[PATCH v4 2/2] hook API: fix v2.36.0 regression: hooks should be connected to a TTY

[Ævar Arnfjörð Bjarmason <avarab@xxxxxxxxx>]

Fix a regression reported[1] in f443246b9f2 (commit: convert
{pre-commit,prepare-commit-msg} hook to hook.h, 2021-12-22): Due to
using the run_process_parallel() API in the earlier 96e7225b310 (hook:
add 'run' subcommand, 2021-12-22) we'd capture the hook's stderr and
stdout, and thus lose the connection to the TTY in the case of
e.g. the "pre-commit" hook.

As a preceding commit notes GNU parallel's similar --ungroup option
also has it emit output faster. While we're unlikely to have hooks
that emit truly massive amounts of output (or where the performance
thereof matters) it's still informative to measure the overhead. In a
similar "seq" test we're now ~30% faster:

	$ cat .git/hooks/seq-hook; git hyperfine -L rev origin/master,HEAD~0 -s 'make CFLAGS=-O3' './git hook run seq-hook'
	#!/bin/sh

	seq 100000000
	Benchmark 1: ./git hook run seq-hook' in 'origin/master
	  Time (mean ± σ):     787.1 ms ±  13.6 ms    [User: 701.6 ms, System: 534.4 ms]
	  Range (min … max):   773.2 ms … 806.3 ms    10 runs

	Benchmark 2: ./git hook run seq-hook' in 'HEAD~0
	  Time (mean ± σ):     603.4 ms ±   1.6 ms    [User: 573.1 ms, System: 30.3 ms]
	  Range (min … max):   601.0 ms … 606.2 ms    10 runs

	Summary
	  './git hook run seq-hook' in 'HEAD~0' ran
	    1.30 ± 0.02 times faster than './git hook run seq-hook' in 'origin/master'

1. https://lore.kernel.org/git/CA+dzEBn108QoMA28f0nC8K21XT+Afua0V2Qv8XkR8rAeqUCCZw@xxxxxxxxxxxxxx/

Reported-by: Anthony Sottile <asottile@xxxxxxxxx>
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@xxxxxxxxx>
---
 hook.c          |  1 +
 t/t1800-hook.sh | 37 +++++++++++++++++++++++++++++++++++++
 2 files changed, 38 insertions(+)

diff --git a/hook.c b/hook.c
index 1d51be3b77a..7451205657a 100644
--- a/hook.c
+++ b/hook.c
@@ -144,6 +144,7 @@ int run_hooks_opt(const char *hook_name, struct run_hooks_opt *options)
 		cb_data.hook_path = abs_path.buf;
 	}
 
+	run_processes_parallel_ungroup = 1;
 	run_processes_parallel_tr2(jobs,
 				   pick_next_hook,
 				   notify_start_failure,
diff --git a/t/t1800-hook.sh b/t/t1800-hook.sh
index 26ed5e11bc8..0b8370d1573 100755
--- a/t/t1800-hook.sh
+++ b/t/t1800-hook.sh
@@ -4,6 +4,7 @@ test_description='git-hook command'
 
 TEST_PASSES_SANITIZE_LEAK=true
 . ./test-lib.sh
+. "$TEST_DIRECTORY"/lib-terminal.sh
 
 test_expect_success 'git hook usage' '
 	test_expect_code 129 git hook &&
@@ -120,4 +121,40 @@ test_expect_success 'git -c core.hooksPath=<PATH> hook run' '
 	test_cmp expect actual
 '
 
+test_hook_tty() {
+	local fd="$1" &&
+
+	cat >expect &&
+
+	test_when_finished "rm -rf repo" &&
+	git init repo &&
+
+	test_hook -C repo pre-commit <<-EOF &&
+	{
+		test -t 1 && echo >&$fd STDOUT TTY || echo >&$fd STDOUT NO TTY &&
+		test -t 2 && echo >&$fd STDERR TTY || echo >&$fd STDERR NO TTY
+	} $fd>actual
+	EOF
+
+	test_commit -C repo A &&
+	test_commit -C repo B &&
+	git -C repo reset --soft HEAD^ &&
+	test_terminal git -C repo commit -m"B.new" &&
+	test_cmp expect repo/actual
+}
+
+test_expect_success TTY 'git hook run: stdout and stderr are connected to a TTY: STDOUT redirect' '
+	test_hook_tty 1 <<-\EOF
+	STDOUT NO TTY
+	STDERR TTY
+	EOF
+'
+
+test_expect_success TTY 'git hook run: stdout and stderr are connected to a TTY: STDERR redirect' '
+	test_hook_tty 2 <<-\EOF
+	STDOUT TTY
+	STDERR NO TTY
+	EOF
+'
+
 test_done
-- 
2.36.1.1103.g036c05811b0