make docker-credential-wincred work like docker-credential-osxkeychain

* fetch credentials for server with matching hostname if scheme, path, or port are not provided
* if the credential request includes specific scheme, path, or port that does not match entry, don't return
* extract url helpers into a package

Signed-off-by: Emily Casey <ecasey@pivotal.io>
Signed-off-by: Danny Joyce <djoyce@pivotal.io>

registryurl/parse.go

@@ -0,0 +1,37 @@
+package registryurl
+
+import (
+	"errors"
+	"net/url"
+	"strings"
+)
+
+// Parse parses and validates a given serverURL to an url.URL, and
+// returns an error if validation failed. Querystring parameters are
+// omitted in the resulting URL, because they are not used in the helper.
+//
+// If serverURL does not have a valid scheme, `//` is used as scheme
+// before parsing. This prevents the hostname being used as path,
+// and the credentials being stored without host.
+func Parse(registryURL string) (*url.URL, error) {
+	// Check if registryURL has a scheme, otherwise add `//` as scheme.
+	if !strings.Contains(registryURL, "://") && !strings.HasPrefix(registryURL, "//") {
+		registryURL = "//" + registryURL
+	}
+
+	u, err := url.Parse(registryURL)
+	if err != nil {
+		return nil, err
+	}
+
+	if u.Scheme != "" && u.Scheme != "https" && u.Scheme != "http" {
+		return nil, errors.New("unsupported scheme: " + u.Scheme)
+	}
+
+	if GetHostname(u) == "" {
+		return nil, errors.New("no hostname in URL")
+	}
+
+	u.RawQuery = ""
+	return u, nil
+}