[PATCH v2 6/8] simplify-merges: never remove all TREESAME parents

[Kevin Bracey <kevin@bracey.fi>]

When simplifying an odd merge, such as one that used "-s ours", we may
find ourselves TREESAME to apparently redundant parents. Prevent
simplify_merges() from removing every TREESAME parent; if this would
happen reinstate the first TREESAME parent - the one that the default
log would have followed.

This avoids producing a totally disjoint history from the default log
when the default log is a better explanation of the end result, and aids
visualisation of odd merges.

Signed-off-by: Kevin Bracey <kevin@bracey.fi>
---
 Documentation/rev-list-options.txt |  3 +-
 revision.c                         | 69 ++++++++++++++++++++++++++++++++++++++
 2 files changed, 71 insertions(+), 1 deletion(-)

diff --git a/Documentation/rev-list-options.txt b/Documentation/rev-list-options.txt
index 1dad341..e23bdb0 100644
--- a/Documentation/rev-list-options.txt
+++ b/Documentation/rev-list-options.txt
@@ -471,7 +471,8 @@ 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 c88ded8..7535757 100644
--- a/revision.c
+++ b/revision.c
@@ -2119,6 +2119,73 @@ static int mark_redundant_parents(struct rev_info *revs, struct commit *commit)
 	return marked;
 }
 
+/*
+ * Awkward naming - this means one parent we are TREESAME to.
+ * cf mark_treesame_root_parents: root parents that are TREESAME (to an
+ * empty tree). Better name suggestions?
+ */
+static int leave_one_treesame_to_parent(struct rev_info *revs, struct commit *commit)
+{
+	struct treesame_state *ts = lookup_decoration(&revs->treesame, &commit->object);
+	struct commit *unmarked = NULL, *marked = NULL;
+	struct commit_list *p;
+	unsigned n;
+
+	for (p = commit->parents, n = 0; p; p = p->next, n++) {
+		if (ts->treesame[n]) {
+			if (p->item->object.flags & TMP_MARK) {
+				if (!marked)
+					marked = p->item;
+			} else {
+				if (!unmarked) {
+					unmarked = p->item;
+					break;
+				}
+			}
+		}
+	}
+
+	/*
+	 * If we are TREESAME to a marked-for-deletion parent, but not to any
+	 * unmarked parents, unmark the first TREESAME parent. This is the
+	 * parent that the default simplify_history==1 scan would have followed,
+	 * and it doesn't make sense to omit that path when asking for a
+	 * simplified full history. Retaining it improves the chances of
+	 * understanding odd missed merges that took an old version of a file.
+	 *
+	 * Example:
+	 *
+	 *   I---------X       A modified the file, but mainline merge X used
+	 *    \       /        "-s ours", so took the version from I. X is
+	 *     `--A--'         TREESAME to I and !TREESAME to A.
+	 *
+	 * Default log from X would produce "I". Without this check,
+	 * --full-history --simplify-merges would produce "I-A-X", showing
+	 * the merge commit X and that it changed A, but not making clear that
+	 * it had just taken the I version. With this check, the topology above
+	 * is retained.
+	 *
+	 * Note that it is possible that the simplification chooses a different
+	 * TREESAME parent from the default, in which case this test doesn't
+	 * activate, and we _do_ drop the default parent. Example:
+	 *
+	 *   I------X         A modified the file, but it was reverted in B,
+	 *    \    /          meaning mainline merge X is TREESAME to both
+	 *     A--B           parents.
+	 *
+	 * Default log would produce "I" by following the first parent;
+	 * --full-history --simplify-merges will produce "I-A-B". But this is a
+	 * reasonable result - it presents a logical full history leading from
+	 * I to X, and X is not an important merge.
+	 */
+	if (!unmarked && marked) {
+		marked->object.flags &= ~TMP_MARK;
+		return 1;
+	}
+
+	return 0;
+}
+
 static int remove_marked_parents(struct rev_info *revs, struct commit *commit)
 {
 	struct commit_list **pp, *p;
@@ -2222,6 +2289,8 @@ static struct commit_list **simplify_one(struct rev_info *revs, struct commit *c
 	if (1 < cnt) {
 		int marked = mark_redundant_parents(revs, commit);
 		if (marked)
+			marked -= leave_one_treesame_to_parent(revs, commit);
+		if (marked)
 			cnt = remove_marked_parents(revs, commit);
 	}
 
-- 
1.8.2.1.632.gd2b1879