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