reflog --expire-unreachable: avoid merge-base computation

The option tells the command to expire older reflog entries that refer to
commits that are no longer reachable from the tip of the ref the reflog is
associated with.  To avoid repeated merge_base() invocations, we used to
mark commits that are known to be reachable by walking the history from
the tip until we hit commits that are older than expire-total (which is
the timestamp before which all the reflog entries are expired).

However, it is a different matter if a commit is _not_ known to be
reachable and the commit is known to be unreachable.  Because you can
rewind a ref to an ancient commit and then reset it back to the original
tip, a recent reflog entry can point at a commit that older than the
expire-total timestamp and we shouldn't expire it.  For that reason, we
had to run merge-base computation when a commit is _not_ known to be
reachable.

This introduces a lazy/on-demand traversal of the history to mark
reachable commits in steps.  As before, we mark commits that are newer
than expire-total to optimize the normal case before walking reflog, but
we dig deeper from the commits the initial step left off when we encounter
a commit that is not known to be reachable.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-reflog.c

@@ -36,6 +36,8 @@ struct expire_reflog_cb {
 	FILE *newlog;
 	const char *ref;
 	struct commit *ref_commit;
+	struct commit_list *mark_list;
+	unsigned long mark_limit;
 	struct cmd_reflog_expire_cb *cmd;
 	unsigned char last_kept_sha1[20];
 };
@@ -210,46 +212,23 @@ static int keep_entry(struct commit **it, unsigned char *sha1)
 	return 1;
 }
 
-static int unreachable(struct expire_reflog_cb *cb, struct commit *commit, unsigned char *sha1)
+/*
+ * Starting from commits in the cb->mark_list, mark commits that are
+ * reachable from them.  Stop the traversal at commits older than
+ * the expire_limit and queue them back, so that the caller can call
+ * us again to restart the traversal with longer expire_limit.
+ */
+static void mark_reachable(struct expire_reflog_cb *cb)
 {
-	/*
-	 * We may or may not have the commit yet - if not, look it
-	 * up using the supplied sha1.
-	 */
-	if (!commit) {
-		if (is_null_sha1(sha1))
-			return 0;
-
-		commit = lookup_commit_reference_gently(sha1, 1);
-
-		/* Not a commit -- keep it */
-		if (!commit)
-			return 0;
-	}
-
-	/* Reachable from the current ref?  Don't prune. */
-	if (commit->object.flags & REACHABLE)
-		return 0;
-	if (in_merge_bases(commit, &cb->ref_commit, 1))
-		return 0;
-
-	/* We can't reach it - prune it. */
-	return 1;
-}
+	struct commit *commit;
+	struct commit_list *pending;
+	unsigned long expire_limit = cb->mark_limit;
+	struct commit_list *leftover = NULL;
 
-static void mark_reachable(struct commit *commit, unsigned long expire_limit)
-{
-	/*
-	 * We need to compute whether the commit on either side of a reflog
-	 * entry is reachable from the tip of the ref for all entries.
-	 * Mark commits that are reachable from the tip down to the
-	 * time threshold first; we know a commit marked thusly is
-	 * reachable from the tip without running in_merge_bases()
-	 * at all.
-	 */
-	struct commit_list *pending = NULL;
+	for (pending = cb->mark_list; pending; pending = pending->next)
+		pending->item->object.flags &= ~REACHABLE;
 
-	commit_list_insert(commit, &pending);
+	pending = cb->mark_list;
 	while (pending) {
 		struct commit_list *entry = pending;
 		struct commit_list *parent;
@@ -261,8 +240,11 @@ static void mark_reachable(struct commit *commit, unsigned long expire_limit)
 		if (parse_commit(commit))
 			continue;
 		commit->object.flags |= REACHABLE;
-		if (commit->date < expire_limit)
+		if (commit->date < expire_limit) {
+			commit_list_insert(commit, &leftover);
 			continue;
+		}
+		commit->object.flags |= REACHABLE;
 		parent = commit->parents;
 		while (parent) {
 			commit = parent->item;
@@ -272,6 +254,36 @@ static void mark_reachable(struct commit *commit, unsigned long expire_limit)
 			commit_list_insert(commit, &pending);
 		}
 	}
+	cb->mark_list = leftover;
+}
+
+static int unreachable(struct expire_reflog_cb *cb, struct commit *commit, unsigned char *sha1)
+{
+	/*
+	 * We may or may not have the commit yet - if not, look it
+	 * up using the supplied sha1.
+	 */
+	if (!commit) {
+		if (is_null_sha1(sha1))
+			return 0;
+
+		commit = lookup_commit_reference_gently(sha1, 1);
+
+		/* Not a commit -- keep it */
+		if (!commit)
+			return 0;
+	}
+
+	/* Reachable from the current ref?  Don't prune. */
+	if (commit->object.flags & REACHABLE)
+		return 0;
+
+	if (cb->mark_list && cb->mark_limit) {
+		cb->mark_limit = 0; /* dig down to the root */
+		mark_reachable(cb);
+	}
+
+	return !(commit->object.flags & REACHABLE);
 }
 
 static int expire_reflog_ent(unsigned char *osha1, unsigned char *nsha1,
@@ -348,8 +360,12 @@ static int expire_reflog(const char *ref, const unsigned char *sha1, int unused,
 	cb.ref_commit = lookup_commit_reference_gently(sha1, 1);
 	cb.ref = ref;
 	cb.cmd = cmd;
-	if (cb.ref_commit)
-		mark_reachable(cb.ref_commit, cmd->expire_total);
+	if (cb.ref_commit) {
+		cb.mark_list = NULL;
+		commit_list_insert(cb.ref_commit, &cb.mark_list);
+		cb.mark_limit = cmd->expire_total;
+		mark_reachable(&cb);
+	}
 	for_each_reflog_ent(ref, expire_reflog_ent, &cb);
 	if (cb.ref_commit)
 		clear_commit_marks(cb.ref_commit, REACHABLE);