[REFACTOR] PKT protocol

- Use `Fprintf` to convert to hex and do padding. Simplifies the code.
- Use `Read()` and `io.ReadFull` instead of `ReadByte()`. Should improve
performance and allows for cleaner code.
- s/pktLineTypeUnknow/pktLineTypeUnknown.
- Disallow empty Pkt line per the specification.
- Disallow too large Pkt line per the specification.
- Add unit tests.

cmd/hook.go

@@ -583,7 +583,7 @@ Forgejo or set your environment appropriately.`, "")
 
 	for {
 		// note: pktLineTypeUnknow means pktLineTypeFlush and pktLineTypeData all allowed
-		rs, err = readPktLine(ctx, reader, pktLineTypeUnknow)
+		rs, err = readPktLine(ctx, reader, pktLineTypeUnknown)
 		if err != nil {
 			return err
 		}
@@ -604,7 +604,7 @@ Forgejo or set your environment appropriately.`, "")
 
 	if hasPushOptions {
 		for {
-			rs, err = readPktLine(ctx, reader, pktLineTypeUnknow)
+			rs, err = readPktLine(ctx, reader, pktLineTypeUnknown)
 			if err != nil {
 				return err
 			}
@@ -699,8 +699,8 @@ Forgejo or set your environment appropriately.`, "")
 type pktLineType int64
 
 const (
-	// UnKnow type
-	pktLineTypeUnknow pktLineType = 0
+	// Unknown type
+	pktLineTypeUnknown pktLineType = 0
 	// flush-pkt "0000"
 	pktLineTypeFlush pktLineType = iota
 	// data line
@@ -714,22 +714,16 @@ type gitPktLine struct {
 	Data   []byte
 }
 
+// Reads an Pkt-Line from `in`. If requestType is not unknown, it will a
 func readPktLine(ctx context.Context, in *bufio.Reader, requestType pktLineType) (*gitPktLine, error) {
-	var (
-		err error
-		r   *gitPktLine
-	)
-
-	// read prefix
+	// Read length prefix
 	lengthBytes := make([]byte, 4)
-	for i := 0; i < 4; i++ {
-		lengthBytes[i], err = in.ReadByte()
-		if err != nil {
-			return nil, fail(ctx, "Protocol: stdin error", "Pkt-Line: read stdin failed : %v", err)
-		}
+	if n, err := in.Read(lengthBytes); n != 4 || err != nil {
+		return nil, fail(ctx, "Protocol: stdin error", "Pkt-Line: read stdin failed : %v", err)
 	}
 
-	r = new(gitPktLine)
+	var err error
+	r := &gitPktLine{}
 	r.Length, err = strconv.ParseUint(string(lengthBytes), 16, 32)
 	if err != nil {
 		return nil, fail(ctx, "Protocol: format parse error", "Pkt-Line format is wrong :%v", err)
@@ -748,11 +742,8 @@ func readPktLine(ctx context.Context, in *bufio.Reader, requestType pktLineType)
 	}
 
 	r.Data = make([]byte, r.Length-4)
-	for i := range r.Data {
-		r.Data[i], err = in.ReadByte()
-		if err != nil {
-			return nil, fail(ctx, "Protocol: data error", "Pkt-Line: read stdin failed : %v", err)
-		}
+	if n, err := io.ReadFull(in, r.Data); uint64(n) != r.Length-4 || err != nil {
+		return nil, fail(ctx, "Protocol: stdin error", "Pkt-Line: read stdin failed : %v", err)
 	}
 
 	r.Type = pktLineTypeData
@@ -768,20 +759,23 @@ func writeFlushPktLine(ctx context.Context, out io.Writer) error {
 	return nil
 }
 
+// Write an Pkt-Line based on `data` to `out` according to the specifcation.
+// https://git-scm.com/docs/protocol-common
 func writeDataPktLine(ctx context.Context, out io.Writer, data []byte) error {
-	hexchar := []byte("0123456789abcdef")
-	hex := func(n uint64) byte {
-		return hexchar[(n)&15]
+	// Implementations SHOULD NOT send an empty pkt-line ("0004").
+	if len(data) == 0 {
+		return fail(ctx, "Protocol: write error", "Not allowed to write empty Pkt-Line")
 	}
 
 	length := uint64(len(data) + 4)
-	tmp := make([]byte, 4)
-	tmp[0] = hex(length >> 12)
-	tmp[1] = hex(length >> 8)
-	tmp[2] = hex(length >> 4)
-	tmp[3] = hex(length)
 
-	lr, err := out.Write(tmp)
+	// The maximum length of a pkt-line’s data component is 65516 bytes.
+	// Implementations MUST NOT send pkt-line whose length exceeds 65520 (65516 bytes of payload + 4 bytes of length data).
+	if length > 65520 {
+		return fail(ctx, "Protocol: write error", "Pkt-Line exceeds maximum of 65520 bytes")
+	}
+
+	lr, err := fmt.Fprintf(out, "%04x", length)
 	if err != nil || lr != 4 {
 		return fail(ctx, "Protocol: write error", "Pkt-Line response failed: %v", err)
 	}