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git-apply: try threeway first when "--3way" is used

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The apply_fragments() method of "git apply"
can silently apply patches incorrectly if
a file has repeating contents. In these
cases a three-way merge is capable of applying
it correctly in more situations, and will
show a conflict rather than applying it
incorrectly. However, because the patches
apply "successfully" using apply_fragments(),
git will never fall back to the merge, even
if the "--3way" flag is used, and the user has
no way to ensure correctness by forcing the
three-way merge method.

Change the behavior so that when "--3way" is used,
git will always try the three-way merge first and
will only fall back to apply_fragments() in cases
where blobs are not available or some other error
(but not in the case of a merge conflict).

Since user-facing results will be different,
this has backwards compatibility implications
for users depending on the old behavior. In
addition, the three-way merge will be slower
than direct patch application.

Signed-off-by: Jerry Zhang <jerry@skydio.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
This commit is contained in:
Jerry Zhang 2021-04-06 16:25:32 -07:00 committed by Junio C Hamano
parent 2e36527f23
commit 923cd87ac8
3 changed files with 28 additions and 10 deletions
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5
Documentation/git-apply.txt
View file

@ -84,9 +84,8 @@ OPTIONS
-3::
--3way::
When the patch does not apply cleanly, fall back on 3-way merge if
the patch records the identity of blobs it is supposed to apply to,
and we have those blobs available locally, possibly leaving the
Attempt 3-way merge if the patch records the identity of blobs it is supposed
to apply to and we have those blobs available locally, possibly leaving the
conflict markers in the files in the working tree for the user to
resolve. This option implies the `--index` option, and is incompatible
with the `--reject` and the `--cached` options.

13
apply.c
View file

@ -3570,10 +3570,10 @@ static int try_threeway(struct apply_state *state,
write_object_file("", 0, blob_type, &pre_oid);
else if (get_oid(patch->old_oid_prefix, &pre_oid) ||
read_blob_object(&buf, &pre_oid, patch->old_mode))
return error(_("repository lacks the necessary blob to fall back on 3-way merge."));
return error(_("repository lacks the necessary blob to perform 3-way merge."));
if (state->apply_verbosity > verbosity_silent)
fprintf(stderr, _("Falling back to three-way merge...\n"));
fprintf(stderr, _("Performing three-way merge...\n"));
img = strbuf_detach(&buf, &len);
prepare_image(&tmp_image, img, len, 1);
@ -3605,7 +3605,7 @@ static int try_threeway(struct apply_state *state,
if (status < 0) {
if (state->apply_verbosity > verbosity_silent)
fprintf(stderr,
_("Failed to fall back on three-way merge...\n"));
_("Failed to perform three-way merge...\n"));
return status;
}
@ -3638,10 +3638,9 @@ static int apply_data(struct apply_state *state, struct patch *patch,
if (load_preimage(state, &image, patch, st, ce) < 0)
return -1;
if (patch->direct_to_threeway ||
apply_fragments(state, &image, patch) < 0) {
if (!state->threeway || try_threeway(state, &image, patch, st, ce) < 0) {
/* Note: with --reject, apply_fragments() returns 0 */
if (!state->threeway || try_threeway(state, &image, patch, st, ce) < 0)
if (patch->direct_to_threeway || apply_fragments(state, &image, patch) < 0)
return -1;
}
patch->result = image.buf;
@ -5018,7 +5017,7 @@ int apply_parse_options(int argc, const char **argv,
OPT_BOOL(0, "apply", force_apply,
N_("also apply the patch (use with --stat/--summary/--check)")),
OPT_BOOL('3', "3way", &state->threeway,
N_( "attempt three-way merge if a patch does not apply")),
N_( "attempt three-way merge, fall back on normal patch if that fails")),
OPT_FILENAME(0, "build-fake-ancestor", &state->fake_ancestor,
N_("build a temporary index based on embedded index information")),
/* Think twice before adding "--nul" synonym to this */

20
t/t4108-apply-threeway.sh
View file

@ -160,4 +160,24 @@ test_expect_success 'apply -3 with add/add conflict (dirty working tree)' '
test_cmp three.save three
'
test_expect_success 'apply -3 with ambiguous repeating file' '
git reset --hard &&
test_write_lines 1 2 1 2 1 2 1 2 1 2 1 >one_two_repeat &&
git add one_two_repeat &&
git commit -m "init one" &&
test_write_lines 1 2 1 2 1 2 1 2 one 2 1 >one_two_repeat &&
git commit -a -m "change one" &&
git diff HEAD~ >Repeat.diff &&
git reset --hard HEAD~ &&
test_write_lines 1 2 1 2 1 2 one 2 1 2 one >one_two_repeat &&
git commit -a -m "change surrounding one" &&
git apply --index --3way Repeat.diff &&
test_write_lines 1 2 1 2 1 2 one 2 one 2 one >expect &&
test_cmp expect one_two_repeat
'
test_done