The atexit module effectively turned itself off if sys.exitfunc already

existed at the time atexit first got imported.  That's a bug, and this
fixes it.

Also reworked test_atexit.py to test for this too, and to stop using
an "expected output" file, and to test what actually happens at exit
instead of just simulating what it thinks atexit will do at exit.

Bugfix candidate, but it's messy so I'll backport to 2.2 myself.

Lib/atexit.py

@@ -29,15 +29,11 @@ def register(func, *targs, **kargs):
     _exithandlers.append((func, targs, kargs))
 
 import sys
-try:
-    x = sys.exitfunc
-except AttributeError:
-    sys.exitfunc = _run_exitfuncs
-else:
-    # if x isn't our own exit func executive, assume it's another
-    # registered exit function - append it to our list...
-    if x != _run_exitfuncs:
-        register(x)
+if hasattr(sys, "exitfunc"):
+    # Assume it's another registered exit function - append it to our list
+    register(sys.exitfunc)
+sys.exitfunc = _run_exitfuncs
+
 del sys
 
 if __name__ == "__main__":

Lib/test/output/test_atexit

-test_atexit
-handler2 (7,) {'kw': 'abc'}
-handler2 () {}
-handler1

Lib/test/test_atexit.py

-# Test the exit module
-from test_support import verbose
+# Test the atexit module.
+from test_support import TESTFN, vereq
+import atexit
+import os
+
+input = """\
 import atexit
 
 def handler1():
@@ -8,17 +12,50 @@ def handler1():
 def handler2(*args, **kargs):
     print "handler2", args, kargs
 
-# save any exit functions that may have been registered as part of the
-# test framework
-_exithandlers = atexit._exithandlers
-atexit._exithandlers = []
-
 atexit.register(handler1)
 atexit.register(handler2)
 atexit.register(handler2, 7, kw="abc")
+"""
+
+fname = TESTFN + ".py"
+f = file(fname, "w")
+f.write(input)
+f.close()
+
+p = os.popen("python " + fname)
+output = p.read()
+p.close()
+vereq(output, """\
+handler2 (7,) {'kw': 'abc'}
+handler2 () {}
+handler1
+""")
+
+input = """\
+def direct():
+    print "direct exit"
+
+import sys
+sys.exitfunc = direct
+
+# Make sure atexit doesn't drop 
+def indirect():
+    print "indirect exit"
+
+import atexit
+atexit.register(indirect)
+"""
+
+f = file(fname, "w")
+f.write(input)
+f.close()
 
-# simulate exit behavior by calling atexit._run_exitfuncs directly...
-atexit._run_exitfuncs()
+p = os.popen("python " + fname)
+output = p.read()
+p.close()
+vereq(output, """\
+indirect exit
+direct exit
+""")
 
-# restore exit handlers
-atexit._exithandlers = _exithandlers
+os.unlink(fname)