[lldb-dev] Proposal for refactoring process launching

Zachary Turner zturner at google.com
Fri Sep 26 09:44:01 PDT 2014

On Wed, Sep 24, 2014 at 10:56 AM, Greg Clayton <gclayton at apple.com> wrote:

> Comments:
> - When running processes one might want to get signals for the process
> that are delivered while the child process is running. There can be any
> amount of signals over the child process' lifetime. This must be included
> in the design. It might not work for windows, but it will need to be worked
> into the design.

Can you clarify?  The goal was to propose this in such a way that it's
functionally equivalent to the current code.  In the existing code, what is
the means by which this currently happens?  There is the ability to send a
signal *to* a process, and that is covered in this design by providing a
Signal() method on HostNativeProcessPosix.  Anyone calling that method is
invoking platform specific functionality, so they would have either already
be inside platform specific code, or write something like
HostProcess->GetNativeProcess()->Signal() wrapped in an appropriate #ifdef.

> - You can change the internal API, but you can't change the public API. If
> you make changes to any of the launch info classes internally, you will
> need to keep the SBLaunchInfo API the same. I know you like cleaning things
> up, but we have clients that use this API and we don't want it broken. You
> will need to carefully check for any other things that get modified in the
> launch info class and make sure any such cases are fixed.
This shouldn't involve a change to the public API.

> - We should make ProcessLauncher a plug-in and be able to request them by
> name. Then we can add things to the launch info to be able to specify the
> process launcher plug-in name ("posix_spawn", "fork", "apple-xpc",
> "default" which can map to the default launcher for a host, etc).
Good idea, probably can be done as a last step.  All of the actual logic
would still live in Host, since it's OS-specific, and the plugin code could
just be a lightweight shim to select the correct one.

> >
> > This approach yields a number of benefits.  For starters, it allows us
> to abstract out the notion of the launching strategy into a reusable piece
> of code so that platforms which have multiple launching strategies have the
> ability to make use of specific strategies in other platform specific code,
> or so that remote process launches can be treated the same as local process
> launches.  As an example, on MacOSX there are three different launching
> strategies:  Launch with XPC services, launch with Applescript, and launch
> with posix_spawn.  Each of these could be independent implementations of
> |ProcessLauncher|, with the default implementation on MacOSX being a fourth
> implementation that chooses one of the first 3 implementations to delegate
> to at runtime.  As a result, the 3 implementations can be used
> independently of each other should the need arise.
> >
> > A second benefit is that it opens up the possibility of treating remote
> process launch transparently with local process launch -- just have a
> |RemoteProcessLauncher| class.
> Uh... Then don't call your class HostProcess. I really would rather not
> have remote launching involved in this at all. Remote launching should
> result in a lldb::pid_t and it won't use the native process layer for
> anything, so I would avoid including remote process launching in this as it
> mucks up what info might need to be in the launch info (remote host/port,
> password, etc). So no remote process support please.
Fair enough, I mentioned that just as something that may or may not be
useful in the future, but the design lends itself nicely to.  I don't want
to choose names based on something that might not happen though, so I think
it still makes sense to go with HostProcess, since for now that's what it
actually is, and gives some naming consistency between this and HostThread.

> >
> > There are additional benefits in the form of overall code health,
> platform separation, and just generally improving the modularity of the
> codebase, although those are perhaps less tangible.
> >
> > Monitoring for process exit
> > It is important to know when a process exits, for 2 reasons.  First, on
> some platforms the process needs to be reaped.  Second, asynchronous exit
> callbacks need to be invoked.  The implementation of the monitoring is
> platform dependent, and a strategy for each platform can be summarized as
> follows:
> >
> >       • Windows
> >               • Create a background thread, use WaitForSingleObject
> >               • [Better, alternative implementation] Create one "static"
> background thread for all processes, use WaitForMultipleObjects() to
> monitor many processes at once, like select().
> >       • MacOSX
> >               • Processes spawned with posix_spawn - Use MacOSX
> "dispatch" library to queue monitoring code on a system thread.
> >               • Processes spawned with XPC - Use MacOSX "dispatch"
> library to queue monitoring code on a system thread.
> >               • Processes spawned with Applescript - No monitoring
> necessary
> >       • Non-MacOSX posix systems
> >               • Processes spawned with posix_spawn - Create a background
> thread, use waitpid in the background thread.
> We also need to be able to deliver signals during child process execution.
Mentioned earlier, but this is already covered.  HostProcess is a wrapper
around a HostNativeProcess, and provides a method just like HostThread
does, which returns the native process.  Since signal is not a generic
operation, it wouldn't live on HostProcess, but on HostNativeProcessPosix.
Anyone wishing to deliver a signal to a child process would write
host_process.GetNativeProcess()->Signal(SIGTERM) or whatever.  If they
write this from generic code it would need to be inside of an #ifdef.  If
they write it from platform-specific code it will just work.
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