nghttpx: Fix request is sent to backend prematurely with http2 upstream

src/shrpx_downstream_queue.cc

@@ -121,6 +121,17 @@ std::unique_ptr<Downstream> DownstreamQueue::pop_pending()
   return downstream;
 }
 
+Downstream* DownstreamQueue::pending_top() const
+{
+  auto i = std::begin(pending_downstreams_);
+
+  if(i == std::end(pending_downstreams_)) {
+    return nullptr;
+  }
+
+  return (*i).second.get();
+}
+
 size_t DownstreamQueue::num_active() const
 {
   return active_downstreams_.size();

src/shrpx_downstream_queue.h

@@ -58,6 +58,9 @@ public:
   // Pops first Downstream object in pending_downstreams_ and returns
   // it.
   std::unique_ptr<Downstream> pop_pending();
+  // Returns first Downstream object in pending_downstreams_.  This
+  // does not pop the first one.  If queue is empty, returns nullptr.
+  Downstream* pending_top() const;
 private:
   // Downstream objects, not processed yet
   std::map<int32_t, std::unique_ptr<Downstream>> pending_downstreams_;

src/shrpx_http2_upstream.cc

@@ -382,6 +382,18 @@ void Http2Upstream::maintain_downstream_concurrency()
 {
   while(get_config()->max_downstream_connections >
         downstream_queue_.num_active()) {
+    if(downstream_queue_.pending_empty()) {
+      break;
+    }
+
+    {
+      auto downstream = downstream_queue_.pending_top();
+      if(downstream->get_request_state() != Downstream::HEADER_COMPLETE &&
+         downstream->get_request_state() != Downstream::MSG_COMPLETE) {
+        break;
+      }
+    }
+
     auto downstream = downstream_queue_.pop_pending();
 
     if(!downstream) {