From @porridge

This is a bug report for perl from marcin@​owsiany.pl,
generated with the help of perlbug 1.39 running under perl 5.10.1.

Reproduce with​:

$ cat <<END > t
#!/usr/bin/perl
open T,"./t|" or die $!; <T>
END
$ chmod +x t
$ (ulimit -u 254; LANG=C ./t)

Expected behaviour​:
- a chain of processes is created, once the limit is reached, the open fails
  in the youngest child, and they all terminate in reverse order.

Actual behaviour​:
- a chain of processes is indeed created​:

32315 pts/0 S 0​:00 /usr/bin/perl ./t
32316 pts/0 S 0​:00 \_ /usr/bin/perl ./t
32317 pts/0 S 0​:00 \_ /usr/bin/perl ./t
32318 pts/0 S 0​:00 \_ /usr/bin/perl ./t
32319 pts/0 S 0​:00 \_ /usr/bin/perl ./t
32320 pts/0 S 0​:00 \_ /usr/bin/perl ./t
32321 pts/0 S 0​:00 \_ /usr/bin/perl ./t
32322 pts/0 S 0​:00 \_ /usr/bin/perl ./t
32323 pts/0 S 0​:00 \_ /usr/bin/perl ./t

- however once the limit is reached, the youngest child enters an infinite
  loop retrying the fork. Running strace shows a the following in a loop​:

  restart_syscall(<... resuming interrupted call ...>) = 0
  clone(child_stack=0, flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD, child_tidptr=0x7f5a94d959d0) = -1 EAGAIN (Resource temporarily unavailable)
  rt_sigprocmask(SIG_BLOCK, [CHLD], [], 8) = 0
  rt_sigaction(SIGCHLD, NULL, {SIG_DFL, [], 0}, 8) = 0
  rt_sigprocmask(SIG_SETMASK, [], NULL, 8) = 0
  nanosleep({5, 0}, 0x7ffff9974210) = 0
 
- this is not very useful behaviour, because if an unprivileged user
  accidentally gets into this situation, he is locked out of the system. Perl
  should instead retry a few times and then back off.

I had a quick look and I can see code that can cause this in the following places​:

pp_sys.c- while ((childpid = PerlProc_fork()) == -1) {
pp_sys.c​: if (errno != EAGAIN) {
pp_sys.c- value = -1;
pp_sys.c- SP = ORIGMARK;
pp_sys.c- XPUSHi(value);
pp_sys.c- if (did_pipes) {
pp_sys.c- PerlLIO_close(pp[0]);
pp_sys.c- PerlLIO_close(pp[1]);
pp_sys.c- }
pp_sys.c-#if (defined(HAS_SIGPROCMASK) && !defined(PERL_MICRO))
pp_sys.c- sigprocmask(SIG_SETMASK, &oldset, NULL);
pp_sys.c-#endif
pp_sys.c- RETURN;
pp_sys.c- }
pp_sys.c- sleep(5);
pp_sys.c- }

util.c- while ((pid = PerlProc_fork()) < 0) {
util.c​: if (errno != EAGAIN) {
util.c- PerlLIO_close(p[This]);
util.c- PerlLIO_close(p[that]);
util.c- if (did_pipes) {
util.c- PerlLIO_close(pp[0]);
util.c- PerlLIO_close(pp[1]);
util.c- }
util.c- return NULL;
util.c- }
util.c- Perl_ck_warner(aTHX_ packWARN(WARN_PIPE), "Can't fork, trying again in 5 seconds");
util.c- sleep(5);
util.c- }

util.c- while ((pid = PerlProc_fork()) < 0) {
util.c​: if (errno != EAGAIN) {
util.c- PerlLIO_close(p[This]);
util.c- PerlLIO_close(p[that]);
util.c- if (did_pipes) {
util.c- PerlLIO_close(pp[0]);
util.c- PerlLIO_close(pp[1]);
util.c- }
util.c- if (!doexec)
util.c- Perl_croak(aTHX_ "Can't fork​: %s", Strerror(errno));
util.c- return NULL;
util.c- }
util.c- Perl_ck_warner(aTHX_ packWARN(WARN_PIPE), "Can't fork, trying again in 5 seconds");
util.c- sleep(5);
util.c- }

Here is how bash deals with the same problem for example​:

$ (ulimit -u 1; LANG=C ./t)
bash​: fork​: retry​: Resource temporarily unavailable
bash​: fork​: retry​: Resource temporarily unavailable
bash​: fork​: retry​: Resource temporarily unavailable
bash​: fork​: retry​: Resource temporarily unavailable
bash​: fork​: Resource temporarily unavailable
Terminated

Flags​:
  category=core
  severity=medium

Site configuration information for perl 5.10.1​:

Configured by Debian Project at Fri Apr 22 18​:53​:20 UTC 2011.

Summary of my perl5 (revision 5 version 10 subversion 1) configuration​:
 
  Platform​:
  osname=linux, osvers=2.6.24-28-server, archname=x86_64-linux-gnu-thread-multi
  uname='linux allspice 2.6.24-28-server #1 smp wed aug 18 21​:17​:51 utc 2010 x86_64 gnulinux '
  config_args='-Dusethreads -Duselargefiles -Dccflags=-DDEBIAN -Dcccdlflags=-fPIC -Darchname=x86_64-linux-gnu -Dprefix=/usr -Dprivlib=/usr/share/perl/5.10 -Darchlib=/usr/lib/perl/5.10 -Dvendorprefix=/usr -Dvendorlib=/usr/share/perl5 -Dvendorarch=/usr/lib/perl5 -Dsiteprefix=/usr/local -Dsitelib=/usr/local/share/perl/5.10.1 -Dsitearch=/usr/local/lib/perl/5.10.1 -Dman1dir=/usr/share/man/man1 -Dman3dir=/usr/share/man/man3 -Dsiteman1dir=/usr/local/man/man1 -Dsiteman3dir=/usr/local/man/man3 -Dman1ext=1 -Dman3ext=3perl -Dpager=/usr/bin/sensible-pager -Uafs -Ud_csh -Ud_ualarm -Uusesfio -Uusenm -DDEBUGGING=-g -Doptimize=-O2 -Duseshrplib -Dlibperl=libperl.so.5.10.1 -Dd_dosuid -des'
  hint=recommended, useposix=true, d_sigaction=define
  useithreads=define, usemultiplicity=define
  useperlio=define, d_sfio=undef, uselargefiles=define, usesocks=undef
  use64bitint=define, use64bitall=define, uselongdouble=undef
  usemymalloc=n, bincompat5005=undef
  Compiler​:
  cc='cc', ccflags ='-D_REENTRANT -D_GNU_SOURCE -DDEBIAN -fno-strict-aliasing -pipe -fstack-protector -I/usr/local/include -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64',
  optimize='-O2 -g',
  cppflags='-D_REENTRANT -D_GNU_SOURCE -DDEBIAN -fno-strict-aliasing -pipe -fstack-protector -I/usr/local/include'
  ccversion='', gccversion='4.4.3', gccosandvers=''
  intsize=4, longsize=8, ptrsize=8, doublesize=8, byteorder=12345678
  d_longlong=define, longlongsize=8, d_longdbl=define, longdblsize=16
  ivtype='long', ivsize=8, nvtype='double', nvsize=8, Off_t='off_t', lseeksize=8
  alignbytes=8, prototype=define
  Linker and Libraries​:
  ld='cc', ldflags =' -fstack-protector -L/usr/local/lib'
  libpth=/usr/local/lib /lib /usr/lib /lib64 /usr/lib64
  libs=-lgdbm -lgdbm_compat -ldb -ldl -lm -lpthread -lc -lcrypt
  perllibs=-ldl -lm -lpthread -lc -lcrypt
  libc=/lib/libc-2.11.1.so, so=so, useshrplib=true, libperl=libperl.so.5.10.1
  gnulibc_version='2.11.1'
  Dynamic Linking​:
  dlsrc=dl_dlopen.xs, dlext=so, d_dlsymun=undef, ccdlflags='-Wl,-E'
  cccdlflags='-fPIC', lddlflags='-shared -O2 -g -L/usr/local/lib -fstack-protector'

Locally applied patches​:
 

@​INC for perl 5.10.1​:
  /etc/perl
  /usr/local/lib/perl/5.10.1
  /usr/local/share/perl/5.10.1
  /usr/lib/perl5
  /usr/share/perl5
  /usr/lib/perl/5.10
  /usr/share/perl/5.10
  /usr/local/lib/site_perl
  .

Environment for perl 5.10.1​:
  HOME=/home/mowsiany
  LANG=pl_PL.utf8
  LANGUAGE (unset)
  LC_COLLATE=C
  LD_LIBRARY_PATH (unset)
  LOGDIR (unset)
  PATH=/home/mowsiany/bin​:/home/mowsiany/bin​:/usr/local/sbin​:/usr/local/bin​:/usr/sbin​:/usr/bin​:/sbin​:/bin
  PERL_BADLANG (unset)
  SHELL=/bin/bash

From @chipdude

Seems to me that EAGAIN should not actually be retried at all, in this case.

The RT System itself - Status changed from 'new' to 'open'

From @Leont

On Sun, Jun 17, 2012 at 8​:00 AM, Chip Salzenberg <rev.chip@​gmail.com> wrote​:

Seems to me that EAGAIN should not actually be retried at all, in this case.

I agree that would be a more sensible approach. Let the user deal with
their own form bomb.

Leon