refactor: extract getting the action handler into getActionHandler

Author: lubieowoce

packages/next/errors.json

@@ -672,5 +672,6 @@
   "671": "Specified images.remotePatterns must have protocol \"http\" or \"https\" received \"%s\".",
   "672": "Expected `telemetry` to be set in globals",
   "673": "Failed to find Server Action \"%s\". This request might be from an older or newer deployment.%s\\nRead more: https://nextjs.org/docs/messages/failed-to-find-server-action",
-  "674": "This function cannot be used in the edge runtime"
+  "674": "This function cannot be used in the edge runtime",
+  "675": "Action handler not found in action module"
 }

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

@@ -866,6 +866,45 @@ export async function handleAction({
     }
   }
 
+  const getActionHandler = async (
+    // eslint-disable-next-line @typescript-eslint/no-shadow
+    actionId: string
+  ): Promise<((...args: unknown[]) => Promise<unknown>) | null> => {
+    // actions.js
+    // app/page.js
+    //   action worker1
+    //     appRender1
+
+    // app/foo/page.js
+    //   action worker2
+    //     appRender
+
+    // / -> fire action -> POST / -> appRender1 -> modId for the action file
+    // /foo -> fire action -> POST /foo -> appRender2 -> modId for the action file
+
+    //  Validate the actionId.
+    let actionModId: string
+    try {
+      actionModId = getActionModIdOrError(actionId, serverModuleMap)
+    } catch (err) {
+      console.error(err)
+      return null
+    }
+
+    const actionMod = (await ComponentMod.__next_app__.require(
+      actionModId
+    )) as Record<string, (...args: unknown[]) => Promise<unknown>>
+
+    const actionHandler = actionMod[actionId]
+    // if the `actionId` is valid, and resolved to a valid `actionModId`, but the module does not contain the action,
+    // then something went very wrong in the bundling process, and we should error.
+    if (!actionHandler) {
+      throw new InvariantError('Action handler not found in action module')
+    }
+
+    return actionHandler
+  }
+
   try {
     return await actionAsyncStorage.run(
       { isAction: true },
@@ -876,15 +915,11 @@ export async function handleAction({
 
         // A fetch action (initiated by the client router).
 
-        // Check the action id. if it's not valid, we can bail out immediately.
-        let actionModId: string
-        try {
-          actionModId = getActionModIdOrError(actionId, serverModuleMap)
-        } catch (err) {
-          console.error(err)
-          return {
-            type: 'not-found',
-          }
+        // Get the action function.
+        const actionHandler = await getActionHandler(actionId)
+        if (!actionHandler) {
+          // We didn't recognize the provided actionId, so we can't run the action.
+          return { type: 'not-found' }
         }
 
         // The temporary reference set is used for parsing the arguments and in the catch handler,
@@ -905,26 +940,7 @@ export async function handleAction({
         const actionArguments =
           await parseFetchActionArguments(temporaryReferences)
 
-        // Get the action function.
-
-        // actions.js
-        // app/page.js
-        //   action worker1
-        //     appRender1
-
-        // app/foo/page.js
-        //   action worker2
-        //     appRender
-
-        // / -> fire action -> POST / -> appRender1 -> modId for the action file
-        // /foo -> fire action -> POST /foo -> appRender2 -> modId for the action file
-
-        const actionMod = (await ComponentMod.__next_app__.require(
-          actionModId
-        )) as Record<string, (...args: unknown[]) => Promise<unknown>>
-
-        const actionHandler = actionMod[actionId]
-
+        // Run the action.
         const returnVal = await executeActionAndPrepareForRender(
           actionHandler,
           actionArguments,