debugger: watch, unwatch, watchers

Fixes #1800.

lib/_debugger.js

@@ -644,6 +644,9 @@ var commands = [
     'clearBreakpoint (cb)',
   ],
   [
+    'watch',
+    'unwatch',
+    'watchers',
     'repl',
     'restart',
     'kill',
@@ -784,6 +787,7 @@ function Interface(stdin, stdout, args) {
     control: []
   };
   this.breakpoints = [];
+  this._watchers = [];
 
   // Run script automatically
   this.pause();
@@ -863,6 +867,8 @@ Interface.prototype.error = function(text) {
 
 // Debugger's `break` event handler
 Interface.prototype.handleBreak = function(r) {
+  var self = this;
+
   this.pause();
 
   // Save execution context's data
@@ -875,10 +881,15 @@ Interface.prototype.handleBreak = function(r) {
   // Print break data
   this.print(SourceInfo(r));
 
-  // And list source
-  this.list(2);
+  // Show watchers' values
+  this.watchers(true, function (err) {
+    if (err) return self.error(err);
 
-  this.resume(true);
+    // And list source
+    self.list(2);
+
+    self.resume(true);
+  });
 };
 
 
@@ -1209,6 +1220,62 @@ Interface.prototype.step = Interface.stepGenerator('in', 1);
 Interface.prototype.out = Interface.stepGenerator('out', 1);
 
 
+// Watch
+Interface.prototype.watch = function(expr) {
+  this._watchers.push(expr);
+};
+
+// Unwatch
+Interface.prototype.unwatch = function(expr) {
+  var index = this._watchers.indexOf(expr);
+
+  // Unwatch by expression
+  // or
+  // Unwatch by watcher number
+  this._watchers.splice(index !== -1 ? index : +expr, 1);
+};
+
+// List watchers
+Interface.prototype.watchers = function() {
+  var self = this,
+      verbose = arguments[0] || false,
+      callback = arguments[1] || function() {},
+      waiting = this._watchers.length,
+      values = [];
+
+  this.pause();
+
+  if (!waiting) {
+    this.resume();
+
+    return callback();
+  }
+
+  this._watchers.forEach(function(watcher, i) {
+    self.debugEval(watcher, null, null, function(err, value) {
+      values[i] = err ? '<error>' : value;
+      wait();
+    });
+  });
+
+  function wait() {
+    if (--waiting === 0) {
+      if (verbose) self.print('Watchers:');
+
+      self._watchers.forEach(function(watcher, i) {
+        self.print(leftPad(i, ' ') + ': ' + watcher + ' = ' +
+                   JSON.stringify(values[i]));
+      });
+
+      if (verbose) self.print('');
+
+      self.resume();
+
+      callback(null);
+    }
+  }
+};
+
 // Add breakpoint
 Interface.prototype.setBreakpoint = function(script, line,
                                              condition, silent) {

test/simple/test-debugger-repl.js

@@ -46,7 +46,7 @@ child.on('line', function(line) {
   assert.ok(expected.length > 0, 'Got unexpected line: ' + line);
 
   var expectedLine = expected[0].lines.shift();
-  assert.ok(line.match(expectedLine) !== null);
+  assert.ok(line.match(expectedLine) !== null, expectedLine);
 
   if (expected[0].lines.length === 0) {
     var callback = expected[0].callback;
@@ -59,6 +59,15 @@ function addTest(input, output) {
   function next() {
     if (expected.length > 0) {
       child.stdin.write(expected[0].input + '\n');
+
+      if (!expected[0].lines) {
+        process.nextTick(function() {
+          var callback = expected[0].callback;
+          expected.shift();
+
+          callback && callback();
+        });
+      }
     } else {
       finish();
     }
@@ -80,12 +89,26 @@ addTest('n', [
   /11/, /12/, /13/, /14/, /15/
 ]);
 
+// Watch
+addTest('watch("\'x\'")');
+
 // Continue
 addTest('c', [
   /break in .*:7/,
+  /Watchers/,
+  /0:\s+'x' = "x"/,
+  /()/,
   /5/, /6/, /7/, /8/, /9/
 ]);
 
+// Show watchers
+addTest('watchers', [
+  /0:\s+'x' = "x"/
+]);
+
+// Unwatch
+addTest('unwatch("\'x\'")');
+
 // Step out
 addTest('o', [
   /break in .*:14/,