internal/pipewire: implement string type

This is still NUL terminated strings, and an extra NUL character on an 8-byte string does cause an extra 7 bytes of padding.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/pipewire/pod.go

@@ -183,6 +183,14 @@ func (e *TrailingGarbageError) Error() string {
 	return "data has extra values starting with type " + strconv.Itoa(int(binary.NativeEndian.Uint32(e.Data[4:])))
 }
 
+// A StringTerminationError describes an incorrectly terminated string
+// encountered during [Unmarshal].
+type StringTerminationError struct{ Value byte }
+
+func (e StringTerminationError) Error() string {
+	return "got byte " + strconv.Itoa(int(e.Value)) + " instead of NUL"
+}
+
 // unmarshalValue implements [Unmarshal] on [reflect.Value].
 func unmarshalValue(data []byte, v reflect.Value, sizeP *Word) error {
 	switch v.Kind() {
@@ -236,6 +244,25 @@ func unmarshalValue(data []byte, v reflect.Value, sizeP *Word) error {
 			return unmarshalValue(data, v.Elem(), sizeP)
 		}
 
+	case reflect.String:
+		if err := unmarshalCheckTypeBounds(&data, SPA_TYPE_String, sizeP); err != nil {
+			return err
+		}
+
+		// string size, one extra NUL byte
+		size := int(*sizeP)
+		if len(data) < size {
+			return io.ErrUnexpectedEOF
+		}
+
+		// the serialised strings still include NUL termination
+		if data[size-1] != 0 {
+			return StringTerminationError{data[size-1]}
+		}
+
+		v.SetString(string(data[:size-1]))
+		return nil
+
 	default:
 		return &UnsupportedTypeError{v.Type()}
 	}

internal/pipewire/pod_test.go

@@ -2,6 +2,7 @@ package pipewire_test
 
 import (
 	"encoding"
+	"encoding/json"
 	"reflect"
 	"testing"
 )
@@ -37,7 +38,7 @@ func (testCases encodingTestCases[V, S]) run(t *testing.T) {
 					t.Fatalf("UnmarshalBinary: error = %v", err)
 				}
 				if !reflect.DeepEqual(&value, &tc.value) {
-					t.Fatalf("UnmarshalBinary: %#v, want %#v", value, tc.value)
+					t.Fatalf("UnmarshalBinary:\n%s\nwant\n%s", mustMarshalJSON(value), mustMarshalJSON(tc.value))
 				}
 			})
 
@@ -53,3 +54,12 @@ func (testCases encodingTestCases[V, S]) run(t *testing.T) {
 		})
 	}
 }
+
+// mustMarshalJSON calls [json.Marshal] and returns the result.
+func mustMarshalJSON(v any) string {
+	if data, err := json.Marshal(v); err != nil {
+		panic(err)
+	} else {
+		return string(data)
+	}
+}