Asynchronously detect if there are duplicate installations of the Lambda library, and log a warning if so.

Author: nhulston

src/index.ts

@@ -20,12 +20,15 @@ import {
   promisifiedHandler,
   setSandboxInit,
   setLogger,
-  setLogLevel,
+  setLogLevel, logWarning,
 } from "./utils";
 import { getEnhancedMetricTags } from "./metrics/enhanced-metrics";
 import { DatadogTraceHeaders } from "./trace/context/extractor";
 import { SpanWrapper } from "./trace/span-wrapper";
 import { SpanOptions, TracerWrapper } from "./trace/tracer-wrapper";
+import path from "path";
+import fs from "fs";
+import * as sea from "node:sea";
 
 // Backwards-compatible export, TODO deprecate in next major
 export { DatadogTraceHeaders as TraceHeaders } from "./trace/context/extractor";
@@ -127,6 +130,13 @@ export function datadog<TEvent, TResult>(
 
   const traceListener = new TraceListener(finalConfig);
 
+  // Check for duplicate installations of the Lambda library
+  detectDuplicateInstallations().then((duplicateFound) => {
+    if (duplicateFound) {
+      logWarning(`Detected duplicate installations of datadog-lambda-js. This can cause: (1) increased cold start times, (2) broken metrics, and (3) other unexpected behavior. Please use either the Lambda layer version or the package in node_modules, but not both. See: https://docs.datadoghq.com/serverless/aws_lambda/installation/nodejs/?tab=custom`)
+    }
+  })
+
   // Only wrap the handler once unless forced
   const _ddWrappedKey = "_ddWrapped";
   if ((handler as any)[_ddWrappedKey] !== undefined && !finalConfig.forceWrap) {
@@ -465,3 +475,23 @@ export async function emitTelemetryOnErrorOutsideHandler(
     await metricsListener.onCompleteInvocation();
   }
 }
+
+async function detectDuplicateInstallations() {
+  try {
+    const layerPath = '/opt/nodejs/node_modules/datadog-lambda-js';
+    const localPath = path.join(process.cwd(), 'node_modules/datadog-lambda-js');
+
+    const checkPathExistsAsync = (libraryPath: string): Promise<boolean> =>
+      new Promise(resolve => fs.access(libraryPath, err => resolve(!err)));
+
+    const [layerExists, localExists] = await Promise.all([
+      checkPathExistsAsync(layerPath),
+      checkPathExistsAsync(localPath)
+    ]);
+
+    return layerExists && localExists;
+  } catch (err) {
+    logDebug('Failed to check for duplicate installations.');
+    return false;
+  }
+}