Middleware!

Convert middleware in the config to be a map of type->[]Middleware

Add support for registry & repository middleware.

Some naming updates as well.

Signed-off-by: Andy Goldstein <agoldste@redhat.com>

docs/middleware/registry/middleware.go

@@ -0,0 +1,39 @@
+package middleware
+
+import (
+	"fmt"
+
+	"github.com/docker/distribution"
+)
+
+// InitFunc is the type of a RegistryMiddleware factory function and is
+// used to register the contsructor for different RegistryMiddleware backends.
+type InitFunc func(registry distribution.Registry, options map[string]interface{}) (distribution.Registry, error)
+
+var middlewares map[string]InitFunc
+
+// Register is used to register an InitFunc for
+// a RegistryMiddleware backend with the given name.
+func Register(name string, initFunc InitFunc) error {
+	if middlewares == nil {
+		middlewares = make(map[string]InitFunc)
+	}
+	if _, exists := middlewares[name]; exists {
+		return fmt.Errorf("name already registered: %s", name)
+	}
+
+	middlewares[name] = initFunc
+
+	return nil
+}
+
+// Get constructs a RegistryMiddleware with the given options using the named backend.
+func Get(name string, options map[string]interface{}, registry distribution.Registry) (distribution.Registry, error) {
+	if middlewares != nil {
+		if initFunc, exists := middlewares[name]; exists {
+			return initFunc(registry, options)
+		}
+	}
+
+	return nil, fmt.Errorf("no registry middleware registered with name: %s", name)
+}