http2: skip trailers round trip for compat responses

The compat layer always responded with waitForTrailers set, so every
response paid for a wantTrailers C++ -> JS callback, an empty
sendTrailers() submission scheduled through setImmediate(), and an
extra empty DATA frame on the wire, even though the vast majority of
responses never register any trailers.

When the headers are flushed as part of response.end() and no
trailers have been registered, there is no further opportunity to
add trailers, so waitForTrailers can be skipped altogether. Headers
flushed early (writeHead, write, flushHeaders) keep the previous
behavior so trailers can still be added while streaming.

Trailers added after response.end() are now silently dropped,
matching the HTTP/1 response.addTrailers() semantics.

Also reuse a shared options object for Http2ServerRequest instances
created without explicit options.

h2load, 1 KiB response payload, -c 4 -m 100, mean of 6 alternating
runs: compat API 43.1k -> 49.9k req/s (+15.7% cumulative vs main).

Signed-off-by: Matteo Collina <hello@matteocollina.com>

Author: mcollina

doc/api/http2.md

@@ -4410,6 +4410,9 @@ added: v8.4.0
 This method adds HTTP trailing headers (a header but at the end of the
 message) to the response.
 
+Trailers must be added before calling [`response.end()`][]; trailers added
+afterwards are silently dropped.
+
 Attempting to set a header field name or value that contains invalid characters
 will result in a [`TypeError`][] being thrown.
 

lib/internal/http2/compat.js

@@ -300,6 +300,10 @@ function onStreamCloseRequest() {
   req.emit('close');
 }
 
+// Shared between all Http2ServerRequest instances created without explicit
+// options; the Readable constructor only reads from it.
+const kDefaultRequestOptions = { autoDestroy: false };
+
 function onStreamTimeoutRequest() {
   this[kRequest].emit('timeout');
 }
@@ -310,7 +314,9 @@ function onStreamTimeoutResponse() {
 
 class Http2ServerRequest extends Readable {
   constructor(stream, headers, options, rawHeaders) {
-    super({ autoDestroy: false, ...options });
+    super(options === undefined ?
+      kDefaultRequestOptions :
+      { autoDestroy: false, ...options });
     this[kState] = {
       closed: false,
       didRead: false,
@@ -473,6 +479,8 @@ class Http2ServerResponse extends Stream {
     this[kState] = {
       closed: false,
       ending: false,
+      finishing: false,
+      hasTrailers: false,
       destroyed: false,
       headRequest: false,
       sendDate: true,
@@ -584,6 +592,7 @@ class Http2ServerResponse extends Stream {
     name = name.trim().toLowerCase();
     assertValidHeader(name, value);
     this[kTrailers][name] = value;
+    this[kState].hasTrailers = true;
   }
 
   addTrailers(headers) {
@@ -839,6 +848,12 @@ class Http2ServerResponse extends Stream {
       return this;
     }
 
+    // If the headers have not been flushed yet, they will be flushed below
+    // as part of ending the response. In that case there is no further
+    // opportunity to add trailers, so the trailers round trip can be
+    // skipped entirely when none have been registered.
+    state.finishing = true;
+
     if (chunk !== null && chunk !== undefined)
       this.write(chunk, encoding);
 
@@ -898,7 +913,11 @@ class Http2ServerResponse extends Stream {
     headers[HTTP2_HEADER_STATUS] = state.statusCode;
     const options = {
       endStream: state.ending,
-      waitForTrailers: true,
+      // Only wait for trailers if some have been registered, or if the
+      // headers are flushed before the response is ended (in which case
+      // trailers may still be added during streaming). Trailers added after
+      // end() are dropped, matching HTTP/1 addTrailers() semantics.
+      waitForTrailers: state.hasTrailers || !state.finishing,
       sendDate: state.sendDate,
     };
     this[kStream].respond(headers, options);