Bug 1629825 - Delete Appcache directory when storage pref is turned off r=necko-reviewers,kershaw

This bug was initially about adding a test that we can still clear appcache
data even when the storage pref is off.
However, due to the fact that we only check the pref at startup and then
are unable to access the storage device if disabled, it's best to just
delete the OfflineCache folder shortly after startup.

Differential Revision: https://phabricator.services.mozilla.com/D90395

netwerk/cache/nsCacheService.cpp

@@ -268,6 +268,23 @@ nsresult nsCacheProfilePrefObserver::ReadPrefs(nsIPrefBranch* branch) {
     return NS_ERROR_FAILURE;
   }
 
+  if (!mOfflineStorageCacheEnabled) {
+    // Dispatch cleanup task
+    nsCOMPtr<nsIRunnable> runnable =
+        NS_NewRunnableFunction("Delete OfflineCache", []() {
+          nsCOMPtr<nsIFile> dir;
+          nsCacheService::GetAppCacheDirectory(getter_AddRefs(dir));
+          bool exists = false;
+          if (dir && NS_SUCCEEDED(dir->Exists(&exists)) && exists) {
+            // Delay delete by 1 minute to avoid IO thrash on startup.
+            CACHE_LOG_INFO(
+                ("Queuing Delete of AppCacheDirectory in 60 seconds"));
+            nsDeleteDir::DeleteDir(dir, false, 60000);
+          }
+        });
+    Unused << nsCacheService::DispatchToCacheIOThread(runnable);
+  }
+
   return NS_OK;
 }