do not break on precompiled templates from Handlebars >=4.0.0 <4.3.0

- The version-range above have compiler version 7 and
  precompiled templates expecte the (block)
  HelperMissing-functions in "helpers" and not in "container.hooks".
- Handlebars now accepts precompiled templates of version 7.
- If a precompiled template with version 7 is loaded,
  the (block)HelperMissing-functions are kept in "helpers"

lib/handlebars/base.js

@@ -6,6 +6,7 @@ import logger from './logger';
 
 export const VERSION = '4.3.0';
 export const COMPILER_REVISION = 8;
+export const LAST_COMPATIBLE_COMPILER_REVISION = 7;
 
 export const REVISION_CHANGES = {
   1: '<= 1.0.rc.2', // 1.0.rc.2 is actually rev2 but doesn't report it

lib/handlebars/runtime.js

@@ -1,23 +1,25 @@
 import * as Utils from './utils';
 import Exception from './exception';
-import {COMPILER_REVISION, createFrame, REVISION_CHANGES} from './base';
+import {COMPILER_REVISION, createFrame, LAST_COMPATIBLE_COMPILER_REVISION, REVISION_CHANGES} from './base';
 import {moveHelperToHooks} from './helpers';
 
 export function checkRevision(compilerInfo) {
   const compilerRevision = compilerInfo && compilerInfo[0] || 1,
         currentRevision = COMPILER_REVISION;
 
-  if (compilerRevision !== currentRevision) {
-    if (compilerRevision < currentRevision) {
-      const runtimeVersions = REVISION_CHANGES[currentRevision],
-            compilerVersions = REVISION_CHANGES[compilerRevision];
-      throw new Exception('Template was precompiled with an older version of Handlebars than the current runtime. ' +
-            'Please update your precompiler to a newer version (' + runtimeVersions + ') or downgrade your runtime to an older version (' + compilerVersions + ').');
-    } else {
-      // Use the embedded version info since the runtime doesn't know about this revision yet
-      throw new Exception('Template was precompiled with a newer version of Handlebars than the current runtime. ' +
-            'Please update your runtime to a newer version (' + compilerInfo[1] + ').');
-    }
+  if (compilerRevision >= LAST_COMPATIBLE_COMPILER_REVISION && compilerRevision <= COMPILER_REVISION) {
+    return;
+  }
+
+  if (compilerRevision < LAST_COMPATIBLE_COMPILER_REVISION) {
+    const runtimeVersions = REVISION_CHANGES[currentRevision],
+          compilerVersions = REVISION_CHANGES[compilerRevision];
+    throw new Exception('Template was precompiled with an older version of Handlebars than the current runtime. ' +
+          'Please update your precompiler to a newer version (' + runtimeVersions + ') or downgrade your runtime to an older version (' + compilerVersions + ').');
+  } else {
+    // Use the embedded version info since the runtime doesn't know about this revision yet
+    throw new Exception('Template was precompiled with a newer version of Handlebars than the current runtime. ' +
+          'Please update your runtime to a newer version (' + compilerInfo[1] + ').');
   }
 }
 
@@ -37,6 +39,9 @@ export function template(templateSpec, env) {
   // for external users to override these as pseudo-supported APIs.
   env.VM.checkRevision(templateSpec.compiler);
 
+  // backwards compatibility for precompiled templates with compiler-version 7 (<4.3.0)
+  const templateWasPrecompiledWithCompilerV7 = templateSpec.compiler && templateSpec.compiler[0] === 7;
+
   function invokePartialWrapper(partial, context, options) {
     if (options.hash) {
       context = Utils.extend({}, context, options.hash);
@@ -163,9 +168,10 @@ export function template(templateSpec, env) {
       }
 
       container.hooks = {};
-      let keepHelper = options.allowCallsToHelperMissing;
-      moveHelperToHooks(container, 'helperMissing', keepHelper);
-      moveHelperToHooks(container, 'blockHelperMissing', keepHelper);
+
+      let keepHelperInHelpers = options.allowCallsToHelperMissing || templateWasPrecompiledWithCompilerV7;
+      moveHelperToHooks(container, 'helperMissing', keepHelperInHelpers);
+      moveHelperToHooks(container, 'blockHelperMissing', keepHelperInHelpers);
 
     } else {
       container.helpers = options.helpers;

spec/regressions.js

@@ -291,4 +291,36 @@ describe('Regressions', function() {
     };
     shouldCompileToWithPartials(string, [{}, {}, partials], true, 'template  block  partial  block  template');
   });
+
+  describe('GH-1561: 4.3.x should still work with precompiled templates from 4.0.0 <= x < 4.3.0', function() {
+
+    it('should compile and execute templates', function() {
+      var newHandlebarsInstance = Handlebars.create();
+
+      registerTemplate(newHandlebarsInstance);
+      newHandlebarsInstance.registerHelper('loud', function(value) {
+        return value.toUpperCase();
+      });
+      let result = newHandlebarsInstance.templates['test.hbs']({name: 'yehuda'});
+      equals(result.trim(), 'YEHUDA');
+    });
+
+    it('should call "helperMissing" if a helper is missing', function() {
+      var newHandlebarsInstance = Handlebars.create();
+
+      shouldThrow(() => {
+        registerTemplate(newHandlebarsInstance);
+        newHandlebarsInstance.templates['test.hbs']({});
+      }, Handlebars.Exception, 'Missing helper: "loud"');
+    });
+
+    // This is a only slightly modified precompiled templated from compiled with 4.2.1
+    function registerTemplate(Handlebars) {
+      var template = Handlebars.template, templates = Handlebars.templates = Handlebars.templates || {};
+      templates['test.hbs'] = template({'compiler': [7, '>= 4.0.0'], 'main': function(container, depth0, helpers, partials, data) {
+          return container.escapeExpression((helpers.loud || (depth0 && depth0.loud) || helpers.helperMissing).call(depth0 != null ? depth0 : (container.nullContext || {}), (depth0 != null ? depth0.name : depth0), {'name': 'loud', 'hash': {}, 'data': data}))
+              + '\n\n';
+        }, 'useData': true});
+    }
+  });
 });

spec/runtime.js

@@ -21,7 +21,7 @@ describe('runtime', function() {
       shouldThrow(function() {
         Handlebars.template({
           main: {},
-          compiler: [Handlebars.COMPILER_REVISION - 1]
+          compiler: [Handlebars.LAST_COMPATIBLE_COMPILER_REVISION - 1]
         });
       }, Error, /Template was precompiled with an older version of Handlebars than the current runtime/);
       shouldThrow(function() {