Re: Possible git-diff bug when using exit-code with diff filters

["René Scharfe" <l.s.r@web.de>]

Am 20.04.24 um 03:13 schrieb German Lashevich:
>> What did you do before the bug happened? (Steps to reproduce your issue)
>
> I configured a diff filter via gitattributes to use a custom script that,
> sometimes, can change the files being compared in a way that there are no
> differences between them.
> Then I used `git diff --exit-code` on the changed file.
>
>> What did you expect to happen? (Expected behavior)
>
> I expected `git diff --exit-code` to return 0, since there are no differences
> between the files after the filter is applied.
>
>> What happened instead? (Actual behavior)
>
> `git diff --exit-code` correctly produces no output, but returns 1.
>
>> What's different between what you expected and what actually happened?
>
> The difference is that `git diff --exit-code`, instead of returning 0, returns
> 1 even when there is no output.
>
>> Anything else you want to add:
>
> I have prepared a repository with a test case that reproduces the issue.
> You can find it at https://github.com/Zebradil/git-diff-exit-code-bug-repro
> The Readme file in the repository contains instructions on how to reproduce
> the issue.

You can more easily reproduce it by setting the environment variable
GIT_EXTERNAL_DIFF or the configuration setting diff.external -- no
attributes needed.

The output of external diffs does not influence the exit code currently.
We don't even have a way to find out if there was any output.

The patch below pipes it through a buffer and sets found_changes only if
something was written.  It fixes the error code for external diffs
producing no output, but breaks several am, pull and merge tests (t4150,
t4151, t5520, t6424, t6430, t7611).  So diff_from_contents doesn't work
quite as I expected.

And slurping in the whole output of external diffs is inefficient, of
course.  That could be fixed later by using a small buffer and writing
directly as we read.

René


diff --git a/diff.c b/diff.c
index 108c187577..76631644e5 100644
--- a/diff.c
+++ b/diff.c
@@ -40,6 +40,7 @@
 #include "setup.h"
 #include "strmap.h"
 #include "ws.h"
+#include "write-or-die.h"

 #ifdef NO_FAST_WORKING_DIRECTORY
 #define FAST_WORKING_DIRECTORY 0
@@ -4404,8 +4405,18 @@ static void run_external_diff(const char *pgm,
 	diff_free_filespec_data(one);
 	diff_free_filespec_data(two);
 	cmd.use_shell = 1;
-	if (run_command(&cmd))
-		die(_("external diff died, stopping at %s"), name);
+	if (o->flags.exit_with_status) {
+		struct strbuf out = STRBUF_INIT;
+		if (capture_command(&cmd, &out, 0))
+			die(_("external diff died, stopping at %s"), name);
+		if (out.len)
+			o->found_changes = 1;
+		write_or_die(1, out.buf, out.len);
+		strbuf_release(&out);
+	} else {
+		if (run_command(&cmd))
+			die(_("external diff died, stopping at %s"), name);
+	}

 	remove_tempfile();
 }
@@ -4851,6 +4862,7 @@ void diff_setup_done(struct diff_options *options)
 	 */

 	if ((options->xdl_opts & XDF_WHITESPACE_FLAGS) ||
+	    options->flags.exit_with_status ||
 	    options->ignore_regex_nr)
 		options->flags.diff_from_contents = 1;
 	else
diff --git a/t/t4020-diff-external.sh b/t/t4020-diff-external.sh
index fdd865f7c3..26430ca66b 100755
--- a/t/t4020-diff-external.sh
+++ b/t/t4020-diff-external.sh
@@ -172,6 +172,14 @@ test_expect_success 'no diff with -diff' '
 	grep Binary out
 '

+test_expect_success 'diff.external and --exit-code with output' '
+	test_expect_code 1 git -c diff.external=echo diff --exit-code
+'
+
+test_expect_success 'diff.external and --exit-code without output' '
+	git -c diff.external=true diff --exit-code
+'
+
 echo NULZbetweenZwords | perl -pe 'y/Z/\000/' > file

 test_expect_success 'force diff with "diff"' '