gh-155888: Fix asyncio writelines() hanging on an empty chunk (GH-155889)

(cherry picked from commit b2c2993)

Co-authored-by: Timofei Ivankov <128279579+deadlovelll@users.noreply.github.com>

Author: deadlovelll

Lib/asyncio/selector_events.py

@@ -1197,8 +1197,13 @@ def writelines(self, list_of_data):
             return
 
         for data in list_of_data:
+            # gh-155888: an empty chunk can never be drained, so never buffer it
+            if not data:
+                continue
             self._buffer.append(memoryview(data))
             self._buffer_size += len(data)
+        if not self._buffer:
+            return
         self._write_ready()
         # If the entire buffer couldn't be written, register a write handler
         if self._buffer:

Lib/test/test_asyncio/test_events.py

@@ -560,6 +560,19 @@ def writer(data):
         r.close()
         self.assertEqual(read, data)
 
+    def test_writelines_empty_chunk(self):
+        # gh-155888: an empty chunk can never be drained, so never buffer it
+        rsock, wsock = socket.socketpair()
+        self.addCleanup(rsock.close)
+
+        async def main():
+            reader, writer = await asyncio.open_connection(sock=wsock)
+            writer.writelines([b'data', b''])
+            writer.close()
+            await asyncio.wait_for(writer.wait_closed(), support.SHORT_TIMEOUT)
+
+        self.loop.run_until_complete(main())
+
     @unittest.skipUnless(hasattr(signal, 'SIGKILL'), 'No SIGKILL')
     def test_add_signal_handler(self):
         caught = 0

Misc/NEWS.d/next/Library/2026-08-16-11-38-17.gh-issue-155888.pO-nAp.rst

@@ -0,0 +1,2 @@
+Fix :meth:`asyncio.WriteTransport.writelines` hanging the transport when the
+last data chunk is empty.