repl: emit 'reset' event when context is reset

Closes #1183.

doc/api/repl.markdown

@@ -135,6 +135,27 @@ Example of listening for `exit`:
     });
 
 
+### Event: 'reset'
+
+`function (context) {}`
+
+Emitted when the REPL's context is reset. This happens when you type `.clear`.
+If you start the repl with `{ useGlobal: true }` then this event will never
+be emitted.
+
+Example of listening for `reset`:
+
+    // Extend the initial repl context.
+    r = repl.start({ options ... });
+    someExtension.extend(r.context);
+
+    // When a new context is created extend it as well.
+    r.on('reset', function (context) {
+      console.log('repl has a new context');
+      someExtension.extend(context);
+    });
+
+
 ## REPL Features
 
 <!-- type=misc -->

lib/repl.js

@@ -356,6 +356,9 @@ REPLServer.prototype.createContext = function() {
 
 REPLServer.prototype.resetContext = function() {
   this.context = this.createContext();
+
+  // Allow REPL extensions to extend the new context
+  this.emit('reset', this.context);
 };
 
 REPLServer.prototype.displayPrompt = function(preserveCursor) {

test/simple/test-repl-reset-event.js

@@ -0,0 +1,73 @@
+// 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');
+common.globalCheck = false;
+
+var assert = require('assert');
+var repl = require('repl');
+var Stream = require('stream');
+
+// create a dummy stream that does nothing
+var dummy = new Stream();
+dummy.write = dummy.pause = dummy.resume = function(){};
+dummy.readable = dummy.writable = true;
+
+function testReset(cb) {
+  var r = repl.start({
+    input: dummy,
+    output: dummy,
+    useGlobal: false
+  });
+  r.context.foo = 42;
+  r.on('reset', function(context) {
+    assert(!!context, 'REPL did not emit a context with reset event');
+    assert.equal(context, r.context, 'REPL emitted incorrect context');
+    assert.equal(context.foo, undefined, 'REPL emitted the previous context, and is not using global as context');
+    context.foo = 42;
+    cb();
+  });
+  r.resetContext();
+}
+
+function testResetGlobal(cb) {
+  var r = repl.start({
+    input: dummy,
+    output: dummy,
+    useGlobal: true
+  });
+  r.context.foo = 42;
+  r.on('reset', function(context) {
+    assert.equal(context.foo, 42, '"foo" property is missing from REPL using global as context');
+    cb();
+  });
+  r.resetContext();
+}
+
+var timeout = setTimeout(function() {
+  assert.fail('Timeout, REPL did not emit reset events');
+}, 5000);
+
+testReset(function() {
+  testResetGlobal(function() {
+    clearTimeout(timeout);
+  });
+});