Preserve unix socket file (#20499)

By default Gitea will always unlink any sockets that are provided using the `LISTEN_FDS` environment variable. This is because it uses this variable to handle passing when it is doing a graceful restart. However, this same mechanism is used by systemd - which explicitly expects that passed in sockets should not be unlinked by the receiving process. 

This PR adjusts Gitea's graceful restart mechanism to use an additional environment variable which tracks if a listening socket was opened by Gitea - and therefore should be unlinked on shutdown by Gitea.

Fix #20490

Co-authored-by: zeripath <art27@cantab.net>

Author: frankli0324

Diff for: modules/graceful/net_unix.go

@@ -23,6 +23,7 @@ import (
 const (
 	listenFDs = "LISTEN_FDS"
 	startFD   = 3
+	unlinkFDs = "GITEA_UNLINK_FDS"
 )
 
 // In order to keep the working directory the same as when we started we record
@@ -33,8 +34,10 @@ var (
 	once  = sync.Once{}
 	mutex = sync.Mutex{}
 
-	providedListeners = []net.Listener{}
-	activeListeners   = []net.Listener{}
+	providedListenersToUnlink = []bool{}
+	activeListenersToUnlink   = []bool{}
+	providedListeners         = []net.Listener{}
+	activeListeners           = []net.Listener{}
 )
 
 func getProvidedFDs() (savedErr error) {
@@ -53,6 +56,16 @@ func getProvidedFDs() (savedErr error) {
 			return
 		}
 
+		fdsToUnlinkStr := strings.Split(os.Getenv(unlinkFDs), ",")
+		providedListenersToUnlink = make([]bool, n)
+		for _, fdStr := range fdsToUnlinkStr {
+			i, err := strconv.Atoi(fdStr)
+			if err != nil || i < 0 || i >= n {
+				continue
+			}
+			providedListenersToUnlink[i] = true
+		}
+
 		for i := startFD; i < n+startFD; i++ {
 			file := os.NewFile(uintptr(i), fmt.Sprintf("listener_FD%d", i))
 
@@ -136,8 +149,11 @@ func GetListenerTCP(network string, address *net.TCPAddr) (*net.TCPListener, err
 	for i, l := range providedListeners {
 		if isSameAddr(l.Addr(), address) {
 			providedListeners = append(providedListeners[:i], providedListeners[i+1:]...)
+			needsUnlink := providedListenersToUnlink[i]
+			providedListenersToUnlink = append(providedListenersToUnlink[:i], providedListenersToUnlink[i+1:]...)
 
 			activeListeners = append(activeListeners, l)
+			activeListenersToUnlink = append(activeListenersToUnlink, needsUnlink)
 			return l.(*net.TCPListener), nil
 		}
 	}
@@ -148,6 +164,7 @@ func GetListenerTCP(network string, address *net.TCPAddr) (*net.TCPListener, err
 		return nil, err
 	}
 	activeListeners = append(activeListeners, l)
+	activeListenersToUnlink = append(activeListenersToUnlink, false)
 	return l, nil
 }
 
@@ -166,9 +183,15 @@ func GetListenerUnix(network string, address *net.UnixAddr) (*net.UnixListener,
 	for i, l := range providedListeners {
 		if isSameAddr(l.Addr(), address) {
 			providedListeners = append(providedListeners[:i], providedListeners[i+1:]...)
+			needsUnlink := providedListenersToUnlink[i]
+			providedListenersToUnlink = append(providedListenersToUnlink[:i], providedListenersToUnlink[i+1:]...)
+
+			activeListenersToUnlink = append(activeListenersToUnlink, needsUnlink)
 			activeListeners = append(activeListeners, l)
 			unixListener := l.(*net.UnixListener)
-			unixListener.SetUnlinkOnClose(true)
+			if needsUnlink {
+				unixListener.SetUnlinkOnClose(true)
+			}
 			return unixListener, nil
 		}
 	}
@@ -189,6 +212,7 @@ func GetListenerUnix(network string, address *net.UnixAddr) (*net.UnixListener,
 	}
 
 	activeListeners = append(activeListeners, l)
+	activeListenersToUnlink = append(activeListenersToUnlink, true)
 	return l, nil
 }
 
@@ -223,3 +247,11 @@ func getActiveListeners() []net.Listener {
 	copy(listeners, activeListeners)
 	return listeners
 }
+
+func getActiveListenersToUnlink() []bool {
+	mutex.Lock()
+	defer mutex.Unlock()
+	listenersToUnlink := make([]bool, len(activeListenersToUnlink))
+	copy(listenersToUnlink, activeListenersToUnlink)
+	return listenersToUnlink
+}

Diff for: modules/graceful/restart_unix.go

@@ -12,6 +12,7 @@ import (
 	"net"
 	"os"
 	"os/exec"
+	"strconv"
 	"strings"
 	"sync"
 	"syscall"
@@ -75,6 +76,20 @@ func RestartProcess() (int, error) {
 	}
 	env = append(env, fmt.Sprintf("%s=%d", listenFDs, len(listeners)))
 
+	sb := &strings.Builder{}
+	for i, unlink := range getActiveListenersToUnlink() {
+		if !unlink {
+			continue
+		}
+		_, _ = sb.WriteString(strconv.Itoa(i))
+		_, _ = sb.WriteString(",")
+	}
+	unlinkStr := sb.String()
+	if len(unlinkStr) > 0 {
+		unlinkStr = unlinkStr[:len(unlinkStr)-1]
+		env = append(env, fmt.Sprintf("%s=%s", unlinkFDs, unlinkStr))
+	}
+
 	allFiles := append([]*os.File{os.Stdin, os.Stdout, os.Stderr}, files...)
 	process, err := os.StartProcess(argv0, os.Args, &os.ProcAttr{
 		Dir:   originalWD,