Make the logic conditional on register_at_fork.

python · gpshead · Sep 14, 2018 · Oct 21, 2017 · Sep 12, 2018 · Sep 13, 2018
commit 6be017a56aef3cd82464e5660e33e01db28bed1e
diff --git a/Lib/logging/__init__.py b/Lib/logging/__init__.py
@@ -227,40 +227,51 @@ def _releaseLock():
 
 
 # Prevent a held logging lock from blocking a child from logging.
-os.register_at_fork(before=_acquireLock,
-                    after_in_child=_releaseLock,
-                    after_in_parent=_releaseLock)
 
+if not hasattr(os, 'register_at_fork'):  # Windows and friends.
+    def _register_at_fork_acquire_release(instance):
+        pass  # no-op when os.register_at_fork does not exist.
+else:  # The os.register_at_fork API exists
+    os.register_at_fork(before=_acquireLock,
+                        after_in_child=_releaseLock,
+                        after_in_parent=_releaseLock)
 
-# A collection of instances with acquire and release methods (logging.Handler)
-# to be called before and after fork.  The weakref avoids us keeping discarded
-# Handler instances alive forever in case an odd program creates and destroys
-# many over its lifetime.
-_at_fork_acquire_release_weakset = weakref.WeakSet()
+    # A collection of instances with acquire and release methods (logging.Handler)
+    # to be called before and after fork.  The weakref avoids us keeping discarded
+    # Handler instances alive forever in case an odd program creates and destroys
+    # many over its lifetime.
+    _at_fork_acquire_release_weakset = weakref.WeakSet()
 
 
-def _at_fork_weak_calls(method_name):
-    for instance in _at_fork_acquire_release_weakset:
-        method = getattr(instance, method_name)
-        try:
-            method()
-        except Exception as err:
-            # Similar to what PyErr_WriteUnraisable does.
-            print("Ignoring exception from logging atfork", instance,
-                  method_name, "method:", err, file=sys.stderr)
+    def _register_at_fork_acquire_release(instance):
+        # We put the instance itself in a single WeakSet as we MUST have only
+        # one atomic weak ref. used by both before and after atfork calls to
+        # guarantee matched pairs of acquire and release calls.
+        _at_fork_acquire_release_weakset.add(instance)
+
+
+    def _at_fork_weak_calls(method_name):
+        for instance in _at_fork_acquire_release_weakset:
+            method = getattr(instance, method_name)
+            try:
+                method()
+            except Exception as err:
+                # Similar to what PyErr_WriteUnraisable does.
+                print("Ignoring exception from logging atfork", instance,
+                      method_name, "method:", err, file=sys.stderr)
 
 
-def _before_at_fork_weak_calls():
-    _at_fork_weak_calls('acquire')
+    def _before_at_fork_weak_calls():
+        _at_fork_weak_calls('acquire')
 
 
-def _after_at_fork_weak_calls():
-    _at_fork_weak_calls('release')
+    def _after_at_fork_weak_calls():
+        _at_fork_weak_calls('release')
 
 
-os.register_at_fork(before=_before_at_fork_weak_calls,
-                    after_in_child=_after_at_fork_weak_calls,
-                    after_in_parent=_after_at_fork_weak_calls)
+    os.register_at_fork(before=_before_at_fork_weak_calls,
+                        after_in_child=_after_at_fork_weak_calls,
+                        after_in_parent=_after_at_fork_weak_calls)
 
 
 #---------------------------------------------------------------------------