Aquire semaphore when network will be accessed (#25)

* Fix: only acquire semaphore if going to fetch

* narrow lock time to not include network request

cache.go

@@ -144,20 +144,23 @@ func (c *Cache) getOrFetch(ctx context.Context, u string, forceRefresh bool) (in
 	}
 	c.mu.RUnlock()
 
-	// Only one goroutine may enter this section.
-	e.acquireSem()
-
 	// has this entry been fetched? (but ignore and do a fetch
 	// if forceRefresh is true)
 	if forceRefresh || !e.hasBeenFetched() {
-		if err := c.queue.fetchAndStore(ctx, e); err != nil {
-			e.releaseSem()
-			return nil, fmt.Errorf(`failed to fetch %q: %w`, u, err)
+		// Only one goroutine may enter this section.
+		// redundant checks allow cached gets avoid the semaphore,
+		// which allows them to return cached data immediately
+		// if the upstream server is slow or not responding
+		e.acquireSem()
+		if forceRefresh || !e.hasBeenFetched() {
+			if err := c.queue.fetchAndStore(ctx, e); err != nil {
+				e.releaseSem()
+				return nil, fmt.Errorf(`failed to fetch %q: %w`, u, err)
+			}
 		}
+		e.releaseSem()
 	}
 
-	e.releaseSem()
-
 	e.mu.RLock()
 	data := e.data
 	e.mu.RUnlock()

queue.go

@@ -318,12 +318,12 @@ func (q *queue) refreshLoop(ctx context.Context, errSink ErrSink) {
 }
 
 func (q *queue) fetchAndStore(ctx context.Context, e *entry) error {
-	e.mu.Lock()
-	defer e.mu.Unlock()
-
+	now := time.Now()
 	// synchronously go fetch
-	e.lastFetch = time.Now()
 	res, err := q.fetch.fetch(ctx, e.request)
+	e.mu.Lock()
+	defer e.mu.Unlock()
+	e.lastFetch = now
 	if err != nil {
 		// Even if the request failed, we need to queue the next fetch
 		q.enqueueNextFetch(nil, e)