Fix merge-base calculation

Fix JGits merge-base calculation in case of inconsistent commit times.
JGit was potentially failing to compute correct merge-bases when the
commit times where inconsistent (a parent commit was younger than a
child commit). The code in MergeBaseGenerator was aware of the fact that
sometimes the discovery of a merge base x can occur after the parents of
x have been seen (see comment in #carryOntoOne()). But in the light of
inconsistent commit times it was possible that these parents of a
merge-base have already been returned as a merge-base.

This commit fixes the bug by buffering all commits generated by
MergeBaseGenerator. It is expected that this buffer will be small
because the number of merge-bases will be small. Additionally a new
flag is used to mark the ancestors of merge-bases. This allows to filter
out the unwanted commits.

Bug: 507584
Change-Id: I9cc140b784c3231b972bd2c3de61a789365237ab

org.eclipse.jgit.test/tst/org/eclipse/jgit/revwalk/RevWalkMergeBaseTest.java

@@ -146,4 +146,29 @@ public void testCrissCross() throws Exception {
 		assertCommit(b, rw.next());
 		assertNull(rw.next());
 	}
-}
+
+	@Test
+	public void testInconsistentCommitTimes() throws Exception {
+		// When commit times are inconsistent (a parent is younger than a child)
+		// make sure that not both, parent and child, are reported as merge
+		// base. In the following repo the merge base between C,D should be B.
+		// But when A is younger than B the MergeBaseGenerator used to generate
+		// A before it detected that B is also a merge base.
+		//
+		//   +---C
+		//  /   /
+		// A---B---D
+
+		final RevCommit a = commit(2);
+		final RevCommit b = commit(-1, a);
+		final RevCommit c = commit(2, b, a);
+		final RevCommit d = commit(1, b);
+
+		rw.setRevFilter(RevFilter.MERGE_BASE);
+		markStart(d);
+		markStart(c);
+		assertCommit(b, rw.next());
+		assertNull(rw.next());
+	}
+
+}

org.eclipse.jgit/src/org/eclipse/jgit/revwalk/MergeBaseGenerator.java

@@ -45,6 +45,7 @@
 
 import java.io.IOException;
 import java.text.MessageFormat;
+import java.util.LinkedList;
 
 import org.eclipse.jgit.errors.IncorrectObjectTypeException;
 import org.eclipse.jgit.errors.MissingObjectException;
@@ -85,12 +86,15 @@ class MergeBaseGenerator extends Generator {
 
 	private int recarryMask;
 
+	private int mergeBaseAncestor = -1;
+	private LinkedList<RevCommit> ret = new LinkedList<RevCommit>();
+
 	MergeBaseGenerator(final RevWalk w) {
 		walker = w;
 		pending = new DateRevQueue();
 	}
 
-	void init(final AbstractRevQueue p) {
+	void init(final AbstractRevQueue p) throws IOException {
 		try {
 			for (;;) {
 				final RevCommit c = p.next();
@@ -98,17 +102,25 @@ void init(final AbstractRevQueue p) {
 					break;
 				add(c);
 			}
-		} finally {
-			// Always free the flags immediately. This ensures the flags
-			// will be available for reuse when the walk resets.
-			//
-			walker.freeFlag(branchMask);
-
 			// Setup the condition used by carryOntoOne to detect a late
 			// merge base and produce it on the next round.
 			//
 			recarryTest = branchMask | POPPED;
 			recarryMask = branchMask | POPPED | MERGE_BASE;
+			mergeBaseAncestor = walker.allocFlag();
+
+			for (;;) {
+				RevCommit c = _next();
+				if (c == null) {
+					break;
+				}
+				ret.add(c);
+			}
+		} finally {
+			// Always free the flags immediately. This ensures the flags
+			// will be available for reuse when the walk resets.
+			//
+			walker.freeFlag(branchMask | mergeBaseAncestor);
 		}
 	}
 
@@ -131,8 +143,7 @@ int outputType() {
 		return 0;
 	}
 
-	@Override
-	RevCommit next() throws MissingObjectException,
+	private RevCommit _next() throws MissingObjectException,
 			IncorrectObjectTypeException, IOException {
 		for (;;) {
 			final RevCommit c = pending.next();
@@ -156,7 +167,7 @@ RevCommit next() throws MissingObjectException,
 				// also flagged as being popped, so that they do not
 				// generate to the caller.
 				//
-				carry |= MERGE_BASE;
+				carry |= MERGE_BASE | mergeBaseAncestor;
 			}
 			carryOntoHistory(c, carry);
 
@@ -179,6 +190,18 @@ RevCommit next() throws MissingObjectException,
 		}
 	}
 
+	@Override
+	RevCommit next() throws MissingObjectException,
+			IncorrectObjectTypeException, IOException {
+		while (!ret.isEmpty()) {
+			RevCommit commit = ret.remove();
+			if ((commit.flags & mergeBaseAncestor) == 0) {
+				return commit;
+			}
+		}
+		return null;
+	}
+
 	private void carryOntoHistory(RevCommit c, final int carry) {
 		for (;;) {
 			final RevCommit[] pList = c.parents;