Add opinionated Plugin interface

pkg/plugin/interfaces.go

@@ -0,0 +1,34 @@
+// Copyright (c) 2019, the Drone Plugins project authors.
+// Please see the AUTHORS file for details. All rights reserved.
+// Use of this source code is governed by an Apache 2.0 license that can be
+// found in the LICENSE file.
+
+package plugin
+
+type (
+	// Plugin is an interface for a Drone plugin written in Go.
+	//
+	// This is a higly opinionated interface for what a Plugin should do. Its
+	// not required that a plugin author follow it.
+	Plugin interface {
+		// Validate checks the inputs to the Plugin and verifies that the
+		// configuration is correct before executing.
+		//
+		// An error is returned if there are any issues with the current
+		// configuration, such as missing information or files not being
+		// present. A Plugin may choose to populate additional information to
+		// ensure a successful execution, for example if a URL is parsed
+		// successfully it can be stored off for later use.
+		//
+		// Validate needs to be called before Exec.
+		Validate() error
+
+		// Exec runs the plugin in the current configuration.
+		//
+		// An error is returned if the Plugin did not run successfully that
+		// describes the runtime error.
+		//
+		// Exec needs to be called after Validate.
+		Exec() error
+	}
+)