p5pRT
commented
14 years ago From cems@mac.com
This is a bug report for perl from cems@lanl.gov\, generated with the help of perlbug 1.39 running under perl v5.8.9.
FROM: charlie strauss\, cems@lanl.gov
perlbug: wait and waitpid can return -1 even when there are running child forks. This happens any time a kill signal is sent to a previously reaped child process ID from inside the parent fork.
tested on platforms:
1) mac osx 10.5.10 Darwin Kernel Version 9.8.0 perl v5.8.9 built for darwin-2level
2) Linux 2.6.31-20-generic #58-Ubuntu SMP
Expected behaviour:
if a parent waits on children then I expect that 1) the wait() command should block till a child exists (or SIGCHLD). 2) the wait() command should never return -1 if there are unreaped or running children.
Observed behaviour:
the above is normally true. But you can get wait() violate both those conditions very easily.
This happens any time a kill signal is sent to a previously reaped child process ID from inside
the parent fork.
In order for the parent to be able to send a kill signal while it is waiting\, this has to happen from a signal handler that is triggered externally.
Steps to reproduce:
see enclosed perl script demo.
basic flow is this:
1) fork off two+ children whose jobs are different lengths: e.g. sleep 1 and sleep 300 2) let the short running job be our $pid1 and the long running job be our $pid2. 3) set a %SIG signal handler for any signal like\, say sig-QUIT\, that sends a kill to our $pid1. 4) wait() for $pid1 to finish and get reaped by parent. 4) parent again waits() for $pid2 to finsish using $r=wait(); 5) send the parent a sigQUIT from another terminal or process. 6) result: wait() unblocks and you find that $r is -1 but $pid2 is still running.
WHY THIS IS A PROBLEM:
most perl man pages suggest a reaper that looks something like this:
do {} while ( waitpid(-1\,WNOHANG) > 0 );
Simmilarly waits in parent processes generally look like:
do {} while ( wait() > -1 ); exit;
or
do { sleep 1 } while ( waitpid(-1\,WNOHANG) > 0 ); exit;
However this fails due to above bug. waitpid and wait can return a -1 even when jobs are still running.
WORKAROUND: the work around is to test the wait() value twice in a row when you get a -1.
do { wait() } while (waitpid(-1\,WNOHANG) > -1 );
this could also fail\, perhaps\, if multiple kill signals came too quickly for the loop to trap them.
DEMO SCRIPT
#!/usr/bin/perl -w use strict;
my @command1 = ("sleep 360"); my @command2 = ("sleep 2");
my $pid1 = fork();
unless ($pid1) { exec @command1; };
my $pid2 = fork();
unless ($pid2) { exec @command2; };
# THIS IS WHAT TRIGGERS THE BUG $SIG{CONT} = sub{ my $sig = $_[0]; kill $sig\, $pid2; warn " dead process $pid2 was sent $sig\n"};
# NEITHER OF THESE WILL TRIGGER THE BUG. thus the bug is due to sending a sigCONT to the dead process. #$SIG{CONT} = sub{ my $sig = $_[0]; kill $sig\, $pid1; warn "live process $pid1 was sent $sig\n"}; #$SIG{CONT} = sub{ my $sig = $_[0]; warn "did nothing with $sig\n"};
warn "processes running:\n";
warn `ps au | grep $pid2 | grep -v grep` ;
warn `ps au | grep $pid1 | grep -v grep` ;
wait(); # reap process #2
warn "\n\ncommand @command2 should have been reaped now\n"; warn "\nprocesses running:\n"; my $r = `ps au | grep $pid2 | grep -v grep` ; warn $r if $r; warn `ps au | grep $pid1 | grep -v grep` ;
warn "\n\nHUMAN SLAVE: To induce the bug\, pull up another terminal and type: kill -CONT $$ then kill -CONT $$ then kill -CONT $$ then kill -CONT $$ then kill -QUIT $pid1
(NOTICE the last one was $pid1 and not $$)\n";
warn "\n\nParent will first use wait()\n";
if ( wait() == -1 ) {warn "perl thinks the child process ended\, but did it?\n";};
if ( kill 0\,$pid1) { warn "\nPerl bug detected!! the process $pid1 is still running!!\n"; warn "you can see this because waitpid is now zero:"\, waitpid(-1\,1)\,"\n"; warn "and you can also see it in the process list as active:"; warn `ps au | grep $pid1 | grep -v grep`; }
warn "\n\nParent will next use waitpid()\n"; if ( waitpid(-1\,0) == -1 ) {warn "perl thinks the child process ended\, but did it?\n";};
if ( kill 0\,$pid1) {
warn "\nPerl bug detected!! the process $pid1 is still running!!\n";
warn "you can see this because waitpid is now zero:"\, waitpid(-1\,1)\,"\n";
warn "and you can also see it in the process list as active:";
warn `ps au | grep $pid1 | grep -v grep`;
}
# here is the only way I know of to work around this bug:
warn "\n\nParent is working around wait() bug by trapping it in a double waitpid wrapper\n"; while ( waitpid(-1\,1) > -1 ) { wait(); warn "perl wait() thinks the child process ended\, but we trapped this bug\n";};
if ( kill 0\,$pid1) { warn "\nPerl bug detected!! the process $pid1 is still running!!\n"; warn "you can see this because waitpid is now zero:"\, waitpid(-1\,1)\,"\n"; warn "and you can also see it in the process list as active:"; warn `ps au | grep $pid1 | grep -v grep`; } else { warn "\n\nThis time the process $pid1 really is ended for real!!!!\n"; warn "you can see this because waitpid is now -1:"\, waitpid(-1\,1)\,"\n"; warn "and you can also see it is NOT in the process list:"; warn `ps au | grep $pid1 | grep -v grep`; }
Flags: category=core severity=high
Site configuration information for perl v5.8.9:
Configured by cems at Wed Sep 16 16:20:28 MDT 2009.
Summary of my perl5 (revision 5 version 8 subversion 9) configuration: Platform: osname=darwin\, osvers=9.7.0\, archname=darwin-2level uname='darwin ocho.lanl.gov 9.7.0 darwin kernel version 9.7.0: tue mar 31 22:52:17 pdt 2009; root:xnu-1228.12.14~1release_i386 i386 ' config_args='-des -D prefix=/opt/local -D scriptdir=/opt/local/bin -D cppflags=-I/opt/local/include -D ldflags=-L/opt/local/lib -D vendorprefix=/opt/local -D man1ext=1pm -D man3ext=3pm -D cc=/usr/bin/gcc-4.0 -D ld=/usr/bin/gcc-4.0 -D man1dir=/opt/local/share/man/man1p -D man3dir=/opt/local/share/man/man3p -D siteman1dir=/opt/local/share/man/man1 -D siteman3dir=/opt/local/share/man/man3 -D vendorman1dir=/opt/local/share/man/man1 -D vendorman3dir=/opt/local/share/man/man3 -D inc_version_list=5.8.8 5.8.8/darwin-2level -U i_bind -U i_gdbm -U i_db' hint=recommended\, useposix=true\, d_sigaction=define usethreads=undef use5005threads=undef useithreads=undef usemultiplicity=undef useperlio=define d_sfio=undef uselargefiles=define usesocks=undef use64bitint=undef use64bitall=undef uselongdouble=undef usemymalloc=n\, bincompat5005=undef Compiler: cc='/usr/bin/gcc-4.0'\, ccflags ='-fno-common -DPERL_DARWIN -I/opt/local/include -no-cpp-precomp -fno-strict-aliasing -pipe -I/usr/local/include -I/opt/local/include'\, optimize='-O3'\, cppflags='-I/opt/local/include -no-cpp-precomp -fno-common -DPERL_DARWIN -I/opt/local/include -no-cpp-precomp -fno-strict-aliasing -pipe -I/usr/local/include -I/opt/local/include' ccversion=''\, gccversion='4.0.1 (Apple Inc. build 5493)'\, gccosandvers='' intsize=4\, longsize=4\, ptrsize=4\, doublesize=8\, byteorder=1234 d_longlong=define\, longlongsize=8\, d_longdbl=define\, longdblsize=16 ivtype='long'\, ivsize=4\, nvtype='double'\, nvsize=8\, Off_t='off_t'\, lseeksize=8 alignbytes=8\, prototype=define Linker and Libraries: ld='env MACOSX_DEPLOYMENT_TARGET=10.3 /usr/bin/gcc-4.0'\, ldflags ='-L/opt/local/lib -L/usr/local/lib' libpth=/usr/local/lib /opt/local/lib /usr/lib libs=-ldbm -ldl -lm -lutil -lc perllibs=-ldl -lm -lutil -lc libc=/usr/lib/libc.dylib\, so=dylib\, useshrplib=false\, libperl=libperl.a gnulibc_version='' Dynamic Linking: dlsrc=dl_dlopen.xs\, dlext=bundle\, d_dlsymun=undef\, ccdlflags=' ' cccdlflags=' '\, lddlflags='-L/opt/local/lib -bundle -undefined dynamic_lookup -L/usr/local/lib'
Locally applied patches:
@INC for perl v5.8.9: /opt/local/lib/perl5/site_perl/5.8.9/darwin-2level /opt/local/lib/perl5/site_perl/5.8.9 /opt/local/lib/perl5/site_perl /opt/local/lib/perl5/vendor_perl/5.8.9/darwin-2level /opt/local/lib/perl5/vendor_perl/5.8.9 /opt/local/lib/perl5/vendor_perl /opt/local/lib/perl5/5.8.9/darwin-2level /opt/local/lib/perl5/5.8.9 .
Environment for perl v5.8.9: DYLD_LIBRARY_PATH (unset) HOME=/Users/cems LANG (unset) LANGUAGE (unset) LD_LIBRARY_PATH (unset) LOGDIR (unset) PATH=/Library/Frameworks/Python.framework/Versions/Current/bin:/opt/local/bin:/opt/local/sbin:/Library/Frameworks/Python.framework/Versions/Current/bin:/Library/Frameworks/Python.framework/Versions/Current/bin:/Library/Frameworks/Python.framework/Versions/Current/bin:/Library/Frameworks/Python.framework/Versions/Current/bin:/Library/Frameworks/Python.framework/Versions/Current/bin:/Library/Frameworks/Python.framework/Versions/Current/bin:/usr/bin:/bin:/usr/sbin:/sbin:/usr/local/bin:/usr/X11/bin PERL_BADLANG (unset) SHELL=/bin/bash
Migrated from rt.perl.org#74854 (status was 'resolved')
Searchable as RT74854$