Detect and warn if duplicate lambda libraries are detected

src/index.spec.ts

@@ -24,6 +24,7 @@ import { TraceSource } from "./trace/trace-context-service";
 import { inflateSync } from "zlib";
 import { MetricsListener } from "./metrics/listener";
 import { SpanOptions, TracerWrapper } from "./trace/tracer-wrapper";
+import fs from "fs";
 
 jest.mock("./metrics/enhanced-metrics");
 
@@ -623,3 +624,64 @@ describe("emitTelemetryOnErrorOutsideHandler", () => {
     expect(mockedStartSpan).toBeCalledTimes(0);
   });
 });
+
+describe("detectDuplicateInstallations", () => {
+  jest.mock("fs");
+
+  let fsAccessMock: jest.SpyInstance;
+  let logWarningMock: jest.SpyInstance;
+
+  beforeEach(() => {
+    jest.resetAllMocks();
+    fsAccessMock = jest.spyOn(fs, "access");
+    logWarningMock = jest.spyOn(require("./utils"), "logWarning").mockImplementation(() => {});
+  });
+
+  it("should log warning when duplicate installations are detected", async () => {
+    // Mock fs.access to simulate both paths exist
+    fsAccessMock.mockImplementation((path: string, callback: any) => {
+      callback(null); // No error = path exists
+    });
+
+    await datadog(async () => {}, { forceWrap: true })();
+    expect(logWarningMock).toHaveBeenCalledWith(expect.stringContaining("Detected duplicate installations"));
+  });
+
+  it("should not log warning when only layer installation exists", async () => {
+    // Simulate layerPath exists, localPath does not exist
+    fsAccessMock.mockImplementation((path: string, callback: any) => {
+      if (path.includes("/opt/nodejs")) {
+        callback(null); // Exists
+      } else {
+        callback(new Error("ENOENT")); // Does not exist
+      }
+    });
+
+    await datadog(async () => {}, { forceWrap: true })();
+    expect(logWarningMock).not.toHaveBeenCalled();
+  });
+
+  it("should not log warning when only local installation exists", async () => {
+    // Simulate localPath exists, layerPath does not exist
+    fsAccessMock.mockImplementation((path: string, callback: any) => {
+      if (path.includes("/opt/nodejs")) {
+        callback(new Error("ENOENT")); // Does not exist
+      } else {
+        callback(null); // Exists
+      }
+    });
+
+    await datadog(async () => {}, { forceWrap: true })();
+    expect(logWarningMock).not.toHaveBeenCalled();
+  });
+
+  it("should not log warning when neither installation exists", async () => {
+    // Simulate neither path exists
+    fsAccessMock.mockImplementation((path: string, callback: any) => {
+      callback(new Error("ENOENT")); // Does not exist
+    });
+
+    await datadog(async () => {}, { forceWrap: true })();
+    expect(logWarningMock).not.toHaveBeenCalled();
+  });
+});

src/index.ts

@@ -21,11 +21,14 @@ import {
   setSandboxInit,
   setLogger,
   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";
 
 // Backwards-compatible export, TODO deprecate in next major
 export { DatadogTraceHeaders as TraceHeaders } from "./trace/context/extractor";
@@ -127,6 +130,15 @@ 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 +477,23 @@ 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;
+  }
+}