Support Proxy protocol (#12527)

This PR adds functionality to allow Gitea to sit behind an
HAProxy and HAProxy protocolled connections directly.

Fix #7508

Signed-off-by: Andrew Thornton <art27@cantab.net>

modules/proxyprotocol/errors.go

@@ -0,0 +1,45 @@
+// Copyright 2020 The Gitea Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package proxyprotocol
+
+import "fmt"
+
+// ErrBadHeader is an error demonstrating a bad proxy header
+type ErrBadHeader struct {
+	Header []byte
+}
+
+func (e *ErrBadHeader) Error() string {
+	return fmt.Sprintf("Unexpected proxy header: %v", e.Header)
+}
+
+// ErrBadAddressType is an error demonstrating a bad proxy header with bad Address type
+type ErrBadAddressType struct {
+	Address string
+}
+
+func (e *ErrBadAddressType) Error() string {
+	return fmt.Sprintf("Unexpected proxy header address type: %s", e.Address)
+}
+
+// ErrBadRemote is an error demonstrating a bad proxy header with bad Remote
+type ErrBadRemote struct {
+	IP   string
+	Port string
+}
+
+func (e *ErrBadRemote) Error() string {
+	return fmt.Sprintf("Unexpected proxy header remote IP and port: %s %s", e.IP, e.Port)
+}
+
+// ErrBadLocal is an error demonstrating a bad proxy header with bad Local
+type ErrBadLocal struct {
+	IP   string
+	Port string
+}
+
+func (e *ErrBadLocal) Error() string {
+	return fmt.Sprintf("Unexpected proxy header local IP and port: %s %s", e.IP, e.Port)
+}