[lldb-dev] Process spawning and shutdown

Zachary Turner zturner at google.com
Wed Sep 17 00:51:18 PDT 2014

Sorry, hit send too soon.

Exposing native OS primitive types like lldb::thread_t, lldb::process_t,
and raw file descriptors to generic code means that people will use them in
ways that aren't actually generic, and this can already been seen in many
places.  The biggest example is currently llgs, which has posix-y stuff all
over and which will be quite difficult to port to Windows as a result, if
and when we get there.  There are other examples too though.
 Platform-specific ideas have made it into the public API, such as
SBHostOS::ThreadDetach, and are used in other places as well, such as a
reliance on TLS destructors (Timer) and thread cancellation.  There's also
select() being used on file descriptors, and probably many things I haven't
even found yet.

I can go fix all of these things on a case-by-case basis, and originally
that was my strategy.  But as I found more and more examples of it, I
started thinking that I want to prevent this type of code from showing up
in the future.  I searched for the thread but was unable to find it, where
Jim (apologies if I'm misquoting or misremembering) said that when the Host
layer was originally written, there was not sufficient time to sit down and
design something future-proof, and that you guys just had to get it
working.  So what I was (and have been) trying to accomplish was exactly
that.  Obviously, such large changes are not without risk, and can create
headaches and introduce bugs, although I think that once the bugs are
resolved the entire codebase and all platforms will benefit from improved
code health as a result.

Ultimately if you don't think these type of changes add value, or you don't
think it's an improvement, then that's that.  It's much easier for me to
write code just for Windows and not have to worry about getting stuff
working on 3 different platforms that I have varying levels of familiarity
with.  I don't think it's necessarily easier in the long term though, as
there will still be no clear separation between generic and platform
specific code, and new things will continue to turn up where an assumption
was made that something was generic when it wasn't.

On Wed, Sep 17, 2014 at 12:27 AM, Zachary Turner <zturner at google.com> wrote:

> On Tue, Sep 16, 2014 at 3:17 PM, Greg Clayton <gclayton at apple.com> wrote:
>> > On Sep 16, 2014, at 12:55 PM, Zachary Turner <zturner at google.com>
>> wrote:
>> >
>> > Just a follow-up.  It seems like there are three use cases for the
>> StartMonitoringChildProcess code-path (poorly named, since FreeBSD and
>> linux process plugins also have a ProcessMonitor class leading to great
>> confusion).
>> >
>> > 1) Some places want to Join() on a process exiting.  They currently do
>> this by calling Join() on a HostThread returned by
>> StartMonitoringChildProcess, but the important thing is just that they want
>> to join, not how it's implemented.
>> Join is not the right word. Reap() is the correct word. Join is just seen
>> because you might spawn a thread whose sole purpose in life is to reap a
>> child process. When you launch a shell command, you want to spawn the
>> process and wait for it to get reaped. If you debug a process, you need to
>> reap your child process if you launch it, but you won't ever call join on a
>> thread that is waiting for it.
> I know Join isn't really the right word, but there's no concept of reaping
> a process on Windows.  My understanding is that on posix, "reaping"
> specifically refers to ensuring that zombie processes don't linger and
> waste system resources which is a logically different operation than "wait
> for this process to exit", even though certain operations that reap also
> wait for the process to exit as a means to an end.  So I'm using join for
> lack of a better term to refer specifically to "wait for this process to
> exit".
> On Tue, Sep 16, 2014 at 3:21 PM, Greg Clayton <gclayton at apple.com> wrote:
>> I would really like to see the Host::LaunchProcess fixed for windows and
>> fix the reaping to just work for windows. I would like to avoid large
>> changes that aren't needed. All of the AppleScript stuff and other details
>> are fine to stay hidden within the MacOSX specific version of
>> Host::LaunchProcess() as long as the contents of the ProcessLaunchInfo are
>> obeyed.
> I had been working on creating a HostProcess abstraction similar to the
> HostThread abstraction that I created.  I think that there's alot of value
> in having a robust set of OS abstractions that can be used by lldb,
> debugserver, and other processes alike.  Exposing native OS primitive types
> like lldb::thread_t and lldb::process_t to generic code
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