Lib/test/test_threaded_import.py

   1 # This is a variant of the very old (early 90's) file
   2 # Demo/threads/bug.py.  It simply provokes a number of threads into
   3 # trying to import the same module "at the same time".
   4 # There are no pleasant failure modes -- most likely is that Python
   5 # complains several times about module random having no attribute
   6 # randrange, and then Python hangs.
   7
   8 import unittest
   9 from test.test_support import verbose, TestFailed, import_module
  10 thread = import_module('thread')
  11
  12 critical_section = thread.allocate_lock()
  13 done = thread.allocate_lock()
  14
  15 def task():
  16     global N, critical_section, done
  17     import random
  18     x = random.randrange(1, 3)
  19     critical_section.acquire()
  20     N -= 1
  21     # Must release critical_section before releasing done, else the main
  22     # thread can exit and set critical_section to None as part of global
  23     # teardown; then critical_section.release() raises AttributeError.
  24     finished = N == 0
  25     critical_section.release()
  26     if finished:
  27         done.release()
  28
  29 def test_import_hangers():
  30     import sys
  31     if verbose:
  32         print "testing import hangers ...",
  33
  34     import test.threaded_import_hangers
  35     try:
  36         if test.threaded_import_hangers.errors:
  37             raise TestFailed(test.threaded_import_hangers.errors)
  38         elif verbose:
  39             print "OK."
  40     finally:
  41         # In case this test is run again, make sure the helper module
  42         # gets loaded from scratch again.
  43         del sys.modules['test.threaded_import_hangers']
  44
  45 # Tricky:  When regrtest imports this module, the thread running regrtest
  46 # grabs the import lock and won't let go of it until this module returns.
  47 # All other threads attempting an import hang for the duration.  Since
  48 # this test spawns threads that do little *but* import, we can't do that
  49 # successfully until after this module finishes importing and regrtest
  50 # regains control.  To make this work, a special case was added to
  51 # regrtest to invoke a module's "test_main" function (if any) after
  52 # importing it.
  53
  54 def test_main():        # magic name!  see above
  55     global N, done
  56
  57     import imp
  58     if imp.lock_held():
  59         # This triggers on, e.g., from test import autotest.
  60         raise unittest.SkipTest("can't run when import lock is held")
  61
  62     done.acquire()
  63     for N in (20, 50) * 3:
  64         if verbose:
  65             print "Trying", N, "threads ...",
  66         for i in range(N):
  67             thread.start_new_thread(task, ())
  68         done.acquire()
  69         if verbose:
  70             print "OK."
  71     done.release()
  72
  73     test_import_hangers()
  74
  75 if __name__ == "__main__":
  76     test_main()