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-rw-r--r--Documentation/git-merge-base.txt38
-rw-r--r--builtin/merge-base.c109
-rwxr-xr-xt/t6010-merge-base.sh28
3 files changed, 173 insertions, 2 deletions
diff --git a/Documentation/git-merge-base.txt b/Documentation/git-merge-base.txt
index 87842e3..808426f 100644
--- a/Documentation/git-merge-base.txt
+++ b/Documentation/git-merge-base.txt
@@ -13,6 +13,7 @@ SYNOPSIS
'git merge-base' [-a|--all] --octopus <commit>...
'git merge-base' --is-ancestor <commit> <commit>
'git merge-base' --independent <commit>...
+'git merge-base' --fork-point <ref> [<commit>]
DESCRIPTION
-----------
@@ -24,8 +25,8 @@ that does not have any better common ancestor is a 'best common
ancestor', i.e. a 'merge base'. Note that there can be more than one
merge base for a pair of commits.
-OPERATION MODE
---------------
+OPERATION MODES
+---------------
As the most common special case, specifying only two commits on the
command line means computing the merge base between the given two commits.
@@ -56,6 +57,14 @@ from linkgit:git-show-branch[1] when used with the `--merge-base` option.
and exit with status 0 if true, or with status 1 if not.
Errors are signaled by a non-zero status that is not 1.
+--fork-point::
+ Find the point at which a branch (or any history that leads
+ to <commit>) forked from another branch (or any reference)
+ <ref>. This does not just look for the common ancestor of
+ the two commits, but also takes into account the reflog of
+ <ref> to see if the history leading to <commit> forked from
+ an earlier incarnation of the branch <ref> (see discussion
+ on this mode below).
OPTIONS
-------
@@ -137,6 +146,31 @@ In modern git, you can say this in a more direct way:
instead.
+Discussion on fork-point mode
+-----------------------------
+
+After working on the `topic` branch created with `git checkout -b
+topic origin/master`, the history of remote-tracking branch
+`origin/master` may have been rewound and rebuilt, leading to a
+history of this shape:
+
+ o---B1
+ /
+ ---o---o---B2--o---o---o---B (origin/master)
+ \
+ B3
+ \
+ Derived (topic)
+
+where `origin/master` used to point at commits B3, B2, B1 and now it
+points at B, and your `topic` branch was started on top of it back
+when `origin/master` was at B3. This mode uses the reflog of
+`origin/master` to find B3 as the fork point, so that the `topic`
+can be rebased on top of the updated `origin/master` by:
+
+ $ fork_point=$(git merge-base --fork-point origin/master topic)
+ $ git rebase --onto origin/master $fork_point topic
+
See also
--------
diff --git a/builtin/merge-base.c b/builtin/merge-base.c
index d39c910..a292323 100644
--- a/builtin/merge-base.c
+++ b/builtin/merge-base.c
@@ -1,6 +1,9 @@
#include "builtin.h"
#include "cache.h"
#include "commit.h"
+#include "refs.h"
+#include "diff.h"
+#include "revision.h"
#include "parse-options.h"
static int show_merge_base(struct commit **rev, int rev_nr, int show_all)
@@ -27,6 +30,7 @@ static const char * const merge_base_usage[] = {
N_("git merge-base [-a|--all] --octopus <commit>..."),
N_("git merge-base --independent <commit>..."),
N_("git merge-base --is-ancestor <commit> <commit>"),
+ N_("git merge-base --fork-point <ref> [<commit>]"),
NULL
};
@@ -85,6 +89,103 @@ static int handle_is_ancestor(int argc, const char **argv)
return 1;
}
+struct rev_collect {
+ struct commit **commit;
+ int nr;
+ int alloc;
+ unsigned int initial : 1;
+};
+
+static void add_one_commit(unsigned char *sha1, struct rev_collect *revs)
+{
+ struct commit *commit;
+
+ if (is_null_sha1(sha1))
+ return;
+
+ commit = lookup_commit(sha1);
+ if (!commit ||
+ (commit->object.flags & TMP_MARK) ||
+ parse_commit(commit))
+ return;
+
+ ALLOC_GROW(revs->commit, revs->nr + 1, revs->alloc);
+ revs->commit[revs->nr++] = commit;
+ commit->object.flags |= TMP_MARK;
+}
+
+static int collect_one_reflog_ent(unsigned char *osha1, unsigned char *nsha1,
+ const char *ident, unsigned long timestamp,
+ int tz, const char *message, void *cbdata)
+{
+ struct rev_collect *revs = cbdata;
+
+ if (revs->initial) {
+ revs->initial = 0;
+ add_one_commit(osha1, revs);
+ }
+ add_one_commit(nsha1, revs);
+ return 0;
+}
+
+static int handle_fork_point(int argc, const char **argv)
+{
+ unsigned char sha1[20];
+ char *refname;
+ const char *commitname;
+ struct rev_collect revs;
+ struct commit *derived;
+ struct commit_list *bases;
+ int i, ret = 0;
+
+ switch (dwim_ref(argv[0], strlen(argv[0]), sha1, &refname)) {
+ case 0:
+ die("No such ref: '%s'", argv[0]);
+ case 1:
+ break; /* good */
+ default:
+ die("Ambiguous refname: '%s'", argv[0]);
+ }
+
+ commitname = (argc == 2) ? argv[1] : "HEAD";
+ if (get_sha1(commitname, sha1))
+ die("Not a valid object name: '%s'", commitname);
+
+ derived = lookup_commit_reference(sha1);
+ memset(&revs, 0, sizeof(revs));
+ revs.initial = 1;
+ for_each_reflog_ent(refname, collect_one_reflog_ent, &revs);
+
+ for (i = 0; i < revs.nr; i++)
+ revs.commit[i]->object.flags &= ~TMP_MARK;
+
+ bases = get_merge_bases_many(derived, revs.nr, revs.commit, 0);
+
+ /*
+ * There should be one and only one merge base, when we found
+ * a common ancestor among reflog entries.
+ */
+ if (!bases || bases->next) {
+ ret = 1;
+ goto cleanup_return;
+ }
+
+ /* And the found one must be one of the reflog entries */
+ for (i = 0; i < revs.nr; i++)
+ if (&bases->item->object == &revs.commit[i]->object)
+ break; /* found */
+ if (revs.nr <= i) {
+ ret = 1; /* not found */
+ goto cleanup_return;
+ }
+
+ printf("%s\n", sha1_to_hex(bases->item->object.sha1));
+
+cleanup_return:
+ free_commit_list(bases);
+ return ret;
+}
+
int cmd_merge_base(int argc, const char **argv, const char *prefix)
{
struct commit **rev;
@@ -100,6 +201,8 @@ int cmd_merge_base(int argc, const char **argv, const char *prefix)
N_("list revs not reachable from others"), 'r'),
OPT_CMDMODE(0, "is-ancestor", &cmdmode,
N_("is the first one ancestor of the other?"), 'a'),
+ OPT_CMDMODE(0, "fork-point", &cmdmode,
+ N_("find where <commit> forked from reflog of <ref>"), 'f'),
OPT_END()
};
@@ -120,6 +223,12 @@ int cmd_merge_base(int argc, const char **argv, const char *prefix)
if (cmdmode == 'r' || cmdmode == 'o')
return handle_octopus(argc, argv, cmdmode == 'r', show_all);
+ if (cmdmode == 'f') {
+ if (argc < 1 || 2 < argc)
+ usage_with_options(merge_base_usage, options);
+ return handle_fork_point(argc, argv);
+ }
+
if (argc < 2)
usage_with_options(merge_base_usage, options);
diff --git a/t/t6010-merge-base.sh b/t/t6010-merge-base.sh
index f80bba8..30a6833 100755
--- a/t/t6010-merge-base.sh
+++ b/t/t6010-merge-base.sh
@@ -230,4 +230,32 @@ test_expect_success 'criss-cross merge-base for octopus-step' '
test_cmp expected.sorted actual.sorted
'
+test_expect_success 'using reflog to find the fork point' '
+ git reset --hard &&
+ git checkout -b base $E &&
+
+ (
+ for count in 1 2 3
+ do
+ git commit --allow-empty -m "Base commit #$count" &&
+ git rev-parse HEAD >expect$count &&
+ git checkout -B derived &&
+ git commit --allow-empty -m "Derived #$count" &&
+ git rev-parse HEAD >derived$count &&
+ git checkout -B base $E || exit 1
+ done
+
+ for count in 1 2 3
+ do
+ git merge-base --fork-point base $(cat derived$count) >actual &&
+ test_cmp expect$count actual || exit 1
+ done
+
+ ) &&
+ # check that we correctly default to HEAD
+ git checkout derived &&
+ git merge-base --fork-point base >actual &&
+ test_cmp expect3 actual
+'
+
test_done