This is not supported. Depending on what's thrown on the ErrorResult (e.g. a JS exception object!) touching it on the worker thread can blow up in all sorts of interesting ways. ErrorResult usage should be thread-local Looks like Gene is not around anymore, so let's try the reviewers from bug 949325 instead....

Comment on attachment 8688591 [details] [diff] [review] ds.patch >+ // This method cannot fail. Then why is it marked Throws in the IDL? Looking at this, it's implemented in JS, so it totally can fail for whatever random reason it wants to. I would much prefer this to look like this: if (NS_WARN_IF(rv.Failed()) { rv.SuppressException(); } just in case it _does_ decide to fail (e.g. because of OOM or out of stack space, or any of the other not-under-your-control reasons JS can throw). This applies to both DataStoreGetStringRunnable and DataStoreGetReadOnlyRunnable. In DataStoreGetRunnable::MainThreadRun, you need to rv.SuppressException(). In DataStorePutRunnable::MainThreadRun, you need to rv.SuppressException(), in two places. In DataStoreAddRunnable::MainThreadRun, same thing, two places. In DataStoreRemoveRunnable::MainThreadRun, need to rv.SuppressException(). Same in DataStoreClearRunnable::MainThreadRun. Same in DataStoreSyncStoreRunnable::MainThreadRun. Same in DataStoreGetLengthRunnable::MainThreadRun. I'm not sure about IsFailed() vs Failed(). Either way, I guess. r=me with the above fixed.