fix: invalid scoped-sync responses for empty flags (#1352)

Fixes an issue where invalid flag payloads were returned on sync
requests with scopes if the flag set was empty.

Below is an example of the bug.

```shell
$ grpcurl -import-path /home/todd/temp -proto sync.proto -plaintext localhost:8015 flagd.sync.v1.FlagSyncService/FetchAllFlags
{
  "flagConfiguration": "{\"flags\":{}}"
}
$ grpcurl -import-path /home/todd/temp -proto sync.proto -plaintext -d '{"selector":"../config/samples/example_flags.flagd.json"}' localhost:8015 flagd.sync.v1.FlagSyncService/FetchAllFlags 
{}
```

---------

Signed-off-by: Todd Baert <todd.baert@dynatrace.com>

flagd/pkg/service/flag-sync/sync-multiplexer.go

@@ -12,6 +12,11 @@ import (
 	"github.com/open-feature/flagd/core/pkg/store"
 )
 
+//nolint:errchkjson
+var emptyConfigBytes, _ = json.Marshal(map[string]map[string]string{
+	"flags": {},
+})
+
 // Multiplexer abstract subscription handling and storage processing.
 // Flag configurations will be lazy loaded using reFill logic upon the calls to publish.
 type Multiplexer struct {
@@ -162,6 +167,10 @@ func (r *Multiplexer) SourcesAsMetadata() string {
 // reFill local configuration values
 func (r *Multiplexer) reFill() error {
 	clear(r.selectorFlags)
+	// start all sources with empty config
+	for _, source := range r.sources {
+		r.selectorFlags[source] = string(emptyConfigBytes)
+	}
 
 	all, err := r.store.GetAll(context.Background())
 	if err != nil {
@@ -170,7 +179,7 @@ func (r *Multiplexer) reFill() error {
 
 	bytes, err := json.Marshal(map[string]interface{}{"flags": all})
 	if err != nil {
-		return fmt.Errorf("error from marshallin: %w", err)
+		return fmt.Errorf("error marshalling: %w", err)
 	}
 
 	r.allFlags = string(bytes)
@@ -188,6 +197,7 @@ func (r *Multiplexer) reFill() error {
 		}
 	}
 
+	// for all flags, sort them into their correct selector
 	for source, flags := range collector {
 		bytes, err := json.Marshal(map[string]interface{}{"flags": flags})
 		if err != nil {

flagd/pkg/service/flag-sync/sync-multiplexer_test.go

@@ -6,8 +6,12 @@ import (
 	"strings"
 	"testing"
 	"time"
+
+	"github.com/stretchr/testify/assert"
 )
 
+const emptyConfigString = "{\"flags\":{}}"
+
 func TestRegistration(t *testing.T) {
 	// given
 	mux, err := NewMux(getSimpleFlagStore())
@@ -17,11 +21,12 @@ func TestRegistration(t *testing.T) {
 	}
 
 	tests := []struct {
-		testName    string
-		id          interface{}
-		source      string
-		connection  chan payload
-		expectError bool
+		testName            string
+		id                  interface{}
+		source              string
+		flagStringValidator func(flagString string, testSource string, testName string)
+		connection          chan payload
+		expectError         bool
 	}{
 		{
 			testName:   "subscribe to all flags",
@@ -29,21 +34,38 @@ func TestRegistration(t *testing.T) {
 			connection: make(chan payload, 1),
 		},
 		{
-			testName:   "subscribe to source A",
-			id:         context.Background(),
-			source:     "A",
+			testName: "subscribe to source A",
+			id:       context.Background(),
+			source:   "A",
+			flagStringValidator: func(flagString string, testSource string, testName string) {
+				assert.Contains(t, flagString, fmt.Sprintf("\"source\":\"%s\"", testSource))
+			},
 			connection: make(chan payload, 1),
 		},
 		{
-			testName:   "subscribe to source B",
-			id:         context.Background(),
-			source:     "B",
+			testName: "subscribe to source B",
+			id:       context.Background(),
+			source:   "B",
+			flagStringValidator: func(flagString string, testSource string, testName string) {
+				assert.Contains(t, flagString, fmt.Sprintf("\"source\":\"%s\"", testSource))
+			},
 			connection: make(chan payload, 1),
 		},
+
+		{
+			testName:   "subscribe to empty",
+			id:         context.Background(),
+			source:     "C",
+			connection: make(chan payload, 1),
+			flagStringValidator: func(flagString string, testSource string, testName string) {
+				assert.Equal(t, flagString, emptyConfigString)
+			},
+			expectError: false,
+		},
 		{
 			testName:    "subscribe to non-existing",
 			id:          context.Background(),
-			source:      "C",
+			source:      "D",
 			connection:  make(chan payload, 1),
 			expectError: true,
 		},
@@ -75,13 +97,8 @@ func TestRegistration(t *testing.T) {
 				break
 			}
 
-			if initSync.flags == "" {
-				t.Fatal("expected non empty flag payload, but got empty")
-			}
-
-			// validate source of flag
-			if test.source != "" && !strings.Contains(initSync.flags, fmt.Sprintf("\"source\":\"%s\"", test.source)) {
-				t.Fatal("expected initial flag response to contain flags from source, but failed to find source")
+			if test.flagStringValidator != nil {
+				test.flagStringValidator(initSync.flags, test.source, test.testName)
 			}
 		})
 	}
@@ -173,4 +190,13 @@ func TestGetAllFlags(t *testing.T) {
 		t.Fatal("expected flags to be scoped")
 		return
 	}
+
+	// when - get all for a flagless-scope
+	flags, err = mux.GetAllFlags("C")
+	if err != nil {
+		t.Fatal("error when retrieving all flags")
+		return
+	}
+
+	assert.Equal(t, flags, emptyConfigString)
 }

flagd/pkg/service/flag-sync/sync_service_test.go

@@ -128,7 +128,7 @@ func TestSyncServiceEndToEnd(t *testing.T) {
 		t.Fatal("expected sources entry in the metadata, but got nil")
 	}
 
-	if asMap["sources"] != "A,B" {
+	if asMap["sources"] != "A,B,C" {
 		t.Fatal("incorrect sources entry in metadata")
 	}
 

flagd/pkg/service/flag-sync/util_test.go

@@ -5,7 +5,7 @@ import (
 	"github.com/open-feature/flagd/core/pkg/store"
 )
 
-// getSimpleFlagStore returns a flag store pre-filled with flags from sources A & B
+// getSimpleFlagStore returns a flag store pre-filled with flags from sources A & B & C, which C empty
 func getSimpleFlagStore() (*store.Flags, []string) {
 	variants := map[string]any{
 		"true":  true,
@@ -28,5 +28,5 @@ func getSimpleFlagStore() (*store.Flags, []string) {
 		Source:         "B",
 	})
 
-	return flagStore, []string{"A", "B"}
+	return flagStore, []string{"A", "B", "C"}
 }