streams: avoid calls to listenerCount

Author: ronag

lib/internal/streams/readable.js

@@ -115,6 +115,7 @@ const kHasFlowing = 1 << 23;
 const kFlowing = 1 << 24;
 const kHasPaused = 1 << 25;
 const kPaused = 1 << 26;
+const kDataListening = 1 << 27;
 
 // TODO(benjamingr) it is likely slower to do it this way than with free functions
 function makeBitMapDescriptor(bit) {
@@ -531,8 +532,7 @@ function canPushMore(state) {
 }
 
 function addChunk(stream, state, chunk, addToFront) {
-  if ((state[kState] & (kFlowing | kSync)) === kFlowing && state.length === 0 &&
-      stream.listenerCount('data') > 0) {
+  if ((state[kState] & (kFlowing | kSync | kDataListening)) === (kFlowing | kDataListening) && state.length === 0) {
     // Use the guard to avoid creating `Set()` repeatedly
     // when we have multiple pipes.
     if ((state[kState] & kMultiAwaitDrain) !== 0) {
@@ -1058,7 +1058,7 @@ function pipeOnDrain(src, dest) {
     }
 
     if ((!state.awaitDrainWriters || state.awaitDrainWriters.size === 0) &&
-      src.listenerCount('data')) {
+      (state[kState] & kDataListening) !== 0) {
       src.resume();
     }
   };
@@ -1105,6 +1105,8 @@ Readable.prototype.on = function(ev, fn) {
   const state = this._readableState;
 
   if (ev === 'data') {
+    state[kState] |= kDataListening;
+
     // Update readableListening so that resume() may be a no-op
     // a few lines down. This is needed to support once('readable').
     state.readableListening = this.listenerCount('readable') > 0;
@@ -1131,6 +1133,8 @@ Readable.prototype.on = function(ev, fn) {
 Readable.prototype.addListener = Readable.prototype.on;
 
 Readable.prototype.removeListener = function(ev, fn) {
+  const state = this._readableState;
+
   const res = Stream.prototype.removeListener.call(this,
                                                    ev, fn);
 
@@ -1142,6 +1146,8 @@ Readable.prototype.removeListener = function(ev, fn) {
     // resume within the same tick will have no
     // effect.
     process.nextTick(updateReadableListening, this);
+  } else if (ev === 'data' && this.listenerCount('data') === 0) {
+    state[kState] &= ~kDataListening;
   }
 
   return res;
@@ -1175,7 +1181,7 @@ function updateReadableListening(self) {
     state[kState] |= kHasFlowing | kFlowing;
 
     // Crude way to check if we should resume.
-  } else if (self.listenerCount('data') > 0) {
+  } else if ((state[kState] & kDataListening) !== 0) {
     self.resume();
   } else if ((state[kState] & kReadableListening) === 0) {
     state[kState] &= ~(kHasFlowing | kFlowing);