Harden compiled/selfie format change detection at launch

Related issue:
- https://github.com/uBlockOrigin/uBlock-issues/issues/759

This commit adds code to rely less on the state of the
cache storage to decide whether filter lists should be
re-compiled or whether the selfie is currently valid
at launch time when a change in compiled/selfie format
is detected.

src/js/background.js

@@ -145,6 +145,16 @@ const µBlock = (( ) => { // jshint ignore:line
             selfieMagic: 23,    // Increase when selfie format changes
         },
 
+        // https://github.com/uBlockOrigin/uBlock-issues/issues/759#issuecomment-546654501
+        //   The assumption is that cache storage state reflects whether
+        //   compiled or selfie assets are available or not. The properties
+        //   below is to no longer rely on this assumption -- though it's still
+        //   not clear how the assumption could be wrong, and it's still not
+        //   clear whether relying on those properties will really solve the
+        //   issue. It's just an attempt at hardening.
+        compiledFormatChanged: false,
+        selfieIsInvalid: false,
+
         restoreBackupSettings: {
             lastRestoreFile: '',
             lastRestoreTime: 0,

src/js/start.js

@@ -173,15 +173,15 @@ const onUserSettingsReady = function(fetched) {
 // Housekeeping, as per system setting changes
 
 const onSystemSettingsReady = function(fetched) {
-    let mustSaveSystemSettings = false;
     if ( fetched.compiledMagic !== µb.systemSettings.compiledMagic ) {
         µb.assets.remove(/^compiled\//);
-        mustSaveSystemSettings = true;
+        µb.compiledFormatChanged = true;
+        µb.selfieIsInvalid = true;
     }
     if ( fetched.selfieMagic !== µb.systemSettings.selfieMagic ) {
-        mustSaveSystemSettings = true;
+        µb.selfieIsInvalid = true;
     }
-    if ( mustSaveSystemSettings ) {
+    if ( µb.selfieIsInvalid ) {
         fetched.selfie = null;
         µb.selfieManager.destroy();
         vAPI.storage.set(µb.systemSettings);

src/js/storage.js

@@ -638,6 +638,7 @@
 
         this.selfieManager.destroy();
         this.lz4Codec.relinquish();
+        this.compiledFormatChanged = false;
 
         loadingPromise = undefined;
     };
@@ -712,10 +713,12 @@
 µBlock.getCompiledFilterList = async function(assetKey) {
     const compiledPath = 'compiled/' + assetKey;
 
-    let compiledDetails = await this.assets.get(compiledPath);
-    if ( compiledDetails.content !== '' ) {
-        compiledDetails.assetKey = assetKey;
-        return compiledDetails;
+    if ( this.compiledFormatChanged === false ) {
+        let compiledDetails = await this.assets.get(compiledPath);
+        if ( compiledDetails.content !== '' ) {
+            compiledDetails.assetKey = assetKey;
+            return compiledDetails;
+        }
     }
 
     const rawDetails = await this.assets.get(assetKey);
@@ -730,7 +733,7 @@
     // Fetching the raw content may cause the compiled content to be
     // generated somewhere else in uBO, hence we try one last time to
     // fetch the compiled content in case it has become available.
-    compiledDetails = await this.assets.get(compiledPath);
+    let compiledDetails = await this.assets.get(compiledPath);
     if ( compiledDetails.content === '' ) {
         compiledDetails.content = this.compileFilters(
             rawDetails.content,
@@ -1053,6 +1056,7 @@
             ),
         ]);
         µb.lz4Codec.relinquish();
+        µb.selfieIsInvalid = false;
     };
 
     const loadMain = async function() {
@@ -1080,7 +1084,7 @@
     };
 
     const load = async function() {
-        if ( destroyTimer !== undefined ) {
+        if ( µb.selfieIsInvalid ) {
             return false;
         }
         try {
@@ -1108,6 +1112,7 @@
     const destroy = function() {
         µb.cacheStorage.remove('selfie'); // TODO: obsolete, remove eventually.
         µb.assets.remove(/^selfie\//);
+        µb.selfieIsInvalid = true;
         createTimer = vAPI.setTimeout(( ) => {
             createTimer = undefined;
             create();
@@ -1127,6 +1132,7 @@
             },
             1019
         );
+        µb.selfieIsInvalid = true;
     };
 
     return { load, destroy: destroyAsync };