identifiers: use common package for identifier validation

A few days ago, we added validation for namespaces. We've decided to
expand these naming rules to include containers. To facilitate this, a
common package `identifiers` now provides a common validation area.
These rules will be extended to apply to task identifiers, snapshot keys
and other areas where user-provided identifiers may be used.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

identifiers/validate_test.go

@@ -0,0 +1,47 @@
+package identifiers
+
+import (
+	"testing"
+)
+
+func TestValidIdentifiers(t *testing.T) {
+	for _, input := range []string{
+		"default",
+		"Default",
+		t.Name(),
+		"default-default",
+		"default--default",
+		"containerd.io",
+		"foo.boo",
+		"swarmkit.docker.io",
+		"zn--e9.org", // or something like it!
+	} {
+		t.Run(input, func(t *testing.T) {
+			if err := Validate(input); err != nil {
+				t.Fatalf("unexpected error: %v != nil", err)
+			}
+		})
+	}
+}
+
+func TestInvalidIdentifiers(t *testing.T) {
+	for _, input := range []string{
+		".foo..foo",
+		"foo/foo",
+		"foo/..",
+		"foo..foo",
+		"foo.-boo",
+		"-foo.boo",
+		"foo.boo-",
+		"foo_foo.boo_underscores", // boo-urns?
+	} {
+
+		t.Run(input, func(t *testing.T) {
+			if err := Validate(input); err == nil {
+				t.Fatal("expected invalid error")
+			} else if !IsInvalid(err) {
+				t.Fatal("error should be an invalid identifier error")
+			}
+		})
+	}
+}