merge-base: fully contaminate the well.

The discussion on the list demonstrated a pathological case where
an ancestor of a merge-base can be left interesting.  This commit
introduces a postprocessing phase to fix it.

Signed-off-by: Junio C Hamano <junkio@cox.net>

diff --git a/merge-base.c b/merge-base.c
index 286bf0e..43a6818 100644 (file)
--- a/merge-base.c
+++ b/merge-base.c
@@ -80,6 +80,45 @@ static struct commit *interesting(struct commit_list *list)
  * Now, list does not have any interesting commit.  So we find the newest
  * commit from the result list that is not marked uninteresting.  Which is
  * commit B.
+ *
+ *
+ * Another pathological example how this thing can fail to mark an ancestor
+ * of a merge base as UNINTERESTING without the postprocessing phase.
+ *
+ *               2
+ *               H
+ *         1    / \
+ *         G   A   \
+ *         |\ /     \ 
+ *         | B       \
+ *         |  \       \
+ *          \  C       F
+ *           \  \     / 
+ *            \  D   /   
+ *             \ |  /
+ *              \| /
+ *               E
+ *
+ *      list                   A B C D E F G H
+ *      G1 H2                  - - - - - - 1 2
+ *      H2 E1 B1               - 1 - - 1 - 1 2
+ *      F2 E1 B1 A2            2 1 - - 1 2 1 2
+ *      E3 B1 A2               2 1 - - 3 2 1 2
+ *      B1 A2                  2 1 - - 3 2 1 2
+ *      C1 A2                  2 1 1 - 3 2 1 2
+ *      D1 A2                  2 1 1 1 3 2 1 2
+ *      A2                     2 1 1 1 3 2 1 2
+ *      B3                     2 3 1 1 3 2 1 2
+ *      C7                     2 3 7 1 3 2 1 2
+ *
+ * At this point, unfortunately, everybody in the list is
+ * uninteresting, so we fail to complete the following two
+ * steps to fully marking uninteresting commits.
+ *
+ *      D7                     2 3 7 7 3 2 1 2
+ *      E7                     2 3 7 7 7 2 1 2
+ *
+ * and we end up showing E as an interesting merge base.
  */
 
 static int show_all = 0;
@@ -88,6 +127,7 @@ static int merge_base(struct commit *rev1, struct commit *rev2)
 {
        struct commit_list *list = NULL;
        struct commit_list *result = NULL;
+       struct commit_list *tmp = NULL;
 
        if (rev1 == rev2) {
                printf("%s\n", sha1_to_hex(rev1->object.sha1));
@@ -104,9 +144,10 @@ static int merge_base(struct commit *rev1, struct commit *rev2)
 
        while (interesting(list)) {
                struct commit *commit = list->item;
-               struct commit_list *tmp = list, *parents;
+               struct commit_list *parents;
                int flags = commit->object.flags & 7;
 
+               tmp = list;
                list = list->next;
                free(tmp);
                if (flags == 3) {
@@ -130,6 +171,41 @@ static int merge_base(struct commit *rev1, struct commit *rev2)
        if (!result)
                return 1;
 
+       /*
+        * Postprocess to fully contaminate the well.
+        */
+       for (tmp = result; tmp; tmp = tmp->next) {
+               struct commit *c = tmp->item;
+               /* Reinject uninteresting ones to list,
+                * so we can scan their parents.
+                */
+               if (c->object.flags & UNINTERESTING)
+                       commit_list_insert(c, &list);
+       }
+       while (list) {
+               struct commit *c = list->item;
+               struct commit_list *parents;
+
+               tmp = list;
+               list = list->next;
+               free(tmp);
+
+               /* Anything taken out of the list is uninteresting, so
+                * mark all its parents uninteresting.  We do not
+                * parse new ones (we already parsed all the relevant
+                * ones).
+                */
+               parents = c->parents;
+               while (parents) {
+                       struct commit *p = parents->item;
+                       parents = parents->next;
+                       if (!(p->object.flags & UNINTERESTING)) {
+                               p->object.flags |= UNINTERESTING;
+                               commit_list_insert(p, &list);
+                       }
+               }
+       }
+
        while (result) {
                struct commit *commit = result->item;
                result = result->next;