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Date:   Fri, 8 Feb 2019 17:35:12 -0500
From:   Jeff King <peff@...f.net>
To:     "Randall S. Becker" <rsbecker@...bridge.com>
Cc:     "'brian m. carlson'" <sandals@...stytoothpaste.net>,
        'Junio C Hamano' <gitster@...ox.com>, git@...r.kernel.org,
        'Linux Kernel' <linux-kernel@...r.kernel.org>,
        git-packagers@...glegroups.com
Subject: Re: [Breakage] Git v2.21.0-rc0 - t5318 (NonStop)

On Fri, Feb 08, 2019 at 05:12:43PM -0500, Randall S. Becker wrote:

> On February 8, 2019 17:07, brian m. carlson wrote:
> > On Fri, Feb 08, 2019 at 02:31:57PM -0500, Jeff King wrote:
> > > > It is available AFAIK on Linux, POSIX, and Windows under Cygwin.
> > > > That's more than /dev/zero has anyway. I have the patch ready if you
> > > > want it.
> > >
> > > Is it POSIX? Certainly truncate() is, but I didn't think the
> > > command-line tool was. If it really is available everywhere, then
> > > yeah, I'd be fine with it.
> > 
> > It's not. POSIX doesn't specify the command, and macOS lacks it, I believe.
> 
> I'm happy to modify the test (it is in one spot), to make a decision based on:
> a) whether /dev/zero exists
> b) whether the system is a NonStop
> c) something else
> 
> What would you all prefer? It doesn't matter to me one way or another,
> as long as I can get the dependency to /dev/zero removed so tests will
> run here.

For the case in t5318, I think we can just put the NULs in a file. Does
this work on your platform?

---
diff --git a/t/t5318-commit-graph.sh b/t/t5318-commit-graph.sh
index 16d10ebce8..6d0ccc7eba 100755
--- a/t/t5318-commit-graph.sh
+++ b/t/t5318-commit-graph.sh
@@ -383,7 +383,8 @@ 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)) &&
+	gen_zero_bytes $(($orig_size - $zero_pos)) >zeroes &&
+	dd if=zeroes of="$objdir/info/commit-graph" bs=1 seek="$zero_pos" &&
 	test_must_fail git commit-graph verify 2>test_err &&
 	grep -v "^+" test_err >err &&
 	test_i18ngrep "$grepstr" err
diff --git a/t/test-lib-functions.sh b/t/test-lib-functions.sh
index 92cf8f812c..4afab14431 100644
--- a/t/test-lib-functions.sh
+++ b/t/test-lib-functions.sh
@@ -1302,3 +1302,8 @@ test_set_port () {
 	port=$(($port + ${GIT_TEST_STRESS_JOB_NR:-0}))
 	eval $var=$port
 }
+
+# Generate an output of $1 bytes of all zeroes (NULs, not ASCII zeroes).
+gen_zero_bytes () {
+	perl -e 'print "\0" x $ARGV[0]' "$@"
+}

For the others that need infinite zeroes, I think using "yes" makes more
sense, though we could also teach this function to accept an "infinity"
parameter.

-Peff

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