Add upstream max-age to optimized image (vercel#26739)

This solves the main use case from Issue vercel#19914.

Previously, we would set the `Cache-Control` header to a constant and rely on the server cache. This would mean the browser would always request the image and the server could response with 304 Not Modified to omit the response body.

This PR changes the behavior such that the `max-age` will propagate from the upstream server to the Next.js Image Optimization Server and allow browser caching. ("upstream" meaning external server or just an internal route to an image)

This PR does not change the `max-age` for static imports which will remain `public, max-age=315360000, immutable`.

#### Pros:
- Fewer HTTP requests after initial browser visit
- User configurable `max-age` via the upstream image `Cache-Control` header

#### Cons:
- ~~Might be annoying for `next dev` when modifying a source image~~ (solved: use `max-age=0` for dev)
- Might cause browser to cache longer than expected (up to 2x longer than the server cache if requested in the last second before expiration)

## Bug

- [x] Related issues linked using `fixes #number`

packages/next/server/image-optimizer.ts

@@ -22,7 +22,7 @@ const PNG = 'image/png'
 const JPEG = 'image/jpeg'
 const GIF = 'image/gif'
 const SVG = 'image/svg+xml'
-const CACHE_VERSION = 2
+const CACHE_VERSION = 3
 const MODERN_TYPES = [/* AVIF, */ WEBP]
 const ANIMATABLE_TYPES = [WEBP, PNG, GIF]
 const VECTOR_TYPES = [SVG]
@@ -35,7 +35,8 @@ export async function imageOptimizer(
   res: ServerResponse,
   parsedUrl: UrlWithParsedQuery,
   nextConfig: NextConfig,
-  distDir: string
+  distDir: string,
+  isDev = false
 ) {
   const imageData: ImageConfig = nextConfig.images || imageConfigDefault
   const { deviceSizes = [], imageSizes = [], domains = [], loader } = imageData
@@ -158,24 +159,24 @@ export async function imageOptimizer(
     if (await fileExists(hashDir, 'directory')) {
       const files = await promises.readdir(hashDir)
       for (let file of files) {
-        const [prefix, etag, extension] = file.split('.')
-        const expireAt = Number(prefix)
+        const [maxAgeStr, expireAtSt, etag, extension] = file.split('.')
+        const maxAge = Number(maxAgeStr)
+        const expireAt = Number(expireAtSt)
         const contentType = getContentType(extension)
         const fsPath = join(hashDir, file)
         if (now < expireAt) {
-          res.setHeader(
-            'Cache-Control',
-            isStatic
-              ? 'public, max-age=315360000, immutable'
-              : 'public, max-age=0, must-revalidate'
+          const result = setResponseHeaders(
+            req,
+            res,
+            etag,
+            maxAge,
+            contentType,
+            isStatic,
+            isDev
           )
-          if (sendEtagResponse(req, res, etag)) {
-            return { finished: true }
+          if (!result.finished) {
+            createReadStream(fsPath).pipe(res)
           }
-          if (contentType) {
-            res.setHeader('Content-Type', contentType)
-          }
-          createReadStream(fsPath).pipe(res)
           return { finished: true }
         } else {
           await promises.unlink(fsPath)
@@ -271,8 +272,22 @@ export async function imageOptimizer(
       const animate =
         ANIMATABLE_TYPES.includes(upstreamType) && isAnimated(upstreamBuffer)
       if (vector || animate) {
-        await writeToCacheDir(hashDir, upstreamType, expireAt, upstreamBuffer)
-        sendResponse(req, res, upstreamType, upstreamBuffer, isStatic)
+        await writeToCacheDir(
+          hashDir,
+          upstreamType,
+          maxAge,
+          expireAt,
+          upstreamBuffer
+        )
+        sendResponse(
+          req,
+          res,
+          maxAge,
+          upstreamType,
+          upstreamBuffer,
+          isStatic,
+          isDev
+        )
         return { finished: true }
       }
 
@@ -342,13 +357,35 @@ export async function imageOptimizer(
       }
 
       if (optimizedBuffer) {
-        await writeToCacheDir(hashDir, contentType, expireAt, optimizedBuffer)
-        sendResponse(req, res, contentType, optimizedBuffer, isStatic)
+        await writeToCacheDir(
+          hashDir,
+          contentType,
+          maxAge,
+          expireAt,
+          optimizedBuffer
+        )
+        sendResponse(
+          req,
+          res,
+          maxAge,
+          contentType,
+          optimizedBuffer,
+          isStatic,
+          isDev
+        )
       } else {
         throw new Error('Unable to optimize buffer')
       }
     } catch (error) {
-      sendResponse(req, res, upstreamType, upstreamBuffer, isStatic)
+      sendResponse(
+        req,
+        res,
+        maxAge,
+        upstreamType,
+        upstreamBuffer,
+        isStatic,
+        isDev
+      )
     }
 
     return { finished: true }
@@ -362,37 +399,64 @@ export async function imageOptimizer(
 async function writeToCacheDir(
   dir: string,
   contentType: string,
+  maxAge: number,
   expireAt: number,
   buffer: Buffer
 ) {
   await promises.mkdir(dir, { recursive: true })
   const extension = getExtension(contentType)
   const etag = getHash([buffer])
-  const filename = join(dir, `${expireAt}.${etag}.${extension}`)
+  const filename = join(dir, `${maxAge}.${expireAt}.${etag}.${extension}`)
   await promises.writeFile(filename, buffer)
 }
 
-function sendResponse(
+function setResponseHeaders(
   req: IncomingMessage,
   res: ServerResponse,
+  etag: string,
+  maxAge: number,
   contentType: string | null,
-  buffer: Buffer,
-  isStatic: boolean
+  isStatic: boolean,
+  isDev: boolean
 ) {
-  const etag = getHash([buffer])
   res.setHeader(
     'Cache-Control',
     isStatic
       ? 'public, max-age=315360000, immutable'
-      : 'public, max-age=0, must-revalidate'
+      : `public, max-age=${isDev ? 0 : maxAge}, must-revalidate`
   )
   if (sendEtagResponse(req, res, etag)) {
-    return
+    // already called res.end() so we're finished
+    return { finished: true }
   }
   if (contentType) {
     res.setHeader('Content-Type', contentType)
   }
-  res.end(buffer)
+  return { finished: false }
+}
+
+function sendResponse(
+  req: IncomingMessage,
+  res: ServerResponse,
+  maxAge: number,
+  contentType: string | null,
+  buffer: Buffer,
+  isStatic: boolean,
+  isDev: boolean
+) {
+  const etag = getHash([buffer])
+  const result = setResponseHeaders(
+    req,
+    res,
+    etag,
+    maxAge,
+    contentType,
+    isStatic,
+    isDev
+  )
+  if (!result.finished) {
+    res.end(buffer)
+  }
 }
 
 function getSupportedMimeType(options: string[], accept = ''): string {

packages/next/server/next-server.ts

@@ -789,7 +789,8 @@ export default class Server {
             res,
             parsedUrl,
             server.nextConfig,
-            server.distDir
+            server.distDir,
+            this.renderOpts.dev
           ),
       },
       {