perf(ext/http): Add a sync phase to http serving (#19321)

Under heavy load, we often have requests queued up that don't need an
async call to retrieve. We can use a fast path sync op to drain this set
of ready requests, and then fall back to the async op once we run out of
work.

This is a .5-1% bump in req/s on an M2 mac. About 90% of the handlers go
through this sync phase (based on a simple instrumentation that is not
included in this PR) and skip the async machinery entirely.

3 files changed, 40 insertions, 1 deletions

diff --git a/ext/http/00_serve.js b/ext/http/00_serve.js
index 2fb36d044..e3926280b 100644
--- a/ext/http/00_serve.js
+++ b/ext/http/00_serve.js
@@ -64,6 +64,7 @@ const {
   op_http_set_response_trailers,
   op_http_upgrade_raw,
   op_http_upgrade_websocket_next,
+  op_http_try_wait,
   op_http_wait,
 } = core.generateAsyncOpHandler(
   "op_http_get_request_headers",
@@ -80,6 +81,7 @@ const {
   "op_http_set_response_trailers",
   "op_http_upgrade_raw",
   "op_http_upgrade_websocket_next",
+  "op_http_try_wait",
   "op_http_wait",
 );
 const _upgraded = Symbol("_upgraded");
@@ -558,7 +560,7 @@ function mapToCallback(responseBodies, context, signal, callback, onError) {
       }
     }
 
-    // Attempt to response quickly to this request, otherwise extract the stream
+    // Attempt to respond quickly to this request, otherwise extract the stream
     const stream = fastSyncResponseOrStream(req, inner.body);
     if (stream !== null) {
       // Handle the stream asynchronously
@@ -671,6 +673,18 @@ function serve(arg1, arg2) {
       const rid = context.serverRid;
       let req;
       try {
+        // Attempt to pull as many requests out of the queue as possible before awaiting. This API is
+        // a synchronous, non-blocking API that returns u32::MAX if anything goes wrong.
+        while ((req = op_http_try_wait(rid)) !== 0xffffffff) {
+          PromisePrototypeCatch(callback(req), (error) => {
+            // Abnormal exit
+            console.error(
+              "Terminating Deno.serve loop due to unexpected error",
+              error,
+            );
+            context.close();
+          });
+        }
         currentPromise = op_http_wait(rid);
         if (!ref) {
           core.unrefOp(currentPromise[promiseIdSymbol]);
diff --git a/ext/http/http_next.rs b/ext/http/http_next.rs
index d479d4a91..7edffed65 100644
--- a/ext/http/http_next.rs
+++ b/ext/http/http_next.rs
@@ -810,6 +810,30 @@ where
   ))
 }
 
+/// Synchronous, non-blocking call to see if there are any further HTTP requests. If anything
+/// goes wrong in this method we return [`SlabId::MAX`] and let the async handler pick up the real error.
+#[op(fast)]
+pub fn op_http_try_wait(state: &mut OpState, rid: ResourceId) -> SlabId {
+  // The resource needs to exist.
+  let Ok(join_handle) = state
+    .resource_table
+    .get::<HttpJoinHandle>(rid) else {
+      return SlabId::MAX;
+  };
+
+  // If join handle is somehow locked, just abort.
+  let Some(mut handle) = RcRef::map(&join_handle, |this| &this.2).try_borrow_mut() else {
+    return SlabId::MAX;
+  };
+
+  // See if there are any requests waiting on this channel. If not, return.
+  let Ok(id) = handle.try_recv() else {
+    return SlabId::MAX;
+  };
+
+  id
+}
+
 #[op]
 pub async fn op_http_wait(
   state: Rc<RefCell<OpState>>,
diff --git a/ext/http/lib.rs b/ext/http/lib.rs
index 7d37c53e1..da007ba39 100644
--- a/ext/http/lib.rs
+++ b/ext/http/lib.rs
@@ -120,6 +120,7 @@ deno_core::extension!(
     http_next::op_http_track,
     http_next::op_http_upgrade_websocket_next,
     http_next::op_http_upgrade_raw,
+    http_next::op_http_try_wait,
     http_next::op_http_wait,
   ],
   esm = ["00_serve.js", "01_http.js"],