[BUGFIX lts] Avoid excessively calling Glimmer AST transforms.

packages/ember-template-compiler/lib/system/compile-options.js

@@ -30,20 +30,28 @@ function wrapLegacyPluginIfNeeded(_plugin) {
   let plugin = _plugin;
   if (_plugin.prototype && _plugin.prototype.transform) {
     plugin = (env) => {
+      let pluginInstantiated = false;
+
       return {
         name: _plugin.constructor && _plugin.constructor.name,
 
         visitor: {
           Program(node) {
-            let plugin = new _plugin(env);
+            if (!pluginInstantiated) {
+
+              pluginInstantiated = true;
+              let plugin = new _plugin(env);
 
-            plugin.syntax = env.syntax;
+              plugin.syntax = env.syntax;
 
-            return plugin.transform(node);
+              return plugin.transform(node);
+            }
           }
         }
       };
     };
+
+    plugin.__raw = _plugin;
   }
 
   return plugin;
@@ -54,6 +62,13 @@ export function registerPlugin(type, _plugin) {
     throw new Error(`Attempting to register ${_plugin} as "${type}" which is not a valid Glimmer plugin type.`);
   }
 
+  for (let i = 0; i < USER_PLUGINS.length; i++) {
+    let PLUGIN = USER_PLUGINS[i];
+    if (PLUGIN === _plugin || PLUGIN.__raw === _plugin) {
+      return;
+    }
+  }
+
   let plugin = wrapLegacyPluginIfNeeded(_plugin);
 
   USER_PLUGINS = [plugin, ...USER_PLUGINS];
@@ -64,5 +79,5 @@ export function unregisterPlugin(type, PluginClass) {
     throw new Error(`Attempting to unregister ${PluginClass} as "${type}" which is not a valid Glimmer plugin type.`);
   }
 
-  USER_PLUGINS = USER_PLUGINS.filter((plugin) => plugin !== PluginClass);
+  USER_PLUGINS = USER_PLUGINS.filter((plugin) => plugin !== PluginClass && plugin.__raw !== PluginClass);
 }

packages/ember-template-compiler/tests/system/compile_options_test.js

@@ -18,8 +18,10 @@ moduleFor('ember-template-compiler: default compile options', class extends Abst
   }
 });
 
+let customTransformCounter = 0;
 class CustomTransform {
   constructor(options) {
+    customTransformCounter++;
     this.options = options;
     this.syntax = null;
   }
@@ -45,13 +47,10 @@ class CustomTransform {
   }
 }
 
-moduleFor('ember-template-compiler: registerPlugin with a custom plugins', class extends RenderingTestCase {
-  beforeEach() {
-    registerPlugin('ast', CustomTransform);
-  }
-
+class CustomPluginsTests extends RenderingTestCase {
   afterEach() {
-    unregisterPlugin('ast', CustomTransform);
+    customTransformCounter = 0;
+    return super.afterEach();
   }
 
   ['@test custom plugins can be used']() {
@@ -62,6 +61,28 @@ moduleFor('ember-template-compiler: registerPlugin with a custom plugins', class
       content: ''
     });
   }
+
+  ['@test wrapped plugins are only invoked once per template'](assert) {
+    this.render('<div>{{#if falsey}}nope{{/if}}</div>');
+    assert.equal(customTransformCounter, 1, 'transform should only be instantiated once');
+  }
+}
+
+moduleFor('ember-template-compiler: registerPlugin with a custom plugins', class extends CustomPluginsTests {
+  beforeEach() {
+    registerPlugin('ast', CustomTransform);
+  }
+
+  afterEach() {
+    unregisterPlugin('ast', CustomTransform);
+    return super.afterEach();
+  }
+
+  ['@test custom registered plugins are deduplicated'](assert) {
+    registerPlugin('ast', CustomTransform);
+    this.registerTemplate('application', '<div data-test="foo" data-blah="derp" class="hahaha"></div>');
+    assert.equal(customTransformCounter, 1, 'transform should only be instantiated once');
+  }
 });
 
 moduleFor('ember-template-compiler: custom plugins passed to compile', class extends RenderingTestCase {
@@ -73,13 +94,4 @@ moduleFor('ember-template-compiler: custom plugins passed to compile', class ext
       }
     });
   }
-
-  ['@test custom plugins can be used']() {
-    this.render('<div data-test="foo" data-blah="derp" class="hahaha"></div>');
-    this.assertElement(this.firstChild, {
-      tagName: 'div',
-      attrs: { class: 'hahaha', 'data-blah': 'derp' },
-      content: ''
-    });
-  }
 });