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/graceful/cleanup.go

@@ -0,0 +1,38 @@
+// 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 graceful
+
+import "sync"
+
+var cleanupWaitGroup sync.WaitGroup
+
+func init() {
+	cleanupWaitGroup = sync.WaitGroup{}
+
+	// There are three places that could inherit sockets:
+	//
+	// * HTTP or HTTPS main listener
+	// * HTTP redirection fallback
+	// * SSH
+	//
+	// If you add an additional place you must increment this number
+	// and add a function to call InformCleanup if it's not going to be used
+	cleanupWaitGroup.Add(3)
+
+	// Wait till we're done getting all of the listeners and then close
+	// the unused ones
+	go func() {
+		cleanupWaitGroup.Wait()
+		// Ignore the error here there's not much we can do with it
+		// They're logged in the CloseProvidedListeners function
+		_ = CloseProvidedListeners()
+	}()
+}
+
+// InformCleanup tells the cleanup wait group that we have either taken a listener
+// or will not be taking a listener
+func InformCleanup() {
+	cleanupWaitGroup.Done()
+}