Support SIGRTMIN+n signals

systemd uses SIGRTMIN+n signals, but containerd didn't support the signals
since Go's sys/unix doesn't support them.

This change introduces SIGRTMIN+n handling by utilizing moby/sys/signal.

Fixes #5402.

https://www.freedesktop.org/software/systemd/man/systemd.html#Signals

Signed-off-by: Kazuyoshi Kato <katokazu@amazon.com>

signals.go

@@ -24,6 +24,7 @@ import (
 
 	"github.com/containerd/containerd/content"
 	"github.com/containerd/containerd/images"
+	"github.com/moby/sys/signal"
 	v1 "github.com/opencontainers/image-spec/specs-go/v1"
 )
 
@@ -40,7 +41,7 @@ func GetStopSignal(ctx context.Context, container Container, defaultSignal sysca
 	}
 
 	if stopSignal, ok := labels[StopSignalLabel]; ok {
-		return ParseSignal(stopSignal)
+		return signal.ParseSignal(stopSignal)
 	}
 
 	return defaultSignal, nil
@@ -48,7 +49,7 @@ func GetStopSignal(ctx context.Context, container Container, defaultSignal sysca
 
 // GetOCIStopSignal retrieves the stop signal specified in the OCI image config
 func GetOCIStopSignal(ctx context.Context, image Image, defaultSignal string) (string, error) {
-	_, err := ParseSignal(defaultSignal)
+	_, err := signal.ParseSignal(defaultSignal)
 	if err != nil {
 		return "", err
 	}
@@ -81,3 +82,12 @@ func GetOCIStopSignal(ctx context.Context, image Image, defaultSignal string) (s
 
 	return config.StopSignal, nil
 }
+
+// ParseSignal parses a given string into a syscall.Signal
+// the rawSignal can be a string with "SIG" prefix,
+// or a signal number in string format.
+//
+// Deprecated: Use github.com/moby/sys/signal instead.
+func ParseSignal(rawSignal string) (syscall.Signal, error) {
+	return signal.ParseSignal(rawSignal)
+}