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

[Greg Clayton]

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