stream: avoid pause with unpipe in buffered write

If a pipe is cleaned up (due to unpipe) during a write that
returned false, the source stream can get stuck in a paused state.

Fixes: https://github.com/nodejs/node/issues/2323
PR-URL: https://github.com/nodejs/node/pull/2325
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>

lib/_stream_readable.js

@@ -497,6 +497,7 @@ Readable.prototype.pipe = function(dest, pipeOpts) {
   var ondrain = pipeOnDrain(src);
   dest.on('drain', ondrain);
 
+  var cleanedUp = false;
   function cleanup() {
     debug('cleanup');
     // cleanup event handlers once the pipe is broken
@@ -509,6 +510,8 @@ Readable.prototype.pipe = function(dest, pipeOpts) {
     src.removeListener('end', cleanup);
     src.removeListener('data', ondata);
 
+    cleanedUp = true;
+
     // if the reader is waiting for a drain event from this
     // specific writer, then it would cause it to never start
     // flowing again.
@@ -524,9 +527,16 @@ Readable.prototype.pipe = function(dest, pipeOpts) {
     debug('ondata');
     var ret = dest.write(chunk);
     if (false === ret) {
-      debug('false write response, pause',
-            src._readableState.awaitDrain);
+      // If the user unpiped during `dest.write()`, it is possible
+      // to get stuck in a permanently paused state if that write
+      // also returned false.
+      if (state.pipesCount === 1 &&
+          state.pipes[0] === dest &&
+          src.listenerCount('data') === 1 &&
+          !cleanedUp) {
+        debug('false write response, pause', src._readableState.awaitDrain);
         src._readableState.awaitDrain++;
+      }
       src.pause();
     }
   }

test/parallel/test-stream-pipe-cleanup-pause.js

@@ -0,0 +1,38 @@
+'use strict';
+const common = require('../common');
+const assert = require('assert');
+const stream = require('stream');
+
+const reader = new stream.Readable();
+const writer1 = new stream.Writable();
+const writer2 = new stream.Writable();
+
+// 560000 is chosen here because it is larger than the (default) highWaterMark
+// and will cause `.write()` to return false
+// See: https://github.com/nodejs/node/issues/2323
+const buffer = new Buffer(560000);
+
+reader._read = function(n) {};
+
+writer1._write = common.mustCall(function(chunk, encoding, cb) {
+  this.emit('chunk-received');
+  cb();
+}, 1);
+writer1.once('chunk-received', function() {
+  reader.unpipe(writer1);
+  reader.pipe(writer2);
+  reader.push(buffer);
+  setImmediate(function() {
+    reader.push(buffer);
+    setImmediate(function() {
+      reader.push(buffer);
+    });
+  });
+});
+
+writer2._write = common.mustCall(function(chunk, encoding, cb) {
+  cb();
+}, 3);
+
+reader.pipe(writer1);
+reader.push(buffer);