fix(gatsby-plugin-netlify): don't add /undefined in generated _headers file (#18925)

* fix(gatsby-plugin-netlify): /undefined in generated _headers file

* Explicitly check that /undefined is not in output of buildHeadersProgram

* Fix matcher

Author: jonathonlui

packages/gatsby-plugin-netlify/src/__tests__/__snapshots__/build-headers-program.js.snap

@@ -284,3 +284,77 @@ exports[`build-headers-program without caching headers 1`] = `
   Link: </component---src-pages-404-js-53e6c51a5a7e73090f50.js>; rel=preload; as=script
 "
 `;
+
+exports[`build-headers-program without manifest['pages-manifest'] 1`] = `
+"## Created with gatsby-plugin-netlify
+
+/*
+  X-Frame-Options: DENY
+  X-XSS-Protection: 1; mode=block
+  X-Content-Type-Options: nosniff
+  Referrer-Policy: same-origin
+/component---node-modules-gatsby-plugin-offline-app-shell-js-78f9e4dea04737fa062d.js
+  Cache-Control: public, max-age=31536000, immutable
+/0-0180cd94ef2497ac7db8.js
+  Cache-Control: public, max-age=31536000, immutable
+/component---src-templates-blog-post-js-517987eae96e75cddbe7.js
+  Cache-Control: public, max-age=31536000, immutable
+/component---src-pages-404-js-53e6c51a5a7e73090f50.js
+  Cache-Control: public, max-age=31536000, immutable
+/component---src-pages-index-js-0bdd01c77ee09ef0224c.js
+  Cache-Control: public, max-age=31536000, immutable
+/webpack-runtime-acaa8994f1f704475e21.js
+  Cache-Control: public, max-age=31536000, immutable
+/styles.1025963f4f2ec7abbad4.css
+  Cache-Control: public, max-age=31536000, immutable
+/styles-565f081c8374bbda155f.js
+  Cache-Control: public, max-age=31536000, immutable
+/app-f33c13590352da20930f.js
+  Cache-Control: public, max-age=31536000, immutable
+/static/*
+  Cache-Control: public, max-age=31536000, immutable
+/sw.js
+  Cache-Control: no-cache
+/offline-plugin-app-shell-fallback/
+  Link: </webpack-runtime-acaa8994f1f704475e21.js>; rel=preload; as=script
+  Link: </styles-565f081c8374bbda155f.js>; rel=preload; as=script
+  Link: </app-f33c13590352da20930f.js>; rel=preload; as=script
+  Link: </component---node-modules-gatsby-plugin-offline-app-shell-js-78f9e4dea04737fa062d.js>; rel=preload; as=script
+/hi-folks/
+  Link: </webpack-runtime-acaa8994f1f704475e21.js>; rel=preload; as=script
+  Link: </styles-565f081c8374bbda155f.js>; rel=preload; as=script
+  Link: </app-f33c13590352da20930f.js>; rel=preload; as=script
+  Link: </0-0180cd94ef2497ac7db8.js>; rel=preload; as=script
+  Link: </component---src-templates-blog-post-js-517987eae96e75cddbe7.js>; rel=preload; as=script
+/my-second-post/
+  Link: </webpack-runtime-acaa8994f1f704475e21.js>; rel=preload; as=script
+  Link: </styles-565f081c8374bbda155f.js>; rel=preload; as=script
+  Link: </app-f33c13590352da20930f.js>; rel=preload; as=script
+  Link: </0-0180cd94ef2497ac7db8.js>; rel=preload; as=script
+  Link: </component---src-templates-blog-post-js-517987eae96e75cddbe7.js>; rel=preload; as=script
+/hello-world/
+  Link: </webpack-runtime-acaa8994f1f704475e21.js>; rel=preload; as=script
+  Link: </styles-565f081c8374bbda155f.js>; rel=preload; as=script
+  Link: </app-f33c13590352da20930f.js>; rel=preload; as=script
+  Link: </0-0180cd94ef2497ac7db8.js>; rel=preload; as=script
+  Link: </component---src-templates-blog-post-js-517987eae96e75cddbe7.js>; rel=preload; as=script
+/404/
+  Link: </webpack-runtime-acaa8994f1f704475e21.js>; rel=preload; as=script
+  Link: </styles-565f081c8374bbda155f.js>; rel=preload; as=script
+  Link: </app-f33c13590352da20930f.js>; rel=preload; as=script
+  Link: </0-0180cd94ef2497ac7db8.js>; rel=preload; as=script
+  Link: </component---src-pages-404-js-53e6c51a5a7e73090f50.js>; rel=preload; as=script
+/
+  Link: </webpack-runtime-acaa8994f1f704475e21.js>; rel=preload; as=script
+  Link: </styles-565f081c8374bbda155f.js>; rel=preload; as=script
+  Link: </app-f33c13590352da20930f.js>; rel=preload; as=script
+  Link: </0-0180cd94ef2497ac7db8.js>; rel=preload; as=script
+  Link: </component---src-pages-index-js-0bdd01c77ee09ef0224c.js>; rel=preload; as=script
+/404.html
+  Link: </webpack-runtime-acaa8994f1f704475e21.js>; rel=preload; as=script
+  Link: </styles-565f081c8374bbda155f.js>; rel=preload; as=script
+  Link: </app-f33c13590352da20930f.js>; rel=preload; as=script
+  Link: </0-0180cd94ef2497ac7db8.js>; rel=preload; as=script
+  Link: </component---src-pages-404-js-53e6c51a5a7e73090f50.js>; rel=preload; as=script
+"
+`;

packages/gatsby-plugin-netlify/src/__tests__/build-headers-program.js

@@ -173,6 +173,28 @@ describe(`build-headers-program`, () => {
     ).toMatchSnapshot()
   })
 
+  it(`without manifest['pages-manifest']`, async () => {
+    const pluginData = await createPluginData()
+
+    // gatsby 2.9+ no longer has a pages-manifest key
+    delete pluginData.manifest[`pages-manifest`]
+
+    const pluginOptions = {
+      ...DEFAULT_OPTIONS,
+      mergeCachingHeaders: true,
+    }
+
+    await buildHeadersProgram(pluginData, pluginOptions, reporter)
+
+    expect(reporter.warn).not.toHaveBeenCalled()
+    const output = await fs.readFile(
+      pluginData.publicFolder(`_headers`),
+      `utf8`
+    )
+    expect(output).toMatchSnapshot()
+    expect(output).not.toMatch(/\/undefined/g)
+  })
+
   it(`without caching headers`, async () => {
     const pluginData = await createPluginData()
 

packages/gatsby-plugin-netlify/src/build-headers-program.js

@@ -280,7 +280,9 @@ const applyCachingHeaders = (
   const cachingHeaders = {}
 
   files.forEach(file => {
-    cachingHeaders[`/` + file] = [IMMUTABLE_CACHING_HEADER]
+    if (typeof file === `string`) {
+      cachingHeaders[`/` + file] = [IMMUTABLE_CACHING_HEADER]
+    }
   })
 
   return defaultMerge(headers, cachingHeaders, CACHING_HEADERS)