http2: strictly limit number on concurrent streams

Strictly limit the number of concurrent streams based on the
current setting of the MAX_CONCURRENT_STREAMS setting

src/node_http2.cc

@@ -418,6 +418,16 @@ inline Http2Stream* Http2Session::FindStream(int32_t id) {
   return s != streams_.end() ? s->second : nullptr;
 }
 
+inline bool Http2Session::CanAddStream() {
+  uint32_t maxConcurrentStreams =
+      nghttp2_session_get_local_settings(
+          session_, NGHTTP2_SETTINGS_MAX_CONCURRENT_STREAMS);
+  size_t maxSize =
+      std::min(streams_.max_size(), static_cast<size_t>(maxConcurrentStreams));
+  // We can add a new stream so long as we are less than the current
+  // maximum on concurrent streams
+  return streams_.size() < maxSize;
+}
 
 inline void Http2Session::AddStream(Http2Stream* stream) {
   streams_[stream->id()] = stream;
@@ -528,7 +538,14 @@ inline int Http2Session::OnBeginHeadersCallback(nghttp2_session* handle,
 
   Http2Stream* stream = session->FindStream(id);
   if (stream == nullptr) {
-    new Http2Stream(session, id, frame->headers.cat);
+    if (session->CanAddStream()) {
+      new Http2Stream(session, id, frame->headers.cat);
+    } else {
+      // Too many concurrent streams being opened
+      nghttp2_submit_rst_stream(**session, NGHTTP2_FLAG_NONE, id,
+                                NGHTTP2_ENHANCE_YOUR_CALM);
+      return NGHTTP2_ERR_TEMPORAL_CALLBACK_FAILURE;
+    }
   } else {
     stream->StartHeaders(frame->headers.cat);
   }

src/node_http2.h

@@ -819,6 +819,8 @@ class Http2Session : public AsyncWrap {
   // Returns pointer to the stream, or nullptr if stream does not exist
   inline Http2Stream* FindStream(int32_t id);
 
+  inline bool CanAddStream();
+
   // Adds a stream instance to this session
   inline void AddStream(Http2Stream* stream);
 

test/parallel/test-http2-too-many-streams.js

@@ -0,0 +1,75 @@
+'use strict';
+
+const common = require('../common');
+if (!common.hasCrypto)
+  common.skip('missing crypto');
+
+const Countdown = require('../common/countdown');
+const http2 = require('http2');
+const assert = require('assert');
+
+const server = http2.createServer({ settings: { maxConcurrentStreams: 1 } });
+
+let c = 0;
+
+server.on('stream', (stream) => {
+  assert.strictEqual(++c, 1);
+  stream.respond();
+  setImmediate(() => {
+    stream.end('ok');
+    assert.strictEqual(--c, 0);
+  });
+});
+
+server.listen(0, common.mustCall(() => {
+  const client = http2.connect(`http://localhost:${server.address().port}`);
+
+  const countdown = new Countdown(3, common.mustCall(() => {
+    server.close();
+    client.destroy();
+  }));
+
+  // Test that the maxConcurrentStreams setting is strictly enforced
+
+  client.on('remoteSettings', common.mustCall(() => {
+    assert.strictEqual(client.remoteSettings.maxConcurrentStreams, 1);
+
+    // This one should be ok
+    {
+      const req = client.request();
+      req.resume();
+      req.on('end', () => {
+        countdown.dec();
+
+        setImmediate(() => {
+          const req = client.request();
+          req.resume();
+          req.on('end', () => countdown.dec());
+        });
+      });
+    }
+
+    // This one should fail
+    {
+      const req = client.request();
+      req.resume();
+      // TODO(jasnell): Still investigating precisely why, but on Windows,
+      //                the error is not emitted because the underlying
+      //                mechanism ensures that only one request is sent
+      //                at a time per the maxConcurrentStreams setting.
+      //                This likely has to do with the way the response
+      //                is being handled on the server side. This is safe
+      //                to ignore on Windows because of the assert.strictEqual
+      //                check in the on('stream') handler which ensures that
+      //                only one request is being handled at any given time.
+      if (!common.isWindows) {
+        req.on('error', common.expectsError({
+          code: 'ERR_HTTP2_STREAM_ERROR',
+          type: Error,
+          message: 'Stream closed with error code 11'
+        }));
+      }
+      req.on('end', () => countdown.dec());
+    }
+  }));
+}));