Ability to Disable SSG Fallback

packages/next/build/index.ts

@@ -478,6 +478,7 @@ export default async function build(dir: string, conf = null): Promise<void> {
       let isStatic = false
       let isHybridAmp = false
       let ssgPageRoutes: string[] | null = null
+      let hasSsgFallback: boolean = false
 
       pagesManifest[page] = bundleRelative.replace(/\\/g, '/')
 
@@ -533,6 +534,9 @@ export default async function build(dir: string, conf = null): Promise<void> {
               additionalSsgPaths.set(page, result.prerenderRoutes)
               ssgPageRoutes = result.prerenderRoutes
             }
+            if (result.prerenderFallback) {
+              hasSsgFallback = true
+            }
           } else if (result.hasServerProps) {
             serverPropsPages.add(page)
           } else if (result.isStatic && customAppGetInitialProps === false) {
@@ -564,6 +568,7 @@ export default async function build(dir: string, conf = null): Promise<void> {
         isSsg,
         isHybridAmp,
         ssgPageRoutes,
+        hasSsgFallback,
       })
     })
   )

packages/next/build/utils.ts

@@ -42,6 +42,7 @@ export interface PageInfo {
   static: boolean
   isSsg: boolean
   ssgPageRoutes: string[] | null
+  hasSsgFallback: boolean
   serverBundle: string
 }
 
@@ -500,7 +501,7 @@ export async function getPageSizeInKb(
 export async function buildStaticPaths(
   page: string,
   unstable_getStaticPaths: Unstable_getStaticPaths
-): Promise<Array<string>> {
+): Promise<{ paths: string[]; fallback: boolean }> {
   const prerenderPaths = new Set<string>()
   const _routeRegex = getRouteRegex(page)
   const _routeMatcher = getRouteMatcher(_routeRegex)
@@ -511,7 +512,7 @@ export async function buildStaticPaths(
   const staticPathsResult = await unstable_getStaticPaths()
 
   const expectedReturnVal =
-    `Expected: { paths: [] }\n` +
+    `Expected: { paths: [], fallback: boolean }\n` +
     `See here for more info: https://err.sh/zeit/next.js/invalid-getstaticpaths-value`
 
   if (
@@ -525,7 +526,7 @@ export async function buildStaticPaths(
   }
 
   const invalidStaticPathKeys = Object.keys(staticPathsResult).filter(
-    key => key !== 'paths'
+    key => !(key === 'paths' || key === 'fallback')
   )
 
   if (invalidStaticPathKeys.length > 0) {
@@ -536,6 +537,13 @@ export async function buildStaticPaths(
     )
   }
 
+  if (typeof staticPathsResult.fallback !== 'boolean') {
+    throw new Error(
+      `The \`fallback\` key must be returned from unstable_getStaticProps in ${page}.\n` +
+        expectedReturnVal
+    )
+  }
+
   const toPrerender = staticPathsResult.paths
 
   if (!Array.isArray(toPrerender)) {
@@ -601,7 +609,7 @@ export async function buildStaticPaths(
     }
   })
 
-  return [...prerenderPaths]
+  return { paths: [...prerenderPaths], fallback: staticPathsResult.fallback }
 }
 
 export async function isPageStatic(
@@ -614,6 +622,7 @@ export async function isPageStatic(
   hasServerProps?: boolean
   hasStaticProps?: boolean
   prerenderRoutes?: string[] | undefined
+  prerenderFallback?: boolean | undefined
 }> {
   try {
     require('../next-server/lib/runtime-config').setConfig(runtimeEnvConfig)
@@ -667,18 +676,20 @@ export async function isPageStatic(
     }
 
     let prerenderRoutes: Array<string> | undefined
+    let prerenderFallback: boolean | undefined
     if (hasStaticProps && hasStaticPaths) {
-      prerenderRoutes = await buildStaticPaths(
-        page,
-        mod.unstable_getStaticPaths
-      )
+      ;({
+        paths: prerenderRoutes,
+        fallback: prerenderFallback,
+      } = await buildStaticPaths(page, mod.unstable_getStaticPaths))
     }
 
     const config = mod.config || {}
     return {
       isStatic: !hasStaticProps && !hasGetInitialProps && !hasServerProps,
       isHybridAmp: config.amp === 'hybrid',
       prerenderRoutes,
+      prerenderFallback,
       hasStaticProps,
       hasServerProps,
     }

packages/next/next-server/server/load-components.ts

@@ -36,6 +36,7 @@ type Unstable_getStaticProps = (ctx: {
 
 export type Unstable_getStaticPaths = () => Promise<{
   paths: Array<string | { params: ParsedUrlQuery }>
+  fallback: boolean
 }>
 
 type Unstable_getServerProps = (context: {

packages/next/next-server/server/next-server.ts

@@ -870,7 +870,7 @@ export default class Server {
     pathname: string,
     { components, query }: FindComponentsResult,
     opts: any
-  ): Promise<string | null> {
+  ): Promise<string | false | null> {
     // we need to ensure the status code if /404 is visited directly
     if (pathname === '/404') {
       res.statusCode = 404
@@ -1029,6 +1029,7 @@ export default class Server {
     // we lazy load the staticPaths to prevent the user
     // from waiting on them for the page to load in dev mode
     let staticPaths: string[] | undefined
+    let hasStaticFallback = false
 
     if (!isProduction && hasStaticPaths) {
       const __getStaticPaths = async () => {
@@ -1040,13 +1041,12 @@ export default class Server {
         )
         return paths
       }
-
-      staticPaths = (
+      ;({ paths: staticPaths, fallback: hasStaticFallback } = (
         await withCoalescedInvoke(__getStaticPaths)(
           `staticPaths-${pathname}`,
           []
         )
-      ).value
+      ).value)
     }
 
     // const isForcedBlocking =
@@ -1074,6 +1074,10 @@ export default class Server {
       // `getStaticPaths`
       (isProduction || !staticPaths || !staticPaths.includes(urlPathname))
     ) {
+      if (!hasStaticFallback) {
+        return false
+      }
+
       let html: string
 
       // Production already emitted the fallback as static HTML.
@@ -1137,13 +1141,16 @@ export default class Server {
     try {
       const result = await this.findPageComponents(pathname, query)
       if (result) {
-        return await this.renderToHTMLWithComponents(
+        const result2 = await this.renderToHTMLWithComponents(
           req,
           res,
           pathname,
           result,
           { ...this.renderOpts, amphtml, hasAmp }
         )
+        if (result2 !== false) {
+          return result2
+        }
       }
 
       if (this.dynamicRoutes) {
@@ -1159,7 +1166,7 @@ export default class Server {
             params
           )
           if (result) {
-            return await this.renderToHTMLWithComponents(
+            const result2 = await this.renderToHTMLWithComponents(
               req,
               res,
               dynamicRoute.page,
@@ -1171,6 +1178,9 @@ export default class Server {
                 hasAmp,
               }
             )
+            if (result2 !== false) {
+              return result2
+            }
           }
         }
       }
@@ -1224,9 +1234,9 @@ export default class Server {
       result = await this.findPageComponents('/_error', query)
     }
 
-    let html
+    let html: string | null
     try {
-      html = await this.renderToHTMLWithComponents(
+      const result2 = await this.renderToHTMLWithComponents(
         req,
         res,
         using404Page ? '/404' : '/_error',
@@ -1236,6 +1246,10 @@ export default class Server {
           err,
         }
       )
+      if (result2 === false) {
+        throw new Error('invariant: failed to render error page')
+      }
+      html = result2
     } catch (err) {
       console.error(err)
       res.statusCode = 500