alts: move ParseFramedMsg out of common

It's only called in one place, and is effectively a method on conn.

Part of #8510.

credentials/alts/internal/conn/common.go

@@ -19,9 +19,7 @@
 package conn
 
 import (
-	"encoding/binary"
 	"errors"
-	"fmt"
 )
 
 const (
@@ -48,33 +46,3 @@ func SliceForAppend(in []byte, n int) (head, tail []byte) {
 	tail = head[len(in):]
 	return head, tail
 }
-
-// ParseFramedMsg parse the provided buffer and returns a frame of the format
-// msgLength+msg and any remaining bytes in that buffer.
-func ParseFramedMsg(b []byte, maxLen uint32) ([]byte, []byte, error) {
-	// If the size field is not complete, return the provided buffer as
-	// remaining buffer.
-	length, sufficientBytes := parseMessageLength(b)
-	if !sufficientBytes {
-		return nil, b, nil
-	}
-	if length > maxLen {
-		return nil, nil, fmt.Errorf("received the frame length %d larger than the limit %d", length, maxLen)
-	}
-	if len(b) < int(length)+4 { // account for the first 4 msg length bytes.
-		// Frame is not complete yet.
-		return nil, b, nil
-	}
-	return b[:MsgLenFieldSize+length], b[MsgLenFieldSize+length:], nil
-}
-
-// parseMessageLength returns the message length based on frame header. It also
-// returns a boolean indicating if the buffer contains sufficient bytes to parse
-// the length header. If there are insufficient bytes, (0, false) is returned.
-func parseMessageLength(b []byte) (uint32, bool) {
-	if len(b) < MsgLenFieldSize {
-		return 0, false
-	}
-	msgLenField := b[:MsgLenFieldSize]
-	return binary.LittleEndian.Uint32(msgLenField), true
-}

credentials/alts/internal/conn/record.go

@@ -144,7 +144,7 @@ func NewConn(c net.Conn, side core.Side, recordProtocol string, key []byte, prot
 func (p *conn) Read(b []byte) (n int, err error) {
 	if len(p.buf) == 0 {
 		var framedMsg []byte
-		framedMsg, p.nextFrame, err = ParseFramedMsg(p.nextFrame, altsRecordLengthLimit)
+		framedMsg, err = p.parseFramedMsg(p.nextFrame, altsRecordLengthLimit)
 		if err != nil {
 			return n, err
 		}
@@ -184,7 +184,7 @@ func (p *conn) Read(b []byte) (n int, err error) {
 				return 0, err
 			}
 			p.protected = p.protected[:len(p.protected)+n]
-			framedMsg, p.nextFrame, err = ParseFramedMsg(p.protected, altsRecordLengthLimit)
+			framedMsg, err = p.parseFramedMsg(p.protected, altsRecordLengthLimit)
 			if err != nil {
 				return 0, err
 			}
@@ -225,6 +225,38 @@ func (p *conn) Read(b []byte) (n int, err error) {
 	return n, nil
 }
 
+// parseFramedMsg parses the provided buffer and returns a frame of the format
+// msgLength+msg iff a full frame is available.
+func (p *conn) parseFramedMsg(b []byte, maxLen uint32) ([]byte, error) {
+	// If the size field is not complete, return the provided buffer as
+	// remaining buffer.
+	p.nextFrame = b
+	length, sufficientBytes := parseMessageLength(b)
+	if !sufficientBytes {
+		return nil, nil
+	}
+	if length > maxLen {
+		return nil, fmt.Errorf("received the frame length %d larger than the limit %d", length, maxLen)
+	}
+	if len(b) < int(length)+4 { // account for the first 4 msg length bytes.
+		// Frame is not complete yet.
+		return nil, nil
+	}
+	p.nextFrame = b[MsgLenFieldSize+length:]
+	return b[:MsgLenFieldSize+length], nil
+}
+
+// parseMessageLength returns the message length based on frame header. It also
+// returns a boolean indicating if the buffer contains sufficient bytes to parse
+// the length header. If there are insufficient bytes, (0, false) is returned.
+func parseMessageLength(b []byte) (uint32, bool) {
+	if len(b) < MsgLenFieldSize {
+		return 0, false
+	}
+	msgLenField := b[:MsgLenFieldSize]
+	return binary.LittleEndian.Uint32(msgLenField), true
+}
+
 // Write encrypts, frames, and writes bytes from b to the underlying connection.
 func (p *conn) Write(b []byte) (n int, err error) {
 	n = len(b)