[Python-checkins] gh-96727: Document restrictions on Handler.emit() with respect to locking. (GH-96948)
vsajip
webhook-mailer at python.org
Tue Sep 20 04:40:15 EDT 2022
https://github.com/python/cpython/commit/6ad47b41a650a13b4a9214309c10239726331eb8
commit: 6ad47b41a650a13b4a9214309c10239726331eb8
branch: main
author: Vinay Sajip <vinay_sajip at yahoo.co.uk>
committer: vsajip <vinay_sajip at yahoo.co.uk>
date: 2022-09-20T09:40:06+01:00
summary:
gh-96727: Document restrictions on Handler.emit() with respect to locking. (GH-96948)
files:
M Doc/library/logging.rst
diff --git a/Doc/library/logging.rst b/Doc/library/logging.rst
index 8793627e119..34e98fc2577 100644
--- a/Doc/library/logging.rst
+++ b/Doc/library/logging.rst
@@ -534,6 +534,22 @@ subclasses. However, the :meth:`__init__` method in subclasses needs to call
is intended to be implemented by subclasses and so raises a
:exc:`NotImplementedError`.
+ .. warning:: This method is called after a handler-level lock is acquired, which
+ is released after this method returns. When you override this method, note
+ that you should be careful when calling anything that invokes other parts of
+ the logging API which might do locking, because that might result in a
+ deadlock. Specifically:
+
+ * Logging configuration APIs acquire the module-level lock, and then
+ individual handler-level locks as those handlers are configured.
+
+ * Many logging APIs lock the module-level lock. If such an API is called
+ from this method, it could cause a deadlock if a configuration call is
+ made on another thread, because that thread will try to acquire the
+ module-level lock *before* the handler-level lock, whereas this thread
+ tries to acquire the module-level lock *after* the handler-level lock
+ (because in this method, the handler-level lock has already been acquired).
+
For a list of handlers included as standard, see :mod:`logging.handlers`.
.. _formatter-objects:
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