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From: "Ævar Arnfjörð Bjarmason" <avarab@gmail.com>
To: "SZEDER Gábor" <szeder.dev@gmail.com>
Cc: git@vger.kernel.org, Junio C Hamano <gitster@pobox.com>,
	Sebastian Staudt <koraktor@gmail.com>,
	Josh Steadmon <steadmon@google.com>,
	Johannes Schindelin <Johannes.Schindelin@gmx.de>
Subject: Re: [PATCH 2/2] commit-graph tests: fix cryptic unportable "dd" invocation
Date: Fri, 22 Feb 2019 15:34:28 +0100
Message-ID: <8736of6gej.fsf@evledraar.gmail.com> (raw)
In-Reply-To: <20190222105053.GU1622@szeder.dev>


On Fri, Feb 22 2019, SZEDER Gábor wrote:

> On Thu, Feb 21, 2019 at 11:26:26PM +0100, Ævar Arnfjörð Bjarmason wrote:
>> >> -	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 $zero_pos &&
>> >
>> > This will make Dscho unhappy :)
>>
>> Sorry Dscho :)
>>
>> Although this is a one-off in one test, as opposed to a new "perl -e" in
>> test-lib-functions.sh
>>
>> > Is there a problem with:
>> >
>> >   dd if=/dev/null of="$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.
>>
>> I tried that as a one-off and it indeed works as a "truncate" on NetBSD
>> & GNU.
>>
>> My reading of POSIX "dd" and "lseek" docs is that we'd need some similar
>> guard if we're going to be paranoid about a $zero_pos value past the end
>> of the file. It doesn't look like that's portable, my assumption from
>> reading the docs is that the seek=* will devolve without a stat() check
>> on some "dd" implementations to an "lseek".
>
> Could you point to the part of the specs where your assumption comes
> from?  The specs are quite clear on what should happen:
>
>   If the size of the seek plus the size of the input file is less than
>   the previous size of the output file, the output file shall be
>   shortened by the copy. If the input file is empty and either the
>   size of the seek is greater than the previous size of the output
>   file or the output file did not previously exist, the size of the
>   output file shall be set to the file offset after the seek.
>
> IOW no such guard is necessary.

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.

> I checked the man pages of FreeBSD's, NetBSD's, OpenBSD's and Solaris'
> 'dd', and they are clearly following the specs in this respect.  I
> tried NetBSD 6.0's and 8.0's 'dd', and both behave as advertised.
>
> And using 'dd' doesn't add a condition after statement...

>> I'm not going to submit a re-roll of this because it works, and I'd
>> still trust Perl's truncate(...) portability over dd.
>>
>> But more importantly because it takes me *ages* to fully re-test
>> anything on the slow BSD VMs I have access to, and I already tore town
>> my one-off hacking env there after testing these patches...
>>
>> >>  	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
>> >>

  reply index

Thread overview: 25+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-02-19 23:29 [ANNOUNCE] Git v2.21.0-rc2 Junio C Hamano
2019-02-20  0:43 ` Randall S. Becker
2019-02-20 19:41   ` Junio C Hamano
2019-02-20 20:46     ` Johannes Schindelin
2019-02-21 13:10     ` Duy Nguyen
2019-02-21 23:55       ` brian m. carlson
2019-02-22  9:13         ` Duy Nguyen
2019-02-21 15:54     ` Randall S. Becker
2019-02-21 20:04       ` Junio C Hamano
2019-02-21 21:06       ` SZEDER Gábor
2019-02-21 21:30         ` Randall S. Becker
2019-02-21 19:59     ` Randall S. Becker
2019-02-21 10:46 ` Git for Windows v2.21.0-rc2, was " Johannes Schindelin
2019-02-21 19:28 ` [PATCH 0/2] BSD portability fixes for 2.21.0-rc2 Ævar Arnfjörð Bjarmason
2019-02-22  0:37   ` Josh Steadmon
2019-02-21 19:28 ` [PATCH 1/2] tests: fix unportable "\?" and "\+" regex syntax Ævar Arnfjörð Bjarmason
2019-02-22  5:00   ` Junio C Hamano
2019-02-21 19:28 ` [PATCH 2/2] commit-graph tests: fix cryptic unportable "dd" invocation Ævar Arnfjörð Bjarmason
2019-02-21 20:43   ` SZEDER Gábor
2019-02-21 22:26     ` Ævar Arnfjörð Bjarmason
2019-02-22 10:50       ` SZEDER Gábor
2019-02-22 14:34         ` Ævar Arnfjörð Bjarmason [this message]
2019-02-22 18:30           ` Junio C Hamano
2019-02-22 18:35             ` Todd Zullinger
2019-02-22 19:23               ` [PATCH v3] commit-graph tests: fix " Junio C Hamano

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