worker: Use buffered input from the pickle stream

On Python 3, "pickle.load" will raise an exception ("_pickle.UnpicklingError:
pickle data was truncated") when it gets a short read, i.e. it receives fewer
bytes than it requested.

On our build machine, Mercurial seems to frequently hit this problem while
updating a mozilla-central clone iff it gets scheduled in batch mode. It is easy
to trigger with:

    #wipe the workdir
    rm -rf *
    hg update null

    chrt -b 0 hg update default

I've also written the following program, which demonstrates the core problem:

    from __future__ import print_function

    import io
    import os
    import pickle
    import time

    obj = {"a": 1, "b": 2}
    obj_data = pickle.dumps(obj)
    assert len(obj_data) > 10

    rfd, wfd = os.pipe()

    pid = os.fork()
    if pid == 0:
        os.close(rfd)

        for _ in range(4):
            time.sleep(0.5)
            print("First write")
            os.write(wfd, obj_data[:10])

            time.sleep(0.5)
            print("Second write")
            os.write(wfd, obj_data[10:])

        os._exit(0)

    try:
        os.close(wfd)

        rfile = os.fdopen(rfd, "rb", 0)

        print("Reading")
        while True:
            try:
                obj_copy = pickle.load(rfile)
                assert obj == obj_copy
            except EOFError:
                break
        print("Success")
    finally:
        os.kill(pid, 15)

The program reliably fails with Python 3.8 and succeeds with Python 2.7.

Providing the unpickler with a buffered reader fixes the issue, so let
"os.fdopen" create one.

https://bugzilla.mozilla.org/show_bug.cgi?id=1604486

Differential Revision: https://phab.mercurial-scm.org/D8051

mercurial/worker.py

@@ -226,7 +226,7 b' def _posixworker(ui, func, staticargs, a'
     selector = selectors.DefaultSelector()
     for rfd, wfd in pipes:
         os.close(wfd)
-        selector.register(os.fdopen(rfd, 'rb', 0), selectors.EVENT_READ)
+        selector.register(os.fdopen(rfd, 'rb'), selectors.EVENT_READ)
 
     def cleanup():
         signal.signal(signal.SIGINT, oldhandler)