Restore Graceful Restarting & Socket Activation (#7274)

* Prevent deadlock in indexer initialisation during graceful restart

* Move from gracehttp to our own service to add graceful ssh

* Add timeout for start of indexers and make hammer time configurable

* Fix issue with re-initialization in indexer during tests

* move the code to detect use of closed to graceful

* Handle logs gracefully - add a pid suffix just before restart

* Move to using a cond and a holder for indexers

* use time.Since

* Add some comments and attribution

* update modules.txt

* Use zero to disable timeout

* Move RestartProcess to its own file

* Add cleanup routine

modules/ssh/ssh_graceful.go

@@ -0,0 +1,30 @@
+// +build !windows
+
+// Copyright 2019 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 ssh
+
+import (
+	"code.gitea.io/gitea/modules/graceful"
+	"code.gitea.io/gitea/modules/log"
+
+	"github.com/gliderlabs/ssh"
+)
+
+func listen(server *ssh.Server) {
+	gracefulServer := graceful.NewServer("tcp", server.Addr)
+
+	err := gracefulServer.ListenAndServe(server.Serve)
+	if err != nil {
+		log.Critical("Failed to start SSH server: %v", err)
+	}
+	log.Info("SSH Listener: %s Closed", server.Addr)
+
+}
+
+// Unused informs our cleanup routine that we will not be using a ssh port
+func Unused() {
+	graceful.InformCleanup()
+}