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

["Ævar Arnfjörð Bjarmason" <avarab@gmail.com>]

Change an unportable invocation of "dd" that truncated the
commit-graph to call Perl's truncate() function instead.

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).

In POSIX the truncate(2) and ftruncate(2) functions are
portable. We've used the latter since 271421cd34 ("Update partial HTTP
transfers.", 2005-09-30), but the truncate(1) command-line tool is
GNU-specific. Thus let's use Perl's version of it. We could also just
introduce a "test-tool truncate" in the future if we wanted to avoid
shelling out to perl.

On Linux and NetBSD we don't need the "if -s $ARGV[0] > $ARGV[1]"
condition I'm adding. We never have a $zero_pos longer than the file
being truncated. But let's have that condition to future-proof the
code, and because "the behavior is undefined if LENGTH is greater than
the length of the file" (perldoc -f truncate).

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

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
---
 t/t5318-commit-graph.sh | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/t/t5318-commit-graph.sh b/t/t5318-commit-graph.sh
index d4bd1522fe..d99bea6cce 100755
--- a/t/t5318-commit-graph.sh
+++ b/t/t5318-commit-graph.sh
@@ -382,7 +382,8 @@ 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 &&
+	perl -we 'truncate $ARGV[0], $ARGV[1] if -s $ARGV[0] > $ARGV[1]' \
+		$objdir/info/commit-graph $zero_pos &&
 	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.rc0.258.g878e2cd30e