Improve TCP backpressure handling on Windows (#4252)

Our prior setting of backpressure for TCP writes on Windows was naive. It was
based purely on the number of buffers currently outstanding on an IOCP socket.
The amount of data didn't matter at all. Whether more data could be accepted or
not, also wasn't taken into consideration. This commit greatly improves the
situation by only applying backpressure when Windows tells us that it is
applying backpressure.

No more guessing.

Two runtime API methods have been updated on Windows.

The Windows version of `pony_os_writev` will now return the number of buffers
accepted or zero if backpressure is encountered. All other errors still cause
an error that must be handled on the Pony side of the API via a `try` block.

The Windows version of `pony_os_send` will now return the number of bytes
accepted or zero if backpressure is encountered. All other errors still cause
an error that must be handled on the Pony side of the API via a `try` block.

I consider these changes  non-breaking as previously, the return values from
both functions had no meaning.

Closes #4081

.release-notes/4081.md

@@ -0,0 +1,15 @@
+## Improve TCP Backpressure on Windows
+
+Our prior setting of backpressure for TCP writes on Windows was naive. It was  based purely on the number of buffers currently outstanding on an IOCP socket. The amount of data didn't matter at all. Whether more data could be accepted or not, also wasn't taken into consideration. We've enhanced the backpressure support at both the Pony level in `TCPConnection` and in the runtime API.
+
+Two runtime API methods have been updated on Windows.
+
+### pony_os_writev
+
+The Windows version of `pony_os_writev` will now return the number of buffers accepted or zero if backpressure is encountered. All other errors still cause an error that must be handled on the Pony side of the API via a `try` block.
+
+### pony_os_send
+
+The Windows version of `pony_os_send` will now return the number of bytes accepted or zero if backpressure is encountered. All other errors still cause an error that must be handled on the Pony side of the API via a `try` block.
+
+The changes are considered non-breaking as previously, the return values from both functions had no meaning.

packages/net/tcp_connection.pony

@@ -472,18 +472,16 @@ actor TCPConnection is AsioEventNotify
           end
 
           // Write as much data as possible.
-          var len =
+          // Returns how many we sent or 0 if we are experiencing backpressure
+          let len =
             @pony_os_writev(_event,
               _pending_writev_windows.cpointer(_pending_sent),
-              num_to_send) ?
+              num_to_send)?
 
-          _pending_sent = _pending_sent + num_to_send.usize()
-
-          if _pending_sent > 32 then
-            // If more than 32 asynchronous writes are scheduled, apply
-            // backpressure. The choice of 32 is rather arbitrary an
-            // probably needs tuning
+          if len == 0 then
             _apply_backpressure()
+          else
+            _pending_sent = _pending_sent + len
           end
         end
       else
@@ -699,16 +697,15 @@ actor TCPConnection is AsioEventNotify
           _pending_writev_windows .> push((data.size(), data.cpointer()))
           _pending_writev_total = _pending_writev_total + data.size()
 
-          @pony_os_writev(_event,
-            _pending_writev_windows.cpointer(_pending_sent), I32(1)) ?
+          // Write as much data as possible
+          // Returns how many we sent or 0 if we are experiencing backpressure
+          let len = @pony_os_writev(_event,
+            _pending_writev_windows.cpointer(_pending_sent), I32(1))?
 
-          _pending_sent = _pending_sent + 1
-
-          if _pending_sent > 32 then
-            // If more than 32 asynchronous writes are scheduled, apply
-            // backpressure. The choice of 32 is rather arbitrary an
-            // probably needs tuning
+          if len == 0 then
             _apply_backpressure()
+          else
+            _pending_sent = _pending_sent + len
           end
         end
       else

src/libponyrt/lang/socket.c

@@ -371,28 +371,6 @@ static bool iocp_accept(asio_event_t* ev)
   return true;
 }
 
-static bool iocp_send(asio_event_t* ev, const char* data, size_t len)
-{
-  SOCKET s = (SOCKET)ev->fd;
-  iocp_t* iocp = iocp_create(IOCP_SEND, ev);
-  DWORD sent;
-
-  WSABUF buf;
-  buf.buf = (char*)data;
-  buf.len = (u_long)len;
-
-  if(WSASend(s, &buf, 1, &sent, 0, &iocp->ov, NULL) != 0)
-  {
-    if(GetLastError() != WSA_IO_PENDING)
-    {
-      iocp_destroy(iocp);
-      return false;
-    }
-  }
-
-  return true;
-}
-
 static bool iocp_recv(asio_event_t* ev, char* data, size_t len)
 {
   SOCKET s = (SOCKET)ev->fd;
@@ -945,14 +923,20 @@ PONY_API size_t pony_os_writev(asio_event_t* ev, LPWSABUF wsa, int wsacnt)
 
   if(WSASend(s, wsa, wsacnt, &sent, 0, &iocp->ov, NULL) != 0)
   {
-    if(GetLastError() != WSA_IO_PENDING)
+    switch (GetLastError())
     {
-      iocp_destroy(iocp);
-      pony_error();
+      case WSA_IO_PENDING:
+        return wsacnt;
+      case WSAEWOULDBLOCK :
+        return 0;
+      default:
+        iocp_destroy(iocp);
+        pony_error();
+        return 0;
     }
   }
 
-  return 0;
+  return wsacnt;
 }
 #else
 PONY_API size_t pony_os_writev(asio_event_t* ev, const struct iovec *iov, int iovcnt)
@@ -974,10 +958,30 @@ PONY_API size_t pony_os_writev(asio_event_t* ev, const struct iovec *iov, int io
 PONY_API size_t pony_os_send(asio_event_t* ev, const char* buf, size_t len)
 {
 #ifdef PLATFORM_IS_WINDOWS
-  if(!iocp_send(ev, buf, len))
-    pony_error();
+  SOCKET s = (SOCKET)ev->fd;
+  iocp_t* iocp = iocp_create(IOCP_SEND, ev);
+  DWORD sent;
 
-  return 0;
+  WSABUF b;
+  b.buf = (char*)buf;
+  b.len = (u_long)len;
+
+  if(WSASend(s, &b, 1, &sent, 0, &iocp->ov, NULL) != 0)
+  {
+    switch (GetLastError())
+    {
+      case WSA_IO_PENDING:
+        return len;
+      case WSAEWOULDBLOCK :
+        return 0;
+      default:
+        iocp_destroy(iocp);
+        pony_error();
+        return 0;
+    }
+  }
+
+  return sent;
 #else
   ssize_t sent = send(ev->fd, buf, len, 0);