quic: further implementation details

PR-URL: https://github.com/nodejs/node/pull/48244
Reviewed-By: Yagiz Nizipli <yagiz.nizipli@sentry.io>
Reviewed-By: Stephen Belanger <admin@stephenbelanger.com>

src/dataqueue/queue.cc

@@ -162,6 +162,27 @@ class DataQueueImpl final : public DataQueue,
         "entries", entries_, "std::vector<std::unique_ptr<Entry>>");
   }
 
+  void addBackpressureListener(BackpressureListener* listener) override {
+    if (idempotent_) return;
+    DCHECK_NOT_NULL(listener);
+    backpressure_listeners_.insert(listener);
+  }
+
+  void removeBackpressureListener(BackpressureListener* listener) override {
+    if (idempotent_) return;
+    DCHECK_NOT_NULL(listener);
+    backpressure_listeners_.erase(listener);
+  }
+
+  void NotifyBackpressure(size_t amount) {
+    if (idempotent_) return;
+    for (auto& listener : backpressure_listeners_) listener->EntryRead(amount);
+  }
+
+  bool HasBackpressureListeners() const noexcept {
+    return !backpressure_listeners_.empty();
+  }
+
   std::shared_ptr<Reader> get_reader() override;
   SET_MEMORY_INFO_NAME(DataQueue)
   SET_SELF_SIZE(DataQueueImpl)
@@ -173,6 +194,8 @@ class DataQueueImpl final : public DataQueue,
   std::optional<uint64_t> capped_size_ = std::nullopt;
   bool locked_to_reader_ = false;
 
+  std::unordered_set<BackpressureListener*> backpressure_listeners_;
+
   friend class DataQueue;
   friend class IdempotentDataQueueReader;
   friend class NonIdempotentDataQueueReader;
@@ -433,6 +456,17 @@ class NonIdempotentDataQueueReader final
             return;
           }
 
+          // If there is a backpressure listener, lets report on how much data
+          // was actually read.
+          if (data_queue_->HasBackpressureListeners()) {
+            // How much did we actually read?
+            size_t read = 0;
+            for (uint64_t n = 0; n < count; n++) {
+              read += vecs[n].len;
+            }
+            data_queue_->NotifyBackpressure(read);
+          }
+
           // Now that we have updated this readers state, we can forward
           // everything on to the outer next.
           std::move(next)(status, vecs, count, std::move(done));