Add more context to missing handler error

Amend the error message shown when a message handler is missing from a
plugin with the expected name of the handler and the name of the plugin
it's missing from.

packages/lib/link/messages.js

@@ -3,6 +3,25 @@
 const libSchema = require("@clusterio/lib/schema");
 const libErrors = require("@clusterio/lib/errors");
 
+class MissingLinkHandlerError extends Error {
+	constructor(type, source, target) {
+		super();
+		this.code = "MISSING_LINK_HANDLER";
+		this.type = type;
+		this.source = source;
+		this.target = target;
+		this.handler = null;
+		this.plugin = null;
+	}
+
+	get message() {
+		let handler = this.handler ? `${this.handler}` : "handler";
+		let plugin = this.plugin ? ` on plugin ${this.plugin}` : "";
+
+		return `Missing ${handler}${plugin} for ${this.type} on ${this.source}-${this.target} link`;
+	}
+}
+
 /**
  * Represents a message that can be sent over the link
  *
@@ -168,7 +187,7 @@ class Request extends Message {
 			}
 
 			if (!handler) {
-				throw new Error(`Missing handler for ${this.requestType} on ${link.source}-${link.target} link`);
+				throw new MissingLinkHandlerError(this.requestType, link.source, link.target);
 			}
 
 			// Check permission if this is a handler for a control connection on the master
@@ -766,7 +785,7 @@ class Event extends Message {
 			}
 
 			if (!handler) {
-				throw new Error(`Missing handler for ${this.eventType} on ${link.source}-${link.target} link`);
+				throw new MissingLinkHandlerError(this.eventType, link.source, link.target);
 			}
 
 			link.setHandler(this.eventType, message => {
@@ -959,7 +978,15 @@ messages.playerEvent = new Event({
  */
 function attachAllMessages(link) {
 	for (let [name, message] of Object.entries(messages)) {
-		message.attach(link, link[name + message.handlerSuffix]);
+		let handler = `${name}${message.handlerSuffix}`;
+		try {
+			message.attach(link, link[handler]);
+		} catch (err) {
+			if (err.code === "MISSING_LINK_HANDLER") {
+				err.handler = handler;
+			}
+			throw err;
+		}
 	}
 }
 

packages/lib/plugin.js

@@ -402,13 +402,23 @@ function attachPluginMessages(link, pluginInfo, plugin) {
 		}
 
 		let handler = name + messageFormat.handlerSuffix;
-		if (plugin === null || !plugin[handler]) {
-			messageFormat.attach(link);
+		try {
+			if (plugin === null || !plugin[handler]) {
+				messageFormat.attach(link);
 
-		} else {
-			messageFormat.attach(link, async function(message, format) {
-				return await plugin[handler](message, format, this);
-			});
+			} else {
+				messageFormat.attach(link, async function(message, format) {
+					return await plugin[handler](message, format, this);
+				});
+			}
+
+		} catch (err) {
+			if (err.code === "MISSING_LINK_HANDLER") {
+				err.plugin = pluginInfo.name;
+				err.handler = handler;
+			}
+
+			throw err;
 		}
 	}
 }

test/lib/plugin.js

@@ -55,7 +55,7 @@ describe("lib/plugin", function() {
 		it("should throw if missing handler for the given message", function() {
 			assert.throws(
 				() => libPlugin.attachPluginMessages(mockLink, { name: "test", messages: { mockEvent }}, {}),
-				new Error("Missing handler for test:test_event on source-target link")
+				new Error("Missing mockEventEventHandler on plugin test for test:test_event on source-target link")
 			);
 		});
 		it("should throw if message starts with the wrong prefix", function() {