issue #155: handle crash in the forking child better.

This code path is probably only necessary during development, but it
prevents tracebacks (etc.) getting written over the Stream socket, which
naturally causes corruption.

Instead keep whatever the parent has for stderr, manually write a
traceback there and hard exit.

mitogen/fork.py

@@ -31,6 +31,7 @@ import os
 import random
 import sys
 import threading
+import traceback
 
 import mitogen.core
 import mitogen.parent
@@ -65,6 +66,18 @@ def break_logging_locks():
             handler.createLock()
 
 
+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' % (
+        os.getpid(),
+        traceback.format_exc(),
+    ))
+    os._exit(1)
+
+
 class Stream(mitogen.parent.Stream):
     #: Reference to the importer, if any, recovered from the parent.
     importer = None
@@ -94,7 +107,13 @@ class Stream(mitogen.parent.Stream):
             return self.pid, fd
         else:
             parentfp.close()
+            self._wrap_child_main(childfp)
+
+    def _wrap_child_main(self, childfp):
+        try:
             self._child_main(childfp)
+        except BaseException, e:
+            handle_child_crash()
 
     def _child_main(self, childfp):
         mitogen.core.Latch._on_fork()
@@ -113,15 +132,16 @@ class Stream(mitogen.parent.Stream):
         # avoid ExternalContext.main() accidentally allocating new files over
         # the standard handles.
         os.dup2(childfp.fileno(), 0)
-        os.dup2(childfp.fileno(), 2)
+        os.dup2(sys.stderr.fileno(), 2)
         childfp.close()
 
         kwargs = self.get_main_kwargs()
         kwargs['core_src_fd'] = None
         kwargs['importer'] = self.importer
         kwargs['setup_package'] = False
+        try:
             mitogen.core.ExternalContext().main(**kwargs)
-
+        finally:
             # Don't trigger atexit handlers, they were copied from the parent.
             os._exit(0)