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/restart.go

@@ -0,0 +1,67 @@
+// 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.
+// This code is heavily inspired by the archived gofacebook/gracenet/net.go handler
+
+package graceful
+
+import (
+	"fmt"
+	"os"
+	"os/exec"
+	"strings"
+)
+
+// RestartProcess starts a new process passing it the active listeners. It
+// doesn't fork, but starts a new process using the same environment and
+// arguments as when it was originally started. This allows for a newly
+// deployed binary to be started. It returns the pid of the newly started
+// process when successful.
+func RestartProcess() (int, error) {
+	listeners := getActiveListeners()
+
+	// Extract the fds from the listeners.
+	files := make([]*os.File, len(listeners))
+	for i, l := range listeners {
+		var err error
+		// Now, all our listeners actually have File() functions so instead of
+		// individually casting we just use a hacky interface
+		files[i], err = l.(filer).File()
+		if err != nil {
+			return 0, err
+		}
+		// Remember to close these at the end.
+		defer files[i].Close()
+	}
+
+	// Use the original binary location. This works with symlinks such that if
+	// the file it points to has been changed we will use the updated symlink.
+	argv0, err := exec.LookPath(os.Args[0])
+	if err != nil {
+		return 0, err
+	}
+
+	// Pass on the environment and replace the old count key with the new one.
+	var env []string
+	for _, v := range os.Environ() {
+		if !strings.HasPrefix(v, listenFDs+"=") {
+			env = append(env, v)
+		}
+	}
+	env = append(env, fmt.Sprintf("%s=%d", listenFDs, len(listeners)))
+
+	allFiles := append([]*os.File{os.Stdin, os.Stdout, os.Stderr}, files...)
+	process, err := os.StartProcess(argv0, os.Args, &os.ProcAttr{
+		Dir:   originalWD,
+		Env:   env,
+		Files: allFiles,
+	})
+	if err != nil {
+		return 0, err
+	}
+	return process.Pid, nil
+}
+
+type filer interface {
+	File() (*os.File, error)
+}