python
diff --git a/‎Lib/asyncio/base_events.py
Lines changed: 5 additions & 2 deletions b/‎Lib/asyncio/base_events.py
Lines changed: 5 additions & 2 deletions
diff --git a/‎Lib/asyncio/constants.py
Lines changed: 4 additions & 0 deletions b/‎Lib/asyncio/constants.py
Lines changed: 4 additions & 0 deletions
diff --git a/‎Lib/test/test_asyncio/test_base_events.py
Lines changed: 3 additions & 0 deletions b/‎Lib/test/test_asyncio/test_base_events.py
Lines changed: 3 additions & 0 deletions
diff --git a/‎Lib/test/test_asyncio/test_events.py
Lines changed: 11 additions & 0 deletions b/‎Lib/test/test_asyncio/test_events.py
Lines changed: 11 additions & 0 deletions
diff --git a/‎Misc/NEWS.d/next/Library/2018-05-28-16-19-35.bpo-32410.Z1DZaF.rst
Lines changed: 1 addition & 0 deletions b/‎Misc/NEWS.d/next/Library/2018-05-28-16-19-35.bpo-32410.Z1DZaF.rst
Lines changed: 1 addition & 0 deletions
@@ -800,7 +800,10 @@ async def _sock_sendfile_native(self, sock, file, offset, count):
     async def _sock_sendfile_fallback(self, sock, file, offset, count):
         if offset:
             file.seek(offset)
-        blocksize = min(count, 16384) if count else 16384
+        blocksize = (
+            min(count, constants.SENDFILE_FALLBACK_READBUFFER_SIZE)
+            if count else constants.SENDFILE_FALLBACK_READBUFFER_SIZE
+        )
         buf = bytearray(blocksize)
         total_sent = 0
         try:
@@ -810,7 +813,7 @@ async def _sock_sendfile_fallback(self, sock, file, offset, count):
                     if blocksize <= 0:
                         break
                 view = memoryview(buf)[:blocksize]
-                read = file.readinto(view)
+                read = await self.run_in_executor(None, file.readinto, view)
                 if not read:
                     break  # EOF
                 await self.sock_sendall(sock, view)
 
@@ -14,6 +14,10 @@
 # Number of seconds to wait for SSL handshake to complete
 SSL_HANDSHAKE_TIMEOUT = 10.0
 
+# Used in sendfile fallback code.  We use fallback for platforms
+# that don't support sendfile, or for TLS connections.
+SENDFILE_FALLBACK_READBUFFER_SIZE = 1024 * 256
+
 # The enum should be here to break circular dependencies between
 # base_events and sslproto
 class _SendfileMode(enum.Enum):
 
@@ -1818,12 +1818,15 @@ async def wait_closed(self):
 
     @classmethod
     def setUpClass(cls):
+        cls.__old_bufsize = constants.SENDFILE_FALLBACK_READBUFFER_SIZE
+        constants.SENDFILE_FALLBACK_READBUFFER_SIZE = 1024 * 16
         with open(support.TESTFN, 'wb') as fp:
             fp.write(cls.DATA)
         super().setUpClass()
 
     @classmethod
     def tearDownClass(cls):
+        constants.SENDFILE_FALLBACK_READBUFFER_SIZE = cls.__old_bufsize
         support.unlink(support.TESTFN)
         super().tearDownClass()
 
 
@@ -2160,6 +2160,17 @@ def connection_lost(self, exc):
         async def wait_closed(self):
             await self.fut
 
+    @classmethod
+    def setUpClass(cls):
+        cls.__old_bufsize = constants.SENDFILE_FALLBACK_READBUFFER_SIZE
+        constants.SENDFILE_FALLBACK_READBUFFER_SIZE = 1024 * 16
+        super().setUpClass()
+
+    @classmethod
+    def tearDownClass(cls):
+        constants.SENDFILE_FALLBACK_READBUFFER_SIZE = cls.__old_bufsize
+        super().tearDownClass()
+
     def set_socket_opts(self, sock):
         # On macOS, SO_SNDBUF is reset by connect(). So this method
         # should be called after the socket is connected.
 
@@ -0,0 +1 @@
+Avoid blocking on file IO in sendfile fallback code