yangerd: Return {} instead of 404 if no match

src/yangerd/internal/ipc/server.go

@@ -131,10 +131,15 @@ func (s *Server) handleGet(conn net.Conn, req *Request) {
 
 	data := s.tree.Get(key)
 	if data == nil {
+		// An absent subtree is a normal answer for operational data --
+		// the feature is simply not active (e.g. NTP unconfigured).
+		// Answer ok with an empty object rather than an error, so every
+		// client gets "no data" without special-casing.  Deliberately
+		// NOT {"<key>": {}}: that would make libyang instantiate the
+		// container, which for presence containers is real data.
 		WriteResponse(conn, &Response{
-			Status:  "error",
-			Code:    404,
-			Message: "path not found: " + path,
+			Status: "ok",
+			Data:   json.RawMessage(`{}`),
 		})
 		return
 	}

src/yangerd/internal/ipc/server_test.go

@@ -33,8 +33,13 @@ func TestServerGetNotFound(t *testing.T) {
 	tr := tree.New()
 	resp := serverRoundTrip(t, tr, true, &Request{Method: "get", Path: "/nonexistent"})
 
-	if resp.Status != "error" || resp.Code != 404 {
-		t.Fatalf("expected 404 error, got %+v", resp)
+	// An absent subtree is "no data", not an error: ok + empty object,
+	// so clients (statd, yangerctl) need no special-casing.
+	if resp.Status != "ok" {
+		t.Fatalf("expected ok, got %+v", resp)
+	}
+	if string(resp.Data) != "{}" {
+		t.Fatalf("expected empty object data, got %s", resp.Data)
 	}
 }