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/listener.go

@@ -0,0 +1,47 @@
+// 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 (
+	"net"
+	"time"
+)
+
+// Listener is used to wrap an underlying listener,
+// whose connections may be using the HAProxy Proxy Protocol (version 1 or 2).
+// If the connection is using the protocol, the RemoteAddr() will return
+// the correct client address.
+//
+// Optionally define ProxyHeaderTimeout to set a maximum time to
+// receive the Proxy Protocol Header. Zero means no timeout.
+type Listener struct {
+	Listener           net.Listener
+	ProxyHeaderTimeout time.Duration
+	AcceptUnknown      bool // allow PROXY UNKNOWN
+}
+
+// Accept implements the Accept method in the Listener interface
+// it waits for the next call and returns a wrapped Conn.
+func (p *Listener) Accept() (net.Conn, error) {
+	// Get the underlying connection
+	conn, err := p.Listener.Accept()
+	if err != nil {
+		return nil, err
+	}
+
+	newConn := NewConn(conn, p.ProxyHeaderTimeout)
+	newConn.acceptUnknown = p.AcceptUnknown
+	return newConn, nil
+}
+
+// Close closes the underlying listener.
+func (p *Listener) Close() error {
+	return p.Listener.Close()
+}
+
+// Addr returns the underlying listener's network address.
+func (p *Listener) Addr() net.Addr {
+	return p.Listener.Addr()
+}