child_process: Fix stdout=null when stdio=['pipe']

Previously, a command with a short stdio array would result in the child's
stdout and stderr objects set to null. For example:

var c = child_process.spawn(cmd, args, {stdio: ['pipe']});
// results in c.stdout === null.

The expected behavior is the above line functioning the same as this one:

var c = child_process.spawn(cmd, args, {stdio: ['pipe', null, null]});
// provides correct (non-null) c.stdout; as does the above, after this fix.

1 files changed, 4 insertions, 3 deletions

diff --git a/lib/child_process.js b/lib/child_process.js
index 34edbc25ad..5b0c3f318b 100644
--- a/lib/child_process.js
+++ b/lib/child_process.js
@@ -709,9 +709,10 @@ ChildProcess.prototype.spawn = function(options) {
   }
 
   // At least 3 stdio will be created
-  if (stdio.length < 3) {
-    stdio = stdio.concat(new Array(3 - stdio.length));
-  }
+  // Don't concat() a new Array() because it would be sparse, and
+  // stdio.reduce() would skip the sparse elements of stdio.
+  // See http://stackoverflow.com/a/5501711/3561
+  while (stdio.length < 3) stdio.push(undefined);
 
   // Translate stdio into C++-readable form
   // (i.e. PipeWraps or fds)