From f12ba7cd0a7370631214ac0b337ab5455ce717b2 Mon Sep 17 00:00:00 2001
From: Andrew Svetlov <andrew.svetlov@gmail.com>
Date: Tue, 14 May 2019 19:09:44 +0300
Subject: bpo-36916: asyncio: Swallow unhandled write() exception (GH-13313)

---
 Lib/asyncio/streams.py                                        | 11 ++++++++++-
 Lib/test/test_asyncio/test_streams.py                         |  3 +++
 .../next/Library/2019-05-14-15-39-34.bpo-36916._GPsTt.rst     |  2 ++
 3 files changed, 15 insertions(+), 1 deletion(-)
 create mode 100644 Misc/NEWS.d/next/Library/2019-05-14-15-39-34.bpo-36916._GPsTt.rst

diff --git a/Lib/asyncio/streams.py b/Lib/asyncio/streams.py
index d9a9f5e..146a338 100644
--- a/Lib/asyncio/streams.py
+++ b/Lib/asyncio/streams.py
@@ -329,6 +329,13 @@ class StreamReaderProtocol(FlowControlMixin, protocols.Protocol):
             closed.exception()
 
 
+def _swallow_unhandled_exception(task):
+    # Do a trick to suppress unhandled exception
+    # if stream.write() was used without await and
+    # stream.drain() was paused and resumed with an exception
+    task.exception()
+
+
 class StreamWriter:
     """Wraps a Transport.
 
@@ -393,7 +400,9 @@ class StreamWriter:
                 # fast path, the stream is not paused
                 # no need to wait for resume signal
                 return self._complete_fut
-        return self._loop.create_task(self.drain())
+        ret = self._loop.create_task(self.drain())
+        ret.add_done_callback(_swallow_unhandled_exception)
+        return ret
 
     def write_eof(self):
         return self._transport.write_eof()
diff --git a/Lib/test/test_asyncio/test_streams.py b/Lib/test/test_asyncio/test_streams.py
index bf93f30..8d6a1d2 100644
--- a/Lib/test/test_asyncio/test_streams.py
+++ b/Lib/test/test_asyncio/test_streams.py
@@ -851,6 +851,8 @@ os.close(fd)
         # where it never gives up the event loop but the socket is
         # closed on the  server side.
 
+        messages = []
+        self.loop.set_exception_handler(lambda loop, ctx: messages.append(ctx))
         q = queue.Queue()
 
         def server():
@@ -883,6 +885,7 @@ os.close(fd)
         # Clean up the thread.  (Only on success; on failure, it may
         # be stuck in accept().)
         thread.join()
+        self.assertEqual([], messages)
 
     def test___repr__(self):
         stream = asyncio.StreamReader(loop=self.loop,
diff --git a/Misc/NEWS.d/next/Library/2019-05-14-15-39-34.bpo-36916._GPsTt.rst b/Misc/NEWS.d/next/Library/2019-05-14-15-39-34.bpo-36916._GPsTt.rst
new file mode 100644
index 0000000..8726bb2
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2019-05-14-15-39-34.bpo-36916._GPsTt.rst
@@ -0,0 +1,2 @@
+Remove a message about an unhandled exception in a task when writer.write()
+is used without await and writer.drain() fails with an exception.
-- 
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