atexit: Terminate with non-zero status after receiving signal

When rclone received a SIGINT (Ctrl+C) or SIGTERM signal while an atexit
function is registered it always terminated with status code 0. Unix
convention is to exit with a non-zero status code. Often it's
`128 + int(signum), but at least not zero.

With this change fatal signals handled by the `atexit` package cause
a non-zero exit code. On Unix systems it's `128 + int(signum)` while
on other systems, such as Windows, it's always 2 ("error not otherwise
categorised").

Resolves #5437.

Signed-off-by: Michael Hanselmann <public@hansmi.ch>

lib/atexit/atexit.go

@@ -51,7 +51,7 @@ func Register(fn func()) FnHandle {
 			fs.Infof(nil, "Signal received: %s", sig)
 			Run()
 			fs.Infof(nil, "Exiting...")
-			os.Exit(0)
+			os.Exit(exitCode(sig))
 		}()
 	})
 

lib/atexit/atexit_other.go

@@ -4,6 +4,12 @@ package atexit
 
 import (
 	"os"
+
+	"github.com/rclone/rclone/lib/exitcode"
 )
 
 var exitSignals = []os.Signal{os.Interrupt}
+
+func exitCode(_ os.Signal) int {
+	return exitcode.UncategorizedError
+}

lib/atexit/atexit_test.go

@@ -0,0 +1,41 @@
+package atexit
+
+import (
+	"os"
+	"runtime"
+	"testing"
+
+	"github.com/rclone/rclone/lib/exitcode"
+	"github.com/stretchr/testify/assert"
+)
+
+type fakeSignal struct{}
+
+func (*fakeSignal) String() string {
+	return "fake"
+}
+
+func (*fakeSignal) Signal() {
+}
+
+var _ os.Signal = (*fakeSignal)(nil)
+
+func TestExitCode(t *testing.T) {
+	switch runtime.GOOS {
+	case "windows", "plan9":
+		for _, i := range []os.Signal{
+			os.Interrupt,
+			os.Kill,
+		} {
+			assert.Equal(t, exitCode(i), exitcode.UncategorizedError)
+		}
+
+	default:
+		// SIGINT (2) and SIGKILL (9) are portable numbers specified by POSIX.
+		assert.Equal(t, exitCode(os.Interrupt), 128+2)
+		assert.Equal(t, exitCode(os.Kill), 128+9)
+	}
+
+	// Never a real signal
+	assert.Equal(t, exitCode(&fakeSignal{}), exitcode.UncategorizedError)
+}

lib/atexit/atexit_unix.go

@@ -5,6 +5,20 @@ package atexit
 import (
 	"os"
 	"syscall"
+
+	"github.com/rclone/rclone/lib/exitcode"
 )
 
 var exitSignals = []os.Signal{syscall.SIGINT, syscall.SIGTERM} // Not syscall.SIGQUIT as we want the default behaviour
+
+// exitCode calculates the exit code for the given signal. Many Unix programs
+// exit with 128+signum if they handle signals. Most shell also implement the
+// same convention if a program is terminated by an uncaught and/or fatal
+// signal.
+func exitCode(sig os.Signal) int {
+	if real, ok := sig.(syscall.Signal); ok && int(real) > 0 {
+		return 128 + int(real)
+	}
+
+	return exitcode.UncategorizedError
+}