unify server action detection logic (#58879)

In anticipation of #58885 (and to avoid adding another spot where we're checking the same headers), this unifies the logic that parses the request headers and determines if the request corresponds with a server action.

There was already some drift between the check in `base-server` and `action-handler` (unsure if this was intentional - let me know if so, in which case maybe separated handling is the better approach. I couldn't think of a good reason why it would be different, though). 

Existing tests should be sufficient for testing this changeset.

packages/next/src/server/app-render/action-handler.ts

@@ -11,7 +11,6 @@ import type { AppRenderContext, GenerateFlight } from './app-render'
 import type { AppPageModule } from '../../server/future/route-modules/app-page/module'
 
 import {
-  ACTION,
   RSC_HEADER,
   RSC_CONTENT_TYPE_HEADER,
 } from '../../client/components/app-router-headers'
@@ -36,6 +35,10 @@ import {
   NEXT_CACHE_REVALIDATED_TAGS_HEADER,
   NEXT_CACHE_REVALIDATE_TAG_TOKEN_HEADER,
 } from '../../lib/constants'
+import {
+  getIsServerAction,
+  getServerActionRequestMetadata,
+} from '../lib/server-action-request-meta'
 
 function formDataFromSearchQueryString(query: string) {
   const searchParams = new URLSearchParams(query)
@@ -271,20 +274,13 @@ export async function handleAction({
       formState?: any
     }
 > {
-  let actionId = req.headers[ACTION.toLowerCase()] as string
   const contentType = req.headers['content-type']
-  const isURLEncodedAction =
-    req.method === 'POST' && contentType === 'application/x-www-form-urlencoded'
-  const isMultipartAction =
-    req.method === 'POST' && contentType?.startsWith('multipart/form-data')
 
-  const isFetchAction =
-    actionId !== undefined &&
-    typeof actionId === 'string' &&
-    req.method === 'POST'
+  const { actionId, isURLEncodedAction, isMultipartAction, isFetchAction } =
+    getServerActionRequestMetadata(req)
 
   // If it's not a Server Action, skip handling.
-  if (!(isFetchAction || isURLEncodedAction || isMultipartAction)) {
+  if (!getIsServerAction(req)) {
     return
   }
 
@@ -528,6 +524,10 @@ To configure the body size limit for Server Actions, see: https://nextjs.org/doc
 
       let actionModId: string
       try {
+        if (!actionId) {
+          throw new Error('Invariant: actionId should be set')
+        }
+
         actionModId = serverModuleMap[actionId].id
       } catch (err) {
         // When this happens, it could be a deployment skew where the action came

packages/next/src/server/base-server.ts

@@ -82,7 +82,6 @@ import {
   RSC_HEADER,
   RSC_VARY_HEADER,
   NEXT_RSC_UNION_QUERY,
-  ACTION,
   NEXT_ROUTER_PREFETCH_HEADER,
 } from '../client/components/app-router-headers'
 import type {
@@ -129,6 +128,7 @@ import {
 } from './future/route-modules/checks'
 import { PrefetchRSCPathnameNormalizer } from './future/normalizers/request/prefetch-rsc'
 import { NextDataPathnameNormalizer } from './future/normalizers/request/next-data'
+import { getIsServerAction } from './lib/server-action-request-meta'
 
 export type FindComponentsResult = {
   components: LoadComponentsReturnType
@@ -1749,15 +1749,7 @@ export default abstract class Server<ServerOptions extends Options = Options> {
     const isAppPath = components.isAppPath === true
     const hasServerProps = !!components.getServerSideProps
     let hasStaticPaths = !!components.getStaticPaths
-    const actionId = req.headers[ACTION.toLowerCase()] as string
-    const contentType = req.headers['content-type']
-    const isMultipartAction =
-      req.method === 'POST' && contentType?.startsWith('multipart/form-data')
-    const isFetchAction =
-      actionId !== undefined &&
-      typeof actionId === 'string' &&
-      req.method === 'POST'
-    const isServerAction = isFetchAction || isMultipartAction
+    const isServerAction = getIsServerAction(req)
     const hasGetInitialProps = !!components.Component?.getInitialProps
     let isSSG = !!components.getStaticProps
 

packages/next/src/server/lib/server-action-request-meta.ts

@@ -0,0 +1,41 @@
+import type { IncomingMessage } from 'http'
+import type { BaseNextRequest } from '../base-http'
+import { ACTION } from '../../client/components/app-router-headers'
+
+export function getServerActionRequestMetadata(
+  req: IncomingMessage | BaseNextRequest
+): {
+  actionId: string | null
+  isURLEncodedAction: boolean
+  isMultipartAction: boolean
+  isFetchAction: boolean
+} {
+  let actionId: string | null
+  let contentType: string | null
+
+  actionId = (req.headers[ACTION.toLowerCase()] as string) ?? null
+  contentType = req.headers['content-type'] ?? null
+
+  const isURLEncodedAction = Boolean(
+    req.method === 'POST' && contentType === 'application/x-www-form-urlencoded'
+  )
+  const isMultipartAction = Boolean(
+    req.method === 'POST' && contentType?.startsWith('multipart/form-data')
+  )
+  const isFetchAction = Boolean(
+    actionId !== undefined &&
+      typeof actionId === 'string' &&
+      req.method === 'POST'
+  )
+
+  return { actionId, isURLEncodedAction, isMultipartAction, isFetchAction }
+}
+
+export function getIsServerAction(
+  req: IncomingMessage | BaseNextRequest
+): boolean {
+  const { isFetchAction, isURLEncodedAction, isMultipartAction } =
+    getServerActionRequestMetadata(req)
+
+  return Boolean(isFetchAction || isURLEncodedAction || isMultipartAction)
+}