feat: Add MeasurementNames method to MeasurementFieldSet (#23173)

tsdb/shard.go

@@ -1939,6 +1939,20 @@ func (fs *MeasurementFieldSet) Bytes() int {
 	return b
 }
 
+// MeasurementNames returns the names of all of the measurements in the field set in
+// lexographical order.
+func (fs *MeasurementFieldSet) MeasurementNames() []string {
+	fs.mu.RLock()
+	defer fs.mu.RUnlock()
+
+	names := make([]string, 0, len(fs.fields))
+	for name := range fs.fields {
+		names = append(names, name)
+	}
+	sort.Strings(names)
+	return names
+}
+
 // Fields returns fields for a measurement by name.
 func (fs *MeasurementFieldSet) Fields(name []byte) *MeasurementFields {
 	fs.mu.RLock()

tsdb/shard_test.go

@@ -28,6 +28,7 @@ import (
 	_ "github.com/influxdata/influxdb/v2/tsdb/engine"
 	_ "github.com/influxdata/influxdb/v2/tsdb/index"
 	"github.com/influxdata/influxql"
+	"github.com/stretchr/testify/assert"
 )
 
 func TestShardWriteAndIndex(t *testing.T) {
@@ -1705,6 +1706,24 @@ func TestMeasurementFieldSet_ConcurrentSave(t *testing.T) {
 	}
 }
 
+func TestMeasurementFieldSet_MeasurementNames(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "fields.idx")
+	mf, err := tsdb.NewMeasurementFieldSet(path)
+	if err != nil {
+		t.Fatalf("NewMeasurementFieldSet error: %v", err)
+	}
+	defer mf.Close()
+
+	mf.CreateFieldsIfNotExists([]byte("cpu"))
+	mf.CreateFieldsIfNotExists([]byte("memory"))
+	mf.CreateFieldsIfNotExists([]byte("disk_usage"))
+
+	exp := []string{"cpu", "disk_usage", "memory"}
+	got := mf.MeasurementNames()
+	assert.Equal(t, exp, got)
+}
+
 func testFieldMaker(t *testing.T, wg *sync.WaitGroup, mf *tsdb.MeasurementFieldSet, measurement string, fieldNames []string) {
 	defer wg.Done()
 	fields := mf.CreateFieldsIfNotExists([]byte(measurement))