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>

namespaces/validate.go

@@ -1,52 +0,0 @@
-package namespaces
-
-import (
-	"regexp"
-
-	"github.com/pkg/errors"
-)
-
-const (
-	label = `[a-z][a-z0-9]+(?:[-]+[a-z0-9]+)*`
-)
-
-func reGroup(s string) string {
-	return `(?:` + s + `)`
-}
-
-func reAnchor(s string) string {
-	return `^` + s + `$`
-}
-
-var (
-	// namespaceRe validates that a namespace matches valid namespaces.
-	//
-	// Rules for domains, defined in RFC 1035, section 2.3.1, are used for
-	// namespaces.
-	namespaceRe = regexp.MustCompile(reAnchor(label + reGroup("[.]"+reGroup(label)) + "*"))
-
-	errNamespaceInvalid = errors.Errorf("invalid namespace, must match %v", namespaceRe)
-)
-
-// IsNamespacesValid return true if the error was due to an invalid namespace
-// name.
-func IsNamespaceInvalid(err error) bool {
-	return errors.Cause(err) == errNamespaceInvalid
-}
-
-// Validate return nil if the string s is a valid namespace name.
-//
-// Namespaces must be valid domain names according to RFC 1035, section 2.3.1.
-// To enforce case insensitvity, all characters must be lower case.
-//
-// In general, namespaces that pass this validation, should be safe for use as
-// a domain name or filesystem path component.
-//
-// Typically, this function is used through NamespacesRequired, rather than
-// directly.
-func Validate(s string) error {
-	if !namespaceRe.MatchString(s) {
-		return errors.Wrapf(errNamespaceInvalid, "namespace %q", s)
-	}
-	return nil
-}