Revalidate series existance after lock switch

head.go

@@ -297,32 +297,38 @@ func (h *headBlock) appendBatch(samples []hashedSample) (int, error) {
 	// After the samples were successfully written to the WAL, there may
 	// be no further failures.
 	if len(newSeries) > 0 {
-		// TODO(fabxc): re-check if we actually have to create a new series
-		// after acquiring the write lock.
-		// If concurrent appenders attempt to create the same series, there's
-		// a semantical race between switching locks.
+		newLabels = make([]labels.Labels, 0, len(newSeries))
+
 		h.mtx.RUnlock()
 		h.mtx.Lock()
 
 		base := len(h.series)
+		i := 0
 
 		for hash, ser := range newSeries {
-			h.create(hash, ser[0].labels)
+			lset := ser[0].labels
+			// We switched locks and have to re-validate that the series were not
+			// created by another goroutine in the meantime.
+			ms := h.get(hash, lset)
+			if ms != nil {
+				for _, s := range ser {
+					s.ref = ms.ref
 				}
+				continue
+			}
+			// Series is still new.
+			newLabels = append(newLabels, lset)
 
-		h.mtx.Unlock()
-		h.mtx.RLock()
-
-		newLabels = make([]labels.Labels, 0, len(newSeries))
-
-		i := 0
-		for _, ser := range newSeries {
-			newLabels = append(newLabels, ser[0].labels)
+			h.create(hash, lset)
+			// Set sample references to the series we just created.
 			for _, s := range ser {
 				s.ref = uint32(base + i)
 			}
 			i++
 		}
+
+		h.mtx.Unlock()
+		h.mtx.RLock()
 	}
 	// Write all new series and samples to the WAL and add it to the
 	// in-mem database on success.