Bug fix: Avoid race condition by deduplicating entries in bucket stores user scan (#6863)

* Make scanUser always return unique user IDs to avoid concurrent map write

Signed-off-by: Justin Jung <jungjust@amazon.com>

* Changelog

Signed-off-by: Justin Jung <jungjust@amazon.com>

* Instead of returning error, just deduplicate

Signed-off-by: Justin Jung <jungjust@amazon.com>

* changelog

Signed-off-by: Justin Jung <jungjust@amazon.com>

---------

Signed-off-by: Justin Jung <jungjust@amazon.com>

Author: justinjung04

CHANGELOG.md

@@ -72,6 +72,7 @@
 * [BUGFIX] Ruler: Prevent counting 2xx and 4XX responses as failed writes. #6785
 * [BUGFIX] Ingester: Allow shipper to skip corrupted blocks. #6786
 * [BUGFIX] Compactor: Delete the prefix `blocks_meta` from the metadata fetcher metrics. #6832
+* [BUGFIX] Store Gateway: Avoid race condition by deduplicating entries in bucket stores user scan. #6863
 
 ## 1.19.0 2025-02-27
 

pkg/storegateway/bucket_stores.go

@@ -458,10 +458,25 @@ func (u *BucketStores) scanUsers(ctx context.Context) ([]string, error) {
 	users := make([]string, 0, len(activeUsers)+len(deletingUsers))
 	users = append(users, activeUsers...)
 	users = append(users, deletingUsers...)
+	users = deduplicateUsers(users)
 
 	return users, err
 }
 
+func deduplicateUsers(users []string) []string {
+	seen := make(map[string]struct{}, len(users))
+	var uniqueUsers []string
+
+	for _, user := range users {
+		if _, ok := seen[user]; !ok {
+			seen[user] = struct{}{}
+			uniqueUsers = append(uniqueUsers, user)
+		}
+	}
+
+	return uniqueUsers
+}
+
 func (u *BucketStores) getStore(userID string) *store.BucketStore {
 	u.storesMu.RLock()
 	defer u.storesMu.RUnlock()

pkg/storegateway/bucket_stores_test.go

@@ -454,6 +454,38 @@ func TestBucketStores_syncUsersBlocks(t *testing.T) {
 	}
 }
 
+func TestBucketStores_scanUsers(t *testing.T) {
+	t.Parallel()
+
+	tests := map[string]struct {
+		scanner     *mockScanner
+		expectedRes []string
+	}{
+		"should return unique users only": {
+			scanner: &mockScanner{
+				res: []string{"user-1", "user-2", "user-1"},
+			},
+			expectedRes: []string{"user-1", "user-2"},
+		},
+	}
+
+	for testName, testData := range tests {
+		testData := testData
+		t.Run(testName, func(t *testing.T) {
+			t.Parallel()
+
+			stores := &BucketStores{
+				userScanner: testData.scanner,
+			}
+
+			users, err := stores.scanUsers(context.Background())
+
+			assert.NoError(t, err)
+			assert.ElementsMatch(t, testData.expectedRes, users)
+		})
+	}
+}
+
 func TestBucketStores_Series_ShouldCorrectlyQuerySeriesSpanningMultipleChunks(t *testing.T) {
 	for _, lazyLoadingEnabled := range []bool{true, false} {
 		t.Run(fmt.Sprintf("lazy loading enabled = %v", lazyLoadingEnabled), func(t *testing.T) {
@@ -996,3 +1028,11 @@ func (f *failFirstGetBucket) Get(ctx context.Context, name string) (io.ReadClose
 
 	return f.Bucket.Get(ctx, name)
 }
+
+type mockScanner struct {
+	res []string
+}
+
+func (m *mockScanner) ScanUsers(_ context.Context) (active, deleting, deleted []string, err error) {
+	return m.res, nil, nil, nil
+}