[libvirt] Strange periodic failures of the commandtest case

[Eric Blake]

On 02/14/2011 06:45 AM, Daniel P. Berrange wrote:
> On Mon, Feb 14, 2011 at 12:32:18PM +0000, Daniel P. Berrange wrote:
>> I'm getting periodic failures of the 'commandtest' case where the diff
>> is
>>
>>> DAEMON:no
>> --
>> < DAEMON:yes
>>
>> For test cases 'test3' through to 'test15', except 'test'4. Tests 1,
>> 2, 4, 16, 17, 18 are all unaffected. I can never reproduce it when
>> I run just that one test case manually, but the automated builds
>> I'm doing hit it > 50% of the time so it is clearly some kind of race
>> condition. The line of code that's getting confused is this
>>
>>     fprintf(log, "DAEMON:%s\n", getpgrp() == getsid(0) ? "yes" : "no");
>>
>> but I'm not clear how this can be going wrong. Ideas ?
> 
> Hmm, this seems to occur when you run the build under cron. Under an
> interactive shell  getpgrp() != getsid() by default. Under non-interactive
> shell, then getpgrp() == getsid() even if we didn't ask to daemonize.
> So this approach to detecting whether we're daemonized isn't reliable.

It's more reliable than the previous test, which was getppid() == 1,
since that was even more racy depending on whether the intermediate
process had exited yet to result in the grandchild being reparented to
init.  But yes, I think you correctly analyzed the problem of running
from cron rather than an interactive shell.

> 
> Perhaps the 'commandtest' program itself needs to call setsid()+setpgid()
> before running the child processes ?

That sounds like a reasonable solution.  Do you want to tackle it, or
shall I?

I'm also seeing that test fail on cygwin; probably due to .exe suffixes.
 But I haven't yet had time to investigate how easy that would be to
patch, yet.

-- 
Eric Blake   eblake at redhat.com    +1-801-349-2682
Libvirt virtualization library http://libvirt.org

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