Subject: [PATCH v2 1/1] cat-file: add mailmap subcommand to --batch-command

From: Siddharth Asthana <[email-masked]>
Date: Sun, 29 Mar 2026 13:58:08 +0530

git-cat-file(1)'s --batch-command works with the --use-mailmap option,
but this option needs to be set when the process is created. This means
we cannot change this option mid-operation.

At GitLab, Gitaly caches git-cat-file processes and it would be useful
if --batch-command supported toggling mailmap dynamically with existing
processes.

Add a `mailmap` subcommand to --batch-command that takes a single
argument: `yes` to enable mailmap and `no` to disable it. When enabled,
mailmap data is loaded from disk on first use and kept in memory so that
toggling back on does not require reloading.

Suggested-by: Junio C Hamano <[email-masked]>
Signed-off-by: Siddharth Asthana <[email-masked]>
---
CI: https://gitlab.com/gitlab-org/git/-/pipelines/2416081861

 Documentation/git-cat-file.adoc |  7 +++++
 builtin/cat-file.c              | 30 ++++++++++++++++++---
 t/t4203-mailmap.sh              | 48 +++++++++++++++++++++++++++++++++
 3 files changed, 81 insertions(+), 4 deletions(-)

diff --git a/Documentation/git-cat-file.adoc b/Documentation/git-cat-file.adoc
index c139f55a16..af32e929a8 100644
--- a/Documentation/git-cat-file.adoc
+++ b/Documentation/git-cat-file.adoc
@@ -174,6 +174,13 @@ flush::
 	since the beginning or since the last flush was issued. When `--buffer`
 	is used, no output will come until a `flush` is issued. When `--buffer`
 	is not used, commands are flushed each time without issuing `flush`.
+
+mailmap <yes|no>::
+	Enable or disable mailmap for subsequent `contents` and `info`
+	commands. When `yes` is given, mailmap data is loaded from disk on
+	first use and kept in memory; passing `yes` again does not reload it.
+	When `no` is given, mailmap is disabled but the data stays in memory
+	so that a later `mailmap yes` does not need to reload it from disk.
 --
 +
 
diff --git a/builtin/cat-file.c b/builtin/cat-file.c
index b6f12f41d6..a53926d2bb 100644
--- a/builtin/cat-file.c
+++ b/builtin/cat-file.c
@@ -54,6 +54,7 @@ static const char *force_path;
 
 static struct string_list mailmap = STRING_LIST_INIT_NODUP;
 static int use_mailmap;
+static int mailmap_loaded;
 
 static char *replace_idents_using_mailmap(char *, size_t *);
 
@@ -692,6 +693,24 @@ static void parse_cmd_info(struct batch_options *opt,
 	batch_one_object(line, output, opt, data);
 }
 
+static void parse_cmd_mailmap(struct batch_options *opt UNUSED,
+			      const char *line,
+			      struct strbuf *output UNUSED,
+			      struct expand_data *data UNUSED)
+{
+	if (!strcmp(line, "yes")) {
+		if (!mailmap_loaded) {
+			read_mailmap(the_repository, &mailmap);
+			mailmap_loaded = 1;
+		}
+		use_mailmap = 1;
+	} else if (!strcmp(line, "no")) {
+		use_mailmap = 0;
+	} else {
+		die(_("mailmap: unknown argument '%s', expected 'yes' or 'no'"), line);
+	}
+}
+
 static void dispatch_calls(struct batch_options *opt,
 		struct strbuf *output,
 		struct expand_data *data,
@@ -725,9 +744,10 @@ static const struct parse_cmd {
 	parse_cmd_fn_t fn;
 	unsigned takes_args;
 } commands[] = {
-	{ "contents", parse_cmd_contents, 1},
-	{ "info", parse_cmd_info, 1},
-	{ "flush", NULL, 0},
+	{ "contents", parse_cmd_contents, 1 },
+	{ "info", parse_cmd_info, 1 },
+	{ "flush", NULL, 0 },
+	{ "mailmap", parse_cmd_mailmap, 1 },
 };
 
 static void batch_objects_command(struct batch_options *opt,
@@ -1127,8 +1147,10 @@ int cmd_cat_file(int argc,
 	opt_cw = (opt == 'c' || opt == 'w');
 	opt_epts = (opt == 'e' || opt == 'p' || opt == 't' || opt == 's');
 
-	if (use_mailmap)
+	if (use_mailmap) {
 		read_mailmap(the_repository, &mailmap);
+		mailmap_loaded = 1;
+	}
 
 	switch (batch.objects_filter.choice) {
 	case LOFC_DISABLED:
diff --git a/t/t4203-mailmap.sh b/t/t4203-mailmap.sh
index 74b7ddccb2..f66637cd86 100755
--- a/t/t4203-mailmap.sh
+++ b/t/t4203-mailmap.sh
@@ -1133,6 +1133,54 @@ test_expect_success 'git cat-file --batch-command returns correct size with --us
 	test_cmp expect actual
 '
 
+test_expect_success 'git cat-file --batch-command mailmap yes enables mailmap mid-stream' '
+	test_when_finished "rm .mailmap" &&
+	cat >.mailmap <<-\EOF &&
+	C O Mitter <[email-masked]> Orig <[email-masked]>
+	EOF
+	commit_sha=$(git rev-parse HEAD) &&
+	git cat-file commit HEAD >commit_no_mailmap.out &&
+	git cat-file --use-mailmap commit HEAD >commit_mailmap.out &&
+	size_no_mailmap=$(wc -c <commit_no_mailmap.out) &&
+	size_mailmap=$(wc -c <commit_mailmap.out) &&
+	printf "info HEAD\nmailmap yes\ninfo HEAD\n" | git cat-file --batch-command >actual &&
+	echo $commit_sha commit $size_no_mailmap >expect &&
+	echo $commit_sha commit $size_mailmap >>expect &&
+	test_cmp expect actual
+'
+
+test_expect_success 'git cat-file --batch-command mailmap no disables mailmap mid-stream' '
+	test_when_finished "rm .mailmap" &&
+	cat >.mailmap <<-\EOF &&
+	C O Mitter <[email-masked]> Orig <[email-masked]>
+	EOF
+	commit_sha=$(git rev-parse HEAD) &&
+	git cat-file commit HEAD >commit_no_mailmap.out &&
+	git cat-file --use-mailmap commit HEAD >commit_mailmap.out &&
+	size_no_mailmap=$(wc -c <commit_no_mailmap.out) &&
+	size_mailmap=$(wc -c <commit_mailmap.out) &&
+	printf "mailmap yes\ninfo HEAD\nmailmap no\ninfo HEAD\n" | git cat-file --batch-command >actual &&
+	echo $commit_sha commit $size_mailmap >expect &&
+	echo $commit_sha commit $size_no_mailmap >>expect &&
+	test_cmp expect actual
+'
+
+test_expect_success 'git cat-file --batch-command mailmap works in --buffer mode' '
+	test_when_finished "rm .mailmap" &&
+	cat >.mailmap <<-\EOF &&
+	C O Mitter <[email-masked]> Orig <[email-masked]>
+	EOF
+	commit_sha=$(git rev-parse HEAD) &&
+	git cat-file commit HEAD >commit_no_mailmap.out &&
+	git cat-file --use-mailmap commit HEAD >commit_mailmap.out &&
+	size_no_mailmap=$(wc -c <commit_no_mailmap.out) &&
+	size_mailmap=$(wc -c <commit_mailmap.out) &&
+	printf "mailmap yes\ninfo HEAD\nmailmap no\ninfo HEAD\nflush\n" | git cat-file --batch-command --buffer >actual &&
+	echo $commit_sha commit $size_mailmap >expect &&
+	echo $commit_sha commit $size_no_mailmap >>expect &&
+	test_cmp expect actual
+'
+
 test_expect_success 'git cat-file --mailmap works with different author and committer' '
 	test_when_finished "rm .mailmap" &&
 	cat >.mailmap <<-\EOF &&
-- 
2.51.0