Fix autogenerated append to original _headers file (#851)

* Fix autogenerated append to original _headers file

Vercel hard links the public directory so when next-on-pages appends to
the vercel output directory, it is also appending to the original
`public/_headers` file too.

This commit fixes the problem by copying the contents to memory before
unlinking the file and creating a new (non-linked) file.

Closes #848

.changeset/breezy-walls-stare.md

@@ -0,0 +1,5 @@
+---
+'@cloudflare/next-on-pages': patch
+---
+
+Fix autogenerated content also getting appended to the original public/\_headers file

packages/next-on-pages/src/buildApplication/buildMetadataFiles.ts

@@ -1,5 +1,5 @@
 import { join } from 'path';
-import { writeFile } from 'fs/promises';
+import { writeFile, stat, unlink, readFile } from 'fs/promises';
 import { nextOnPagesVersion, readJsonFile } from '../utils';
 import { getPhaseRoutes, getVercelConfig } from './getVercelConfig';
 import { cliError } from '../cli';
@@ -40,22 +40,37 @@ async function buildNextStaticHeaders(
 	);
 	const nextStaticHeaders = (nextStaticRoute as VercelSource)?.headers;
 
-	if (nextStaticHeaders) {
-		await writeFile(
-			join(outputDir, '_headers'),
-			`
+	if (!nextStaticHeaders) {
+		return;
+	}
+
+	const headersPath = join(outputDir, '_headers');
+
+	const nopContent = `
 # === START AUTOGENERATED @cloudflare/next-on-pages IMMUTABLE HEADERS ===
 ${nextStaticPath}/*
 ${Object.entries(nextStaticHeaders)
 	.map(([header, value]) => `  ${header}: ${value}`)
 	.join('\n')}
-# === END AUTOGENERATED @cloudflare/next-on-pages IMMUTABLE HEADERS ===\n`,
-			{
-				// in case someone configured redirects already, append to the end
-				flag: 'a',
-			},
-		);
+# === END AUTOGENERATED @cloudflare/next-on-pages IMMUTABLE HEADERS ===\n`;
+
+	try {
+		// Vercel hard links the static files directory:
+		// https://github.com/vercel/vercel/blob/8e20fed/packages/cli/src/util/build/write-build-result.ts#L336
+		// So we need to unlink before we append data, otherwise we will mutate
+		// the original file found in the public directory.
+		const stats = await stat(headersPath);
+		if (stats.nlink > 1) {
+			const data = await readFile(headersPath);
+			await unlink(headersPath);
+			await writeFile(headersPath, data);
+		}
+	} catch (e) {
+		if ((e as { code?: string }).code !== 'ENOENT') {
+			throw e;
+		}
 	}
+	await writeFile(headersPath, nopContent, { flag: 'a' });
 }
 
 /**