quic: add blockCount statistic

Count the number of times sending stream data is blocked by
flow control

PR-URL: #213
Reviewed-By: Anna Henningsen <anna@addaleax.net>

doc/api/quic.md

@@ -441,6 +441,19 @@ added: REPLACEME
 A `BigInt` representing the total number of bidirectional streams
 created for this `QuicSession`.
 
+#### quicsession.blockCount
+<!-- YAML
+added: REPLACEME
+-->
+
+* Type: {BigInt}
+
+A `BigInt` representing the total number of times the `QuicSession` has
+been blocked from sending stream data due to flow control.
+
+Such blocks indicate that transmitted stream data is not being consumed
+quickly enough by the connected peer.
+
 #### quicsession.bytesInFlight
 <!-- YAML
 added: REPLACEME

lib/internal/quic/core.js

@@ -153,6 +153,7 @@ const {
     IDX_QUIC_SESSION_STATS_LOSS_RETRANSMIT_COUNT,
     IDX_QUIC_SESSION_STATS_ACK_DELAY_RETRANSMIT_COUNT,
     IDX_QUIC_SESSION_STATS_MAX_BYTES_IN_FLIGHT,
+    IDX_QUIC_SESSION_STATS_BLOCK_COUNT,
     ERR_INVALID_REMOTE_TRANSPORT_PARAMS,
     ERR_INVALID_TLS_SESSION_TICKET,
     NGTCP2_PATH_VALIDATION_RESULT_FAILURE,
@@ -1702,6 +1703,11 @@ class QuicSession extends EventEmitter {
       this[kHandle].state[IDX_QUIC_SESSION_STATE_BYTES_IN_FLIGHT] : 0;
   }
 
+  get blockCount() {
+    return this[kHandle] ?
+      this[kHandle].state[IDX_QUIC_SESSION_STATS_BLOCK_COUNT] : 0;
+  }
+
   get authenticated() {
     // Specifically check for null. Undefined means the check has not
     // been performed yet, another other value other than null means

src/node_quic.cc

@@ -213,6 +213,7 @@ void Initialize(Local<Object> target,
   NODE_DEFINE_CONSTANT(constants,
                        IDX_QUIC_SESSION_STATS_PATH_VALIDATION_FAILURE_COUNT);
   NODE_DEFINE_CONSTANT(constants, IDX_QUIC_SESSION_STATS_MAX_BYTES_IN_FLIGHT);
+  NODE_DEFINE_CONSTANT(constants, IDX_QUIC_SESSION_STATS_BLOCK_COUNT);
 
   NODE_DEFINE_CONSTANT(constants, IDX_HTTP3_QPACK_MAX_TABLE_CAPACITY);
   NODE_DEFINE_CONSTANT(constants, IDX_HTTP3_QPACK_BLOCKED_STREAMS);

src/node_quic_default_application.cc

@@ -177,6 +177,7 @@ bool DefaultApplication::SendStreamData(QuicStream* stream) {
           return false;
         case NGTCP2_ERR_STREAM_DATA_BLOCKED:
           Debug(stream, "Stream data blocked");
+          Session()->StreamDataBlocked(stream->GetID());
           return true;
         case NGTCP2_ERR_STREAM_SHUT_WR:
           Debug(stream, "Stream writable side is closed");

src/node_quic_http3_application.cc

@@ -460,6 +460,7 @@ bool Http3Application::SendPendingData() {
     if (nwrite < 0) {
       switch (nwrite) {
         case NGTCP2_ERR_STREAM_DATA_BLOCKED:
+          Session()->StreamDataBlocked(stream_id);
           if (Session()->GetMaxDataLeft() == 0)
             return true;
           // Fall through

src/node_quic_session.cc

@@ -1378,6 +1378,11 @@ void QuicSession::RemoveListener(QuicSessionListener* listener) {
   listener->previous_listener_ = nullptr;
 }
 
+void QuicSession::StreamDataBlocked(int64_t stream_id) {
+  // Increments the block count
+  IncrementStat(1, &session_stats_, &session_stats::block_count);
+}
+
 std::string QuicSession::diagnostic_name() const {
   return std::string("QuicSession ") +
       (IsServer() ? "Server" : "Client") +

src/node_quic_session.h

@@ -197,7 +197,8 @@ enum QuicSessionStatsIdx : int {
   IDX_QUIC_SESSION_STATS_ACK_DELAY_RETRANSMIT_COUNT,
   IDX_QUIC_SESSION_STATS_PATH_VALIDATION_SUCCESS_COUNT,
   IDX_QUIC_SESSION_STATS_PATH_VALIDATION_FAILURE_COUNT,
-  IDX_QUIC_SESSION_STATS_MAX_BYTES_IN_FLIGHT
+  IDX_QUIC_SESSION_STATS_MAX_BYTES_IN_FLIGHT,
+  IDX_QUIC_SESSION_STATS_BLOCK_COUNT,
 };
 
 class QuicSessionListener {
@@ -933,6 +934,9 @@ class QuicSession : public AsyncWrap,
 
   void SetConnectionIDStrategory(ConnectionIDStrategy* strategy);
 
+  // Report that the stream data is flow control blocked
+  void StreamDataBlocked(int64_t stream_id);
+
   // Tracks whether or not we are currently within an ngtcp2 callback
   // function. Certain ngtcp2 APIs are not supposed to be called when
   // within a callback. We use this as a gate to check.
@@ -1370,6 +1374,9 @@ class QuicSession : public AsyncWrap,
     uint64_t path_validation_failure_count;
     // The max number of in flight bytes recorded
     uint64_t max_bytes_in_flight;
+    // The total number of times the session has been
+    // flow control blocked.
+    uint64_t block_count;
   };
   session_stats session_stats_{};