backend,termstatus: Unify foreground/background detection

PR #5358 reintroduced a version of the TIOCGPGRP ioctl call that works
on all Unix platforms except Linux, due to a bug/inconsistency in
x/sys/unix. This commit fixes that by introducing termstatus.Tcgetpgrp.

It also introduces termstatus.Getpgrp and termstatus.Tcsetpgrp to deal
with the different signature of unix.Getpgrp in Solaris vs. all other
Unix platforms and an int-overflowing constant on AIX, so that some
AIX/Solaris-specific code can be removed elsewhere and
foreground/background detection is done the same everywhere except on
Windows.

internal/backend/util/foreground_sysv.go

@@ -1,28 +0,0 @@
-//go:build aix || solaris
-// +build aix solaris
-
-package util
-
-import (
-	"os/exec"
-	"syscall"
-
-	"github.com/restic/restic/internal/errors"
-)
-
-func startForeground(cmd *exec.Cmd) (bg func() error, err error) {
-	// run the command in its own process group so that SIGINT
-	// is not sent to it.
-	cmd.SysProcAttr = &syscall.SysProcAttr{
-		Setpgid: true,
-	}
-
-	// start the process
-	err = cmd.Start()
-	if err != nil {
-		return nil, errors.Wrap(err, "cmd.Start")
-	}
-
-	bg = func() error { return nil }
-	return bg, nil
-}

internal/backend/util/foreground_unix.go

@@ -1,5 +1,4 @@
-//go:build !aix && !solaris && !windows
-// +build !aix,!solaris,!windows
+//go:build unix
 
 package util
 
@@ -10,20 +9,11 @@ import (
 
 	"github.com/restic/restic/internal/debug"
 	"github.com/restic/restic/internal/errors"
+	"github.com/restic/restic/internal/ui/termstatus"
 
 	"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.
-	return unix.IoctlSetPointerInt(fd, unix.TIOCSPGRP, pid)
-}
-
 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.
@@ -39,15 +29,15 @@ func startForeground(cmd *exec.Cmd) (bg func() error, err error) {
 	}
 
 	// only move child process to foreground if restic is in the foreground
-	prev, err := tcgetpgrp(int(tty.Fd()))
+	prev, err := termstatus.Tcgetpgrp(int(tty.Fd()))
 	if err != nil {
 		_ = tty.Close()
 		return nil, err
 	}
 
-	self := unix.Getpgrp()
+	self := termstatus.Getpgrp()
 	if prev != self {
-		debug.Log("restic is not controlling the tty")
+		debug.Log("restic is not controlling the tty; err = %v", err)
 		if err := tty.Close(); err != nil {
 			return nil, err
 		}
@@ -66,7 +56,7 @@ func startForeground(cmd *exec.Cmd) (bg func() error, err error) {
 	}
 
 	// move the command's process group into the foreground
-	err = tcsetpgrp(int(tty.Fd()), cmd.Process.Pid)
+	err = termstatus.Tcsetpgrp(int(tty.Fd()), cmd.Process.Pid)
 	if err != nil {
 		_ = tty.Close()
 		return nil, err
@@ -77,7 +67,7 @@ func startForeground(cmd *exec.Cmd) (bg func() error, err error) {
 		signal.Reset(unix.SIGTTOU)
 
 		// reset the foreground process group
-		err = tcsetpgrp(int(tty.Fd()), prev)
+		err = termstatus.Tcsetpgrp(int(tty.Fd()), prev)
 		if err != nil {
 			_ = tty.Close()
 			return err

internal/ui/termstatus/background_linux.go

@@ -1,27 +0,0 @@
-package termstatus
-
-import (
-	"github.com/restic/restic/internal/debug"
-
-	"golang.org/x/sys/unix"
-)
-
-// IsProcessBackground reports whether the current process is running in the
-// background. fd must be a file descriptor for the terminal.
-func IsProcessBackground(fd uintptr) bool {
-	bg, err := isProcessBackground(fd)
-	if err != nil {
-		debug.Log("Can't check if we are in the background. Using default behaviour. Error: %s\n", err.Error())
-		return false
-	}
-	return bg
-}
-
-func isProcessBackground(fd uintptr) (bool, error) {
-	// We need to use IoctlGetUint32 here, because pid_t is 32-bit even on
-	// 64-bit Linux. IoctlGetInt doesn't work on big-endian platforms:
-	// https://github.com/golang/go/issues/45585
-	// https://github.com/golang/go/issues/60429
-	pid, err := unix.IoctlGetUint32(int(fd), unix.TIOCGPGRP)
-	return int(pid) != unix.Getpgrp(), err
-}

internal/ui/termstatus/background_unix.go

@@ -0,0 +1,24 @@
+//go:build unix
+
+package termstatus
+
+import "github.com/restic/restic/internal/debug"
+
+// IsProcessBackground reports whether the current process is running in the
+// background. fd must be a file descriptor for the terminal.
+func IsProcessBackground(fd uintptr) bool {
+	bg, err := isProcessBackground(int(fd))
+	if err != nil {
+		debug.Log("Can't check if we are in the background. Using default behaviour. Error: %s\n", err.Error())
+		return false
+	}
+	return bg
+}
+
+func isProcessBackground(fd int) (bg bool, err error) {
+	pgid, err := Tcgetpgrp(fd)
+	if err != nil {
+		return false, err
+	}
+	return pgid != Getpgrp(), nil
+}

internal/ui/termstatus/background_unix_test.go

@@ -1,3 +1,5 @@
+//go:build unix
+
 package termstatus
 
 import (
@@ -13,7 +15,7 @@ func TestIsProcessBackground(t *testing.T) {
 		t.Skipf("can't open terminal: %v", err)
 	}
 
-	_, err = isProcessBackground(tty.Fd())
+	_, err = isProcessBackground(int(tty.Fd()))
 	rtest.OK(t, err)
 
 	_ = tty.Close()

internal/ui/termstatus/background_windows.go

@@ -1,6 +1,3 @@
-//go:build !linux
-// +build !linux
-
 package termstatus
 
 // IsProcessBackground reports whether the current process is running in the

internal/ui/termstatus/getpgrp_solaris.go

@@ -0,0 +1,8 @@
+package termstatus
+
+import "golang.org/x/sys/unix"
+
+func Getpgrp() int {
+	pid, _ := unix.Getpgrp()
+	return pid
+}

internal/ui/termstatus/getpgrp_unix.go

@@ -0,0 +1,7 @@
+//go:build unix && !solaris
+
+package termstatus
+
+import "golang.org/x/sys/unix"
+
+func Getpgrp() int { return unix.Getpgrp() }

internal/ui/termstatus/tcgetpgrp_linux.go

@@ -0,0 +1,12 @@
+package termstatus
+
+import "golang.org/x/sys/unix"
+
+func Tcgetpgrp(ttyfd int) (int, error) {
+	// We need to use IoctlGetUint32 here, because pid_t is 32-bit even on
+	// 64-bit Linux. IoctlGetInt doesn't work on big-endian platforms:
+	// https://github.com/golang/go/issues/45585
+	// https://github.com/golang/go/issues/60429
+	pid, err := unix.IoctlGetUint32(ttyfd, unix.TIOCGPGRP)
+	return int(pid), err
+}

internal/ui/termstatus/tcgetpgrp_unix.go

@@ -0,0 +1,9 @@
+//go:build unix && !linux
+
+package termstatus
+
+import "golang.org/x/sys/unix"
+
+func Tcgetpgrp(ttyfd int) (int, error) {
+	return unix.IoctlGetInt(ttyfd, unix.TIOCGPGRP)
+}

internal/ui/termstatus/tcsetpgrp_aix.go

@@ -0,0 +1,10 @@
+package termstatus
+
+import "golang.org/x/sys/unix"
+
+func Tcsetpgrp(fd int, pid int) error {
+	// The second argument to IoctlSetPointerInt has type int on AIX,
+	// but the constant overflows 64-bit int, hence the two-step cast.
+	req := uint(unix.TIOCSPGRP)
+	return unix.IoctlSetPointerInt(fd, int(req), pid)
+}

internal/ui/termstatus/tcsetpgrp_unix.go

@@ -0,0 +1,9 @@
+//go:build unix && !aix
+
+package termstatus
+
+import "golang.org/x/sys/unix"
+
+func Tcsetpgrp(fd int, pid int) error {
+	return unix.IoctlSetPointerInt(fd, unix.TIOCSPGRP, pid)
+}