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merge-base: update the clean-up postprocessing
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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>
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Junio C Hamano committed Jul 9, 2006
1 parent ff4c848 commit f324943
Showing 1 changed file with 90 additions and 190 deletions.
280 changes: 90 additions & 190 deletions commit.c
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Original file line number Diff line number Diff line change
Expand Up @@ -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)
{
Expand All @@ -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;
}
Expand All @@ -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;
}

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