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 >> >>
next prev parent reply other threads:[~2019-02-22 14:34 UTC|newest] 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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