switch to use gliderlabs/ssh for builtin server (#7250)

resolves git conflicts from #3896 (credit to @belak, in case github doesn't keep original author during squash)

Co-Authored-By: Matti Ranta <techknowlogick@gitea.io>

vendor/github.com/gliderlabs/ssh/agent.go

@@ -0,0 +1,83 @@
+package ssh
+
+import (
+	"io"
+	"io/ioutil"
+	"net"
+	"path"
+	"sync"
+
+	gossh "golang.org/x/crypto/ssh"
+)
+
+const (
+	agentRequestType = "auth-agent-req@openssh.com"
+	agentChannelType = "auth-agent@openssh.com"
+
+	agentTempDir    = "auth-agent"
+	agentListenFile = "listener.sock"
+)
+
+// contextKeyAgentRequest is an internal context key for storing if the
+// client requested agent forwarding
+var contextKeyAgentRequest = &contextKey{"auth-agent-req"}
+
+// SetAgentRequested sets up the session context so that AgentRequested
+// returns true.
+func SetAgentRequested(ctx Context) {
+	ctx.SetValue(contextKeyAgentRequest, true)
+}
+
+// AgentRequested returns true if the client requested agent forwarding.
+func AgentRequested(sess Session) bool {
+	return sess.Context().Value(contextKeyAgentRequest) == true
+}
+
+// NewAgentListener sets up a temporary Unix socket that can be communicated
+// to the session environment and used for forwarding connections.
+func NewAgentListener() (net.Listener, error) {
+	dir, err := ioutil.TempDir("", agentTempDir)
+	if err != nil {
+		return nil, err
+	}
+	l, err := net.Listen("unix", path.Join(dir, agentListenFile))
+	if err != nil {
+		return nil, err
+	}
+	return l, nil
+}
+
+// ForwardAgentConnections takes connections from a listener to proxy into the
+// session on the OpenSSH channel for agent connections. It blocks and services
+// connections until the listener stop accepting.
+func ForwardAgentConnections(l net.Listener, s Session) {
+	sshConn := s.Context().Value(ContextKeyConn).(gossh.Conn)
+	for {
+		conn, err := l.Accept()
+		if err != nil {
+			return
+		}
+		go func(conn net.Conn) {
+			defer conn.Close()
+			channel, reqs, err := sshConn.OpenChannel(agentChannelType, nil)
+			if err != nil {
+				return
+			}
+			defer channel.Close()
+			go gossh.DiscardRequests(reqs)
+			var wg sync.WaitGroup
+			wg.Add(2)
+			go func() {
+				io.Copy(conn, channel)
+				conn.(*net.UnixConn).CloseWrite()
+				wg.Done()
+			}()
+			go func() {
+				io.Copy(channel, conn)
+				channel.CloseWrite()
+				wg.Done()
+			}()
+			wg.Wait()
+		}(conn)
+	}
+}