gh-156378: Stop logging.config.listen() spinning on a truncated request

Lib/logging/config.py

@@ -972,7 +972,12 @@ def handle(self):
                     slen = struct.unpack(">L", chunk)[0]
                     chunk = self.connection.recv(slen)
                     while len(chunk) < slen:
-                        chunk = chunk + conn.recv(slen - len(chunk))
+                        data = conn.recv(slen - len(chunk))
+                        if not data:
+                            # The peer closed before sending the whole
+                            # configuration, so there is nothing to apply.
+                            return
+                        chunk = chunk + data
                     if self.server.verify is not None:
                         chunk = self.server.verify(chunk)
                     if chunk is not None:   # verified, can process

Lib/test/test_logging.py

@@ -3740,6 +3740,35 @@ def setup_via_listener(self, text, verify=None):
             logging.config.stopListening()
             threading_helper.join_thread(t)
 
+    @support.requires_working_socket()
+    def test_listen_truncated_request_does_not_spin(self):
+        # gh-156378: a client that declares a length and then closes without
+        # sending the body used to leave handle() calling recv() forever,
+        # burning a CPU core, because recv() keeps returning b'' at EOF.
+        t = logging.config.listen(0)
+        t.start()
+        self.assertTrue(t.ready.wait(support.LONG_TIMEOUT),
+                        msg='the listener did not start')
+        port = t.port
+        t.ready.clear()
+        before = threading.active_count()
+        try:
+            sock = socket.create_connection(('localhost', port), timeout=2.0)
+            # Announce one byte of configuration, then hang up without it.
+            sock.sendall(struct.pack('>L', 1))
+            sock.close()
+            # The handler must notice EOF and finish. Wait rather than assert
+            # immediately, so a slow machine does not make this flaky; only a
+            # handler that never exits fails here.
+            deadline = time.monotonic() + support.SHORT_TIMEOUT
+            while threading.active_count() > before:
+                if time.monotonic() > deadline:
+                    self.fail('the request handler did not exit after EOF')
+                time.sleep(0.01)
+        finally:
+            logging.config.stopListening()
+            threading_helper.join_thread(t)
+
     @support.requires_working_socket()
     def test_listen_config_10_ok(self):
         with support.captured_stdout() as output:

Misc/NEWS.d/next/Library/2026-08-25-19-12-40.gh-issue-156378.Lk8Rw2.rst

@@ -0,0 +1,3 @@
+Fix :func:`logging.config.listen` looping forever and consuming a CPU core
+when a client announces a configuration length and then closes the connection
+without sending the data.