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echo interrupted by SIGCHLD from a dying coprocess
From: |
Tomáš Trnka |
Subject: |
echo interrupted by SIGCHLD from a dying coprocess |
Date: |
Wed, 24 Mar 2010 20:30:00 +0100 |
User-agent: |
KMail/1.13.1 (Linux/2.6.32.9-70.fc12.x86_64; KDE/4.4.1; x86_64; ; ) |
Configuration Information [Automatically generated, do not change]:
Machine: x86_64
OS: linux-gnu
Compiler: gcc
Compilation CFLAGS: -DPROGRAM='bash' -DCONF_HOSTTYPE='x86_64' -
DCONF_OSTYPE='linux-gnu' -DCONF_MACHTYPE='x86_64-unknown-linux-gnu' -
DCONF_VENDOR='unknown' -DLOCALEDIR='/home/trnka/opt/share/locale' -
DPACKAGE='bash' -DSHELL -DHAVE_CONFIG_H -I. -I. -I./include -I./lib -g -
O2
uname output: Linux a324-2 2.6.24.2 #1 SMP Wed Feb 20 12:36:17 CET 2008 x86_64
GNU/Linux
Machine Type: x86_64-unknown-linux-gnu
Bash Version: 4.1
Patch Level: 2
Release Status: release
Description:
I've started using coprocesses heavily and I've found a nasty problem related
(but not limited) to them: After the coprocess finishes its job, the resultant
SIGCHLD is not properly blocked by bash signal processing logic and interferes
with script I/O. In my case, I've been using something like:
read var1 var2 < <( a | long | pipeline | here)
echo "var1=$var1"
echo "var2=$var2"
Sometimes, the SIGCHLD arrived just when one of the echos were doing output
and the result was:
echo: write error: Interrupted system call
As this is a bit of a race, it occurs only when the stars are right, i.e.
during normal usage the probability of the SIGCHLD hitting exactly the echo is
quite low. However, as soon as anything causes the I/O to take significantly
longer, the bug appears. I've been hitting quite often (30%?) when running the
script over SSH.
This bug has probably been reported years ago here:
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=382798
Repeat-By:
I've reduced one of my scripts to this (nothing exceptionally intelligent, but
it does the job):
#!/bin/bash
while [[ 1 ]]; do
set +e
read tmp tmp2 < <( echo "blabla" | wc | tr -s " " "\n" | tail -n 2 | tr "\n" "
")
set -e
echo $tmp
echo $tmp2
echo $tmp
echo $tmp2
echo $tmp
echo $tmp2
echo $tmp
echo $tmp2
echo $tmp
echo $tmp2
echo $tmp
echo $tmp2
echo $tmp
echo $tmp2
echo $tmp
echo $tmp2
echo $tmp
echo $tmp2
done
Using this script I can reliably reproduce the bug (i.e. get a Interrupted
system call error) using bash 4.1.2 (compiled myself from vanilla tarball) and
3.1.17 (Debian lenny) over SSH and 4.0.35 (stock Fedora 12) under strace.
Fix:
Applying the following simple patch (against 4.1.2) fixes the bug:
--- builtins/echo.def.orig 2010-03-24 19:40:54.000000000 +0100
+++ builtins/echo.def 2010-03-24 19:47:07.000000000 +0100
@@ -27,6 +27,7 @@
#include "../bashansi.h"
+#include <signal.h>
#include <stdio.h>
#include "../shell.h"
@@ -108,6 +109,7 @@
{
int display_return, do_v9, i, len;
char *temp, *s;
+ sigset_t nmask, omask;
do_v9 = xpg_echo;
display_return = 1;
@@ -159,6 +161,10 @@
clearerr (stdout); /* clear error before writing and testing success */
+ sigemptyset(&nmask);
+ sigaddset(&nmask, SIGCHLD);
+ sigprocmask(SIG_BLOCK, &nmask, &omask);
+
terminate_immediately++;
while (list)
{
@@ -193,6 +199,8 @@
if (display_return)
putchar ('\n');
+ sigprocmask(SIG_SETMASK, &omask, NULL);
+
terminate_immediately--;
return (sh_chkwrite (EXECUTION_SUCCESS));
}
- echo interrupted by SIGCHLD from a dying coprocess,
Tomáš Trnka <=