fix: create cache entries for fallback pages to support next@canary

Author: pieh

src/build/content/prerendered.ts

@@ -1,6 +1,7 @@
 import { existsSync } from 'node:fs'
 import { mkdir, readFile, writeFile } from 'node:fs/promises'
 import { join } from 'node:path'
+import { join as posixJoin } from 'node:path/posix'
 
 import { trace } from '@opentelemetry/api'
 import { wrapTracer } from '@opentelemetry/api/experimental'
@@ -41,17 +42,28 @@ const writeCacheEntry = async (
 }
 
 /**
- * Normalize routes by stripping leading slashes and ensuring root path is index
+ * Normalize routes by ensuring leading slashes and ensuring root path is index
  */
-const routeToFilePath = (path: string) => (path === '/' ? '/index' : path)
+const routeToFilePath = (path: string) => {
+  if (path === '/') {
+    return '/index'
+  }
+
+  if (path.startsWith('/')) {
+    return path
+  }
+
+  return `/${path}`
+}
 
 const buildPagesCacheValue = async (
   path: string,
   shouldUseEnumKind: boolean,
+  shouldSkipJson = false,
 ): Promise<NetlifyCachedPageValue> => ({
   kind: shouldUseEnumKind ? 'PAGES' : 'PAGE',
   html: await readFile(`${path}.html`, 'utf-8'),
-  pageData: JSON.parse(await readFile(`${path}.json`, 'utf-8')),
+  pageData: shouldSkipJson ? {} : JSON.parse(await readFile(`${path}.json`, 'utf-8')),
   headers: undefined,
   status: undefined,
 })
@@ -146,8 +158,8 @@ export const copyPrerenderedContent = async (ctx: PluginContext): Promise<void>
           })
         : false
 
-      await Promise.all(
-        Object.entries(manifest.routes).map(
+      await Promise.all([
+        ...Object.entries(manifest.routes).map(
           ([route, meta]): Promise<void> =>
             limitConcurrentPrerenderContentHandling(async () => {
               const lastModified = meta.initialRevalidateSeconds
@@ -195,7 +207,41 @@ export const copyPrerenderedContent = async (ctx: PluginContext): Promise<void>
               await writeCacheEntry(key, value, lastModified, ctx)
             }),
         ),
-      )
+        ...Object.entries(manifest.dynamicRoutes).map(async ([route, meta]) => {
+          // fallback can be `string | false | null`
+          //  - `string` - when user use pages router with `fallback: true`, and then it's html file path
+          //  - `null` - when user use pages router with `fallback: 'block'` or app router with `export const dynamicParams = true`
+          //  - `false` - when user use pages router with `fallback: false` or app router with `export const dynamicParams = false`
+          if (typeof meta.fallback === 'string') {
+            // https://github.com/vercel/next.js/pull/68603 started using route cache to serve fallbacks
+            // so we have to seed blobs with fallback entries
+
+            // create cache entry for pages router with `fallback: true` case
+            await limitConcurrentPrerenderContentHandling(async () => {
+              // dynamic routes don't have entries for each locale so we have to generate them
+              // ourselves. If i18n is not used we use empty string as "locale" to be able to use
+              // same handling wether i18n is used or not
+              const locales = ctx.buildConfig.i18n?.locales ?? ['']
+
+              const lastModified = Date.now()
+              for (const locale of locales) {
+                const key = routeToFilePath(posixJoin(locale, route))
+                const value = await buildPagesCacheValue(
+                  join(ctx.publishDir, 'server/pages', key),
+                  shouldUseEnumKind,
+                  true, // there is no corresponding json file for fallback, so we are skipping it for this entry
+                )
+                // Netlify Forms are not support and require a workaround
+                if (value.kind === 'PAGE' || value.kind === 'PAGES' || value.kind === 'APP_PAGE') {
+                  verifyNetlifyForms(ctx, value.html)
+                }
+
+                await writeCacheEntry(key, value, lastModified, ctx)
+              }
+            })
+          }
+        }),
+      ])
 
       // app router 404 pages are not in the prerender manifest
       // so we need to check for them manually