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70 lines
2.2 KiB
Python
70 lines
2.2 KiB
Python
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# The following self-contained little program usually freezes with most
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# threads reporting
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#
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# Unhandled exception in thread:
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# Traceback (innermost last):
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# File "importbug.py", line 6
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# x = whrandom.randint(1,3)
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# AttributeError: randint
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#
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# Here's the program; it doesn't use anything from the attached module:
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import thread
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def task():
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global N
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import whrandom
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x = whrandom.randint(1,3)
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a.acquire()
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N = N - 1
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if N == 0: done.release()
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a.release()
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a = thread.allocate_lock()
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done = thread.allocate_lock()
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N = 10
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done.acquire()
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for i in range(N):
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thread.start_new_thread(task, ())
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done.acquire()
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print 'done'
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# Sticking an acquire/release pair around the 'import' statement makes the
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# problem go away.
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#
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# I believe that what happens is:
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#
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# 1) The first thread to hit the import atomically reaches, and executes
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# most of, get_module. In particular, it finds Lib/whrandom.pyc,
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# installs its name in sys.modules, and executes
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#
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# v = eval_code(co, d, d, d, (object *)NULL);
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#
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# to initialize the module.
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#
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# 2) eval_code "ticker"-slices the 1st thread out, and gives another thread
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# a chance. When this 2nd thread hits the same 'import', import_module
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# finds 'whrandom' in sys.modules, so just proceeds.
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#
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# 3) But the 1st thread is still "in the middle" of executing whrandom.pyc.
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# So the 2nd thread has a good chance of trying to look up 'randint'
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# before the 1st thread has placed it in whrandom's dict.
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#
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# 4) The more threads there are, the more likely that at least one of them
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# will do this before the 1st thread finishes the import work.
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#
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# If that's right, a perhaps not-too-bad workaround would be to introduce a
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# static "you can't interrupt this thread" flag in ceval.c, check it before
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# giving up interpreter_lock, and have IMPORT_NAME set it & restore (plain
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# clearing would not work) it around its call to import_module. To its
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# credit, there's something wonderfully perverse about fixing a race via an
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# unprotected static <grin>.
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#
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# as-with-most-other-things-(pseudo-)parallel-programming's-more-fun-
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# in-python-too!-ly y'rs - tim
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#
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# Tim Peters tim@ksr.com
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# not speaking for Kendall Square Research Corp
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