feat: warn user when a instrumented package was already required open…

…-telemetry#636

packages/opentelemetry-node/src/instrumentation/PluginLoader.ts

@@ -78,6 +78,16 @@ export class PluginLoader {
         return this;
       }
 
+      const alreadyRequiredModules = Object.keys(require.cache);
+      const requiredModulesToHook = modulesToHook.filter(name =>
+        alreadyRequiredModules.some(cached => cached.includes(name))
+      );
+      if (requiredModulesToHook.length > 0) {
+        this.logger.warn(
+          `Some modules (${requiredModulesToHook}) were already required when their respective plugin were loaded. They might not work.`
+        );
+      }
+
       // Enable the require hook.
       hook(modulesToHook, (exports, name, baseDir) => {
         if (this._hookState !== HookState.ENABLED) return exports;

packages/opentelemetry-node/test/instrumentation/PluginLoader.test.ts

@@ -85,6 +85,13 @@ const notSupportedVersionPlugins: Plugins = {
   },
 };
 
+const alreadyRequiredPlugins: Plugins = {
+  'already-require-module': {
+    enabled: true,
+    path: '@opentelemetry/plugin-supported-module',
+  },
+};
+
 describe('PluginLoader', () => {
   const tracer = new NoopTracer();
   const logger = new NoopLogger();
@@ -218,6 +225,23 @@ describe('PluginLoader', () => {
       assert.strictEqual(require('simple-module').value(), 0);
       pluginLoader.unload();
     });
+
+    it(`should warn when module was already loaded`, callback => {
+      const verifyWarnLogger = {
+        error: logger.error,
+        info: logger.info,
+        debug: logger.debug,
+        warn: (message: string, ...args: unknown[]) => {
+          assert(message.match(/were already required when/));
+          assert(message.match(/(already-require-module)/));
+          return callback();
+        },
+      };
+      require('already-require-module');
+      const pluginLoader = new PluginLoader(tracer, verifyWarnLogger);
+      pluginLoader.load(alreadyRequiredPlugins);
+      pluginLoader.unload();
+    });
   });
 
   describe('.unload()', () => {