reftable: fix lookup by ID in merged reftables

On changing a ref, the old SHA1 is not updated in the object => ref
mapping. This means search by object ID may still turn up a ref from
deeper within the stack. To fix this, check all refs produced by the
merged iterator against the merged reftables.

Signed-off-by: Han-Wen Nienhuys <hanwen@google.com>
Change-Id: I41e9cd395b0608eedeeaead0a9fd997238d747c9

org.eclipse.jgit.test/tst/org/eclipse/jgit/internal/storage/reftable/MergedReftableTest.java

@@ -187,6 +187,19 @@ public class MergedReftableTest {
 		}
 	}
 
+	@Test
+	public void tableByIDDeletion() throws IOException {
+		List<Ref> delta1 = Arrays.asList(
+				ref("refs/heads/apple", 1),
+				ref("refs/heads/master", 2));
+		List<Ref> delta2 = Arrays.asList(ref("refs/heads/master", 3));
+
+		MergedReftable mr = merge(write(delta1), write(delta2));
+		try (RefCursor rc = mr.byObjectId(id(2))) {
+			assertFalse(rc.next());
+		}
+	}
+
 	@SuppressWarnings("boxing")
 	@Test
 	public void fourTableScan() throws IOException {

org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/reftable/MergedReftable.java

@@ -131,7 +131,7 @@ public class MergedReftable extends Reftable {
 	/** {@inheritDoc} */
 	@Override
 	public RefCursor byObjectId(AnyObjectId name) throws IOException {
-		MergedRefCursor m = new MergedRefCursor();
+		MergedRefCursor m = new FilteringMergedRefCursor(name);
 		for (int i = 0; i < tables.length; i++) {
 			m.add(new RefQueueEntry(tables[i].byObjectId(name), i));
 		}
@@ -250,6 +250,42 @@ public class MergedReftable extends Reftable {
 		}
 	}
 
+	private class FilteringMergedRefCursor extends MergedRefCursor {
+		final AnyObjectId filterId;
+		Ref filteredRef;
+
+		FilteringMergedRefCursor(AnyObjectId id) {
+			filterId = id;
+			filteredRef = null;
+		}
+
+		@Override
+		public Ref getRef() {
+			return filteredRef;
+		}
+
+		@Override
+		public boolean next() throws IOException {
+			for (;;) {
+				boolean ok = super.next();
+				if (!ok) {
+					return false;
+				}
+
+				String name = super.getRef().getName();
+
+				try (RefCursor c = seekRef(name)) {
+					if (c.next()) {
+						if (filterId.equals(c.getRef().getObjectId())) {
+							filteredRef = c.getRef();
+							return true;
+						}
+					}
+				}
+			}
+		}
+	}
+
 	private static class RefQueueEntry {
 		static int compare(RefQueueEntry a, RefQueueEntry b) {
 			int cmp = a.name().compareTo(b.name());