fix(plc4go): correct BitBuffer EOF behavior and empty-buffer nil semantics

ReadBits returned io.ErrUnexpectedEOF on exhaustion, but the old bitio
library returned io.EOF. ReadOptionalField only catches io.EOF to detect
absent optional fields, so the wrong sentinel caused parse errors in the
CBUS MMI path (CALData additionalData recursion hit EOF and propagated
instead of being silently absent).

NewWriteBitBuffer(0) initialised buf with make([]byte,0,0), producing a
non-nil empty slice where the old bytes.Buffer returned nil for an empty
buffer. Serialising an empty object now correctly returns nil again.

Author: sruehl

plc4go/spi/utils/BitBuffer.go

@@ -85,7 +85,7 @@ func (b *ReadBitBuffer) ReadBits(n uint8) (uint64, error) {
 		return 0, errors.New("cannot read more than 64 bits at once")
 	}
 	if uint64(n) > b.BitsRemaining() {
-		return 0, io.ErrUnexpectedEOF
+		return 0, io.EOF
 	}
 
 	var result uint64
@@ -134,7 +134,11 @@ type WriteBitBuffer struct {
 
 // NewWriteBitBuffer creates a WriteBitBuffer with the given initial capacity hint.
 func NewWriteBitBuffer(initialCap int) *WriteBitBuffer {
-	return &WriteBitBuffer{buf: make([]byte, 0, initialCap)}
+	var buf []byte
+	if initialCap > 0 {
+		buf = make([]byte, 0, initialCap)
+	}
+	return &WriteBitBuffer{buf: buf}
 }
 
 // WriteBool writes a single bit.

plc4go/spi/utils/BitBuffer_test.go

@@ -109,7 +109,7 @@ func TestReadBitBuffer_Read(t *testing.T) {
 func TestReadBitBuffer_ReadBits_EOF(t *testing.T) {
 	b := NewReadBitBuffer([]byte{0xFF})
 	_, err := b.ReadBits(9)
-	assert.ErrorIs(t, err, io.ErrUnexpectedEOF)
+	assert.ErrorIs(t, err, io.EOF)
 }
 
 func TestReadBitBuffer_ReadBits_Zero(t *testing.T) {
@@ -139,7 +139,7 @@ func TestReadBitBuffer_EmptyBuffer(t *testing.T) {
 	b := NewReadBitBuffer([]byte{})
 	assert.Equal(t, uint64(0), b.BitsRemaining())
 	_, err := b.ReadBits(1)
-	assert.ErrorIs(t, err, io.ErrUnexpectedEOF)
+	assert.ErrorIs(t, err, io.EOF)
 }
 
 func TestReadBitBuffer_Read_Partial(t *testing.T) {
@@ -240,14 +240,14 @@ func TestWriteBitBuffer_TryWriteByte(t *testing.T) {
 func TestWriteBitBuffer_WriteBits_Zero(t *testing.T) {
 	w := NewWriteBitBuffer(0)
 	require.NoError(t, w.WriteBits(0xFF, 0))
-	assert.Equal(t, []byte{}, w.Bytes())
+	assert.Nil(t, w.Bytes())
 }
 
 func TestWriteBitBuffer_WriteBits_TooMany(t *testing.T) {
 	w := NewWriteBitBuffer(0)
 	err := w.WriteBits(0, 65)
 	require.Error(t, err)
-	assert.Equal(t, []byte{}, w.Bytes(), "buffer must not change on error")
+	assert.Nil(t, w.Bytes(), "buffer must not change on error")
 }
 
 func TestWriteBitBuffer_WriteBits_64(t *testing.T) {

plc4go/spi/utils/WriteBufferByteBased_test.go

@@ -166,7 +166,7 @@ func Test_byteWriteBuffer_GetBytes(t *testing.T) {
 			fields: fields{
 				bits: NewWriteBitBuffer(0),
 			},
-			want: []byte{},
+			want: nil,
 		},
 	}
 	for _, tt := range tests {