yaml: fix surgical Patch() for the empty-streams config round-trip

internal/app/config.go

@@ -34,6 +34,10 @@ func PatchConfig(path []string, value any) error {
 
 	b, err := yaml.Patch(b, path, value)
 	if err != nil {
+		// deleting a path that doesn't exist is a no-op, not an error
+		if value == nil && errors.Is(err, yaml.ErrPathNotExist) {
+			return nil
+		}
 		return err
 	}
 

pkg/yaml/yaml.go

@@ -92,6 +92,22 @@ func patch(in []byte, path []string, value any) ([]byte, error) {
 	} else {
 		// parent value is nil (use parent indent + 2)
 		paste = addIndent(paste, pKey.Column+1)
+
+		// An explicit null scalar on the key's line (e.g. "streams: null", as emitted by
+		// yaml.Marshal of an empty map) can't hold a child: pasting one after it yields
+		// "streams: null\n  x: y", which fails to parse ("mapping values are not allowed
+		// in this context"). Strip the scalar so the key becomes a bare mapping parent,
+		// then append the child. A bare/implicit-null value ("streams:") carries no token
+		// (pVal.Value == "") and keeps the original behaviour below.
+		if value != nil && pVal.Value != "" && pVal.Line == pKey.Line {
+			vOff := lineOffset(in, pVal.Line) + pVal.Column - 1
+			colon := bytes.LastIndexByte(in[:vOff], ':')
+			eol := lineOffset(in, pKey.Line+1)
+			if eol < 0 {
+				eol = len(in)
+			}
+			return join(in[:colon+1], paste, in[eol:]), nil
+		}
 	}
 
 	_, i1 := nodeBounds(in, pKey)
@@ -137,9 +153,11 @@ func nodeBounds(in []byte, node *yaml.Node) (offset0, offset1 int) {
 	return
 }
 
+var ErrPathNotExist = errors.New("yaml: path not exist")
+
 func addToEnd(in []byte, path []string, value any) ([]byte, error) {
 	if len(path) != 2 || value == nil {
-		return nil, errors.New("yaml: path not exist")
+		return nil, ErrPathNotExist
 	}
 
 	v := map[string]map[string]any{

pkg/yaml/yaml_test.go

@@ -98,6 +98,22 @@ func TestPatch(t *testing.T) {
 			value:  []string{"val1"},
 			expect: "streams:\n  camera1: url1\nhomekit:\n  camera1:\n    name: dummy\n    pairings:\n      - val1\n",
 		},
+		{
+			// yaml.Marshal of an empty streams map emits the explicit token "streams: null";
+			// adding the first stream back must not produce "streams: null\n  camera1: ...".
+			name:   "explicit null scalar parent",
+			src:    "streams: null\nwebrtc:\n  listen: \":8555\"\n",
+			path:   []string{"streams", "camera1"},
+			value:  "val1",
+			expect: "streams:\n  camera1: val1\nwebrtc:\n  listen: \":8555\"\n",
+		},
+		{
+			name:   "explicit null scalar only",
+			src:    "streams: null\n",
+			path:   []string{"streams", "camera1"},
+			value:  "val1",
+			expect: "streams:\n  camera1: val1\n",
+		},
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
@@ -107,3 +123,11 @@ func TestPatch(t *testing.T) {
 		})
 	}
 }
+
+func TestPatchDeleteMissing(t *testing.T) {
+	// Deleting a path whose parent does not exist routes through addToEnd and
+	// must return the ErrPathNotExist sentinel so callers can treat a missing
+	// delete target as a no-op via errors.Is.
+	_, err := Patch([]byte("streams:\n  camera1: url1\n"), []string{"homekit", "camera1"}, nil)
+	require.ErrorIs(t, err, ErrPathNotExist)
+}