Introduce pgproto3 package

pgproto3 will wrap the message encoding and decoding for the PostgreSQL
frontend/backend protocol version 3.

command_complete.go

@@ -0,0 +1,47 @@
+package pgproto3
+
+import (
+	"bytes"
+	"encoding/json"
+)
+
+type CommandComplete struct {
+	CommandTag string
+}
+
+func (*CommandComplete) Backend() {}
+
+func (dst *CommandComplete) UnmarshalBinary(src []byte) error {
+	buf := bytes.NewBuffer(src)
+
+	b, err := buf.ReadBytes(0)
+	if err != nil {
+		return err
+	}
+	dst.CommandTag = string(b[:len(b)-1])
+
+	return nil
+}
+
+func (src *CommandComplete) MarshalBinary() ([]byte, error) {
+	var bigEndian BigEndianBuf
+	buf := &bytes.Buffer{}
+
+	buf.WriteByte('C')
+	buf.Write(bigEndian.Uint32(uint32(4 + len(src.CommandTag) + 1)))
+
+	buf.WriteString(src.CommandTag)
+	buf.WriteByte(0)
+
+	return buf.Bytes(), nil
+}
+
+func (src *CommandComplete) MarshalJSON() ([]byte, error) {
+	return json.Marshal(struct {
+		Type       string
+		CommandTag string
+	}{
+		Type:       "CommandComplete",
+		CommandTag: src.CommandTag,
+	})
+}