RE: [Patch v1 2/3] t5318: replace use of /dev/zero with generate_zero_bytes
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> -----Original Message-----
> From: Junio C Hamano <jch2355@gmail.com> On Behalf Of Junio C Hamano
> Sent: February 13, 2019 12:25
> To: Eric Sunshine <sunshine@sunshineco.us>
> Cc: randall.s.becker@rogers.com; Git List <git@vger.kernel.org>; Randall
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> Becker <rsbecker@nexbridge.com>
> Subject: Re: [Patch v1 2/3] t5318: replace use of /dev/zero with
> generate_zero_bytes
>
> Junio C Hamano <gitster@pobox.com> writes:
>
> > Eric Sunshine <sunshine@sunshineco.com> writes:
> >
> >> On Sat, Feb 9, 2019 at 2:00 PM <randall.s.becker@rogers.com> wrote:
> >>> This change removes the dependency on /dev/zero with
> >>> generate_zero_bytes appending NUL values to blocks generating wrong
> signatures for test cases.
> >>
> >> This commit message says what the patch does but not _why_. At
> >> minimum, it should explain that /dev/zero is not available on all
> >> platforms, therefore, not portable, and (perhaps) cite NonStop as an
> >> example.
> >
> > Does sombody want to do the honors? [PATCH 1/3] would become wasted
> > effort until that happens. On the other hand, if this is not urgent
> > (it is only urgent for those without /dev/zero, and to others it may
> > be distraction/disruption this close to the final release to add
> > increased risk of fat finger mistakes), obviously I can wait.
>
> So, before I lose the access to my primary screen (I was told that somehow
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> need to reimage the workstation today X-<), here is what I have now.
>
> -- >8 --
> From: "Randall S. Becker" <rsbecker@nexbridge.com>
> Date: Sat, 9 Feb 2019 13:59:29 -0500
> Subject: [PATCH] t5318: replace use of /dev/zero with generate_zero_bytes
>
> There are platforms (e.g. NonStop) that lack /dev/zero; use the
> generate_zero_bytes helper we just introduced to append stream of NULs at
> the end of the file.
>
> The original, even though it uses "dd seek=... count=..." to make it look
> is overwriting the middle part of an existing file, has truncated the file
> this step with another use of "dd", which may make it tricky to see why
> rewrite is a correct one.
Here is how I interpret the test - might be wrong, but yanno... The first dd copies something looking like reasonable data from the test case. The second dd copies zeros from seek to the end of a fixed size block.
My first attempt at a fix used truncate that extended the first to the correct size (filling with zeros). My worry there is that I'm not sure there is a guarantee of zeros, but that shouldn't matter for the test which just wants a signature mismatch.
Others suggested using yes to fill in junk.
My second attempt was to create the generate_zero_bytes function to replace exactly what the second dd was doing but not user /dev/zero. The fix was not to change the conditions of the test - not debating the correctness of that here - but to simply replicate the use of /dev/zero in context. So the resulting file contains [reasonable-stuff]{seek}[0]{orig_size-seek}, which is sufficient to satisfy the conditions of the test.
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>
> Signed-off-by: Randall S. Becker <rsbecker@nexbridge.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
> 16d10ebce8..d4bd1522fe 100755
> --- a/t/t5318-commit-graph.sh
> +++ b/t/t5318-commit-graph.sh
> @@ -383,7 +383,7 @@ corrupt_graph_and_verify() {
> 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 if=/dev/zero of="$objdir/info/commit-graph" bs=1
> seek="$zero_pos" count=$(($orig_size - $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 &&
> test_i18ngrep "$grepstr" err
