D8051: worker: Use buffered input from the pickle stream

[heftig (Jan Alexander Steffens)]

heftig created this revision.
Herald added a subscriber: mercurial-devel.
Herald added a reviewer: hg-reviewers.

REVISION SUMMARY
  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:
  
  1. 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

REPOSITORY
  rHG Mercurial

BRANCH
  stable

REVISION DETAIL
  https://phab.mercurial-scm.org/D8051

AFFECTED FILES
  mercurial/worker.py

CHANGE DETAILS

diff --git a/mercurial/worker.py b/mercurial/worker.py
--- a/mercurial/worker.py
+++ b/mercurial/worker.py
@@ -226,7 +226,7 @@
     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)



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