Add optional experimentalStreamingLambda field for prerender (#10476)

This adds a new `experimentalStreamingLambda` field to Prerender
outputs, allowing references to an optional streaming lambda path.

.changeset/olive-trees-applaud.md

@@ -0,0 +1,5 @@
+---
+'@vercel/build-utils': minor
+---
+
+Add new optional prerender field: experimentalStreamingLambdaPath

packages/build-utils/src/prerender.ts

@@ -12,6 +12,7 @@ interface PrerenderOptions {
   initialStatus?: number;
   passQuery?: boolean;
   sourcePath?: string;
+  experimentalStreamingLambdaPath?: string;
 }
 
 export class Prerender {
@@ -26,6 +27,7 @@ export class Prerender {
   public initialStatus?: number;
   public passQuery?: boolean;
   public sourcePath?: string;
+  public experimentalStreamingLambdaPath?: string;
 
   constructor({
     expiration,
@@ -38,6 +40,7 @@ export class Prerender {
     initialStatus,
     passQuery,
     sourcePath,
+    experimentalStreamingLambdaPath,
   }: PrerenderOptions) {
     this.type = 'Prerender';
     this.expiration = expiration;
@@ -130,5 +133,14 @@ export class Prerender {
       }
       this.allowQuery = allowQuery;
     }
+
+    if (experimentalStreamingLambdaPath !== undefined) {
+      if (typeof experimentalStreamingLambdaPath !== 'string') {
+        throw new Error(
+          'The `experimentalStreamingLambdaPath` argument for `Prerender` must be a string.'
+        );
+      }
+      this.experimentalStreamingLambdaPath = experimentalStreamingLambdaPath;
+    }
   }
 }

packages/build-utils/test/unit.test.ts

@@ -387,6 +387,42 @@ it('should support passQuery correctly', async () => {
   );
 });
 
+it('should support experimentalStreamingLambdaPath correctly', async () => {
+  new Prerender({
+    expiration: 1,
+    fallback: null,
+    group: 1,
+    bypassToken: 'some-long-bypass-token-to-make-it-work',
+    experimentalStreamingLambdaPath: undefined,
+  });
+  new Prerender({
+    expiration: 1,
+    fallback: null,
+    group: 1,
+    bypassToken: 'some-long-bypass-token-to-make-it-work',
+    experimentalStreamingLambdaPath: '/some/path/to/lambda',
+  });
+  new Prerender({
+    expiration: 1,
+    fallback: null,
+    group: 1,
+    bypassToken: 'some-long-bypass-token-to-make-it-work',
+  });
+
+  expect(() => {
+    new Prerender({
+      expiration: 1,
+      fallback: null,
+      group: 1,
+      bypassToken: 'some-long-bypass-token-to-make-it-work',
+      // @ts-expect-error testing invalid field
+      experimentalStreamingLambdaPath: 1,
+    });
+  }).toThrowError(
+    `The \`experimentalStreamingLambdaPath\` argument for \`Prerender\` must be a string.`
+  );
+});
+
 it('should support require by path for legacy builders', () => {
   const index = require('../');