Intermittent failures retrieving process exit codes

[Tom Honermann]

I spent most of the week debugging this issue.  This appears to be a 
defect in Windows.  I can reproduce the issue without Cygwin.  I can't 
rule out other third party kernel mode software possibly contributing to 
the issue.  A simple change to Cygwin works around the problem for me.

I don't know which Windows releases are affected by this.  I've only 
reproduced the problem (outside of Cygwin) with Wow64 processes running 
on 64-bit Windows 7.  I haven't yet tried elsewhere.

The problem appears to be a race condition involving concurrent calls to 
TerminateProcess() and ExitThread().  The example code below minimally 
mimics the threads created and exit process/thread calls that are 
performed when running Cygwin's false.exe.  The primary thread exits the 
process via TerminateProcess() ala pinfo::exit() in 
winsup/cygwin/pinfo.cc.  The secondary thread exits itself via 
ExitThread() ala Cygwin's signal processing thread function, wait_sig(), 
in winsup/cygwin/sigproc.cc.

When the race condition results in the undesirable outcome, the exit 
code for the process is set to the exit code for the secondary thread's 
call to ExitThread().  I can only speculate at this point, but my guess 
is that the TerminateProcess() code disassociates the calling thread 
from the process before other threads are stopped such that 
ExitThread(), concurrently running in another thread, may determine that 
the calling thread is the last thread of the process and overwrite the 
process exit code.

The issue also reproduces if ExitProcess() is called in place of 
TerminateProcess().  The test case below only uses TerminateProcess() 
because that is what Cygwin does.

Source code to reproduce the issue follows.  Again, Cygwin is not 
required to reproduce the problem.  For my own testing, I compiled the 
code using Microsoft's Visual Studio 2010 x86 compiler with the command 
'cl /Fetest-exit-code.exe test-exit-code.cpp'

test-exit-code.cpp:

#include <windows.h>
#include <stdio.h>
#include <stdlib.h>

DWORD WINAPI SecondaryThread(
     LPVOID lpParameter)
{
     Sleep(1);
     ExitThread(2);
}

int main() {
     HANDLE hSecondaryThread = CreateThread(
         NULL,                               // lpThreadAttributes
         0,                                  // dwStackSize
         SecondaryThread,                    // lpStartAddress
         (LPVOID)0,                          // lpParameter
         0,                                  // dwCreationFlags
         NULL);                              // lpThreadId
     if (!hSecondaryThread) {
         fprintf(stderr, "CreateThread failed.  GLE=%lu\n",
             (unsigned long)GetLastError());
         exit(127);
     }

     Sleep(1);

     if (!TerminateProcess(GetCurrentProcess(), 1)) {
         fprintf(stderr, "TerminateProcess failed.  GLE=%lu\n",
             (unsigned long)GetLastError());
         exit(127);
     }

     return 0;
}


To run the test, a simple .bat file is used:

test.bat:

@echo off
setlocal

:loop
echo test...
test-exit-code.exe
if %ERRORLEVEL% NEQ 1 (
     echo test-exit-code.exe returned %ERRORLEVEL%
     exit /B 1
)
goto loop


test.bat should run indefinitely.  The amount of time it takes to fail 
on my machine (64-bit Windows 7 running in a VMware Workstation 8 VM 
under Kubuntu 12.04 on a Lenovo T420 Intel i7-2640M 2 processor laptop) 
varies considerably.  I had one run fail in less than 10 iterations, but 
most of the time it has taken upwards of 5 minutes to get a failure.

The workaround I implemented within Cygwin was simple and sloppy.  I 
added a call to Sleep(1000) immediately before the call to ExitThread() 
in wait_sig() in winsup/cygwin/sigproc.cc.  Since this thread (probably) 
doesn't exit until the process is exiting anyway, the call to Sleep() 
does not adversely affect shutdown.  The thread just gets terminated 
while in the call to Sleep() instead of exiting before the process is 
terminated or getting terminated while still in the call to 
ExitThread().  A better solution might be to avoid the thread exiting at 
all (so long as it can't get terminated while holding critical 
resources), or to have the process exiting thread wait on it.  Neither 
of these is ideal.  Orderly shutdown of multi-threaded processes is 
really hard to do correctly on Windows.

Since the exit code for the signal processing thread is not used, having 
the wait_sig() thread (and any other threads that could potentially 
concurrently exit with another thread) exit with a special status value 
such as STATUS_THREAD_IS_TERMINATING (0xC000004BL) would enable 
diagnosis of this issue as any process exit code matching this would be 
a likely indicator that this issue was encountered.

As is, when this race condition results in the undesirable outcome, 
since the signal processing thread exits with a status of 0, the exit 
status of the process is 0.  This explains why false.exe works so well 
to reproduce the issue.  It would be impossible to produce a negative 
test using true.exe.

Tom.


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