Fix StringSliceCSV treating explicit empty string as a one-element list

CHANGELOG.md

@@ -93,6 +93,7 @@
 * [BUGFIX] Memberlist: Add `-memberlist.packet-read-timeout`, `-memberlist.max-packet-size`, and `-memberlist.max-concurrent-connections` flags to bound inbound gossip TCP connections, preventing slow-read, OOM, and connection-flood attacks on the gossip port. #7518
 * [BUGFIX] Distributor: Add `WrappedHistogram` with configurable size limit (`-validation.max-native-histogram-size-bytes`, default 16 KB) to cap native histogram protobuf size before unmarshalling, preventing memory amplification attacks via packed varint deltas. #7570
 * [BUGFIX] Distributor: Fix a panic (`slice bounds out of range`) in the stream push path when the context deadline expires while the worker goroutine is still marshalling a `WriteRequest`. #7541
+* [BUGFIX] Config: Fix CSV-list flags/YAML fields (e.g. `-compactor.enabled-tenants`, `-compactor.disabled-tenants`) treating an explicitly empty string (`""`) as a one-element list containing an empty tenant name instead of an empty list. This caused every tenant to be skipped when `enabled_tenants: ""` was set in config, since only the (nonexistent) empty-string tenant matched the allow-list. #6581
 
 ## 1.21.0 2026-04-24
 

pkg/util/flagext/stringslicecsv.go

@@ -13,6 +13,10 @@ func (v StringSliceCSV) String() string {
 
 // Set implements flag.Value
 func (v *StringSliceCSV) Set(s string) error {
+	if s == "" {
+		*v = nil
+		return nil
+	}
 	*v = strings.Split(s, ",")
 	return nil
 }

pkg/util/flagext/stringslicecsv_test.go

@@ -32,3 +32,21 @@ func Test_StringSliceCSV(t *testing.T) {
 	assert.Nil(t, err)
 	assert.Equal(t, testStruct, testStruct2)
 }
+
+func Test_StringSliceCSV_Empty(t *testing.T) {
+	var v StringSliceCSV
+	assert.Nil(t, v.Set(""))
+	assert.Equal(t, []string(nil), []string(v))
+	assert.Len(t, v, 0)
+}
+
+func Test_StringSliceCSV_UnmarshalYAML_Empty(t *testing.T) {
+	type TestStruct struct {
+		CSV StringSliceCSV `yaml:"csv"`
+	}
+
+	var testStruct TestStruct
+	err := yaml.Unmarshal([]byte(`csv: ""`), &testStruct)
+	assert.Nil(t, err)
+	assert.Len(t, testStruct.CSV, 0)
+}