stream: do only fake drain when unpiped stream is the source

If the destination had multiply read streams piped to it,
they would all decrease the awaitDrain state and thereby
start the flow

lib/_stream_readable.js

@@ -327,16 +327,17 @@ Readable.prototype.pipe = function(dest, pipeOpts) {
   var ondrain = pipeOnDrain(src);
   dest.on('drain', ondrain);
   dest.on('unpipe', function cleanup(readable) {
-    if (readable === src)
+    if (readable === src) {
       dest.removeListener('unpipe', cleanup);
 
-    // if the reader is waiting for a drain event from this
-    // specific writer, then it would cause it to never start
-    // flowing again.
-    // So, if this is awaiting a drain, then we just call it now.
-    // If we don't know, then assume that we are waiting for one.
-    if (!dest._writableState || dest._writableState.needDrain)
-      ondrain();
+      // if the reader is waiting for a drain event from this
+      // specific writer, then it would cause it to never start
+      // flowing again.
+      // So, if this is awaiting a drain, then we just call it now.
+      // If we don't know, then assume that we are waiting for one.
+      if (!dest._writableState || dest._writableState.needDrain)
+        ondrain();
+    }
   });
 
   // if the dest has an error, then stop piping into it.

test/simple/test-stream2-unpipe-drain.js

@@ -0,0 +1,76 @@
+// Copyright Joyent, Inc. and other Node contributors.
+//
+// Permission is hereby granted, free of charge, to any person obtaining a
+// copy of this software and associated documentation files (the
+// "Software"), to deal in the Software without restriction, including
+// without limitation the rights to use, copy, modify, merge, publish,
+// distribute, sublicense, and/or sell copies of the Software, and to permit
+// persons to whom the Software is furnished to do so, subject to the
+// following conditions:
+//
+// The above copyright notice and this permission notice shall be included
+// in all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
+// OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+// MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN
+// NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM,
+// DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
+// OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE
+// USE OR OTHER DEALINGS IN THE SOFTWARE.
+
+
+var common = require('../common.js');
+var assert = require('assert');
+var stream = require('stream');
+var crypto = require('crypto');
+
+var util = require('util');
+
+function TestWriter() {
+    stream.Writable.call(this);
+}
+util.inherits(TestWriter, stream.Writable);
+
+TestWriter.prototype._write = function (buffer, callback) {
+  console.log('write called');
+  // super slow write stream (callback never called)
+};
+
+var dest = new TestWriter();
+
+function TestReader(id) {
+    stream.Readable.call(this);
+    this.reads = 0;
+}
+util.inherits(TestReader, stream.Readable);
+
+TestReader.prototype._read = function (size, callback) {
+  this.reads += 1;
+  crypto.randomBytes(size, callback);
+};
+
+var src1 = new TestReader();
+var src2 = new TestReader();
+
+src1.pipe(dest);
+
+src1.once('readable', function () {
+  process.nextTick(function () {
+
+    src2.pipe(dest);
+
+    src2.once('readable', function () {
+      process.nextTick(function () {
+
+        src1.unpipe(dest);
+      });
+    });
+  });
+});
+
+
+process.on('exit', function () {
+  assert.equal(src1.reads, 2);
+  assert.equal(src2.reads, 2);
+});