issue #477: make mitogen.fork unsupported on Py<2.6.

docs/api.rst

@@ -115,6 +115,9 @@ Connection Methods
     and router, and responds to function calls identically to children
     created using other methods.
 
+    The use of this method is strongly discouraged. It requires Python 2.6 or
+    newer, as older Pythons made no effort to reset threading state upon fork.
+
     For long-lived processes, :meth:`local` is always better as it
     guarantees a pristine interpreter state that inherited little from the
     parent. Forking should only be used in performance-sensitive scenarios
@@ -158,7 +161,9 @@ Connection Methods
 
     * Locks held in the parent causing random deadlocks in the child, such
       as when another thread emits a log entry via the :mod:`logging`
-      package concurrent to another thread calling :meth:`fork`.
+      package concurrent to another thread calling :meth:`fork`, or when a C
+      extension module calls the C library allocator, or when a thread is using
+      the C library DNS resolver, for example via :func:`socket.gethostbyname`.
 
     * Objects existing in Thread-Local Storage of every non-:meth:`fork`
       thread becoming permanently inaccessible, and never having their

mitogen/fork.py

@@ -39,6 +39,18 @@ import mitogen.parent
 
 LOG = logging.getLogger('mitogen')
 
+# Python 2.4/2.5 cannot support fork+threads whatsoever, it doesn't even fix up
+# interpreter state. So 2.4/2.5 interpreters start .local() contexts for
+# isolation instead. Since we don't have any crazy memory sharing problems to
+# avoid, there is no virginal fork parent either. The child is started directly
+# from the login/become process. In future this will be default everywhere,
+# fork is brainwrong from the stone age.
+FORK_SUPPORTED = sys.version_info >= (2, 6)
+
+
+class Error(mitogen.core.StreamError):
+    pass
+
 
 def fixup_prngs():
     """
@@ -113,9 +125,19 @@ class Stream(mitogen.parent.Stream):
     #: User-supplied function for cleaning up child process state.
     on_fork = None
 
+    python_version_msg = (
+        "The mitogen.fork method is not supported on Python versions "
+        "prior to 2.6, since those versions made no attempt to repair "
+        "critical interpreter state following a fork. Please use the "
+        "local() method instead."
+    )
+
     def construct(self, old_router, max_message_size, on_fork=None,
                   debug=False, profiling=False, unidirectional=False,
                   on_start=None):
+        if not FORK_SUPPORTED:
+            raise Error(self.python_version_msg)
+
         # fork method only supports a tiny subset of options.
         super(Stream, self).construct(max_message_size=max_message_size,
                                       debug=debug, profiling=profiling,

tests/fork_test.py

@@ -1,12 +1,25 @@
 
-import _ssl
-import ctypes
 import os
 import random
-import ssl
 import struct
 import sys
 
+try:
+    import _ssl
+except ImportError:
+    _ssl = None
+
+try:
+    import ssl
+except ImportError:
+    ssl = None
+
+try:
+    import ctypes
+except ImportError:
+    # Python 2.4
+    ctypes = None
+
 import mitogen
 import unittest2
 
@@ -29,13 +42,14 @@ def _find_ssl_darwin():
             return bits[1]
 
 
-if sys.platform.startswith('linux'):
+if ctypes and sys.platform.startswith('linux'):
     LIBSSL_PATH = _find_ssl_linux()
-elif sys.platform == 'darwin':
+elif ctypes and sys.platform == 'darwin':
     LIBSSL_PATH = _find_ssl_darwin()
 else:
-    assert 0, "Don't know how to find libssl on this platform"
+    LIBSSL_PATH = None
 
+if ctypes and LIBSSL_PATH:
     c_ssl = ctypes.CDLL(LIBSSL_PATH)
     c_ssl.RAND_pseudo_bytes.argtypes = [ctypes.c_char_p, ctypes.c_int]
     c_ssl.RAND_pseudo_bytes.restype = ctypes.c_int
@@ -64,6 +78,12 @@ def exercise_importer(n):
     return simple_pkg.a.subtract_one_add_two(n)
 
 
+skipIfUnsupported = unittest2.skipIf(
+    condition=(not mitogen.fork.FORK_SUPPORTED),
+    reason="mitogen.fork unsupported on this platform"
+)
+
+
 class ForkTest(testlib.RouterMixin, testlib.TestCase):
     def test_okay(self):
         context = self.router.fork()
@@ -74,6 +94,10 @@ class ForkTest(testlib.RouterMixin, testlib.TestCase):
         context = self.router.fork()
         self.assertNotEqual(context.call(random_random), random_random())
 
+    @unittest2.skipIf(
+        condition=LIBSSL_PATH is None or ctypes is None,
+        reason='cant test libssl on this platform',
+    )
     def test_ssl_module_diverges(self):
         # Ensure generator state is initialized.
         RAND_pseudo_bytes()
@@ -93,6 +117,8 @@ class ForkTest(testlib.RouterMixin, testlib.TestCase):
         context = self.router.fork(on_start=on_start)
         self.assertEquals(123, recv.get().unpickle())
 
+ForkTest = skipIfUnsupported(ForkTest)
+
 
 class DoubleChildTest(testlib.RouterMixin, testlib.TestCase):
     def test_okay(self):
@@ -115,6 +141,8 @@ class DoubleChildTest(testlib.RouterMixin, testlib.TestCase):
         c2 = self.router.fork(name='c2', via=c1)
         self.assertEqual(2, c2.call(exercise_importer, 1))
 
+DoubleChildTest = skipIfUnsupported(DoubleChildTest)
+
 
 if __name__ == '__main__':
     unittest2.main()