another deadlock in free() called from a signal handler

[Kamil Dudka <kdudka@xxxxxxxxxx>]

As already reported at <http://www.zsh.org/mla/workers/2015/msg00496.html>,
zsh may occasionally call free() from a signal handler, which causes deadlock.

The latest upstream fix for this issue now appears to be incomplete:

http://sourceforge.net/p/zsh/code/ci/a4ff8e69

Wouldn't it be safer to wrap [z]free() internally by the signal queuing macros?

Such an approach was already considered when applying the previous patch:

http://www.zsh.org/mla/workers/2015/msg00514.html

A backtrace of the deadlock (captured with zsh-4.3.11-3.el6) follows:

#0  __lll_lock_wait_private () at ../nptl/sysdeps/unix/sysv/linux/x86_64/lowlevellock.S:97
#1  0x000000380167d0a0 in _L_lock_5189 () from /lib64/libc-2.12.so
#2  0x00000038016789fb in _int_free (av=0x380198fe80, p=0x1217d40, have_lock=0) at malloc.c:4959
#3  0x000000000044340d in freejob (jn=0x1200420, deleting=1) at jobs.c:1103
#4  0x000000000044464d in printjob (jn=0x1200420, lng=0, synch=0) at jobs.c:1066
#5  0x00000000004470ba in update_job (jn=0x1200420) at jobs.c:508
#6  0x000000000047354b in wait_for_processes () at signals.c:502
#7  0x0000000000473e15 in zhandler (sig=17) at signals.c:584
#8  <signal handler called>
#9  0x00000038016787cd in _int_free (av=0x380198fe80, p=0x1217c30, have_lock=0) at malloc.c:5013
#10 0x000000000044a339 in lexrestore () at lex.c:342
#11 0x000000000044aa68 in parse_subscript (s=<value optimized out>, sub=1, endchar=<value optimized out>) at lex.c:1679
#12 0x000000000045c2db in isident (s=0x7fc4402de570) at params.c:1016
#13 0x0000000000460279 in assignsparam (s=0x7fc4402de570, val=0x1217b10 "true", flags=0) at params.c:2583
#14 0x0000000000424154 in addvars (state=<value optimized out>, pc=<value optimized out>, addflags=<value optimized out>) at exec.c:2204
#15 0x0000000000424321 in execsimple (state=<value optimized out>) at exec.c:1061
#16 0x000000000042c023 in execlist (state=0x7fff4a707070, dont_change_job=1, exiting=0) at exec.c:1175
#17 0x000000000044b212 in execif (state=0x7fff4a707070, do_exec=0) at loop.c:515
#18 0x0000000000429d04 in execcmd (state=<value optimized out>, input=0, output=0, how=0, last1=2) at exec.c:3124
#19 0x000000000042ad56 in execpline2 (state=0x7fff4a707070, pcode=<value optimized out>, how=18, input=0, output=0, last1=0) at exec.c:1640
#20 0x000000000042b118 in execpline (state=0x7fff4a707070, slcode=<value optimized out>, how=18, last1=0) at exec.c:1424
...
#101 0x000000000042b118 in execpline (state=0x7fff4a70d420, slcode=<value optimized out>, how=18, last1=0) at exec.c:1424
#102 0x000000000042c467 in execlist (state=0x7fff4a70d420, dont_change_job=0, exiting=0) at exec.c:1207
#103 0x000000000042c7b6 in execode (p=0x7fc4402e8818, dont_change_job=0, exiting=0, context=0x48e5be "toplevel") at exec.c:1025
#104 0x000000000043dae2 in loop (toplevel=1, justonce=0) at init.c:185
#105 0x000000000043f7de in zsh_main (argc=<value optimized out>, argv=<value optimized out>) at init.c:1508
#106 0x000000380161ed1d in __libc_start_main (main=0x40de30 <main>, argc=2, ...) at libc-start.c:226
#107 0x000000000040dd69 in _start ()