Bug 1916277 - Part 1: Add pref dom.multiple_import_maps.enabled. r=jonco

Add a pref to enable support for multiple import maps.

Differential Revision: https://phabricator.services.mozilla.com/D278654

Author: allstarschh

js/loader/ImportMap.cpp

@@ -387,6 +387,12 @@ static bool GetOwnProperty(JSContext* aCx, Handle<JSObject*> aObj,
   return true;
 }
 
+// static
+bool ImportMap::IsMultipleImportMapsSupported() {
+  return NS_IsMainThread() &&
+         mozilla::StaticPrefs::dom_multiple_import_maps_enabled();
+}
+
 // https://html.spec.whatwg.org/multipage/webappapis.html#parse-an-import-map-string
 // static
 UniquePtr<ImportMap> ImportMap::ParseString(

js/loader/ImportMap.h

@@ -71,6 +71,14 @@ class ImportMap {
         mScopes(std::move(aScopes)),
         mIntegrity(std::move(aIntegrity)) {}
 
+  /**
+   * A helper function to get the "dom.multiple_import_maps.enabled" pref.
+   * The pref's type is of non-atomic, which can be only accessed on the main
+   * thread.
+   * If this function is called from a non-main thread, it safely returns false.
+   */
+  static bool IsMultipleImportMapsSupported();
+
   /**
    * Parse the JSON string from the Import map script.
    * This function will throw a TypeError if there's any invalid key or value in

modules/libpref/init/StaticPrefList.yaml

@@ -3766,6 +3766,12 @@
   value: true
   mirror: always
 
+# Enable mutiple import maps.
+- name: dom.multiple_import_maps.enabled
+  type: bool
+  value: false
+  mirror: always
+
 # Limit of location change caused by content scripts in a time span per
 # BrowsingContext. This includes calls to History and Location APIs.
 - name: dom.navigation.navigationRateLimit.count