json: Implement json.Unmarshaler for messages.

This will allow using pgmockproxy output as ingestion data for pgmock.

error_response.go

@@ -3,6 +3,8 @@ package pgproto3
 import (
 	"bytes"
 	"encoding/binary"
+	"encoding/json"
+	"fmt"
 	"strconv"
 )
 
@@ -225,3 +227,68 @@ func (src *ErrorResponse) marshalBinary(typeByte byte) []byte {
 
 	return buf.Bytes()
 }
+
+// UnmarshalJSON implements encoding/json.Unmarshaler.
+func (dst *ErrorResponse) UnmarshalJSON(data []byte) error {
+	// Ignore null, like in the main JSON package.
+	if string(data) == "null" {
+		return nil
+	}
+
+	var msg struct {
+		Severity            string
+		SeverityUnlocalized string // only in 9.6 and greater
+		Code                string
+		Message             string
+		Detail              string
+		Hint                string
+		Position            int32
+		InternalPosition    int32
+		InternalQuery       string
+		Where               string
+		SchemaName          string
+		TableName           string
+		ColumnName          string
+		DataTypeName        string
+		ConstraintName      string
+		File                string
+		Line                int32
+		Routine             string
+
+		UnknownFields map[string]string
+	}
+	if err := json.Unmarshal(data, &msg); err != nil {
+		return err
+	}
+
+	dst.Severity = msg.Severity
+	dst.SeverityUnlocalized = msg.SeverityUnlocalized
+	dst.Code = msg.Code
+	dst.Message = msg.Message
+	dst.Detail = msg.Detail
+	dst.Hint = msg.Hint
+	dst.Position = msg.Position
+	dst.InternalPosition = msg.InternalPosition
+	dst.InternalQuery = msg.InternalQuery
+	dst.Where = msg.Where
+	dst.SchemaName = msg.SchemaName
+	dst.TableName = msg.TableName
+	dst.ColumnName = msg.ColumnName
+	dst.DataTypeName = msg.DataTypeName
+	dst.ConstraintName = msg.ConstraintName
+	dst.File = msg.File
+	dst.Line = msg.Line
+	dst.Routine = msg.Routine
+
+	if msg.UnknownFields != nil {
+		dst.UnknownFields = map[byte]string{}
+	}
+	for k, v := range msg.UnknownFields {
+		if len(k) != 1 {
+			return fmt.Errorf("invalid UnknownFields field %q value", k)
+		}
+		dst.UnknownFields[k[0]] = v
+	}
+
+	return nil
+}