUpdate.Result.NEW, result);
+
+		stats = gc.getStatistics();
+		assertEquals(4, stats.numberOfPackedObjects);
+		assertEquals(1, stats.numberOfPackFiles);
+	}
+
 	private PackConfig configureGc(GC myGc, boolean aggressive) {
 		PackConfig pconfig = new PackConfig(repo);
 		if (aggressive) {