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

From: SZEDER Gábor <[email-masked]>
Date: Thu, 21 Feb 2019 21:43:10 +0100

On Thu, Feb 21, 2019 at 08:28:49PM +0100, Ævar Arnfjörð Bjarmason wrote:
> 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 <[email-masked]>
> ---
>  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 &&

This will make Dscho unhappy :)

Is there a problem with:

  dd if=/dev/null of="$objdir/info/commit-graph" bs=1 seek="$zero_pos"

?

To my understanding of the specs it's well-defined what it should do,
even when $zero_pos is larget than the file size,  it's shorter,
simpler, and doesn't introduce yet another Perl dependency.

>  	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
>