Fix {{#with}} over a data context that is mutated

In `Spacebars.With` we embox the data context. This commit makes
that emboxing happen modulo `safeEquals`. So now if you
{{#with}} over a helper that returns an object, any time
that helper gets invalidated we re-run the computations in the block.

Fixes #2046 (though notably that example mutates the data context
from within a helper, which could lead to other types of unintended
behavior; it's probably in this particular example -- the data context
just gets added properties)

packages/spacebars-tests/template_tests.html

@@ -712,3 +712,9 @@ Hi there!
     {{bar}}
   --}}
 </template>
+
+<template name="spacebars_test_with_mutated_data_context">
+  {{#with foo}}
+    {{value}}
+  {{/with}}
+</template>

packages/spacebars-tests/template_tests.js

@@ -1877,7 +1877,7 @@ Tinytest.add(
   }
 );
 
-Tinytest.add("spacebars - templates - UI.toHTML", function (test) {
+Tinytest.add("spacebars - template - UI.toHTML", function (test) {
   // run once, verifying that autoruns are stopped
   var once = function (tmplToRender, tmplForHelper, helper, val) {
     var count = 0;
@@ -1919,3 +1919,24 @@ Tinytest.add(
     test.equal(canonicalizeHtml(div.innerHTML), '');
   }
 );
+
+// Originally reported at https://github.com/meteor/meteor/issues/2046
+Tinytest.add(
+  "spacebars - template - {{#with}} with mutated data context",
+  function (test) {
+    var tmpl = Template.spacebars_test_with_mutated_data_context;
+    var foo = {value: 0};
+    var dep = new Deps.Dependency;
+    tmpl.foo = function () {
+      dep.depend();
+      return foo;
+    };
+
+    var div = renderToDiv(tmpl);
+    test.equal(canonicalizeHtml(div.innerHTML), '0');
+
+    foo.value = 1;
+    dep.changed();
+    Deps.flush();
+    test.equal(canonicalizeHtml(div.innerHTML), '1');
+  });

packages/spacebars/spacebars-runtime.js

@@ -216,7 +216,7 @@ Spacebars.dot = function (value, id1/*, id2, ...*/) {
 Spacebars.With = function (argFunc, contentBlock, elseContentBlock) {
   return UI.Component.extend({
     init: function () {
-      this.v = UI.emboxValue(argFunc);
+      this.v = UI.emboxValue(argFunc, UI.safeEquals);
     },
     render: function () {
       return UI.If(this.v, UI.With(this.v, contentBlock), elseContentBlock);

packages/ui/builtins.js

@@ -38,7 +38,7 @@ UI.Unless = function (argFunc, contentBlock, elseContentBlock) {
 // (Because then, they may be equal references to an object that was mutated,
 // and we'll never know.  We save only a reference to the old object; we don't
 // do any deep-copying or diffing.)
-var safeEquals = function (a, b) {
+UI.safeEquals = function (a, b) {
   if (a !== b)
     return false;
   else
@@ -67,7 +67,7 @@ UI.With = function (argFunc, contentBlock) {
   };
 
   block.init = function () {
-    this.data = UI.emboxValue(argFunc, safeEquals);
+    this.data = UI.emboxValue(argFunc, UI.safeEquals);
   };
 
   block.materialized = function () {