From 2353d77fad7ed9d11d8a4d66b5dd1306cdb94125 Mon Sep 17 00:00:00 2001
From: Irit Katriel <iritkatriel@yahoo.com>
Date: Sun, 16 Aug 2020 16:10:13 +0100
Subject: [PATCH] bpo-41503: Fix race between setTarget and flush in
 logging.handlers.MemoryHandler (GH-21765)

---
 Lib/logging/handlers.py                       |  6 +++++-
 Lib/test/test_logging.py                      | 21 +++++++++++++++++++
 .../2020-08-07-15-18-16.bpo-41503.IYftcu.rst  |  1 +
 3 files changed, 27 insertions(+), 1 deletion(-)
 create mode 100644 Misc/NEWS.d/next/Library/2020-08-07-15-18-16.bpo-41503.IYftcu.rst

diff --git a/Lib/logging/handlers.py b/Lib/logging/handlers.py
index 4a120e9f1ec..867ef4ebc76 100644
--- a/Lib/logging/handlers.py
+++ b/Lib/logging/handlers.py
@@ -1324,7 +1324,11 @@ class MemoryHandler(BufferingHandler):
         """
         Set the target handler for this handler.
         """
-        self.target = target
+        self.acquire()
+        try:
+            self.target = target
+        finally:
+            self.release()
 
     def flush(self):
         """
diff --git a/Lib/test/test_logging.py b/Lib/test/test_logging.py
index eb5b926908a..d8b3727eb11 100644
--- a/Lib/test/test_logging.py
+++ b/Lib/test/test_logging.py
@@ -1160,6 +1160,27 @@ class MemoryHandlerTest(BaseTest):
         # assert that no new lines have been added
         self.assert_log_lines(lines)  # no change
 
+    def test_race_between_set_target_and_flush(self):
+        class MockRaceConditionHandler:
+            def __init__(self, mem_hdlr):
+                self.mem_hdlr = mem_hdlr
+
+            def removeTarget(self):
+                self.mem_hdlr.setTarget(None)
+
+            def handle(self, msg):
+                t = threading.Thread(target=self.removeTarget)
+                t.daemon = True
+                t.start()
+
+        target = MockRaceConditionHandler(self.mem_hdlr)
+        self.mem_hdlr.setTarget(target)
+
+        for _ in range(10):
+            time.sleep(0.005)
+            self.mem_logger.info("not flushed")
+            self.mem_logger.warning("flushed")
+
 
 class ExceptionFormatter(logging.Formatter):
     """A special exception formatter."""
diff --git a/Misc/NEWS.d/next/Library/2020-08-07-15-18-16.bpo-41503.IYftcu.rst b/Misc/NEWS.d/next/Library/2020-08-07-15-18-16.bpo-41503.IYftcu.rst
new file mode 100644
index 00000000000..c34996d8819
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2020-08-07-15-18-16.bpo-41503.IYftcu.rst
@@ -0,0 +1 @@
+Fixed a race between setTarget and flush in logging.handlers.MemoryHandler.
\ No newline at end of file