ref(nextjs): Exclude cross-platform tracing code from bundles (#3978)

The `@sentry/tracing` package contains shared code, code which only applies to browser, and code which only applies to node. At the moment, webpack is unable to treeshake the former out of the server build and the latter out of the client build. This PR does that treeshaking manually, taking advantage of webpack 5’s new ability[1] to replace a module with an empty object by setting `resolve.alias.<someModule> = false` in the webpack config.

Notes:

- All node tracing integrations have been moved into a `node` folder, so we don't need to worry that at some point a non-node integration might get added to `integrations` and be accidentally excluded.

- Normally we only care about bundle size on the browser side, but since Vercel turns server routes into serverless functions, bundle size matters on the backend, too, so the change was made in both directions.

- As a point of reference, on my test app it took the sentry portion of the client `_app` bundle down from ~32.3 kb to ~30.6 kb, which is a little over a 5% savings.

[1] https://webpack.js.org/configuration/resolve/#resolvealias

packages/nextjs/src/config/types.ts

@@ -42,6 +42,9 @@ export type WebpackConfigObject = {
   output: { filename: string; path: string };
   target: string;
   context: string;
+  resolve?: {
+    alias?: { [key: string]: string | boolean };
+  };
 } & {
   // other webpack options
   [key: string]: unknown;

packages/nextjs/src/config/webpack.ts

@@ -60,6 +60,24 @@ export function constructWebpackConfigFunction(
     const origEntryProperty = newConfig.entry;
     newConfig.entry = async () => addSentryToEntryProperty(origEntryProperty, buildContext);
 
+    // In webpack 5, you can get webpack to replace any module you'd like with an empty object, just by setting its
+    // `resolve.alias` value to `false`. Not much of our code is neatly separated into "things node needs" and "things
+    // the browser needs," but where it is, we can save ~1.6 kb in eventual bundle size by excluding code we know we
+    // don't need. (Normally this would only matter for the client side, but because vercel turns backend code into
+    // serverless functions, it's worthwhile to do it for both.)
+    if (buildContext.webpack.version.startsWith('5')) {
+      const excludedTracingDir = buildContext.isServer ? 'browser' : 'integrations/node';
+      newConfig.resolve = {
+        ...newConfig.resolve,
+        alias: {
+          ...newConfig.resolve?.alias,
+          [path.resolve(buildContext.dir, `./node_modules/@sentry/tracing/esm/${excludedTracingDir}`)]: false,
+          // TODO It's not clear if it will ever pull from `dist` (in testing it never does), so we may not need this.
+          [path.resolve(buildContext.dir, `./node_modules/@sentry/tracing/dist/${excludedTracingDir}`)]: false,
+        },
+      };
+    }
+
     // Enable the Sentry plugin (which uploads source maps to Sentry when not in dev) by default
     const enableWebpackPlugin = buildContext.isServer
       ? !userNextConfig.sentry?.disableServerWebpackPlugin

packages/tracing/src/integrations/index.ts

@@ -1,4 +1,4 @@
-export { Express } from './express';
-export { Postgres } from './postgres';
-export { Mysql } from './mysql';
-export { Mongo } from './mongo';
+export { Express } from './node/express';
+export { Postgres } from './node/postgres';
+export { Mysql } from './node/mysql';
+export { Mongo } from './node/mongo';

packages/tracing/test/integrations/mongo.test.ts → packages/tracing/test/integrations/node/mongo.test.ts

@@ -1,8 +1,8 @@
 /* eslint-disable @typescript-eslint/unbound-method */
 import { Hub, Scope } from '@sentry/hub';
 
-import { Mongo } from '../../src/integrations/mongo';
-import { Span } from '../../src/span';
+import { Mongo } from '../../../src/integrations/node/mongo';
+import { Span } from '../../../src/span';
 
 class Collection {
   public collectionName: string = 'mockedCollectionName';

packages/tracing/test/integrations/postgres.test.ts → packages/tracing/test/integrations/node/postgres.test.ts

@@ -1,8 +1,8 @@
 /* eslint-disable @typescript-eslint/unbound-method */
 import { Hub, Scope } from '@sentry/hub';
 
-import { Postgres } from '../../src/integrations/postgres';
-import { Span } from '../../src/span';
+import { Postgres } from '../../../src/integrations/node/postgres';
+import { Span } from '../../../src/span';
 
 class PgClient {
   // https://node-postgres.com/api/client#clientquery