error on empty securityType and more efficient locks

internal/open_figi.go

@@ -18,7 +18,10 @@ type OpenFIGI struct {
 	client         *http.Client
 	mappingLimiter *rate.Limiter
 
-	mu                sync.RWMutex
+	mu sync.RWMutex
+	// TODO: there's no eviction policy at the moment as this is only used by short-lived application
+	// which processes a relatively small amount of records. We need to consider using an external
+	// cache lib (like golang-lru or go-cache) if this becomes a problem or implement this ourselves.
 	securityTypeCache map[string]string
 }
 
@@ -39,6 +42,16 @@ func (of *OpenFIGI) SecurityTypeByISIN(ctx context.Context, isin string) (string
 	}
 	of.mu.RUnlock()
 
+	of.mu.Lock()
+	defer of.mu.Unlock()
+
+	// we check again because there could be more than one concurrent cache miss and we want only one
+	// of them to result in an actual request. When the first one releases the lock the following
+	// reads will hit the cache.
+	if secType, ok := of.securityTypeCache[isin]; ok {
+		return secType, nil
+	}
+
 	if len(isin) != 12 || countries.ByName(isin[:2]) == countries.Unknown {
 		return "", fmt.Errorf("invalid ISIN: %s", isin)
 	}
@@ -90,10 +103,11 @@ func (of *OpenFIGI) SecurityTypeByISIN(ctx context.Context, isin string) (string
 	// It is not possible that an isin is assign to diferent security types, therefore we can assume
 	// all entries have the same securityType value.
 	secType := resBody[0].Data[0].SecurityType
+	if secType == "" {
+		return "", fmt.Errorf("empty security type returned for ISIN: %s", isin)
+	}
 
-	of.mu.Lock()
 	of.securityTypeCache[isin] = secType
-	of.mu.Unlock()
 
 	return secType, nil
 }