encoding: MarshalPacket shall take a buffer hint now

encoding/ssh/filexfer/buffer.go

@@ -29,16 +29,19 @@ func NewBuffer(buf []byte) *Buffer {
 	}
 }
 
-// NewMarshalBuffer creates an initializes a new Buffer ready to start marshaling a Packet into.
+// NewMarshalBuffer creates a new Buffer ready to start marshaling a Packet into.
-// It prepopulates 4 bytes for length, the 1-byte packetType, and the 4-byte requestID.
+// It preallocates enough space for uint32(length), uint8(type), uint32(request-id) and size more bytes.
-// It preallocates enough space for an additional size bytes of data above the prepopulated values.
+func NewMarshalBuffer(size int) *Buffer {
-func NewMarshalBuffer(packetType PacketType, requestID uint32, size int) *Buffer {
+	return NewBuffer(make([]byte, 4+1+4+size))
-	buf := NewBuffer(make([]byte, 4, 4+1+4+size))
+}
 
-	buf.AppendUint8(uint8(packetType))
+// StartPacket resets and initializes the Buffer to be ready to start marshaling a Packet body into.
-	buf.AppendUint32(requestID)
+// It truncates the buffer, reserves space for uint32(length), then appends the packetType and requestID.
+func (b *Buffer) StartPacket(packetType PacketType, requestID uint32) {
+	b.b = append(b.b[:0], make([]byte, 4)...)
 
-	return buf
+	b.AppendUint8(uint8(packetType))
+	b.AppendUint32(requestID)
 }
 
 // Bytes returns a slice of length b.Len() holding the unconsumed bytes in the Buffer.
@@ -65,9 +68,11 @@ func (b *Buffer) Packet(payload []byte) (header, payloadPassThru []byte, err err
 }
 
 // Len returns the number of unconsumed bytes in the Buffer.
-func (b *Buffer) Len() int {
+func (b *Buffer) Len() int { return len(b.b) - b.off }
-	return len(b.b) - b.off
+
-}
+// Cap returns the capacity of the Buffer’s underlying byte slice,
+// that is, the total space allocated for the buffer’s data.
+func (b *Buffer) Cap() int { return cap(b.b) }
 
 // ConsumeUint8 consumes a single byte from the Buffer.
 // If Buffer does not have enough data, it will return ErrShortPacket.

encoding/ssh/filexfer/extended_packets.go

@@ -50,12 +50,15 @@ type ExtendedPacket struct {
 // MarshalPacket returns p as a two-part binary encoding of p.
 //
 // The Data is marshaled into binary, and returned as the payload.
-func (p *ExtendedPacket) MarshalPacket(reqid uint32) (header, payload []byte, err error) {
+func (p *ExtendedPacket) MarshalPacket(reqid uint32, b []byte) (header, payload []byte, err error) {
-	size := 4 + len(p.ExtendedRequest) // string(extended-request)
+	buf := NewBuffer(b)
+	if buf.Cap() < 9 {
+		size := 4 + len(p.ExtendedRequest) // string(extended-request)
+		buf = NewMarshalBuffer(size)
+	}
 
-	b := NewMarshalBuffer(PacketTypeExtended, reqid, size)
+	buf.StartPacket(PacketTypeExtended, reqid)
-
+	buf.AppendString(p.ExtendedRequest)
-	b.AppendString(p.ExtendedRequest)
 
 	if p.Data != nil {
 		payload, err = p.Data.MarshalBinary()
@@ -64,7 +67,7 @@ func (p *ExtendedPacket) MarshalPacket(reqid uint32) (header, payload []byte, er
 		}
 	}
 
-	return b.Packet(payload)
+	return buf.Packet(payload)
 }
 
 // UnmarshalPacketBody unmarshals the packet body from the given Buffer.
@@ -93,8 +96,13 @@ type ExtendedReplyPacket struct {
 // MarshalPacket returns p as a two-part binary encoding of p.
 //
 // The Data is marshaled into binary, and returned as the payload.
-func (p *ExtendedReplyPacket) MarshalPacket(reqid uint32) (header, payload []byte, err error) {
+func (p *ExtendedReplyPacket) MarshalPacket(reqid uint32, b []byte) (header, payload []byte, err error) {
-	b := NewMarshalBuffer(PacketTypeExtendedReply, reqid, 0)
+	buf := NewBuffer(b)
+	if buf.Cap() < 9 {
+		buf = NewMarshalBuffer(0)
+	}
+
+	buf.StartPacket(PacketTypeExtendedReply, reqid)
 
 	if p.Data != nil {
 		payload, err = p.Data.MarshalBinary()
@@ -103,7 +111,7 @@ func (p *ExtendedReplyPacket) MarshalPacket(reqid uint32) (header, payload []byt
 		}
 	}
 
-	return b.Packet(payload)
+	return buf.Packet(payload)
 }
 
 // UnmarshalPacketBody unmarshals the packet body from the given Buffer.

encoding/ssh/filexfer/extended_packets_test.go

@@ -42,7 +42,7 @@ func TestExtendedPacketNoData(t *testing.T) {
 		ExtendedRequest: extendedRequest,
 	}
 
-	data, err := ComposePacket(p.MarshalPacket(id))
+	data, err := ComposePacket(p.MarshalPacket(id, nil))
 	if err != nil {
 		t.Fatal("unexpected error:", err)
 	}
@@ -86,7 +86,7 @@ func TestExtendedPacketTestData(t *testing.T) {
 		},
 	}
 
-	data, err := ComposePacket(p.MarshalPacket(id))
+	data, err := ComposePacket(p.MarshalPacket(id, nil))
 	if err != nil {
 		t.Fatal("unexpected error:", err)
 	}
@@ -153,7 +153,7 @@ func TestExtendedReplyNoData(t *testing.T) {
 
 	p := &ExtendedReplyPacket{}
 
-	data, err := ComposePacket(p.MarshalPacket(id))
+	data, err := ComposePacket(p.MarshalPacket(id, nil))
 	if err != nil {
 		t.Fatal("unexpected error:", err)
 	}
@@ -190,7 +190,7 @@ func TestExtendedReplyPacketTestData(t *testing.T) {
 		},
 	}
 
-	data, err := ComposePacket(p.MarshalPacket(id))
+	data, err := ComposePacket(p.MarshalPacket(id, nil))
 	if err != nil {
 		t.Fatal("unexpected error:", err)
 	}

encoding/ssh/filexfer/filexfer.go

@@ -1,8 +1,23 @@
+// Package filexfer implements the wire encoding for secsh-filexfer as described in https://tools.ietf.org/html/draft-ietf-secsh-filexfer-02
 package filexfer
 
 // Packet defines the behavior of an SFTP packet.
 type Packet interface {
-	MarshalPacket(reqid uint32) (header, payload []byte, err error)
+	// MarshalPacket is the primary intended way to encode a packet.
+	// The request-id for the packet is set from reqid.
+	//
+	// An optional buffer may be given in b.
+	// If the buffer has a minimum capacity, it shall be truncated and used to marshal the header into.
+	// The minimum capacity for the packet must be a constant expression, and should be at least 9.
+	//
+	// It shall return the main body of the encoded packet in header,
+	// and may optionally return an additional payload to be written immediately after the header.
+	//
+	// It shall encode in the first 4-bytes of the header the proper length of the rest of the header+payload.
+	MarshalPacket(reqid uint32, b []byte) (header, payload []byte, err error)
+
+	// UnmarshalPacketBody decodes a packet body from the given Buffer.
+	// It is assumed that the common header values of the length, type and request-id have already been consumed.
 	UnmarshalPacketBody(buf *Buffer) error
 }
 

encoding/ssh/filexfer/handle_packets.go

@@ -6,14 +6,17 @@ type ClosePacket struct {
 }
 
 // MarshalPacket returns p as a two-part binary encoding of p.
-func (p *ClosePacket) MarshalPacket(reqid uint32) (header, payload []byte, err error) {
+func (p *ClosePacket) MarshalPacket(reqid uint32, b []byte) (header, payload []byte, err error) {
-	size := 4 + len(p.Handle) // string(handle)
+	buf := NewBuffer(b)
+	if buf.Cap() < 9 {
+		size := 4 + len(p.Handle) // string(handle)
+		buf = NewMarshalBuffer(size)
+	}
 
-	b := NewMarshalBuffer(PacketTypeClose, reqid, size)
+	buf.StartPacket(PacketTypeClose, reqid)
+	buf.AppendString(p.Handle)
 
-	b.AppendString(p.Handle)
+	return buf.Packet(payload)
-
-	return b.Packet(payload)
 }
 
 // UnmarshalPacketBody unmarshals the packet body from the given Buffer.
@@ -34,17 +37,20 @@ type ReadPacket struct {
 }
 
 // MarshalPacket returns p as a two-part binary encoding of p.
-func (p *ReadPacket) MarshalPacket(reqid uint32) (header, payload []byte, err error) {
+func (p *ReadPacket) MarshalPacket(reqid uint32, b []byte) (header, payload []byte, err error) {
-	// string(handle) + uint64(offset) + uint32(len)
+	buf := NewBuffer(b)
-	size := 4 + len(p.Handle) + 8 + 4
+	if buf.Cap() < 9 {
+		// string(handle) + uint64(offset) + uint32(len)
+		size := 4 + len(p.Handle) + 8 + 4
+		buf = NewMarshalBuffer(size)
+	}
 
-	b := NewMarshalBuffer(PacketTypeRead, reqid, size)
+	buf.StartPacket(PacketTypeRead, reqid)
+	buf.AppendString(p.Handle)
+	buf.AppendUint64(p.Offset)
+	buf.AppendUint32(p.Len)
 
-	b.AppendString(p.Handle)
+	return buf.Packet(payload)
-	b.AppendUint64(p.Offset)
-	b.AppendUint32(p.Len)
-
-	return b.Packet(payload)
 }
 
 // UnmarshalPacketBody unmarshals the packet body from the given Buffer.
@@ -73,17 +79,20 @@ type WritePacket struct {
 }
 
 // MarshalPacket returns p as a two-part binary encoding of p.
-func (p *WritePacket) MarshalPacket(reqid uint32) (header, payload []byte, err error) {
+func (p *WritePacket) MarshalPacket(reqid uint32, b []byte) (header, payload []byte, err error) {
-	// string(handle) + uint64(offset) + uint32(len(data)); data content in payload
+	buf := NewBuffer(b)
-	size := 4 + len(p.Handle) + 8 + 4
+	if buf.Cap() < 9 {
+		// string(handle) + uint64(offset) + uint32(len(data)); data content in payload
+		size := 4 + len(p.Handle) + 8 + 4
+		buf = NewMarshalBuffer(size)
+	}
 
-	b := NewMarshalBuffer(PacketTypeWrite, reqid, size)
+	buf.StartPacket(PacketTypeWrite, reqid)
+	buf.AppendString(p.Handle)
+	buf.AppendUint64(p.Offset)
+	buf.AppendUint32(uint32(len(p.Data)))
 
-	b.AppendString(p.Handle)
+	return buf.Packet(p.Data)
-	b.AppendUint64(p.Offset)
-	b.AppendUint32(uint32(len(p.Data)))
-
-	return b.Packet(p.Data)
 }
 
 // UnmarshalPacketBody unmarshals the packet body from the given Buffer.
@@ -110,14 +119,17 @@ type FStatPacket struct {
 }
 
 // MarshalPacket returns p as a two-part binary encoding of p.
-func (p *FStatPacket) MarshalPacket(reqid uint32) (header, payload []byte, err error) {
+func (p *FStatPacket) MarshalPacket(reqid uint32, b []byte) (header, payload []byte, err error) {
-	size := 4 + len(p.Handle) // string(handle)
+	buf := NewBuffer(b)
+	if buf.Cap() < 9 {
+		size := 4 + len(p.Handle) // string(handle)
+		buf = NewMarshalBuffer(size)
+	}
 
-	b := NewMarshalBuffer(PacketTypeFStat, reqid, size)
+	buf.StartPacket(PacketTypeFStat, reqid)
+	buf.AppendString(p.Handle)
 
-	b.AppendString(p.Handle)
+	return buf.Packet(payload)
-
-	return b.Packet(payload)
 }
 
 // UnmarshalPacketBody unmarshals the packet body from the given Buffer.
@@ -137,16 +149,19 @@ type FSetstatPacket struct {
 }
 
 // MarshalPacket returns p as a two-part binary encoding of p.
-func (p *FSetstatPacket) MarshalPacket(reqid uint32) (header, payload []byte, err error) {
+func (p *FSetstatPacket) MarshalPacket(reqid uint32, b []byte) (header, payload []byte, err error) {
-	size := 4 + len(p.Handle) + p.Attrs.Len() // string(handle) + ATTRS(attrs)
+	buf := NewBuffer(b)
+	if buf.Cap() < 9 {
+		size := 4 + len(p.Handle) + p.Attrs.Len() // string(handle) + ATTRS(attrs)
+		buf = NewMarshalBuffer(size)
+	}
 
-	b := NewMarshalBuffer(PacketTypeFSetstat, reqid, size)
+	buf.StartPacket(PacketTypeFSetstat, reqid)
+	buf.AppendString(p.Handle)
 
-	b.AppendString(p.Handle)
+	p.Attrs.MarshalInto(buf)
 
-	p.Attrs.MarshalInto(b)
+	return buf.Packet(payload)
-
-	return b.Packet(payload)
 }
 
 // UnmarshalPacketBody unmarshals the packet body from the given Buffer.
@@ -165,14 +180,17 @@ type ReadDirPacket struct {
 }
 
 // MarshalPacket returns p as a two-part binary encoding of p.
-func (p *ReadDirPacket) MarshalPacket(reqid uint32) (header, payload []byte, err error) {
+func (p *ReadDirPacket) MarshalPacket(reqid uint32, b []byte) (header, payload []byte, err error) {
-	size := 4 + len(p.Handle) // string(handle)
+	buf := NewBuffer(b)
+	if buf.Cap() < 9 {
+		size := 4 + len(p.Handle) // string(handle)
+		buf = NewMarshalBuffer(size)
+	}
 
-	b := NewMarshalBuffer(PacketTypeReadDir, reqid, size)
+	buf.StartPacket(PacketTypeReadDir, reqid)
+	buf.AppendString(p.Handle)
 
-	b.AppendString(p.Handle)
+	return buf.Packet(payload)
-
-	return b.Packet(payload)
 }
 
 // UnmarshalPacketBody unmarshals the packet body from the given Buffer.

encoding/ssh/filexfer/handle_packets_test.go

@@ -17,7 +17,7 @@ func TestClosePacket(t *testing.T) {
 		Handle: "somehandle",
 	}
 
-	data, err := ComposePacket(p.MarshalPacket(id))
+	data, err := ComposePacket(p.MarshalPacket(id, nil))
 	if err != nil {
 		t.Fatal("unexpected error:", err)
 	}
@@ -61,7 +61,7 @@ func TestReadPacket(t *testing.T) {
 		Len:    length,
 	}
 
-	data, err := ComposePacket(p.MarshalPacket(id))
+	data, err := ComposePacket(p.MarshalPacket(id, nil))
 	if err != nil {
 		t.Fatal("unexpected error:", err)
 	}
@@ -116,7 +116,7 @@ func TestWritePacket(t *testing.T) {
 		Data:   payload,
 	}
 
-	data, err := ComposePacket(p.MarshalPacket(id))
+	data, err := ComposePacket(p.MarshalPacket(id, nil))
 	if err != nil {
 		t.Fatal("unexpected error:", err)
 	}
@@ -166,7 +166,7 @@ func TestFStatPacket(t *testing.T) {
 		Handle: "somehandle",
 	}
 
-	data, err := ComposePacket(p.MarshalPacket(id))
+	data, err := ComposePacket(p.MarshalPacket(id, nil))
 	if err != nil {
 		t.Fatal("unexpected error:", err)
 	}
@@ -211,7 +211,7 @@ func TestFSetstatPacket(t *testing.T) {
 		},
 	}
 
-	data, err := ComposePacket(p.MarshalPacket(id))
+	data, err := ComposePacket(p.MarshalPacket(id, nil))
 	if err != nil {
 		t.Fatal("unexpected error:", err)
 	}
@@ -253,7 +253,7 @@ func TestReadDirPacket(t *testing.T) {
 		Handle: "somehandle",
 	}
 
-	data, err := ComposePacket(p.MarshalPacket(id))
+	data, err := ComposePacket(p.MarshalPacket(id, nil))
 	if err != nil {
 		t.Fatal("unexpected error:", err)
 	}

encoding/ssh/filexfer/open_packets.go

@@ -18,18 +18,21 @@ type OpenPacket struct {
 }
 
 // MarshalPacket returns p as a two-part binary encoding of p.
-func (p *OpenPacket) MarshalPacket(reqid uint32) (header, payload []byte, err error) {
+func (p *OpenPacket) MarshalPacket(reqid uint32, b []byte) (header, payload []byte, err error) {
-	// string(filename) + uint32(pflags) + ATTRS(attrs)
+	buf := NewBuffer(b)
-	size := 4 + len(p.Filename) + 4 + p.Attrs.Len()
+	if buf.Cap() < 9 {
+		// string(filename) + uint32(pflags) + ATTRS(attrs)
+		size := 4 + len(p.Filename) + 4 + p.Attrs.Len()
+		buf = NewMarshalBuffer(size)
+	}
 
-	b := NewMarshalBuffer(PacketTypeOpen, reqid, size)
+	buf.StartPacket(PacketTypeOpen, reqid)
+	buf.AppendString(p.Filename)
+	buf.AppendUint32(p.PFlags)
 
-	b.AppendString(p.Filename)
+	p.Attrs.MarshalInto(buf)
-	b.AppendUint32(p.PFlags)
 
-	p.Attrs.MarshalInto(b)
+	return buf.Packet(payload)
-
-	return b.Packet(payload)
 }
 
 // UnmarshalPacketBody unmarshals the packet body from the given Buffer.
@@ -52,14 +55,17 @@ type OpenDirPacket struct {
 }
 
 // MarshalPacket returns p as a two-part binary encoding of p.
-func (p *OpenDirPacket) MarshalPacket(reqid uint32) (header, payload []byte, err error) {
+func (p *OpenDirPacket) MarshalPacket(reqid uint32, b []byte) (header, payload []byte, err error) {
-	size := 4 + len(p.Path) // string(path)
+	buf := NewBuffer(b)
+	if buf.Cap() < 9 {
+		size := 4 + len(p.Path) // string(path)
+		buf = NewMarshalBuffer(size)
+	}
 
-	b := NewMarshalBuffer(PacketTypeOpenDir, reqid, size)
+	buf.StartPacket(PacketTypeOpenDir, reqid)
+	buf.AppendString(p.Path)
 
-	b.AppendString(p.Path)
+	return buf.Packet(payload)
-
-	return b.Packet(payload)
 }
 
 // UnmarshalPacketBody unmarshals the packet body from the given Buffer.

encoding/ssh/filexfer/open_packets_test.go

@@ -23,7 +23,7 @@ func TestOpenPacket(t *testing.T) {
 		},
 	}
 
-	data, err := ComposePacket(p.MarshalPacket(id))
+	data, err := ComposePacket(p.MarshalPacket(id, nil))
 	if err != nil {
 		t.Fatal("unexpected error:", err)
 	}
@@ -78,7 +78,7 @@ func TestOpenDirPacket(t *testing.T) {
 		Path: path,
 	}
 
-	data, err := ComposePacket(p.MarshalPacket(id))
+	data, err := ComposePacket(p.MarshalPacket(id, nil))
 	if err != nil {
 		t.Fatal("unexpected error:", err)
 	}

encoding/ssh/filexfer/openssh/hardlink.go

@@ -27,13 +27,13 @@ type HardlinkExtendedPacket struct {
 }
 
 // MarshalPacket returns ep as a two-part binary encoding of the full extended packet.
-func (ep *HardlinkExtendedPacket) MarshalPacket(reqid uint32) (header, payload []byte, err error) {
+func (ep *HardlinkExtendedPacket) MarshalPacket(reqid uint32, b []byte) (header, payload []byte, err error) {
 	p := &sshfx.ExtendedPacket{
 		ExtendedRequest: extensionHardlink,
 
 		Data: ep,
 	}
-	return p.MarshalPacket(reqid)
+	return p.MarshalPacket(reqid, b)
 }
 
 // MarshalInto encodes ep into the binary encoding of the hardlink@openssh.com extended packet-specific data.

encoding/ssh/filexfer/openssh/openssh.go

@@ -0,0 +1,2 @@
+// Package openssh implements the openssh secsh-filexfer extensions as described in https://github.com/openssh/openssh-portable/blob/master/PROTOCOL
+package openssh

encoding/ssh/filexfer/openssh/posix-rename.go

@@ -27,13 +27,13 @@ type PosixRenameExtendedPacket struct {
 }
 
 // MarshalPacket returns ep as a two-part binary encoding of the full extended packet.
-func (ep *PosixRenameExtendedPacket) MarshalPacket(reqid uint32) (header, payload []byte, err error) {
+func (ep *PosixRenameExtendedPacket) MarshalPacket(reqid uint32, b []byte) (header, payload []byte, err error) {
 	p := &sshfx.ExtendedPacket{
 		ExtendedRequest: extensionPosixRename,
 
 		Data: ep,
 	}
-	return p.MarshalPacket(reqid)
+	return p.MarshalPacket(reqid, b)
 }
 
 // MarshalInto encodes ep into the binary encoding of the hardlink@openssh.com extended packet-specific data.

encoding/ssh/filexfer/openssh/statvfs.go

@@ -26,13 +26,13 @@ type StatVFSExtendedPacket struct {
 }
 
 // MarshalPacket returns ep as a two-part binary encoding of the full extended packet.
-func (ep *StatVFSExtendedPacket) MarshalPacket(reqid uint32) (header, payload []byte, err error) {
+func (ep *StatVFSExtendedPacket) MarshalPacket(reqid uint32, b []byte) (header, payload []byte, err error) {
 	p := &sshfx.ExtendedPacket{
 		ExtendedRequest: extensionStatVFS,
 
 		Data: ep,
 	}
-	return p.MarshalPacket(reqid)
+	return p.MarshalPacket(reqid, b)
 }
 
 // MarshalInto encodes ep into the binary encoding of the statvfs@openssh.com extended packet-specific data.
@@ -89,13 +89,13 @@ type FStatVFSExtendedPacket struct {
 }
 
 // MarshalPacket returns ep as a two-part binary encoding of the full extended packet.
-func (ep *FStatVFSExtendedPacket) MarshalPacket(reqid uint32) (header, payload []byte, err error) {
+func (ep *FStatVFSExtendedPacket) MarshalPacket(reqid uint32, b []byte) (header, payload []byte, err error) {
 	p := &sshfx.ExtendedPacket{
 		ExtendedRequest: extensionFStatVFS,
 
 		Data: ep,
 	}
-	return p.MarshalPacket(reqid)
+	return p.MarshalPacket(reqid, b)
 }
 
 // MarshalInto encodes ep into the binary encoding of the statvfs@openssh.com extended packet-specific data.
@@ -153,11 +153,11 @@ type StatVFSExtendedReplyPacket struct {
 }
 
 // MarshalPacket returns ep as a two-part binary encoding of the full extended reply packet.
-func (ep *StatVFSExtendedReplyPacket) MarshalPacket(reqid uint32) (header, payload []byte, err error) {
+func (ep *StatVFSExtendedReplyPacket) MarshalPacket(reqid uint32, b []byte) (header, payload []byte, err error) {
 	p := &sshfx.ExtendedReplyPacket{
 		Data: ep,
 	}
-	return p.MarshalPacket(reqid)
+	return p.MarshalPacket(reqid, b)
 }
 
 // MarshalInto encodes ep into the binary encoding of the (f)statvfs@openssh.com extended reply packet-specific data.

encoding/ssh/filexfer/packets.go

@@ -64,15 +64,20 @@ type RawPacket struct {
 // MarshalPacket returns p as a two-part binary encoding of p.
 //
 // The internal p.RequestID is overridden by the reqid argument.
-func (p *RawPacket) MarshalPacket(reqid uint32) (header, payload []byte, err error) {
+func (p *RawPacket) MarshalPacket(reqid uint32, b []byte) (header, payload []byte, err error) {
-	b := NewMarshalBuffer(p.Type, reqid, 0)
+	buf := NewBuffer(b)
+	if buf.Cap() < 9 {
+		buf = NewMarshalBuffer(0)
+	}
 
-	return b.Packet(p.Data.Bytes())
+	buf.StartPacket(p.Type, reqid)
+
+	return buf.Packet(p.Data.Bytes())
 }
 
 // MarshalBinary returns p as the binary encoding of p.
 func (p *RawPacket) MarshalBinary() ([]byte, error) {
-	return ComposePacket(p.MarshalPacket(p.RequestID))
+	return ComposePacket(p.MarshalPacket(p.RequestID, nil))
 }
 
 // UnmarshalFrom decodes a RawPacket from the given Buffer into p.
@@ -139,7 +144,8 @@ func (p *RawPacket) ReadFrom(r io.Reader, b []byte) error {
 	return p.UnmarshalBinary(b)
 }
 
-// RequestPacket decodes a full request packet from the internal Data based on the Type.
+// RequestPacket decodes a full RequestPacket from the internal Data based on the Type.
+// Prefer using a RequestPacket directly, rather than going indirectly through RawPacket.
 func (p *RawPacket) RequestPacket() (*RequestPacket, error) {
 	packet, err := newPacketFromType(p.Type)
 	if err != nil {
@@ -173,17 +179,17 @@ func (p *RequestPacket) Reset() {
 // MarshalPacket returns p as a two-part binary encoding of p.
 //
 // The internal p.RequestID is overridden by the reqid argument.
-func (p *RequestPacket) MarshalPacket(reqid uint32) (header, payload []byte, err error) {
+func (p *RequestPacket) MarshalPacket(reqid uint32, b []byte) (header, payload []byte, err error) {
 	if p.Request == nil {
 		return nil, nil, errors.New("empty request packet")
 	}
 
-	return p.Request.MarshalPacket(reqid)
+	return p.Request.MarshalPacket(reqid, b)
 }
 
 // MarshalBinary returns p as the binary encoding of p.
 func (p *RequestPacket) MarshalBinary() ([]byte, error) {
-	return ComposePacket(p.MarshalPacket(p.RequestID))
+	return ComposePacket(p.MarshalPacket(p.RequestID, nil))
 }
 
 // UnmarshalFrom decodes a RequestPacket from the given Buffer into p.

encoding/ssh/filexfer/path_packets.go

@@ -6,14 +6,17 @@ type LStatPacket struct {
 }
 
 // MarshalPacket returns p as a two-part binary encoding of p.
-func (p *LStatPacket) MarshalPacket(reqid uint32) (header, payload []byte, err error) {
+func (p *LStatPacket) MarshalPacket(reqid uint32, b []byte) (header, payload []byte, err error) {
-	size := 4 + len(p.Path) // string(path)
+	buf := NewBuffer(b)
+	if buf.Cap() < 9 {
+		size := 4 + len(p.Path) // string(path)
+		buf = NewMarshalBuffer(size)
+	}
 
-	b := NewMarshalBuffer(PacketTypeLStat, reqid, size)
+	buf.StartPacket(PacketTypeLStat, reqid)
+	buf.AppendString(p.Path)
 
-	b.AppendString(p.Path)
+	return buf.Packet(payload)
-
-	return b.Packet(payload)
 }
 
 // UnmarshalPacketBody unmarshals the packet body from the given Buffer.
@@ -33,16 +36,19 @@ type SetstatPacket struct {
 }
 
 // MarshalPacket returns p as a two-part binary encoding of p.
-func (p *SetstatPacket) MarshalPacket(reqid uint32) (header, payload []byte, err error) {
+func (p *SetstatPacket) MarshalPacket(reqid uint32, b []byte) (header, payload []byte, err error) {
-	size := 4 + len(p.Path) + p.Attrs.Len() // string(path) + ATTRS(attrs)
+	buf := NewBuffer(b)
+	if buf.Cap() < 9 {
+		size := 4 + len(p.Path) + p.Attrs.Len() // string(path) + ATTRS(attrs)
+		buf = NewMarshalBuffer(size)
+	}
 
-	b := NewMarshalBuffer(PacketTypeSetstat, reqid, size)
+	buf.StartPacket(PacketTypeSetstat, reqid)
+	buf.AppendString(p.Path)
 
-	b.AppendString(p.Path)
+	p.Attrs.MarshalInto(buf)
 
-	p.Attrs.MarshalInto(b)
+	return buf.Packet(payload)
-
-	return b.Packet(payload)
 }
 
 // UnmarshalPacketBody unmarshals the packet body from the given Buffer.
@@ -61,14 +67,17 @@ type RemovePacket struct {
 }
 
 // MarshalPacket returns p as a two-part binary encoding of p.
-func (p *RemovePacket) MarshalPacket(reqid uint32) (header, payload []byte, err error) {
+func (p *RemovePacket) MarshalPacket(reqid uint32, b []byte) (header, payload []byte, err error) {
-	size := 4 + len(p.Path) // string(path)
+	buf := NewBuffer(b)
+	if buf.Cap() < 9 {
+		size := 4 + len(p.Path) // string(path)
+		buf = NewMarshalBuffer(size)
+	}
 
-	b := NewMarshalBuffer(PacketTypeRemove, reqid, size)
+	buf.StartPacket(PacketTypeRemove, reqid)
+	buf.AppendString(p.Path)
 
-	b.AppendString(p.Path)
+	return buf.Packet(payload)
-
-	return b.Packet(payload)
 }
 
 // UnmarshalPacketBody unmarshals the packet body from the given Buffer.
@@ -88,16 +97,19 @@ type MkdirPacket struct {
 }
 
 // MarshalPacket returns p as a two-part binary encoding of p.
-func (p *MkdirPacket) MarshalPacket(reqid uint32) (header, payload []byte, err error) {
+func (p *MkdirPacket) MarshalPacket(reqid uint32, b []byte) (header, payload []byte, err error) {
-	size := 4 + len(p.Path) + p.Attrs.Len() // string(path) + ATTRS(attrs)
+	buf := NewBuffer(b)
+	if buf.Cap() < 9 {
+		size := 4 + len(p.Path) + p.Attrs.Len() // string(path) + ATTRS(attrs)
+		buf = NewMarshalBuffer(size)
+	}
 
-	b := NewMarshalBuffer(PacketTypeMkdir, reqid, size)
+	buf.StartPacket(PacketTypeMkdir, reqid)
+	buf.AppendString(p.Path)
 
-	b.AppendString(p.Path)
+	p.Attrs.MarshalInto(buf)
 
-	p.Attrs.MarshalInto(b)
+	return buf.Packet(payload)
-
-	return b.Packet(payload)
 }
 
 // UnmarshalPacketBody unmarshals the packet body from the given Buffer.
@@ -116,14 +128,17 @@ type RmdirPacket struct {
 }
 
 // MarshalPacket returns p as a two-part binary encoding of p.
-func (p *RmdirPacket) MarshalPacket(reqid uint32) (header, payload []byte, err error) {
+func (p *RmdirPacket) MarshalPacket(reqid uint32, b []byte) (header, payload []byte, err error) {
-	size := 4 + len(p.Path) // string(path)
+	buf := NewBuffer(b)
+	if buf.Cap() < 9 {
+		size := 4 + len(p.Path) // string(path)
+		buf = NewMarshalBuffer(size)
+	}
 
-	b := NewMarshalBuffer(PacketTypeRmdir, reqid, size)
+	buf.StartPacket(PacketTypeRmdir, reqid)
+	buf.AppendString(p.Path)
 
-	b.AppendString(p.Path)
+	return buf.Packet(payload)
-
-	return b.Packet(payload)
 }
 
 // UnmarshalPacketBody unmarshals the packet body from the given Buffer.
@@ -142,14 +157,17 @@ type RealPathPacket struct {
 }
 
 // MarshalPacket returns p as a two-part binary encoding of p.
-func (p *RealPathPacket) MarshalPacket(reqid uint32) (header, payload []byte, err error) {
+func (p *RealPathPacket) MarshalPacket(reqid uint32, b []byte) (header, payload []byte, err error) {
-	size := 4 + len(p.Path) // string(path)
+	buf := NewBuffer(b)
+	if buf.Cap() < 9 {
+		size := 4 + len(p.Path) // string(path)
+		buf = NewMarshalBuffer(size)
+	}
 
-	b := NewMarshalBuffer(PacketTypeRealPath, reqid, size)
+	buf.StartPacket(PacketTypeRealPath, reqid)
+	buf.AppendString(p.Path)
 
-	b.AppendString(p.Path)
+	return buf.Packet(payload)
-
-	return b.Packet(payload)
 }
 
 // UnmarshalPacketBody unmarshals the packet body from the given Buffer.
@@ -168,14 +186,17 @@ type StatPacket struct {
 }
 
 // MarshalPacket returns p as a two-part binary encoding of p.
-func (p *StatPacket) MarshalPacket(reqid uint32) (header, payload []byte, err error) {
+func (p *StatPacket) MarshalPacket(reqid uint32, b []byte) (header, payload []byte, err error) {
-	size := 4 + len(p.Path) // string(path)
+	buf := NewBuffer(b)
+	if buf.Cap() < 9 {
+		size := 4 + len(p.Path) // string(path)
+		buf = NewMarshalBuffer(size)
+	}
 
-	b := NewMarshalBuffer(PacketTypeStat, reqid, size)
+	buf.StartPacket(PacketTypeStat, reqid)
+	buf.AppendString(p.Path)
 
-	b.AppendString(p.Path)
+	return buf.Packet(payload)
-
-	return b.Packet(payload)
 }
 
 // UnmarshalPacketBody unmarshals the packet body from the given Buffer.
@@ -195,16 +216,19 @@ type RenamePacket struct {
 }
 
 // MarshalPacket returns p as a two-part binary encoding of p.
-func (p *RenamePacket) MarshalPacket(reqid uint32) (header, payload []byte, err error) {
+func (p *RenamePacket) MarshalPacket(reqid uint32, b []byte) (header, payload []byte, err error) {
-	// string(oldpath) + string(newpath)
+	buf := NewBuffer(b)
-	size := 4 + len(p.OldPath) + 4 + len(p.NewPath)
+	if buf.Cap() < 9 {
+		// string(oldpath) + string(newpath)
+		size := 4 + len(p.OldPath) + 4 + len(p.NewPath)
+		buf = NewMarshalBuffer(size)
+	}
 
-	b := NewMarshalBuffer(PacketTypeRename, reqid, size)
+	buf.StartPacket(PacketTypeRename, reqid)
+	buf.AppendString(p.OldPath)
+	buf.AppendString(p.NewPath)
 
-	b.AppendString(p.OldPath)
+	return buf.Packet(payload)
-	b.AppendString(p.NewPath)
-
-	return b.Packet(payload)
 }
 
 // UnmarshalPacketBody unmarshals the packet body from the given Buffer.
@@ -227,14 +251,17 @@ type ReadLinkPacket struct {
 }
 
 // MarshalPacket returns p as a two-part binary encoding of p.
-func (p *ReadLinkPacket) MarshalPacket(reqid uint32) (header, payload []byte, err error) {
+func (p *ReadLinkPacket) MarshalPacket(reqid uint32, b []byte) (header, payload []byte, err error) {
-	size := 4 + len(p.Path) // string(path)
+	buf := NewBuffer(b)
+	if buf.Cap() < 9 {
+		size := 4 + len(p.Path) // string(path)
+		buf = NewMarshalBuffer(size)
+	}
 
-	b := NewMarshalBuffer(PacketTypeReadLink, reqid, size)
+	buf.StartPacket(PacketTypeReadLink, reqid)
+	buf.AppendString(p.Path)
 
-	b.AppendString(p.Path)
+	return buf.Packet(payload)
-
-	return b.Packet(payload)
 }
 
 // UnmarshalPacketBody unmarshals the packet body from the given Buffer.
@@ -258,17 +285,21 @@ type SymlinkPacket struct {
 }
 
 // MarshalPacket returns p as a two-part binary encoding of p.
-func (p *SymlinkPacket) MarshalPacket(reqid uint32) (header, payload []byte, err error) {
+func (p *SymlinkPacket) MarshalPacket(reqid uint32, b []byte) (header, payload []byte, err error) {
-	// string(targetpath) + string(linkpath)
+	buf := NewBuffer(b)
-	size := 4 + len(p.TargetPath) + 4 + len(p.LinkPath)
+	if buf.Cap() < 9 {
+		// string(targetpath) + string(linkpath)
+		size := 4 + len(p.TargetPath) + 4 + len(p.LinkPath)
+		buf = NewMarshalBuffer(size)
+	}
 
-	b := NewMarshalBuffer(PacketTypeSymlink, reqid, size)
+	buf.StartPacket(PacketTypeSymlink, reqid)
 
 	// Arguments were inadvertently reversed.
-	b.AppendString(p.TargetPath)
+	buf.AppendString(p.TargetPath)
-	b.AppendString(p.LinkPath)
+	buf.AppendString(p.LinkPath)
 
-	return b.Packet(payload)
+	return buf.Packet(payload)
 }
 
 // UnmarshalPacketBody unmarshals the packet body from the given Buffer.

encoding/ssh/filexfer/path_packets_test.go

@@ -17,7 +17,7 @@ func TestLStatPacket(t *testing.T) {
 		Path: path,
 	}
 
-	data, err := ComposePacket(p.MarshalPacket(id))
+	data, err := ComposePacket(p.MarshalPacket(id, nil))
 	if err != nil {
 		t.Fatal("unexpected error:", err)
 	}
@@ -62,7 +62,7 @@ func TestSetstatPacket(t *testing.T) {
 		},
 	}
 
-	data, err := ComposePacket(p.MarshalPacket(id))
+	data, err := ComposePacket(p.MarshalPacket(id, nil))
 	if err != nil {
 		t.Fatal("unexpected error:", err)
 	}
@@ -112,7 +112,7 @@ func TestRemovePacket(t *testing.T) {
 		Path: path,
 	}
 
-	data, err := ComposePacket(p.MarshalPacket(id))
+	data, err := ComposePacket(p.MarshalPacket(id, nil))
 	if err != nil {
 		t.Fatal("unexpected error:", err)
 	}
@@ -157,7 +157,7 @@ func TestMkdirPacket(t *testing.T) {
 		},
 	}
 
-	data, err := ComposePacket(p.MarshalPacket(id))
+	data, err := ComposePacket(p.MarshalPacket(id, nil))
 	if err != nil {
 		t.Fatal("unexpected error:", err)
 	}
@@ -207,7 +207,7 @@ func TestRmdirPacket(t *testing.T) {
 		Path: path,
 	}
 
-	data, err := ComposePacket(p.MarshalPacket(id))
+	data, err := ComposePacket(p.MarshalPacket(id, nil))
 	if err != nil {
 		t.Fatal("unexpected error:", err)
 	}
@@ -247,7 +247,7 @@ func TestRealPathPacket(t *testing.T) {
 		Path: path,
 	}
 
-	data, err := ComposePacket(p.MarshalPacket(id))
+	data, err := ComposePacket(p.MarshalPacket(id, nil))
 	if err != nil {
 		t.Fatal("unexpected error:", err)
 	}
@@ -287,7 +287,7 @@ func TestStatPacket(t *testing.T) {
 		Path: path,
 	}
 
-	data, err := ComposePacket(p.MarshalPacket(id))
+	data, err := ComposePacket(p.MarshalPacket(id, nil))
 	if err != nil {
 		t.Fatal("unexpected error:", err)
 	}
@@ -329,7 +329,7 @@ func TestRenamePacket(t *testing.T) {
 		NewPath: newpath,
 	}
 
-	data, err := ComposePacket(p.MarshalPacket(id))
+	data, err := ComposePacket(p.MarshalPacket(id, nil))
 	if err != nil {
 		t.Fatal("unexpected error:", err)
 	}
@@ -374,7 +374,7 @@ func TestReadLinkPacket(t *testing.T) {
 		Path: path,
 	}
 
-	data, err := ComposePacket(p.MarshalPacket(id))
+	data, err := ComposePacket(p.MarshalPacket(id, nil))
 	if err != nil {
 		t.Fatal("unexpected error:", err)
 	}
@@ -416,7 +416,7 @@ func TestSymlinkPacket(t *testing.T) {
 		TargetPath: targetpath,
 	}
 
-	data, err := ComposePacket(p.MarshalPacket(id))
+	data, err := ComposePacket(p.MarshalPacket(id, nil))
 	if err != nil {
 		t.Fatal("unexpected error:", err)
 	}

encoding/ssh/filexfer/response_packets.go

@@ -10,17 +10,20 @@ type StatusPacket struct {
 }
 
 // MarshalPacket returns p as a two-part binary encoding of p.
-func (p *StatusPacket) MarshalPacket(reqid uint32) (header, payload []byte, err error) {
+func (p *StatusPacket) MarshalPacket(reqid uint32, b []byte) (header, payload []byte, err error) {
-	// uint32(error/status code) + string(error message) + string(language tag)
+	buf := NewBuffer(b)
-	size := 4 + 4 + len(p.ErrorMessage) + 4 + len(p.LanguageTag)
+	if buf.Cap() < 9 {
+		// uint32(error/status code) + string(error message) + string(language tag)
+		size := 4 + 4 + len(p.ErrorMessage) + 4 + len(p.LanguageTag)
+		buf = NewMarshalBuffer(size)
+	}
 
-	b := NewMarshalBuffer(PacketTypeStatus, reqid, size)
+	buf.StartPacket(PacketTypeStatus, reqid)
+	buf.AppendUint32(uint32(p.StatusCode))
+	buf.AppendString(p.ErrorMessage)
+	buf.AppendString(p.LanguageTag)
 
-	b.AppendUint32(uint32(p.StatusCode))
+	return buf.Packet(payload)
-	b.AppendString(p.ErrorMessage)
-	b.AppendString(p.LanguageTag)
-
-	return b.Packet(payload)
 }
 
 // UnmarshalPacketBody unmarshals the packet body from the given Buffer.
@@ -49,14 +52,17 @@ type HandlePacket struct {
 }
 
 // MarshalPacket returns p as a two-part binary encoding of p.
-func (p *HandlePacket) MarshalPacket(reqid uint32) (header, payload []byte, err error) {
+func (p *HandlePacket) MarshalPacket(reqid uint32, b []byte) (header, payload []byte, err error) {
-	size := 4 + len(p.Handle) // string(handle)
+	buf := NewBuffer(b)
+	if buf.Cap() < 9 {
+		size := 4 + len(p.Handle) // string(handle)
+		buf = NewMarshalBuffer(size)
+	}
 
-	b := NewMarshalBuffer(PacketTypeHandle, reqid, size)
+	buf.StartPacket(PacketTypeHandle, reqid)
+	buf.AppendString(p.Handle)
 
-	b.AppendString(p.Handle)
+	return buf.Packet(payload)
-
-	return b.Packet(payload)
 }
 
 // UnmarshalPacketBody unmarshals the packet body from the given Buffer.
@@ -75,14 +81,17 @@ type DataPacket struct {
 }
 
 // MarshalPacket returns p as a two-part binary encoding of p.
-func (p *DataPacket) MarshalPacket(reqid uint32) (header, payload []byte, err error) {
+func (p *DataPacket) MarshalPacket(reqid uint32, b []byte) (header, payload []byte, err error) {
-	size := 4 // uint32(len(data)); data content in payload
+	buf := NewBuffer(b)
+	if buf.Cap() < 9 {
+		size := 4 // uint32(len(data)); data content in payload
+		buf = NewMarshalBuffer(size)
+	}
 
-	b := NewMarshalBuffer(PacketTypeData, reqid, size)
+	buf.StartPacket(PacketTypeData, reqid)
+	buf.AppendUint32(uint32(len(p.Data)))
 
-	b.AppendUint32(uint32(len(p.Data)))
+	return buf.Packet(p.Data)
-
-	return b.Packet(p.Data)
 }
 
 // UnmarshalPacketBody unmarshals the packet body from the given Buffer.
@@ -101,22 +110,26 @@ type NamePacket struct {
 }
 
 // MarshalPacket returns p as a two-part binary encoding of p.
-func (p *NamePacket) MarshalPacket(reqid uint32) (header, payload []byte, err error) {
+func (p *NamePacket) MarshalPacket(reqid uint32, b []byte) (header, payload []byte, err error) {
-	size := 4 // uint32(len(entries))
+	buf := NewBuffer(b)
+	if buf.Cap() < 9 {
+		size := 4 // uint32(len(entries))
 
-	for _, e := range p.Entries {
+		for _, e := range p.Entries {
-		size += e.Len()
+			size += e.Len()
+		}
+
+		buf = NewMarshalBuffer(size)
 	}
 
-	b := NewMarshalBuffer(PacketTypeName, reqid, size)
+	buf.StartPacket(PacketTypeName, reqid)
-
+	buf.AppendUint32(uint32(len(p.Entries)))
-	b.AppendUint32(uint32(len(p.Entries)))
 
 	for _, e := range p.Entries {
-		e.MarshalInto(b)
+		e.MarshalInto(buf)
 	}
 
-	return b.Packet(payload)
+	return buf.Packet(payload)
 }
 
 // UnmarshalPacketBody unmarshals the packet body from the given Buffer.
@@ -147,14 +160,17 @@ type AttrsPacket struct {
 }
 
 // MarshalPacket returns p as a two-part binary encoding of p.
-func (p *AttrsPacket) MarshalPacket(reqid uint32) (header, payload []byte, err error) {
+func (p *AttrsPacket) MarshalPacket(reqid uint32, b []byte) (header, payload []byte, err error) {
-	size := p.Attrs.Len() // ATTRS(attrs)
+	buf := NewBuffer(b)
+	if buf.Cap() < 9 {
+		size := p.Attrs.Len() // ATTRS(attrs)
+		buf = NewMarshalBuffer(size)
+	}
 
-	b := NewMarshalBuffer(PacketTypeAttrs, reqid, size)
+	buf.StartPacket(PacketTypeAttrs, reqid)
+	p.Attrs.MarshalInto(buf)
 
-	p.Attrs.MarshalInto(b)
+	return buf.Packet(payload)
-
-	return b.Packet(payload)
 }
 
 // UnmarshalPacketBody unmarshals the packet body from the given Buffer.

encoding/ssh/filexfer/response_packets_test.go

@@ -21,7 +21,7 @@ func TestStatusPacket(t *testing.T) {
 		LanguageTag:  languageTag,
 	}
 
-	data, err := ComposePacket(p.MarshalPacket(id))
+	data, err := ComposePacket(p.MarshalPacket(id, nil))
 	if err != nil {
 		t.Fatal("unexpected error:", err)
 	}
@@ -71,7 +71,7 @@ func TestHandlePacket(t *testing.T) {
 		Handle: "somehandle",
 	}
 
-	data, err := ComposePacket(p.MarshalPacket(id))
+	data, err := ComposePacket(p.MarshalPacket(id, nil))
 	if err != nil {
 		t.Fatal("unexpected error:", err)
 	}
@@ -112,7 +112,7 @@ func TestDataPacket(t *testing.T) {
 		Data: payload,
 	}
 
-	data, err := ComposePacket(p.MarshalPacket(id))
+	data, err := ComposePacket(p.MarshalPacket(id, nil))
 	if err != nil {
 		t.Fatal("unexpected error:", err)
 	}
@@ -171,7 +171,7 @@ func TestNamePacket(t *testing.T) {
 		},
 	}
 
-	data, err := ComposePacket(p.MarshalPacket(id))
+	data, err := ComposePacket(p.MarshalPacket(id, nil))
 	if err != nil {
 		t.Fatal("unexpected error:", err)
 	}
@@ -240,7 +240,7 @@ func TestAttrsPacket(t *testing.T) {
 		},
 	}
 
-	data, err := ComposePacket(p.MarshalPacket(id))
+	data, err := ComposePacket(p.MarshalPacket(id, nil))
 	if err != nil {
 		t.Fatal("unexpected error:", err)
 	}