[lldb-dev] Process monitoring Host/Plugin interactions

[Greg Clayton]

> On Oct 24, 2014, at 11:53 AM, Zachary Turner <zturner at google.com> wrote:
> 
> I'm going to tackle implementing process monitoring for Windows next, and I've started going over how this works for other platforms.
> 
> There's a bit of a confusing interaction between Host, Target, and the individual plugins, and I'm wondering if it might make more sense to put the monitoring code somewhere other than Host.
> 
> From what I can tell, launching a process for debugging goes something like this:
> 
> 1) Call ProcessPlugin::DoLaunch
> 2) some system calls to actually launch the process
> 3) Call Host::StartMonitoringChildProcess with callback set to somewhere in your plugin.
> 4) The monitoring thread (in Host) periodically calls back into ProcessPlugin
> 
> For starters, this seems to be broken in the case where the ProcessLaunchInfo has pre-set an m_monitor_callback because the plugins end up just ignoring this and using their own callback.
> 
> Ultimately, how a process is monitored is going to depend on the plugin, so shouldn't StartMonitoringChildProcess also be part of the plugin?  In what situation would you ever want to monitor a child process that is not under the control of some process plugin?

Any process you launch that you aren't going to debug. Like launching a shell command or just say running "debugserver" in order for _it_ to be able to debug your application along with ProcessGDBRemote. ProcessGDBRemote is very interested if the "debugserver" process dies because it crashes, so it can kill the debug session and say "debugserver exited with status 12". Also when running a shell command you need to be able to run the shell command and find out when the shell command exits.

So the monitoring of the process definitely will stay in the host layer. The host layer can and should be used to launch any process (though some plug-ins might not be respecting that and they might be doing it themselves. The idea is we write the process launching code once in the host layer, then lldb-gdbserver, lldb-platform, and lldb can all use that one copy of the code to correctly launch processes and monitor (for signals) and reap them (get exit status and know that any stdout/stderr can now be forwarded onto whomever needs it.

Greg