Warn about modules loaded before trace agent

Add additional logging to report any modules loaded before the trace
agent even if they are not modules that we patch directly.

Fixes #187.

index.js

@@ -16,6 +16,8 @@
 
 'use strict';
 
+var filesLoadedBeforeTrace = Object.keys(require.cache);
+
 // Load continuation-local-storage first to ensure the core async APIs get
 // patched before any user-land modules get loaded.
 require('continuation-local-storage');
@@ -26,6 +28,17 @@ var semver = require('semver');
 var constants = require('./lib/constants.js');
 var path = require('path');
 
+var modulesLoadedBeforeTrace = [];
+var moduleRegex =
+  new RegExp('.*?node_modules\\' + path.sep + '([^\\' + path.sep + ']*).*');
+for (var i = 0; i < filesLoadedBeforeTrace.length; i++) {
+  var matches = moduleRegex.exec(filesLoadedBeforeTrace[i]);
+  if (matches && matches.length > 1 &&
+      modulesLoadedBeforeTrace.indexOf(matches[1]) === -1) {
+    modulesLoadedBeforeTrace.push(matches[1]);
+  }
+}
+
 /**
  * Phantom implementation of the trace agent. This allows API users to decouple
  * the enable/disable logic from the calls to the tracing API. The phantom API
@@ -121,6 +134,12 @@ var publicAgent = {
     var headers = {};
     headers[constants.TRACE_AGENT_REQUEST_HEADER] = 1;
 
+    if (modulesLoadedBeforeTrace.length > 0) {
+      logger.warn('Tracing might not work as the following modules ' +
+        ' were loaded before the trace agent was initialized: ' +
+        JSON.stringify(modulesLoadedBeforeTrace));
+    }
+
     if (typeof config.projectId === 'undefined') {
       // Queue the work to acquire the projectNumber (potentially from the
       // network.)
@@ -172,6 +191,6 @@ var publicAgent = {
 module.exports = global._google_trace_agent = publicAgent;
 
 // If the module was --require'd from the command line, start the agent.
-if (module.parent.id === 'internal/preload') {
+if (module.parent && module.parent.id === 'internal/preload') {
   module.exports.start();
 }

test/fixtures/start-agent.js

@@ -0,0 +1,20 @@
+/**
+ * Copyright 2015 Google Inc. All Rights Reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+'use strict';
+
+process.env.GCLOUD_TRACE_LOGLEVEL = 4;
+require('glob'); // Load a module before agent
+require('../..').start();

test/standalone/test-modules-loaded-before-agent.js

@@ -0,0 +1,29 @@
+/**
+ * Copyright 2015 Google Inc. All Rights Reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+'use strict';
+
+var assert = require('assert');
+var cp = require('child_process');
+
+describe('modules loaded before agent', function() {
+  it('should log if modules were loaded before agent', function() {
+    var output =
+      cp.execSync('node test/fixtures/start-agent.js');
+    console.log(output.toString());
+    assert(output.toString().match(/Tracing might not work.*"glob".*/));
+  });
+});