Add a monkey-patch for __import__ that properly locks multiple greenlets....

Add a monkey-patch for __import__ that properly locks multiple greenlets. Apply it automatically under python 2 (not needed on Py3). Fixes #108.

changelog.rst

@@ -50,6 +50,11 @@ Unreleased
   ``FileObjectPosix``. This unifies the code with the Python 3
   implementation, and fixes problems with using ``seek()``. See
   :issue:`151`.
+- Under Python 2, importing a module that uses gevent blocking
+  functions at its top level from multiple greenlets no longer
+  produces import errors (Python 3 handles this case natively).
+  Reported in :issue:`108` by shaun and initial fix based on code by
+  Sylvain Zimmer.
 
 1.1a2 (Jul 8, 2015)
 ===================

gevent/monkey.py

@@ -280,8 +280,18 @@ def patch_subprocess():
     patch_module('subprocess')
 
 
+def patch_builtins():
+    """Make the builtin __import__ function greenlet safe under Python 2"""
+    # https://github.com/gevent/gevent/issues/108
+    # Note that this is only needed in Python 2; under Python 3 (at least the versions
+    # we support) import locks are not global, they're per-module.
+    if sys.version_info[:2] < (3, 3):
+        patch_module('builtins')
+
+
 def patch_all(socket=True, dns=True, time=True, select=True, thread=True, os=True, ssl=True, httplib=False,
-              subprocess=True, sys=False, aggressive=True, Event=False):
+              subprocess=True, sys=False, aggressive=True, Event=False,
+              builtins=True):
     """Do all of the default monkey patching (calls every other applicable function in this module)."""
     # order is important
     if os:
@@ -304,6 +314,8 @@ def patch_all(socket=True, dns=True, time=True, select=True, thread=True, os=Tru
         raise ValueError('gevent.httplib is no longer provided, httplib must be False')
     if subprocess:
         patch_subprocess()
+    if builtins:
+        patch_builtins()
 
 
 if __name__ == '__main__':

greentest/_blocks_at_top_level.py

+from gevent import sleep
+sleep(0.01)
+x = "done"

greentest/test__import_blocking_in_greenlet.py

+#!/usr/bin/python
+# See https://github.com/gevent/gevent/issues/108
+import gevent
+from gevent import monkey
+monkey.patch_all()
+
+import_errors = []
+
+
+def some_func():
+    try:
+        from _blocks_at_top_level import x
+        assert x == 'done'
+    except ImportError as e:
+        import_errors.append(e)
+
+gs = [gevent.spawn(some_func) for i in range(2)]
+gevent.joinall(gs)
+
+assert not import_errors, import_errors