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Date: Sun, 28 Aug 2005 14:28:13 +0300
From: Pavel Tsekov <ptsekov AT gmx DOT net>
X-X-Sender: ptsekov AT mordor
cc: cygwin AT cygwin DOT com
Subject: Re: zsh: command not found => hangs
In-Reply-To: <Pine.LNX.4.60.0508261329430.19032@gremlin.fruitbat.org>
Message-ID: <Pine.CYG.4.58.0508281417030.624@mordor>
References: <20050823205318 DOT GF6716 AT bouh DOT ens-lyon DOT fr>   <Pine DOT LNX DOT 4 DOT 60 DOT 0508241620460 DOT 19032 AT gremlin DOT fruitbat DOT org>   <20050825001137 DOT GI7338 AT bouh DOT ens-lyon DOT fr> <Pine DOT LNX DOT 4 DOT 60 DOT 0508251406580 DOT 19032 AT gremlin DOT fruitbat DOT org>   <20050825220454 DOT GR7662 AT bouh DOT ens-lyon DOT fr> <Pine DOT LNX DOT 4 DOT 60 DOT 0508251537490 DOT 19032 AT gremlin DOT fruitbat DOT org>   <20050826005349 DOT GA4087 AT trixie DOT casa DOT cgf DOT cx> <Pine DOT LNX DOT 4 DOT 60 DOT 0508261033310 DOT 19032 AT gremlin DOT fruitbat DOT org>   <20050826191429 DOT GA2034 AT trixie DOT casa DOT cgf DOT cx> <Pine DOT LNX DOT 4 DOT 60 DOT 0508261329430 DOT 19032 AT gremlin DOT fruitbat DOT org>
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Hello,

I did trace this problem an it looks like a race in Cygwin but I may be
wrong. Here I am providing two backtraces from a debug session - the first
one shows what happens normally and the second one shows the hang:

=== In this case zsh doesn't hang ===
(gdb) break 593
Breakpoint 1 at 0x10049315: file /home/ptsekov/src/zsh-4.2.4/Src/signals.c, line 593.
(gdb) c
Continuing.
[Switching to thread 1708.0x1a8]

Breakpoint 1, zhandler (sig=20) at /home/ptsekov/src/zsh-4.2.4/Src/signals.c:601
601     } /* handler */
(gdb) n
0x6109259c in sigreturn () at ../../../../src/winsup/cygwin/cygtls.h:239
239           _myfault = NULL;
Current language:  auto; currently c++
(gdb)
set_process_mask (newmask=524288) at ../../../../src/winsup/cygwin/exceptions.cc:912
912     {
(gdb)
913       set_signal_mask (newmask, myself->getsigmask ());
(gdb)
914     }
(gdb)
0x610925b1 in sigreturn () at ../../../../src/winsup/cygwin/cygtls.h:239
239           _myfault = NULL;
(gdb)
signal_suspend (sig=20, sig2=0) at /home/ptsekov/src/zsh-4.2.4/Src/signals.c:401
401     }
Current language:  auto; currently c
(gdb)
zwaitjob (job=-1, sig=0) at /home/ptsekov/src/zsh-4.2.4/Src/jobs.c:1152
1152                if (subsh) {

=== End ===

=== In this case zsh hangs ===
Breakpoint 1, zhandler (sig=20) at /home/ptsekov/src/zsh-4.2.4/Src/signals.c:601
601     } /* handler */
(gdb) n
_cygtls::call_signal_handler (this=0x22f074) at ../../../../src/winsup/cygwin/exceptions.cc:1227
1227          incyg++;
Current language:  auto; currently c++
(gdb)
1228          set_signal_mask (this_oldmask, myself->getsigmask ());
(gdb)
1229          if (this_errno >= 0)
(gdb)
1213      while (sig)
(gdb) n
1233      return this_sa_flags & SA_RESTART;
(gdb)
1234    }
(gdb)
0x610926e4 in stabilize_sig_stack () at ../../../../src/winsup/cygwin/cygtls.h:239
239           _myfault = NULL;
(gdb)
pthread::is_good_object (thread=0x22ed0c) at ../../../../src/winsup/cygwin/thread.cc:234
234       return true;
(gdb)
235     }
(gdb)
cancelable_wait (object=0x350, timeout=4294967295, cancel_action=cw_cancel_self, sig_wait=cw_sig_nosig)
    at ../../../../src/winsup/cygwin/thread.cc:830
830           cancel_n = WAIT_OBJECT_0 + num++;
(gdb)
831           wait_objects[cancel_n] = thread->cancel_event;
(gdb)
835       if (sig_wait == cw_sig_nosig || &_my_tls != _main_tls)
(gdb)
836         sig_n = (DWORD) -1;
(gdb)
845           res = WaitForMultipleObjects (num, wait_objects, FALSE, timeout);
(gdb)

(gdb) bt
#0  cancelable_wait (object=0x350, timeout=4294967295, cancel_action=cw_cancel_self, sig_wait=cw_sig_nosig)
        at ../../../../src/winsup/cygwin/thread.cc:845
#1  0x6101cac8 in handle_sigsuspend (tempmask=4294443006) at ../../../../src/winsup/cygwin/exceptions.cc:599
#2  0x6109527f in sigsuspend (set=0x22ed9c) at ../../../../src/winsup/cygwin/signal.cc:450
#3  0x6109254f in _sigfe () at ../../../../src/winsup/cygwin/cygtls.h:239
#4  0x0022eda8 in ?? ()
#5  0xfff7fffe in ?? ()
#6  0x00000001 in ?? ()
#7  0x10120234 in ?? ()
#8  0x0022edd8 in ?? ()
#9  0x100280e4 in zwaitjob (job=852, sig=0) at /home/ptsekov/src/zsh-4.2.4/Src/jobs.c:1145
#10 0x00000000 in ?? () from
=== End ===

First I determined (by using strace) the place in zsh where the hang
occurs to be in or after zhandler() when it processes SIGCLD. Then I
built debugging zsh and after debugging for a while it turned out that
the hang occures after leaving the signal handler.

P.S. While looking at this I noticed that Cygwin's wait family of
functions won't return 0 if WNOHANG is passed and no children are found
that match the wait criteria  - JFYI.

Hope this info helps.

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