Initial revision

Demo/threads/bug.py

+# 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