internal/rest/resources: Reserve the /core namespace

...for CoreAPI servers

Fixes canonical#227

Signed-off-by: Wesley Hershberger <[email protected]>

internal/rest/resources/resources.go

@@ -95,6 +95,15 @@ func ValidateEndpoints(extensionServers map[string]rest.Server, coreAddress stri
 			serverAddresses[server.Address.String()] = true
 		}
 
+		// Reserve the /core namespace for Microcluster
+		if server.CoreAPI {
+			for _, resource := range server.Resources {
+				if firstPathElement(string(resource.PathPrefix)) == endpoints.EndpointsCore {
+					return fmt.Errorf("Resource path prefix from server %q conflicts with reserved namespace /%s", serverName, endpoints.EndpointsCore)
+				}
+			}
+		}
+
 		// Ensure no endpoint path conflicts with another endpoint on the same server.
 		// If a server is an extension to the core API, it will be compared against all core API paths.
 		for _, resource := range server.Resources {
@@ -124,3 +133,18 @@ func ValidateEndpoints(extensionServers map[string]rest.Server, coreAddress stri
 
 	return nil
 }
+
+// "/core"          -> "core"
+// "/core/internal" -> "core"
+// "/"              -> ""
+// Returns the first non-root path element in `path`.
+func firstPathElement(path string) string {
+	dir, file := filepath.Split(path)
+	if dir == "" || dir == "/" {
+		return file
+	}
+	if file == "" {
+		return filepath.Base(dir)
+	}
+	return firstPathElement(dir)
+}