[RFC/PATCH 2/3] simplify-merges: never remove all TREESAME parents

[Kevin Bracey <kevin@bracey.fi>]

In the event of an odd merge, we may find ourselves TREESAME to
apparently redundant parents. Prevent simplify_merges() from removing
every TREESAME parent - in the event of such a merge it's useful to see
where we came actually from came.

Signed-off-by: Kevin Bracey <kevin@bracey.fi>
---
 Documentation/rev-list-options.txt |  3 ++-
 revision.c                         | 24 ++++++++++++++++++++++++
 2 files changed, 26 insertions(+), 1 deletion(-)

diff --git a/Documentation/rev-list-options.txt b/Documentation/rev-list-options.txt
index 380db48..0832004 100644
--- a/Documentation/rev-list-options.txt
+++ b/Documentation/rev-list-options.txt
@@ -472,7 +472,8 @@ history according to the following rules:
 +
 * Replace each parent `P` of `C'` with its simplification `P'`.  In
   the process, drop parents that are ancestors of other parents, and
-  remove duplicates.
+  remove duplicates, but take care to never drop all parents that
+  we are TREESAME to.
 +
 * If after this parent rewriting, `C'` is a root or merge commit (has
   zero or >1 parents), a boundary commit, or !TREESAME, it remains.
diff --git a/revision.c b/revision.c
index 176eb7b..4e27c9a 100644
--- a/revision.c
+++ b/revision.c
@@ -2106,8 +2106,32 @@ static int remove_marked_parents(struct rev_info *revs, struct commit *commit)
 {
 	struct treesame_state *ts = lookup_decoration(&revs->treesame, &commit->object);
 	struct commit_list **pp, *p;
+	struct commit *su = NULL, *sm = NULL;
 	int n, removed = 0;
 
+	/* Prescan - look for both marked and unmarked TREESAME parents */
+	for (p = commit->parents, n = 0; p; p = p->next, n++) {
+		if (ts->treesame[n]) {
+			if (p->item->object.flags & TMP_MARK) {
+				if (!sm) sm = p->item;
+			}
+			else {
+				if (!su) {
+					su = p->item;
+					break;
+				}
+			}
+		}
+	}
+
+	/* If we are TREESAME to a marked-for-deletion parent, but not to any
+	 * unmarked parents, unmark the first TREESAME parent. We don't want
+	 * to remove our "real" parent in the event of an "-s ours" type
+	 * merge.
+	 */
+	if (!su && sm)
+		sm->object.flags &= ~TMP_MARK;
+
 	pp = &commit->parents;
 	n = 0;
 	while ((p = *pp) != NULL) {
-- 
1.8.2.1.632.gd2b1879