issue #155: fix double-fork behaviour and test it this time.

docs/getting_started.rst

@@ -262,7 +262,6 @@ Recovering Mitogen Object References In Children
         ...
 
 
-
 Recursion
 ---------
 

mitogen/fork.py

@@ -71,10 +71,12 @@ def handle_child_crash():
     Respond to _child_main() crashing by ensuring the relevant exception is
     logged to /dev/tty.
     """
-    sys.stderr.write('\n\nFORKED CHILD PID %d CRASHED\n%s\n\n' % (
+    tty = open('/dev/tty', 'wb')
+    tty.write('\n\nFORKED CHILD PID %d CRASHED\n%s\n\n' % (
         os.getpid(),
         traceback.format_exc(),
     ))
+    tty.close()
     os._exit(1)
 
 
@@ -134,7 +136,13 @@ class Stream(mitogen.parent.Stream):
         # avoid ExternalContext.main() accidentally allocating new files over
         # the standard handles.
         os.dup2(childfp.fileno(), 0)
-        os.dup2(sys.stderr.fileno(), 2)
+
+        # Avoid corrupting the stream on fork crash by dupping /dev/null over
+        # stderr. Instead, handle_child_crash() uses /dev/tty to log errors.
+        devnull = os.open('/dev/null', os.O_WRONLY)
+        if devnull != 2:
+            os.dup2(devnull, 2)
+            os.close(devnull)
         childfp.close()
 
         kwargs = self.get_main_kwargs()

tests/fork_test.py

@@ -26,6 +26,10 @@ c_ssl.RAND_pseudo_bytes.argtypes = [ctypes.c_char_p, ctypes.c_int]
 c_ssl.RAND_pseudo_bytes.restype = ctypes.c_int
 
 
+def ping():
+    return 123
+
+
 def random_random():
     return random.random()
 
@@ -54,5 +58,22 @@ class ForkTest(testlib.RouterMixin, unittest2.TestCase):
                             RAND_pseudo_bytes())
 
 
+class DoubleChildTest(testlib.RouterMixin, unittest2.TestCase):
+    def test_okay(self):
+        # When forking from the master process, Mitogen had nothing to do with
+        # setting up stdio -- that was inherited wherever the Master is running
+        # (supervisor, iTerm, etc). When forking from a Mitogen child context
+        # however, Mitogen owns all of fd 0, 1, and 2, and during the fork
+        # procedure, it deletes all of these descriptors. That leaves the
+        # process in a weird state that must be handled by some combination of
+        # fork.py and ExternalContext.main().
+
+        # Below we simply test whether ExternalContext.main() managed to boot
+        # successfully. In future, we need lots more tests.
+        c1 = self.router.fork()
+        c2 = self.router.fork(via=c1)
+        self.assertEquals(123, c2.call(ping))
+
+
 if __name__ == '__main__':
     unittest2.main()