Re: [PATCH 2/2] commit-graph tests: fix cryptic unportable "dd" invocation

[Junio C Hamano <gitster@pobox.com>]

Ævar Arnfjörð Bjarmason <avarab@gmail.com> writes:

> It was my reading of the seek=* section ("the implementation shall seek
> to the specified offset"). I didn't spot that bit covered in of=*. Yeah,
> I see that's defined & safe after reading that.

OK, so...

-- >8 --
From: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Date: Thu, 21 Feb 2019 20:28:49 +0100
Subject: [PATCH] commit-graph tests: fix cryptic unportable "dd" invocation

Change an unportable invocation of "dd" with count=0, that wanted to
truncate the commit-graph file.  In POSIX it is unspecified what
happens when count=0 is provided[1]. The NetBSD "dd" behavior
differs from GNU (and seemingly other BSDs), which as left this test
broken since d2b86fbaa1 ("commit-graph: fix buffer read-overflow",
2019-01-15).

Copying from /dev/null would seek/truncate to seek=$zero_pos and
stop immediately after that (without being able to copy anything),
which is the right way to truncate the file.

1. http://pubs.opengroup.org/onlinepubs/9699919799/utilities/dd.html

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Helped-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
---
 t/t5318-commit-graph.sh | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/t/t5318-commit-graph.sh b/t/t5318-commit-graph.sh
index d4bd1522fe..561796f280 100755
--- a/t/t5318-commit-graph.sh
+++ b/t/t5318-commit-graph.sh
@@ -382,7 +382,7 @@ corrupt_graph_and_verify() {
 	test_when_finished mv commit-graph-backup $objdir/info/commit-graph &&
 	cp $objdir/info/commit-graph commit-graph-backup &&
 	printf "$data" | dd of="$objdir/info/commit-graph" bs=1 seek="$pos" conv=notrunc &&
-	dd of="$objdir/info/commit-graph" bs=1 seek="$zero_pos" count=0 &&
+	dd of="$objdir/info/commit-graph" bs=1 seek="$zero_pos" if=/dev/null &&
 	generate_zero_bytes $(($orig_size - $zero_pos)) >>"$objdir/info/commit-graph" &&
 	test_must_fail git commit-graph verify 2>test_err &&
 	grep -v "^+" test_err >err &&
-- 
2.21.0-rc2