stream: Fix double pipe error emit

If an error listener is added to a stream using once() before it is
piped, it is invoked and removed during pipe() but before pipe() sees it
which causes it to be emitted again.

Fixes #4155 #4978

1 files changed, 3 insertions, 1 deletions

diff --git a/lib/_stream_readable.js b/lib/_stream_readable.js
index 2259d2e778..f35599f455 100644..100755
--- a/lib/_stream_readable.js
+++ b/lib/_stream_readable.js
@@ -511,9 +511,11 @@ Readable.prototype.pipe = function(dest, pipeOpts) {
 
   // if the dest has an error, then stop piping into it.
   // however, don't suppress the throwing behavior for this.
+  // check for listeners before emit removes one-time listeners.
+  var errListeners = EE.listenerCount(dest, 'error');
   function onerror(er) {
     unpipe();
-    if (EE.listenerCount(dest, 'error') === 0)
+    if (errListeners === 0 && EE.listenerCount(dest, 'error') === 0)
       dest.emit('error', er);
   }
   dest.once('error', onerror);