Fix race condition

utils/hasher/hasher.go

@@ -3,6 +3,7 @@ package hasher
 import (
 	"hash/maphash"
 	"strconv"
+	"sync"
 
 	"github.com/navidrome/navidrome/utils/random"
 )
@@ -23,6 +24,7 @@ func HashFunc() func(id, str string) uint64 {
 
 type Hasher struct {
 	seeds    map[string]string
+	mutex    sync.RWMutex
 	hashSeed maphash.Seed
 }
 
@@ -35,6 +37,8 @@ func NewHasher() *Hasher {
 
 // SetSeed sets a seed for the given id
 func (h *Hasher) SetSeed(id string, seed string) {
+	h.mutex.Lock()
+	defer h.mutex.Unlock()
 	h.seeds[id] = seed
 }
 
@@ -45,16 +49,17 @@ func (h *Hasher) Reseed(id string) {
 
 func (h *Hasher) reseed(id string) string {
 	seed := strconv.FormatUint(random.Uint64(), 36)
-	h.seeds[id] = seed
+	h.SetSeed(id, seed)
 	return seed
 }
 
 // HashFunc returns a function that hashes a string using the seed for the given id
 func (h *Hasher) HashFunc() func(id, str string) uint64 {
 	return func(id, str string) uint64 {
-		var seed string
-		var ok bool
-		if seed, ok = h.seeds[id]; !ok {
+		h.mutex.RLock()
+		seed, ok := h.seeds[id]
+		h.mutex.RUnlock()
+		if !ok {
 			seed = h.reseed(id)
 		}
 

utils/hasher/hasher_test.go

@@ -1,6 +1,7 @@
 package hasher_test
 
 import (
+	"strconv"
 	"testing"
 
 	"github.com/navidrome/navidrome/utils/hasher"
@@ -54,4 +55,14 @@ var _ = Describe("HashFunc", func() {
 		hasher.SetSeed(id, "original_seed")
 		Expect(sum).To(Equal(hashFunc(id, input)))
 	})
+
+	It("does not cause race conditions", func() {
+		for i := 0; i < 1000; i++ {
+			go func() {
+				hashFunc := hasher.HashFunc()
+				sum := hashFunc(strconv.Itoa(i), input)
+				Expect(sum).ToNot(BeZero())
+			}()
+		}
+	})
 })