Pull request: home: fix exe path finding

Closes #4735.

Squashed commit of the following:

commit 8228e5f82c9d8056d5567a7f1b13b1365346c4d4
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Mon Jul 11 17:41:19 2022 +0300

    home: fix exe path finding

internal/home/controlupdate.go

@@ -117,7 +117,18 @@ func handleUpdate(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	err := Context.updater.Update()
+	// Retain the current absolute path of the executable, since the updater is
+	// likely to change the position current one to the backup directory.
+	//
+	// See https://github.com/AdguardTeam/AdGuardHome/issues/4735.
+	execPath, err := os.Executable()
+	if err != nil {
+		aghhttp.Error(r, w, http.StatusInternalServerError, "getting path: %s", err)
+
+		return
+	}
+
+	err = Context.updater.Update()
 	if err != nil {
 		aghhttp.Error(r, w, http.StatusInternalServerError, "%s", err)
 
@@ -129,13 +140,10 @@ func handleUpdate(w http.ResponseWriter, r *http.Request) {
 		f.Flush()
 	}
 
-	// The background context is used because the underlying functions wrap
+	// The background context is used because the underlying functions wrap it
-	// it with timeout and shut down the server, which handles current
+	// with timeout and shut down the server, which handles current request.  It
-	// request. It also should be done in a separate goroutine due to the
+	// also should be done in a separate goroutine for the same reason.
-	// same reason.
+	go finishUpdate(context.Background(), execPath)
-	go func() {
-		finishUpdate(context.Background())
-	}()
 }
 
 // versionResponse is the response for /control/version.json endpoint.
@@ -174,45 +182,46 @@ func tlsConfUsesPrivilegedPorts(c *tlsConfigSettings) (ok bool) {
 }
 
 // finishUpdate completes an update procedure.
-func finishUpdate(ctx context.Context) {
+func finishUpdate(ctx context.Context, execPath string) {
-	log.Info("Stopping all tasks")
+	var err error
+
+	log.Info("stopping all tasks")
+
 	cleanup(ctx)
 	cleanupAlways()
 
-	curBinName, err := os.Executable()
-	if err != nil {
-		log.Fatalf("executable path request failed: %s", err)
-	}
-
 	if runtime.GOOS == "windows" {
 		if Context.runningAsService {
-			// Note:
+			// NOTE: We can't restart the service via "kardianos/service"
-			// we can't restart the service via "kardianos/service" package - it kills the process first
+			// package, because it kills the process first we can't start a new
-			// we can't start a new instance - Windows doesn't allow it
+			// instance, because Windows doesn't allow it.
+			//
+			// TODO(a.garipov): Recheck the claim above.
 			cmd := exec.Command("cmd", "/c", "net stop AdGuardHome & net start AdGuardHome")
 			err = cmd.Start()
 			if err != nil {
-				log.Fatalf("exec.Command() failed: %s", err)
+				log.Fatalf("restarting: stopping: %s", err)
 			}
+
 			os.Exit(0)
 		}
 
-		cmd := exec.Command(curBinName, os.Args[1:]...)
+		cmd := exec.Command(execPath, os.Args[1:]...)
-		log.Info("Restarting: %v", cmd.Args)
+		log.Info("restarting: %q %q", execPath, os.Args[1:])
 		cmd.Stdin = os.Stdin
 		cmd.Stdout = os.Stdout
 		cmd.Stderr = os.Stderr
 		err = cmd.Start()
 		if err != nil {
-			log.Fatalf("exec.Command() failed: %s", err)
+			log.Fatalf("restarting:: %s", err)
 		}
+
 		os.Exit(0)
-	} else {
+	}
-		log.Info("Restarting: %v", os.Args)
+
-		err = syscall.Exec(curBinName, os.Args, os.Environ())
+	log.Info("restarting: %q %q", execPath, os.Args[1:])
-		if err != nil {
+	err = syscall.Exec(execPath, os.Args, os.Environ())
-			log.Fatalf("syscall.Exec() failed: %s", err)
+	if err != nil {
-		}
+		log.Fatalf("restarting: %s", err)
-		// Unreachable code
 	}
 }