Add 'pipe' event

doc/api/streams.markdown

@@ -125,6 +125,12 @@ Emitted on error with the exception `exception`.
 
 Emitted when the underlying file descriptor has been closed.
 
+### Event: 'pipe'
+
+`function (src) { }`
+
+Emitted when the stream is passed to a readable stream's pipe method.
+
 ### stream.writable
 
 A boolean that is `true` by default, but turns `false` after an `'error'`

lib/stream.js

@@ -67,4 +67,6 @@ Stream.prototype.pipe = function(dest, options) {
   dest.on('resume', function() {
     if (source.readable) source.resume();
   });
+
+  dest.emit('pipe', source);
 };

test/simple/test-stream-pipe-event.js

@@ -0,0 +1,27 @@
+var stream = require('stream');
+var assert = require('assert');
+var util = require('util');
+
+function Writable () {
+  this.writable = true;
+  stream.Stream.call(this);
+}
+util.inherits(Writable, stream.Stream);
+
+function Readable () {
+  this.readable = true;
+  stream.Stream.call(this);
+}
+util.inherits(Readable, stream.Stream);
+
+var passed = false;
+
+var w = new Writable();
+w.on('pipe', function (src) {
+  passed = true;
+});
+
+var r = new Readable();
+r.pipe(w);
+
+assert.ok(passed)