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Re: Log for attempted build of gzip_1.6-1 on m68k (dist=unstable)

From: Paul Eggert
Subject: Re: Log for attempted build of gzip_1.6-1 on m68k (dist=unstable)
Date: Wed, 19 Jun 2013 08:52:52 -0700
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130510 Thunderbird/17.0.6

On 06/19/2013 07:18 AM, Antonio Diaz Diaz wrote:
> Thorsten Glaser wrote:
>> what do I do to investigate this:
>> zgrep-signal: set-up failure: signal handling busted on this host
>> ERROR: zgrep-signal
> FWIW, I have found this same bug on an i686 machine, with gzip 1.5 and 1.6, 
> but never found the cause. (My shell is bash).

Thanks for confirming it.  This prompted me to look at the code again
(my! I haven't used Perl in a year and a half! time to brush off
some of that rust...) and I found a race condition that could explain
the bug.  I pushed the following patch; can you please give it a try?
(Hmm, I now see a typo "becausee" in a comment; I'll fix that.)

>From 4c0bc4089a762ffef20ad9e0311e634e44b4cfdd Mon Sep 17 00:00:00 2001
From: Paul Eggert <address@hidden>
Date: Wed, 19 Jun 2013 08:46:29 -0700
Subject: [PATCH] tests: zgrep-signal race condition fix

* tests/zgrep-signal: Check that Perl supports dup2.
(exec_with_SIGPIPE_SIGDFL): Remove.
(write_to_dangling_pipe): Simplify by moving more of it into Perl.
Fix race condition, where subcommand writes to a pipe before the ":"
command exits.  Problem reported by Thorsten Glaser in
 tests/zgrep-signal | 38 +++++++++++++++++++++++---------------
 1 file changed, 23 insertions(+), 15 deletions(-)

diff --git a/tests/zgrep-signal b/tests/zgrep-signal
index 3a9c089..5475e5e 100755
--- a/tests/zgrep-signal
+++ b/tests/zgrep-signal
@@ -23,34 +23,42 @@
 echo a | gzip -c > f.gz || framework_failure_
 test "x$PERL" = x && PERL=perl
-("$PERL" -e 'use warnings') >/dev/null 2>&1 || skip_ "no suitable perl found"
+("$PERL" -e 'use POSIX qw(dup2)') >/dev/null 2>&1 ||
+   skip_ "no suitable perl found"
-exec_with_SIGPIPE_SIG_DFL () {
+# Run the arguments as a command, in a process where stdout is a
+# dangling pipe and SIGPIPE has the default signal-handling action.
+# This can't be done portably in the shell, becausee if SIGPIPE is
+# ignored when the shell is entered, the shell might refuse to trap
+# it.  Fall back on Perl+POSIX, if available.  Take care to close the
+# pipe's read end before running the program; the equivalent of the
+# shell's "command | :" has a race condition in that COMMAND could
+# write before ":" exits.
+write_to_dangling_pipe () {
   for arg; do
     args="$args, '$arg'"
-  "$PERL" -e "\$SIG{PIPE} = 'DEFAULT'; exec '$program'$args"
+  "$PERL" -e '
+     use POSIX qw(dup2);
+     $SIG{PIPE} = "DEFAULT";
+     pipe my ($read_end, $write_end) or die "pipe: $!\n";
+     dup2 fileno $write_end, 1 or die "dup2: $!\n";
+     close $read_end or die "close: $!\n";
+     exec '"'$program'$args"';
+  '
-write_to_dangling_pipe () {
-  exec 3>&1
-  (
-    exec_with_SIGPIPE_SIG_DFL "$@"
-    echo $? >&3
-  ) | : || framework_failure_
-signal_status=$(write_to_dangling_pipe cat f.gz f.gz)
+write_to_dangling_pipe cat f.gz f.gz
 test 128 -lt $signal_status ||
   framework_failure_ 'signal handling busted on this host'
-st=$(write_to_dangling_pipe zgrep a f.gz f.gz)
-test $st = $signal_status || fail=1
+write_to_dangling_pipe zgrep a f.gz f.gz
+test $? -eq $signal_status || fail=1
 Exit $fail

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