Mind potential race condition when dynamically registering scriptlets

gorhill · gorhill · commit 15e832da8a1d · 2025-04-11T09:35:38.000-04:00
In Firefox, scriptlets are dynamically registered as content scripts
to ensure they execute in a timely manner.

The race condition could lead to scriptlet injection failing at
browser launch time in Firefox when the setting "Suspend network
activity until all filter lists are loaded" had been disabled[1],
even after forcing a page reload. Causing the filter lists to
reload would make the issue go away.

[1] Default is enabled in Firefox and it is strongly advised to NOT
    change this.
diff --git a/src/js/scriptlet-filtering.js b/src/js/scriptlet-filtering.js
@@ -38,6 +38,7 @@ import µb from './background.js';
 /******************************************************************************/
 
 const contentScriptRegisterer = {
+    id: 1,
     hostnameToDetails: new Map(),
     register(hostname, code) {
         if ( browser.contentScripts === undefined ) { return false; }
@@ -47,22 +48,25 @@ const contentScriptRegisterer = {
             if ( code === details.code ) {
                 return details.handle instanceof Promise === false;
             }
-            details.handle.unregister();
+            this.unregisterHandle(details.handle);
             this.hostnameToDetails.delete(hostname);
         }
+        const id = this.id++;
         const promise = browser.contentScripts.register({
             js: [ { code } ],
             allFrames: true,
             matches: [ `*://*.${hostname}/*` ],
             matchAboutBlank: true,
             runAt: 'document_start',
         }).then(handle => {
-            this.hostnameToDetails.set(hostname, { handle, code });
-            return handle;
+            const details = this.hostnameToDetails.get(hostname);
+            if ( details === undefined ) { return; }
+            if ( details.id !== id ) { return; }
+            details.handle = handle;
         }).catch(( ) => {
             this.hostnameToDetails.delete(hostname);
         });
-        this.hostnameToDetails.set(hostname, { handle: promise, code });
+        this.hostnameToDetails.set(hostname, { id, handle: promise, code });
         return false;
     },
     unregister(hostname) {
