2.36 gitk/diff-tree --stdin regression fix

This only surfaced as a regression after 2.36 release, but the
breakage was already there with us for at least a year.

The diff_free() call is to be used after we completely finished with
a diffopt structure.  After "git diff A B" finishes producing
output, calling it before process exit is fine.  But there are
commands that prepares diff_options struct once, compares two sets
of paths, releases resources that were used to do the comparison,
then reuses the same diff_option struct to go on to compare the next
two sets of paths, like "git log -p".

After "git log -p" finishes showing a single commit, calling it
before it goes on to the next commit is NOT fine.  There is a
mechanism, the .no_free member in diff_options struct, to help "git
log" to avoid calling diff_free() after showing each commit and
instead call it just one.  When the mechanism was introduced in
e900d494 (diff: add an API for deferred freeing, 2021-02-11),
however, we forgot to do the same to "diff-tree --stdin", which *is*
a moral equivalent to "git log".

During 2.36 release cycle, we started clearing the pathspec in
diff_free(), so programs like gitk that runs

    git diff-tree --stdin -- <pathspec>

downstream of a pipe, processing one commit after another, started
showing irrelevant comparison outside the given <pathspec> from the
second commit.  The same commit, by forgetting to teach the .no_free
mechanism, broke "diff-tree --stdin -I<regexp>" and nobody noticed
it for over a year, presumably because it is so seldom used an
option.

But <pathspec> is a different story.  The breakage was very
prominently visible and was reported immediately after 2.36 was
released.

Fix this breakage by mimicking how "git log" utilizes the .no_free
member so that "diff-tree --stdin" behaves more similarly to "log".

Protect the fix with a few new tests.

Reported-by: Matthias Aßhauer <mha1993@live.de>
Helped-by: René Scharfe <l.s.r@web.de>
Helped-by: Phillip Wood <phillip.wood123@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 3 insertions, 0 deletions

diff --git a/builtin/diff-tree.c b/builtin/diff-tree.c
index 0e0ac1f167..116097a404 100644
--- a/builtin/diff-tree.c
+++ b/builtin/diff-tree.c
@@ -195,6 +195,7 @@ int cmd_diff_tree(int argc, const char **argv, const char *prefix)
 		int saved_dcctc = 0;
 
 		opt->diffopt.rotate_to_strict = 0;
+		opt->diffopt.no_free = 1;
 		if (opt->diffopt.detect_rename) {
 			if (!the_index.cache)
 				repo_read_index(the_repository);
@@ -217,6 +218,8 @@ int cmd_diff_tree(int argc, const char **argv, const char *prefix)
 		}
 		opt->diffopt.degraded_cc_to_c = saved_dcctc;
 		opt->diffopt.needed_rename_limit = saved_nrl;
+		opt->diffopt.no_free = 0;
+		diff_free(&opt->diffopt);
 	}
 
 	return diff_result_code(&opt->diffopt, 0);