ctr: make kill use stop-signal by default

The OCI image specification includes a `StopSignal` field in the image
configuration, denoting the system call signal to be sent to the
container to exit.  This commit adds a new `WithImageStopSignal` container
option that can be used for storing the `StopSignal` field as a label on
the container.  This commit also adjusts `ctr run` to call
`WithImageStopSignal` and `ctr tasks kill` to send the signal stored in
that label by default.

Signed-off-by: Samuel Karp <skarp@amazon.com>

signals.go

@@ -17,12 +17,73 @@
 package containerd
 
 import (
+	"context"
+	"encoding/json"
 	"fmt"
 	"strconv"
 	"strings"
 	"syscall"
+
+	"github.com/containerd/containerd/content"
+	"github.com/containerd/containerd/images"
+	"github.com/opencontainers/image-spec/specs-go/v1"
 )
 
+// StopSignalLabel is a well-known containerd label for storing the stop
+// signal specified in the OCI image config
+const StopSignalLabel = "io.containerd.image.config.stop-signal"
+
+// GetStopSignal retrieves the container stop signal, specified by the
+// well-known containerd label (StopSignalLabel)
+func GetStopSignal(ctx context.Context, container Container, defaultSignal syscall.Signal) (syscall.Signal, error) {
+	labels, err := container.Labels(ctx)
+	if err != nil {
+		return -1, err
+	}
+
+	if stopSignal, ok := labels[StopSignalLabel]; ok {
+		return ParseSignal(stopSignal)
+	}
+
+	return defaultSignal, nil
+}
+
+// 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)
+	if err != nil {
+		return "", err
+	}
+	ic, err := image.Config(ctx)
+	if err != nil {
+		return "", err
+	}
+	var (
+		ociimage v1.Image
+		config   v1.ImageConfig
+	)
+	switch ic.MediaType {
+	case v1.MediaTypeImageConfig, images.MediaTypeDockerSchema2Config:
+		p, err := content.ReadBlob(ctx, image.ContentStore(), ic)
+		if err != nil {
+			return "", err
+		}
+
+		if err := json.Unmarshal(p, &ociimage); err != nil {
+			return "", err
+		}
+		config = ociimage.Config
+	default:
+		return "", fmt.Errorf("unknown image config media type %s", ic.MediaType)
+	}
+
+	if config.StopSignal == "" {
+		return defaultSignal, nil
+	}
+
+	return config.StopSignal, nil
+}
+
 // ParseSignal parses a given string into a syscall.Signal
 // it checks that the signal exists in the platform-appropriate signalMap
 func ParseSignal(rawSignal string) (syscall.Signal, error) {