From 8ea34d2bc7f4401b1b49534e01d32fea245aece7 Mon Sep 17 00:00:00 2001
From: tzickel <tzickel@users.noreply.github.com>
Date: Wed, 18 Mar 2020 17:39:24 +0200
Subject: [PATCH] bpo-40007: Make asyncio.transport.writelines on selector use
 sendmsg

---
 Lib/asyncio/selector_events.py                | 102 +++++++++++++++++-
 .../2020-03-19-12-40-38.bpo-40007.hLaPs4.rst  |   1 +
 2 files changed, 100 insertions(+), 3 deletions(-)
 create mode 100644 Misc/NEWS.d/next/Library/2020-03-19-12-40-38.bpo-40007.hLaPs4.rst

diff --git a/Lib/asyncio/selector_events.py b/Lib/asyncio/selector_events.py
index a05cbb6bdd6..4e615cf0b9a 100644
--- a/Lib/asyncio/selector_events.py
+++ b/Lib/asyncio/selector_events.py
@@ -29,6 +29,9 @@ from . import trsock
 from .log import logger
 
 
+sendmsg = getattr(socket.socket, "sendmsg", False)
+
+
 def _test_selector_event(selector, fd, event):
     # Test if the selector is monitoring 'event' events
     # for the file descriptor 'fd'.
@@ -746,6 +749,7 @@ class _SelectorTransport(transports._FlowControlMixin,
 
 class _SelectorSocketTransport(_SelectorTransport):
 
+    _buffer_factory = list
     _start_tls_compatible = True
     _sendfile_compatible = constants._SendfileMode.TRY_NATIVE
 
@@ -921,7 +925,62 @@ class _SelectorSocketTransport(_SelectorTransport):
             self._loop._add_writer(self._sock_fd, self._write_ready)
 
         # Add it to the buffer.
-        self._buffer.extend(data)
+        self._buffer.append(data)
+        self._maybe_pause_protocol()
+
+    @staticmethod
+    def _calculate_leftovers(n, items):
+        leftovers = []
+        whole = False
+        for item in items:
+            if whole:
+                leftovers.append(item)
+                continue
+            n -= len(item)
+            if n >= 0:
+                continue
+            leftovers.append(memoryview(item)[n:])
+            whole = True
+        return leftovers
+
+    def writelines(self, lines):
+        if not sendmsg:
+            return self.write(b''.join(lines))
+        if self._eof:
+            raise RuntimeError('Cannot call write() after write_eof()')
+        if self._empty_waiter is not None:
+            raise RuntimeError('unable to write; sendfile is in progress')
+        if not lines:
+            return
+
+        if self._conn_lost:
+            if self._conn_lost >= constants.LOG_THRESHOLD_FOR_CONNLOST_WRITES:
+                logger.warning('socket.send() raised exception.')
+            self._conn_lost += 1
+            return
+
+        if not self._buffer:
+            # Optimization: try to send now.
+            try:
+                n = self._sock.sendmsg(lines)
+            except OSError:
+                return self.write(b''.join(lines))
+            except (BlockingIOError, InterruptedError):
+                pass
+            except (SystemExit, KeyboardInterrupt):
+                raise
+            except BaseException as exc:
+                self._fatal_error(exc, 'Fatal write error on socket transport')
+                return
+            else:
+                lines = self._calculate_leftovers(n, lines)
+                if not lines:
+                    return
+            # Not all was written; register write handler.
+            self._loop._add_writer(self._sock_fd, self._write_ready)
+
+        # Add it to the buffer.
+        self._buffer.extend(lines)
         self._maybe_pause_protocol()
 
     def _write_ready(self):
@@ -929,8 +988,13 @@ class _SelectorSocketTransport(_SelectorTransport):
 
         if self._conn_lost:
             return
+
+        if sendmsg:
+            return self._write_vectored_self()
+
         try:
-            n = self._sock.send(self._buffer)
+            tmp = b''.join(self._buffer)
+            n = self._sock.send(tmp)
         except (BlockingIOError, InterruptedError):
             pass
         except (SystemExit, KeyboardInterrupt):
@@ -943,7 +1007,7 @@ class _SelectorSocketTransport(_SelectorTransport):
                 self._empty_waiter.set_exception(exc)
         else:
             if n:
-                del self._buffer[:n]
+                self._buffer = [tmp[:n]]
             self._maybe_resume_protocol()  # May append to buffer.
             if not self._buffer:
                 self._loop._remove_writer(self._sock_fd)
@@ -954,6 +1018,38 @@ class _SelectorSocketTransport(_SelectorTransport):
                 elif self._eof:
                     self._sock.shutdown(socket.SHUT_WR)
 
+    def _write_vectored_self(self):
+        try:
+            try:
+                n = self._sock.sendmsg(self._buffer)
+            except OSError:
+                self._buffer = [b''.join(self._buffer)]
+                n = self._sock.sendmsg(self._buffer)
+        except (BlockingIOError, InterruptedError):
+            pass
+        except (SystemExit, KeyboardInterrupt):
+            raise
+        except BaseException as exc:
+            self._loop._remove_writer(self._sock_fd)
+            self._buffer.clear()
+            self._fatal_error(exc, 'Fatal write error on socket transport')
+            if self._empty_waiter is not None:
+                self._empty_waiter.set_exception(exc)
+        else:
+            self._buffer = self._calculate_leftovers(n, self._buffer)
+            self._maybe_resume_protocol()  # May append to buffer.
+            if not self._buffer:
+                self._loop._remove_writer(self._sock_fd)
+                if self._empty_waiter is not None:
+                    self._empty_waiter.set_result(None)
+                if self._closing:
+                    self._call_connection_lost(None)
+                elif self._eof:
+                    self._sock.shutdown(socket.SHUT_WR)
+
+    def get_write_buffer_size(self):
+        return sum(len(data) for data in self._buffer)
+
     def write_eof(self):
         if self._closing or self._eof:
             return
diff --git a/Misc/NEWS.d/next/Library/2020-03-19-12-40-38.bpo-40007.hLaPs4.rst b/Misc/NEWS.d/next/Library/2020-03-19-12-40-38.bpo-40007.hLaPs4.rst
new file mode 100644
index 00000000000..48a140fe273
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2020-03-19-12-40-38.bpo-40007.hLaPs4.rst
@@ -0,0 +1 @@
+Make asyncio.transport.writelines on selector use sendmsg