dgram: use `uv_udp_try_send()`

This improves dgram performance by avoiding unnecessary async
operations.

One issue with this commit is that it seems hard to actually create
conditions under which the fallback path to the async case is
actually taken, for all supported OS, so an internal CLI option
is used for testing that path.

Another caveat is that the lack of an async operation means
that there are slight timing differences (essentially `nextTick()`
rather than `setImmediate()` for the send callback).

PR-URL: https://github.com/nodejs/node/pull/29832
Reviewed-By: David Carlier <devnexen@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>

1 files changed, 8 insertions, 0 deletions

diff --git a/lib/dgram.js b/lib/dgram.js
index 923463cc2e..cc61d4d274 100644
--- a/lib/dgram.js
+++ b/lib/dgram.js
@@ -666,6 +666,14 @@ function doSend(ex, self, ip, list, address, port, callback) {
   else
     err = state.handle.send(req, list, list.length, !!callback);
 
+  if (err >= 1) {
+    // Synchronous finish. The return code is msg_length + 1 so that we can
+    // distinguish between synchronous success and asynchronous success.
+    if (callback)
+      process.nextTick(callback, null, err - 1);
+    return;
+  }
+
   if (err && callback) {
     // Don't emit as error, dgram_legacy.js compatibility
     const ex = exceptionWithHostPort(err, 'send', address, port);