[pantsd] Multiple pantsd runs piped by xargs cause a spurious exception.

[blorente]

When I pipe two pants runs together with an xargs in between them, I get an exception when the NailgunStreamWriter of the first run tries to remove a file descriptor twice (because it errored):

$ ./pants --pantsd list src/scala:: | xargs ./pants --pantsd filedeps
Exception in thread NailgunStreamWriter:
Traceback (most recent call last):
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/threading.py", line 917, in _bootstrap_inner
    self.run()
  File "/Users/bescobar/workspace/otherpants/src/python/pants/java/nailgun_io.py", line 229, in run
    self.do_run(readable_fds, errored_fds)
  File "/Users/bescobar/workspace/otherpants/src/python/pants/java/nailgun_io.py", line 247, in do_run
    self.stop_reading_from_fd(fileno)
  File "/Users/bescobar/workspace/otherpants/src/python/pants/java/nailgun_io.py", line 224, in stop_reading_from_fd
    self._in_fds.remove(fileno)
ValueError: list.remove(x): x not in list

/Users/bescobar/workspace/otherpants/3rdparty/jvm/org/pantsbuild/buck/util/zip/BUILD
... (correct output)

This doesn't happen if we pipe to xargs but not another pants run:

$ ./pants --pantsd filedeps src/scala:: | parallel grep "BUILD" {} \;

# This BUILD file defines common external library dependencies used across many modules.  Modules
# If you're looking at this BUILD file for inspiration organizing your own project's 3rdparty

The overall exit code is 0 in both cases.

My current hypothesis is that the second pants run closes the socket ahead of time, but I don't really know.

[stuhood]

This will deadlock in many cases, unfortunately (although with a nice error message now!). So while cleaning up the exception would be good, we need to be discouraging this pattern.

[blorente]

Thanks!

[stuhood]

This was fixed by #11147. Hooray!

Avoiding the (potential) deadlock of the first case on large runs will be resolved by #7654.