[lldb-dev] Process monitoring Host/Plugin interactions

[Zachary Turner]

In the patch I posted, there actually isn't any duplicated code.  All the
Windows specific stuff about launching a process is in
Host/windows/ProcessLauncherWindows.  It has all the stdio redirection and
everything else.  In Host::LaunchProcess there's an #ifdef that on Windows
makes a ProcessLauncherWindows and uses it.  In ProcessWindows::DoLaunch I
create a DebugProcessLauncher, which has all the knowledge about launching
processes on different threads, debug monitoring, etc, and at some point in
it also makes a ProcessLauncherWindows and re-uses it.

So the platform specific part of launching processes on Windows is already
re-used between the Host::LaunchProcess and ProcessWindows::DoLaunch
codepaths.  It's just that Host::LaunchProcess and ProcessWindows::DoLaunch
reuse the same thing, rather than ProcessWindows::DoLaunch directly reusing
Host::LaunchProcess.

Another tricky part is that Host::LaunchProcess returns a HostThread and
users of the API expect to be able to join on this thread to wait for the
process to exit, and that might not work because you might have the same
monitor thread debugging multiple processes.

On Fri Oct 31 2014 at 1:01:17 PM Greg Clayton <gclayton at apple.com> wrote:

> How about this solution: on windows in Host::LaunchProcess() if
> eLaunchFlagDebug is set, then return an error if the monitor callback is
> set. If it is set, don't monitor the process with the usual method that
> would be used (again only on windows).
>
> There is a lot that goes into launching a process for the arguments,
> environment, working directory, re-direct stdin/out/err and others, so it
> would be a shame to duplicate that code especially since the
> Host::LaunchProcess is so close.
>
> So:
> 1 - no new launch flag
> 2 - on windows watch for eLaunchFlagDebug and don't monitor the process if
> it is set
> 3 - modify ProcessWindows::DoLaunch() to use Host::LaunchProcess() and
> then monitor it in the different way for debugging.
>
> I don't see the need for a separate implementation unless I am missing
> something.
>
> Greg
> > On Oct 31, 2014, at 12:00 PM, Zachary Turner <zturner at google.com> wrote:
> >
> > So this email originally went through while I was still trying to come
> up with a working solution.  So before I had arrived at the solution I came
> up with in the patches I uploaded.
> >
> > Anyway, some rambling ahead.
> >
> > Ultimately I ended up not going through Host::LaunchProcess, so perhaps
> most of the questions I asked are moot now.  In any case, the biggest
> problem I had when trying to make ProcessWindows::DoLaunch use
> Host::LaunchProcess is that the interface wasn't really flexible enough.
> It seems that based on the distinction you describe between a process
> status monitor and a debug monitor, Windows doesn't ever need to "monitor"
> a process for anything.  At least not when debugging.
> >
> > Passing it through as a flag like eLaunchFlagDontMonitor as you
> suggested probably would work, but it seems like at that point we're
> sinking platform specific flags into a generic interface.  If
> eLaunchFlagDontMonitor is set if and only if we're launching a process for
> debugging on Windows, then imagine if it were called
> eLaunchFlagWindowsProcess instead.
> >
> > Part of me thinks that launching processes is just complicated, and
> maybe we shouldn't try to mash it into one generic function (e.g.
> Host::LaunchProcess).  That was the driving idea behind the ProcessLauncher
> abstraction.  It allows launching strategies that are plugin specific, or
> platform specific, to be implemented in plugin-specific or
> platform-specific ways, or even in multiple different ways.
> >
> > Of course at some level generic code needs to be able to have one
> function that just says "hey, launch this process", but that function could
> simply look at the flags in the ProcessLaunchInfo and create the correct
> type of ProcessLauncher based on the flags that are specified.
> >
> > A concrete example of how this is useful is because right now, it's
> unclear what would happen in ProcessWindows if someone passed a monitor
> callback / baton pair, because I just ignore it.  Really that just means
> that the idea of the monitor callback and baton isn't actually generic
> enough, and maybe would be better somewhere else.  But if you search
> through the code, there's only a few very well-defined places where it's
> actually set.  The most obvious example is running a shell command.  So
> here, you would just have ShellProcessLauncher that does something
> different than RegularProcessLauncher, or whatever.  You could remove the
> callback and baton from ProcessLaunchInfo entirely.
> >
> > Anyway, these are all just high level ideas.  FWIW, one of the reasons
> I'm going out of my way and make great effort to create these abstractions
> in a way that they're as reusable and generic as possible, is because it
> would be great if everything I'm doing now just fits into llgs in the
> future without a tremendous porting effort.  Not having posix compatibility
> in Windows breaks alot of the assumptions that LLDB originally made, so
> hopefully incrementally tackling these assumptions over time provides some
> benefit in the long run.
> >
> > On Fri Oct 31 2014 at 11:31:48 AM Greg Clayton <gclayton at apple.com>
> wrote:
> >
> > > On Oct 27, 2014, at 4:16 PM, Zachary Turner <zturner at google.com>
> wrote:
> > >
> > > I keep coming back to this interface solution.  The issue I'm having
> is that I need completely different monitoring algorithms for when the
> process is launched as a debug target versus something like a shell command
> or utility process where I only want to know when it exits.  In both cases
> it will spawn a thread and run some code in the thread, but the code that
> runs in the thread will be different.
> > >
> > > On the other hand, Process launching and monitoring should be
> initiated by Host, since it is common functionality, and as you said
> previously process plugins should be using Host::LaunchProcess but may not
> be respecting that.
> > >
> > > So the issue is: I need my process plugin to be able call
> Host::LaunchProcess in such a way that Host::LaunchProcess knows to use a
> different monitoring algorithm.
> > >
> > > But then things get tricky.  The algorithm it needs to use in the case
> of a debug target does not really belong in Host, because it will allow me
> to detect events like dll loads / unloads, thread creation, child process
> spawning, exceptions, and other things.  So this code belongs in the
> process plugin.  The cleanest way I can come up with to really handle this
> is to let Host implement this interface in such a way that it only monitors
> for exit status, and let my plugin implement it in such a way that it
> monitors for all the other stuff as well.
> > >
> > > If you're opposed to this, I can "make it work" it's going to involve
> implementing ProcessWindows::DoLaunch without the help of
> Host::LaunchProcess(), which is what I was trying to avoid.
> >
> > We could pass the launch flags (which would contain eLaunchFlagDebug and
> could be used to "do the right thing") into the process monitor. Would that
> solve the issue?
> >
> > >
> > > I think the high level notion of a process status monitor (note this
> what I'm calling a process status monitor is much more narrowly defined
> than what is encompassed by the ProcessMonitor class in Linux and FreeBSD)
> applies to all platform, regardless of whether we're doing local debugging,
> lldb-gdbserver, or even no debugging at all (e.g. running a shell command),
> so I think it's generally useful.
> >
> > I do think there is usually a difference between the "process status
> monitor" and the "I have a special connection to a process because I am
> debugging it and need all exceptions and other very low level stuff.". So I
> am not sure it is a great idea to try and merge the two.
> >
> > >
> > > I can do this in such a way that (for now anyway) only Windows
> actually makes use of this interface and other platforms' logic remains
> untouched, with the idea of using it in the future (after llgs is more
> cemented, for example).
> > >
> > > What are your thoughts?  If you need to see some code to make things
> concrete, I can upload a patch.
> >
> > Think about the difference between monitoring the process and debugging
> a process. They are quite different for MacOSX. I believe they are as well
> on linux (waitpid() is not used as the main way to figure out if a process
> has stopped due to a breakpoint for example, and windows seems to be quite
> different).
> >
> > A simpler approach might be to allow clients to specify if a process
> should be monitored when launching with a new eLaunchFlagDontMonitor. Only
> windows would set this from its ProcessWindows::DoLaunch() and then it can
> run the more complex thread that tracks a process for debugging. On Linux
> and MacOSX, it is quite ok to monitor a process with waitpid(), and also be
> debugging it with ptrace() or being attached to a task port with MacOSX.
> >
> > Greg
>
>
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