stream: fix disparity between buffer and the count

This changes the disparity of bufferedRequestCount and the actual buffer
on file _stream_writable.js

PR-URL: nodejs/node#15661
Fixes: nodejs/node#6758
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

lib/_stream_writable.js

@@ -503,6 +503,7 @@ function clearBuffer(stream, state) {
       corkReq.finish = onCorkedFinish.bind(undefined, corkReq, state);
       state.corkedRequestsFree = corkReq;
     }
+    state.bufferedRequestCount = 0;
   } else {
     // Slow case, write chunks one-by-one
     while (entry) {
@@ -513,6 +514,7 @@ function clearBuffer(stream, state) {
 
       doWrite(stream, state, false, len, chunk, encoding, cb);
       entry = entry.next;
+      state.bufferedRequestCount--;
       // if we didn't call the onwrite immediately, then
       // it means that we need to wait until it does.
       // also, that means that the chunk and cb are currently
@@ -526,7 +528,6 @@ function clearBuffer(stream, state) {
       state.lastBufferedRequest = null;
   }
 
-  state.bufferedRequestCount = 0;
   state.bufferedRequest = entry;
   state.bufferProcessing = false;
 }

test/sequential/test-stream-writable-clear-buffer.js

@@ -0,0 +1,34 @@
+'use strict';
+const common = require('../common');
+const Stream = require('stream');
+// This test ensures that the _writeableState.bufferedRequestCount and
+// the actual buffered request count are the same
+const assert = require('assert');
+
+class StreamWritable extends Stream.Writable {
+  constructor() {
+    super({ objectMode: true });
+  }
+
+  // We need a timeout like on the original issue thread
+  // otherwise the code will never reach our test case
+  // this means this should go on the sequential folder.
+  _write(chunk, encoding, cb) {
+    setTimeout(cb, common.platformTimeout(10));
+  }
+}
+
+const testStream = new StreamWritable();
+testStream.cork();
+
+for (let i = 1; i <= 5; i++) {
+  testStream.write(i, function() {
+    assert.strictEqual(
+      testStream._writableState.bufferedRequestCount,
+      testStream._writableState.getBuffer().length,
+      'bufferedRequestCount variable is different from the actual length of' +
+      ' the buffer');
+  });
+}
+
+testStream.end();