[lldb-dev] Process attach and pgid of forked process under Linux

[Todd Fiala]

Ah nice.  I'll definitely run this again soon.  Thanks for getting a test
around it - we can't have enough of that around important expectations.


On Wed, Mar 26, 2014 at 2:06 PM, Andrew MacPherson <andrew.macp at gmail.com>wrote:

> Ok that makes sense. I've updated the patch I sent yesterday with a unit
> test that exposes the problem as well as a couple of other process attach
> issues that were fixed in the last few days. It was in fact this getpgid()
> issue that was causing problems when trying to create a unit test earlier
> as the problem scenario occurs when the unit test script forks a subprocess
> with popen().
>
>
> On Wed, Mar 26, 2014 at 7:20 PM, Greg Clayton <gclayton at apple.com> wrote:
>
>>
>> On Mar 25, 2014, at 12:09 PM, Andrew MacPherson <andrew.macp at gmail.com>
>> wrote:
>>
>> > Ok great, I'm attaching a patch here that just uses getpgid() to
>> determine the pgid to use with waitpid(). I get the same unit test results
>> before and after the patch (there are a few tests that fail consistently
>> for me). I tried using a simple -1 with waitpid() but this results in a
>> hang in the unit tests. This is probably something worth investigating but
>> for now this patch does resolve the issues around waitpid() when attaching.
>>
>> We should never use -1 with waitpid() as you could end up reaping a
>> process that someone else is already trying to reap. You must only try to
>> reap the process that you launched using some pid for you process. Many
>> things launch processes within LLDB like ProcessGDBRemote launching
>> "debugserver" binaries. The ProcessGDBRemote must be notified when and if
>> "debugserver" crashes, so if anyone else does waitpid(-1, ...)
>> ProcessGDBRemote might never find out if debugserver crashes for some
>> reason.
>>
>> > I mentioned a few issues I ran into with the unit tests in the email I
>> just pinged with a Linux detach patch, basically there appears to be a bug
>> or limitation somewhere in that doing a "continue" in asynchronous mode in
>> a unit test breaks things and this is needed for most attach-related debug
>> testing. I can probably look into this (along with the couple of failing
>> tests I have) but likely won't be able to this week. I haven't filed a bug
>> for the consistently-failing tests on my end since it doesn't appear to be
>> happening for others and I haven't had a chance to investigate here, let me
>> know if I should file a bug anyway.
>> >
>> >
>> > On Tue, Mar 25, 2014 at 6:30 PM, Todd Fiala <tfiala at google.com> wrote:
>> > I'd suggest trying the change, running the tests, and see if anything
>> new fails.
>> >
>> > And if this fixes currently broken behavior, add a test to make sure we
>> don't regress it.  I'm pretty sure if it breaks something we'll know pretty
>> quickly (either via bugs or our own usage).  At which point - we should add
>> a test so we don't regress in the future and perhaps add a few comments as
>> to the reason.
>> >
>> >
>> > On Tue, Mar 25, 2014 at 9:54 AM, <jingham at apple.com> wrote:
>> > Why are you waiting for process groups?  That's not something we have
>> to do on Mac OS X.
>> >
>> > Jim
>> >
>> >
>> > On Mar 25, 2014, at 1:17 AM, Andrew MacPherson <andrew.macp at gmail.com>
>> wrote:
>> >
>> > > Currently under Linux if you attach to a process whose process group
>> id is not equal to its process id (such as the child process of a fork()
>> call) the calls to waitpid() that pass -1*pid will return ECHILD since the
>> pid argument refers to a process group that doesn't exist. These calls
>> occur in Host::MonitorChildProcessThreadFunction() and the Linux
>> ProcessMonitor.
>> > >
>> > > Changing -1*pid to simply -1 or to -1*getpgid(pid) resolves the issue
>> but it's not clear if this is the right fix as I'm unsure how other OSes
>> deal with this scenario.
>> > >
>> > > Any thoughts?
>> > >
>> > > Thanks,
>> > > Andrew
>> > > _______________________________________________
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>> > > lldb-dev at cs.uiuc.edu
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>> >
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>> >
>> >
>> >
>> > --
>> > Todd Fiala |   Software Engineer |     tfiala at google.com |
>> 650-943-3180
>> >
>> >
>> > <waitpid-getpgid.patch>_______________________________________________
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>> > lldb-dev at cs.uiuc.edu
>> > http://lists.cs.uiuc.edu/mailman/listinfo/lldb-dev
>>
>>
>


-- 
Todd Fiala | Software Engineer | tfiala at google.com | 650-943-3180
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