Labels are consistently validated across services

* The combined size of a key/value pair cannot exceed 4096 bytes

Signed-off-by: Jess Valarezo <valarezo.jessica@gmail.com>

labels/validate_test.go

@@ -0,0 +1,37 @@
+package labels
+
+import (
+	"strings"
+	"testing"
+
+	"github.com/containerd/containerd/errdefs"
+)
+
+func TestValidLabels(t *testing.T) {
+	shortStr := "s"
+	longStr := strings.Repeat("s", maxSize-1)
+
+	for key, value := range map[string]string{
+		"some":   "value",
+		shortStr: longStr,
+	} {
+		if err := Validate(key, value); err != nil {
+			t.Fatalf("unexpected error: %v != nil", err)
+		}
+	}
+}
+
+func TestInvalidLabels(t *testing.T) {
+	addOneStr := "s"
+	maxSizeStr := strings.Repeat("s", maxSize)
+
+	for key, value := range map[string]string{
+		maxSizeStr: addOneStr,
+	} {
+		if err := Validate(key, value); err == nil {
+			t.Fatal("expected invalid error")
+		} else if !errdefs.IsInvalidArgument(err) {
+			t.Fatal("error should be an invalid label error")
+		}
+	}
+}