Add a workaround for the not-found app directory issue (cloudflare#418)

src/buildApplication/processVercelFunctions/index.ts

@@ -4,7 +4,7 @@ import { processEdgeFunctions } from './edgeFunctions';
 import { processPrerenderFunctions } from './prerenderFunctions';
 import type { CollectedFunctionIdentifiers } from './dedupeEdgeFunctions';
 import { dedupeEdgeFunctions } from './dedupeEdgeFunctions';
-import { checkForInvalidFunctions } from './invalidFunctions';
+import { checkInvalidFunctions } from './invalidFunctions';
 
 /**
  * Processes and dedupes the Vercel build output functions directory.
@@ -23,7 +23,7 @@ export async function processVercelFunctions(
 
 	await processEdgeFunctions(collectedFunctions);
 
-	await checkForInvalidFunctions(collectedFunctions);
+	await checkInvalidFunctions(collectedFunctions);
 
 	const identifiers = await dedupeEdgeFunctions(collectedFunctions, opts);
 

src/buildApplication/processVercelFunctions/invalidFunctions.ts

@@ -1,19 +1,25 @@
 import { gtr as versionGreaterThan, coerce } from 'semver';
-import { cliError } from '../../cli';
+import { cliError, cliWarn } from '../../cli';
 import { getPackageVersion } from '../packageManagerUtils';
 import { stripFuncExtension } from '../../utils';
 import type { CollectedFunctions, FunctionInfo } from './configs';
+import { join, resolve } from 'path';
 
 /**
  * Checks if there are any invalid functions from the Vercel build output.
  *
- * If there are any invalid functions, an error message will be printed and the process will exit.
+ * If there are any invalid functions it will try to see if they are amendable and if the
+ * build output can still be used.
+ *
+ * If however the build output can't be used, an error message will be printed and the process will exit.
  *
  * @param collectedFunctions Collected functions from the Vercel build output.
  */
-export async function checkForInvalidFunctions(
+export async function checkInvalidFunctions(
 	collectedFunctions: CollectedFunctions,
 ): Promise<void> {
+	await tryToFixNotFoundRoute(collectedFunctions);
+
 	if (collectedFunctions.invalidFunctions.size > 0) {
 		await printInvalidFunctionsErrorMessage(
 			collectedFunctions.invalidFunctions,
@@ -22,6 +28,49 @@ export async function checkForInvalidFunctions(
 	}
 }
 
+/**
+ * Tries to fix potential not-found invalid functions from the Vercel build output.
+ *
+ * Static app/not-found.(jsx|tsx) pages generate an _not-found.func serverless function,
+ * that can be removed as we can fallback to the statically generated 404 page
+ *
+ * If the app/not-found.(jsx|tsx) contains runtime logic alongside the _not-found.func serverless
+ * function also an _error.func will be generated, in such a case we can only warn the user about
+ * it.
+ * (
+ *  That's the only option because:
+ *    - removing the _not-found.func and _error.func doesn't result in a working application
+ *    - we don't have a guarantee that the _error.func hasn't been generated by something else
+ *      and that the _not-found.func is that of a static app/not-found route
+ * )
+ *
+ * @param collectedFunctions Collected functions from the Vercel build output.
+ */
+async function tryToFixNotFoundRoute(
+	collectedFunctions: CollectedFunctions,
+): Promise<void> {
+	const functionsDir = resolve('.vercel', 'output', 'functions');
+	const notFoundDir = join(functionsDir, '_not-found.func');
+	const errorDir = join(functionsDir, '_error.func');
+
+	const invalidNotFound = collectedFunctions.invalidFunctions.get(notFoundDir);
+	const invalidError = collectedFunctions.invalidFunctions.get(errorDir);
+
+	if (invalidNotFound && !invalidError) {
+		collectedFunctions.invalidFunctions.delete(notFoundDir);
+		const notFoundRscDir = join(functionsDir, '_not-found.rsc.func');
+		collectedFunctions.invalidFunctions.delete(notFoundRscDir);
+	}
+
+	if (invalidNotFound && invalidError) {
+		cliWarn(`
+			Warning: your app/not-found route might contain runtime logic, this is currently
+			not supported by @cloudflare/next-on-pages, if that's actually the case please
+			remove the runtime logic from your not-found route
+		`);
+	}
+}
+
 /**
  * Prints an error message for the invalid functions from the Vercel build output.
  *