Bug 1591877 - Share the same cached pref value of dom.storage.next_gen across all relevant processes; r=dom-storage-reviewers,jstutte a=pascalc

Differential Revision: https://phabricator.services.mozilla.com/D119711

dom/ipc/ContentChild.cpp

@@ -4359,6 +4359,13 @@ mozilla::ipc::IPCResult ContentChild::RecvDecoderSupportedMimeTypes(
   return IPC_OK();
 }
 
+mozilla::ipc::IPCResult ContentChild::RecvInitNextGenLocalStorageEnabled(
+    const bool& aEnabled) {
+  mozilla::dom::RecvInitNextGenLocalStorageEnabled(aEnabled);
+
+  return IPC_OK();
+}
+
 /* static */ void ContentChild::DispatchBeforeUnloadToSubtree(
     BrowsingContext* aStartingAt,
     const DispatchBeforeUnloadToSubtreeResolver& aResolver) {

dom/ipc/ContentChild.h

@@ -820,6 +820,9 @@ class ContentChild final : public PContentChild,
   mozilla::ipc::IPCResult RecvDecoderSupportedMimeTypes(
       nsTArray<nsCString>&& aSupportedTypes);
 
+  mozilla::ipc::IPCResult RecvInitNextGenLocalStorageEnabled(
+      const bool& aEnabled);
+
  public:
   static void DispatchBeforeUnloadToSubtree(
       BrowsingContext* aStartingAt,

dom/ipc/ContentParent.cpp

@@ -1715,6 +1715,8 @@ void ContentParent::Init() {
     Unused << NS_WARN_IF(!SendPreferenceUpdate(pref));
   }
   mQueuedPrefs.Clear();
+
+  Unused << SendInitNextGenLocalStorageEnabled(NextGenLocalStorageEnabled());
 }
 
 void ContentParent::MaybeBeginShutDown(uint32_t aExpectedBrowserCount,

dom/ipc/PContent.ipdl

@@ -964,6 +964,11 @@ child:
   // Send the list of the supported mimetypes in the given process. GeckoView-specific
   async DecoderSupportedMimeTypes(nsCString[] supportedTypes);
 
+  // Used to initialize the global variable in content processes with the
+  // latched value in the parent process. See dom/LocalStorageCommon.h for more
+  // details.
+  async InitNextGenLocalStorageEnabled(bool enabled);
+
 parent:
 
     async SynchronizeLayoutHistoryState(MaybeDiscardedBrowsingContext aContext,

dom/localstorage/LocalStorageCommon.cpp

@@ -72,14 +72,15 @@ bool NextGenLocalStorageEnabled() {
     return !!gNextGenLocalStorageEnabled;
   }
 
+  return CachedNextGenLocalStorageEnabled();
+}
+
+void RecvInitNextGenLocalStorageEnabled(const bool aEnabled) {
+  MOZ_ASSERT(!XRE_IsParentProcess());
   MOZ_ASSERT(NS_IsMainThread());
+  MOZ_ASSERT(gNextGenLocalStorageEnabled == -1);
 
-  if (gNextGenLocalStorageEnabled == -1) {
-    bool enabled = Preferences::GetBool("dom.storage.next_gen", false);
-    gNextGenLocalStorageEnabled = enabled ? 1 : 0;
-  }
-
-  return !!gNextGenLocalStorageEnabled;
+  gNextGenLocalStorageEnabled = aEnabled ? 1 : 0;
 }
 
 bool CachedNextGenLocalStorageEnabled() {

dom/localstorage/LocalStorageCommon.h

@@ -230,14 +230,23 @@ void MaybeEnableNextGenLocalStorage();
 
 /**
  * A check of LSNG being enabled, the value is latched once initialized so
- * changing the preference during runtime has no effect.
- * May be called on any thread in the parent process, but you should call
+ * changing the preference during runtime has no effect. May be called on any
+ * thread in the parent process, but you should call
  * CachedNextGenLocalStorageEnabled if you know that NextGenLocalStorageEnabled
- * was already called because it is faster.
- * May be called on the main thread only in a content process.
+ * was already called because it is faster. May be called on any thread in
+ * content processes, but you should call CachedNextGenLocalStorageEnabled
+ * directly if you know you are in a content process because it is slightly
+ * faster.
  */
 bool NextGenLocalStorageEnabled();
 
+/**
+ * Called by ContentChild during content process initialization to initialize
+ * the global variable in the content process with the latched value in the
+ * parent process."
+ */
+void RecvInitNextGenLocalStorageEnabled(const bool aEnabled);
+
 /**
  * Cached any-thread version of NextGenLocalStorageEnabled().
  */