Merge pull request #5358 from MichaelEischer/fix-tty-capture-in-background

Fix tty capture when started in background

changelog/unreleased/issue-5354

@@ -0,0 +1,12 @@
+Bugfix: Allow use of rclone/sftp backend when running restic in background
+
+When starting restic in the background, this could result in unexpected behavior
+when using the rclone or sftp backend.
+
+For example running `restic -r rclone:./example --insecure-no-password init &`
+could cause the calling `bash` shell to exit unexpectedly.
+
+This has been fixed.
+
+https://github.com/restic/restic/issues/5354
+https://github.com/restic/restic/pull/5358

internal/backend/util/foreground_unix.go

@@ -14,6 +14,10 @@ import (
 	"golang.org/x/sys/unix"
 )
 
+func tcgetpgrp(fd int) (int, error) {
+	return unix.IoctlGetInt(fd, unix.TIOCGPGRP)
+}
+
 func tcsetpgrp(fd int, pid int) error {
 	// IoctlSetPointerInt silently casts to int32 internally,
 	// so this assumes pid fits in 31 bits.
@@ -21,24 +25,39 @@ func tcsetpgrp(fd int, pid int) error {
 }
 
 func startForeground(cmd *exec.Cmd) (bg func() error, err error) {
+	// run the command in its own process group
+	// this ensures that sending ctrl-c to restic will not immediately stop the backend process.
+	cmd.SysProcAttr = &unix.SysProcAttr{
+		Setpgid: true,
+	}
+
 	// open the TTY, we need the file descriptor
 	tty, err := os.OpenFile("/dev/tty", os.O_RDWR, 0)
 	if err != nil {
 		debug.Log("unable to open tty: %v", err)
-		bg = func() error {
-			return nil
-		}
-		return bg, cmd.Start()
+		return startFallback(cmd)
 	}
 
+	// only move child process to foreground if restic is in the foreground
+	prev, err := tcgetpgrp(int(tty.Fd()))
+	if err != nil {
+		_ = tty.Close()
+		return nil, err
+	}
+
+	self := unix.Getpgrp()
+	if prev != self {
+		debug.Log("restic is not controlling the tty")
+		if err := tty.Close(); err != nil {
+			return nil, err
+		}
+		return startFallback(cmd)
+	}
+
+	// Prevent getting suspended when interacting with the tty
 	signal.Ignore(unix.SIGTTIN)
 	signal.Ignore(unix.SIGTTOU)
 
-	// run the command in its own process group
-	cmd.SysProcAttr = &unix.SysProcAttr{
-		Setpgid: true,
-	}
-
 	// start the process
 	err = cmd.Start()
 	if err != nil {
@@ -47,7 +66,6 @@ func startForeground(cmd *exec.Cmd) (bg func() error, err error) {
 	}
 
 	// move the command's process group into the foreground
-	prev := unix.Getpgrp()
 	err = tcsetpgrp(int(tty.Fd()), cmd.Process.Pid)
 	if err != nil {
 		_ = tty.Close()
@@ -70,3 +88,11 @@ func startForeground(cmd *exec.Cmd) (bg func() error, err error) {
 
 	return bg, nil
 }
+
+func startFallback(cmd *exec.Cmd) (bg func() error, err error) {
+	bg = func() error {
+		return nil
+	}
+
+	return bg, cmd.Start()
+}