feat(scope): better logging of infinite digest error

Feedback team has often problems debugging inifinite digest errors, this change
should reveal info about what watchers are causing the infinite loop

src/Scope.js

@@ -260,7 +260,8 @@ Scope.prototype = {
         watcher = {
           fn: listenFn,
           last: Number.NaN, // NaN !== NaN. We used this to force $watch to fire on first run.
-          get: get
+          get: get,
+          exp: watchExp
         };
 
     if (!array) {
@@ -325,7 +326,8 @@ Scope.prototype = {
         asyncQueue,
         length,
         dirty, ttl = 100,
-        next, current, target = this;
+        next, current, target = this,
+        watchLog = [];
 
     if (target.$$phase) {
       throw Error(target.$$phase + ' already in progress');
@@ -356,6 +358,14 @@ Scope.prototype = {
                 dirty = true;
                 watch.last = copy(value);
                 watch.fn(current, value, last);
+                if (ttl < 5) {
+                  if (!watchLog[4-ttl]) watchLog[4-ttl] = [];
+                  if (isFunction(watch.exp)) {
+                    watchLog[4-ttl].push('fn: ' + (watch.exp.name || watch.exp.toString()));
+                  } else {
+                    watchLog[4-ttl].push(watch.exp);
+                  }
+                }
               }
             } catch (e) {
               current.$service('$exceptionHandler')(e);
@@ -376,7 +386,8 @@ Scope.prototype = {
       } while ((current = next));
 
       if(!(ttl--)) {
-        throw Error('100 $digest() iterations reached. Aborting!');
+        throw Error('100 $digest() iterations reached. Aborting!\n' +
+            'Watchers fired in the last 5 iterations: ' + toJson(watchLog));
       }
     } while (dirty);
   },

test/ScopeSpec.js

@@ -218,14 +218,34 @@ describe('Scope', function() {
     });
 
 
-    it('should prevent infinite recursion', function() {
-      root.$watch('a', function(self, v){self.b++;});
-      root.$watch('b', function(self, v){self.a++;});
+    it('should prevent infinite recursion and print watcher expression', function() {
+      root.$watch('a', function(self){self.b++;});
+      root.$watch('b', function(self){self.a++;});
       root.a = root.b = 0;
 
       expect(function() {
         root.$digest();
-      }).toThrow('100 $digest() iterations reached. Aborting!');
+      }).toThrow('100 $digest() iterations reached. Aborting!\n'+
+          'Watchers fired in the last 5 iterations: ' +
+          '[["a","b"],["a","b"],["a","b"],["a","b"],["a","b"]]');
+    });
+
+
+    it('should prevent infinite recurcion and print print watcher function name or body',
+        function() {
+      root.$watch(function watcherA() {return root.a;}, function(self){self.b++;});
+      root.$watch(function() {return root.b;}, function(self){self.a++;});
+      root.a = root.b = 0;
+
+      expect(function() {
+        root.$digest();
+      }).toThrow('100 $digest() iterations reached. Aborting!\n'+
+          'Watchers fired in the last 5 iterations: ' +
+          '[["fn: watcherA","fn: function () {return root.b;}"],'+
+           '["fn: watcherA","fn: function () {return root.b;}"],'+
+           '["fn: watcherA","fn: function () {return root.b;}"],'+
+           '["fn: watcherA","fn: function () {return root.b;}"],'+
+           '["fn: watcherA","fn: function () {return root.b;}"]]');
     });