merge-base: update the clean-up postprocessing

This removes the "contaminate the well even more" approach
taken in the current merge-base postprosessing code.  Instead,
when there are more than one merge-base results, we compute the
merge-base between them and see if one is a fast-forward of the
other, in which case the ancestor is removed from the result.

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

diff --git a/commit.c b/commit.c
index c6bf10d045a45a38352af0a7dc0b3732b7493182..522a6f3aca644f968b825abf83e1fa6474b559eb 100644 (file)
--- a/commit.c
+++ b/commit.c
@@ -854,6 +854,7 @@ void sort_in_topological_order_fn(struct commit_list ** list, int lifo,
 #define PARENT1                (1u<< 8)
 #define PARENT2                (1u<< 9)
 #define STALE          (1u<<10)
+#define RESULT         (1u<<11)
 
 static struct commit *interesting(struct commit_list *list)
 {
@@ -867,183 +868,42 @@ static struct commit *interesting(struct commit_list *list)
        return NULL;
 }
 
-/*
- * A pathological example of how this thing works.
- *
- * Suppose we had this commit graph, where chronologically
- * the timestamp on the commit are A <= B <= C <= D <= E <= F
- * and we are trying to figure out the merge base for E and F
- * commits.
- *
- *                  F
- *                 / \
- *            E   A   D
- *             \ /   /
- *              B   /
- *               \ /
- *                C
- *
- * First we push E and F to list to be processed.  E gets bit 1
- * and F gets bit 2.  The list becomes:
- *
- *     list=F(2) E(1), result=empty
- *
- * Then we pop F, the newest commit, from the list.  Its flag is 2.
- * We scan its parents, mark them reachable from the side that F is
- * reachable from, and push them to the list:
- *
- *     list=E(1) D(2) A(2), result=empty
- *
- * Next pop E and do the same.
- *
- *     list=D(2) B(1) A(2), result=empty
- *
- * Next pop D and do the same.
- *
- *     list=C(2) B(1) A(2), result=empty
- *
- * Next pop C and do the same.
- *
- *     list=B(1) A(2), result=empty
- *
- * Now it is B's turn.  We mark its parent, C, reachable from B's side,
- * and push it to the list:
- *
- *     list=C(3) A(2), result=empty
- *
- * Now pop C and notice it has flags==3.  It is placed on the result list,
- * and the list now contains:
- *
- *     list=A(2), result=C(3)
- *
- * We pop A and do the same.
- *
- *     list=B(3), result=C(3)
- *
- * Next, we pop B and something very interesting happens.  It has flags==3
- * so it is also placed on the result list, and its parents are marked
- * stale, retroactively, and placed back on the list:
- *
- *    list=C(7), result=C(7) B(3)
- *
- * Now, list does not have any interesting commit.  So we find the newest
- * commit from the result list that is not marked stale.  Which is
- * commit B.
- *
- *
- * Another pathological example how this thing used to fail to mark an
- * ancestor of a merge base as STALE before we introduced the
- * postprocessing phase (mark_reachable_commits).
- *
- *               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
- * stale, so we fail to complete the following two
- * steps to fully marking stale commits.
- *
- *      D7                     2 3 7 7 3 2 1 2
- *      E7                     2 3 7 7 7 2 1 2
- *
- * and we ended up showing E as an interesting merge base.
- * The postprocessing phase re-injects C and continues traversal
- * to contaminate D and E.
- */
-
-static void mark_reachable_commits(struct commit_list *result,
-                                  struct commit_list *list)
-{
-       struct commit_list *tmp;
-
-       /*
-        * Postprocess to fully contaminate the well.
-        */
-       for (tmp = result; tmp; tmp = tmp->next) {
-               struct commit *c = tmp->item;
-               /* Reinject stale ones to list,
-                * so we can scan their parents.
-                */
-               if (c->object.flags & STALE)
-                       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 stale, so
-                * mark all its parents stale.  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 & STALE)) {
-                               p->object.flags |= STALE;
-                               commit_list_insert(p, &list);
-                       }
-               }
-       }
-}
-
-struct commit_list *get_merge_bases(struct commit *rev1, struct commit *rev2,
-                                    int cleanup)
+static struct commit_list *merge_bases(struct commit *one, struct commit *two)
 {
        struct commit_list *list = NULL;
        struct commit_list *result = NULL;
-       struct commit_list *tmp = NULL;
 
-       if (rev1 == rev2)
-               return commit_list_insert(rev1, &result);
+       if (one == two)
+               /* We do not mark this even with RESULT so we do not
+                * have to clean it up.
+                */
+               return commit_list_insert(one, &result);
 
-       parse_commit(rev1);
-       parse_commit(rev2);
+       parse_commit(one);
+       parse_commit(two);
 
-       rev1->object.flags |= PARENT1;
-       rev2->object.flags |= PARENT2;
-       insert_by_date(rev1, &list);
-       insert_by_date(rev2, &list);
+       one->object.flags |= PARENT1;
+       two->object.flags |= PARENT2;
+       insert_by_date(one, &list);
+       insert_by_date(two, &list);
 
        while (interesting(list)) {
-               struct commit *commit = list->item;
+               struct commit *commit;
                struct commit_list *parents;
-               int flags = commit->object.flags
-                       & (PARENT1 | PARENT2 | STALE);
+               struct commit_list *n;
+               int flags;
 
-               tmp = list;
-               list = list->next;
-               free(tmp);
-               if (flags == (PARENT1 | PARENT2)) {
-                       insert_by_date(commit, &result);
+               commit = list->item;
+               n = list->next;
+               free(list);
+               list = n;
 
+               flags = commit->object.flags & (PARENT1 | PARENT2 | STALE);
+               if (flags == (PARENT1 | PARENT2)) {
+                       if (!(commit->object.flags & RESULT)) {
+                               commit->object.flags |= RESULT;
+                               insert_by_date(commit, &result);
+                       }
                        /* Mark parents of a found merge stale */
                        flags |= STALE;
                }
@@ -1059,35 +919,75 @@ struct commit_list *get_merge_bases(struct commit *rev1, struct commit *rev2,
                }
        }
 
-       if (!result)
-               goto finish;
-
-       if (result->next && list)
-               mark_reachable_commits(result, list);
+       /* Clean up the result to remove stale ones */
+       list = result; result = NULL;
+       while (list) {
+               struct commit_list *n = list->next;
+               if (!(list->item->object.flags & STALE))
+                       insert_by_date(list->item, &result);
+               free(list);
+               list = n;
+       }
+       return result;
+}
 
-       /* cull duplicates */
-       for (tmp = result, list = NULL; tmp; ) {
-               struct commit *commit = tmp->item;
-               struct commit_list *next = tmp->next;
-               if (commit->object.flags & STALE) {
-                       if (list != NULL)
-                               list->next = next;
-                       free(tmp);
-               } else {
-                       if (list == NULL)
-                               result = tmp;
-                       list = tmp;
-                       commit->object.flags |= STALE;
+struct commit_list *get_merge_bases(struct commit *one,
+                                   struct commit *two,
+                                    int cleanup)
+{
+       const unsigned all_flags = (PARENT1 | PARENT2 | STALE | RESULT);
+       struct commit_list *list;
+       struct commit **rslt;
+       struct commit_list *result;
+       int cnt, i, j;
+
+       result = merge_bases(one, two);
+       if (one == two)
+               return result;
+       if (!result || !result->next) {
+               if (cleanup) {
+                       clear_commit_marks(one, all_flags);
+                       clear_commit_marks(two, all_flags);
                }
-
-               tmp = next;
+               return result;
        }
 
- finish:
-       if (cleanup) {
-               clear_commit_marks(rev1, PARENT1 | PARENT2 | STALE);
-               clear_commit_marks(rev2, PARENT1 | PARENT2 | STALE);
+       /* There are more than one */
+       cnt = 0;
+       list = result;
+       while (list) {
+               list = list->next;
+               cnt++;
+       }
+       rslt = xcalloc(cnt, sizeof(*rslt));
+       for (list = result, i = 0; list; list = list->next)
+               rslt[i++] = list->item;
+       free_commit_list(result);
+
+       clear_commit_marks(one, all_flags);
+       clear_commit_marks(two, all_flags);
+       for (i = 0; i < cnt - 1; i++) {
+               for (j = i+1; j < cnt; j++) {
+                       if (!rslt[i] || !rslt[j])
+                               continue;
+                       result = merge_bases(rslt[i], rslt[j]);
+                       clear_commit_marks(rslt[i], all_flags);
+                       clear_commit_marks(rslt[j], all_flags);
+                       for (list = result; list; list = list->next) {
+                               if (rslt[i] == list->item)
+                                       rslt[i] = NULL;
+                               if (rslt[j] == list->item)
+                                       rslt[j] = NULL;
+                       }
+               }
        }
 
+       /* Surviving ones in rslt[] are the independent results */
+       result = NULL;
+       for (i = 0; i < cnt; i++) {
+               if (rslt[i])
+                       insert_by_date(rslt[i], &result);
+       }
+       free(rslt);
        return result;
 }