1 files changed, 82 insertions, 0 deletions
diff --git a/packages/astro/src/vite-plugin-load-fallback/index.ts b/packages/astro/src/vite-plugin-load-fallback/index.ts
new file mode 100644
index 000000000..02385207d
--- /dev/null
+++ b/packages/astro/src/vite-plugin-load-fallback/index.ts
@@ -0,0 +1,82 @@
+import nodeFs from 'node:fs';
+import npath from 'node:path';
+import type * as vite from 'vite';
+import { slash } from '../core/path.js';
+import { cleanUrl } from '../vite-plugin-utils/index.js';
+
+type NodeFileSystemModule = typeof nodeFs;
+
+interface LoadFallbackPluginParams {
+	fs?: NodeFileSystemModule;
+	root: URL;
+}
+
+export default function loadFallbackPlugin({
+	fs,
+	root,
+}: LoadFallbackPluginParams): vite.Plugin[] | false {
+	// Only add this plugin if a custom fs implementation is provided.
+	// Also check for `fs.default` because `import * as fs from 'node:fs'` will
+	// export as so, which only it's `.default` would === `nodeFs`.
+	// @ts-expect-error check default
+	if (!fs || fs === nodeFs || fs.default === nodeFs) {
+		return false;
+	}
+
+	const tryLoadModule = async (id: string) => {
+		try {
+			// await is necessary for the catch
+			return await fs.promises.readFile(cleanUrl(id), 'utf-8');
+		} catch {
+			try {
+				return await fs.promises.readFile(id, 'utf-8');
+			} catch {
+				try {
+					const fullpath = new URL('.' + id, root);
+					return await fs.promises.readFile(fullpath, 'utf-8');
+				} catch {
+					// Let fall through to the next
+				}
+			}
+		}
+	};
+
+	return [
+		{
+			name: 'astro:load-fallback',
+			enforce: 'post',
+			async resolveId(id, parent) {
+				// See if this can be loaded from our fs
+				if (parent) {
+					const candidateId = npath.posix.join(npath.posix.dirname(slash(parent)), id);
+					try {
+						// Check to see if this file exists and is not a directory.
+						const stats = await fs.promises.stat(candidateId);
+						if (!stats.isDirectory()) {
+							return candidateId;
+						}
+					} catch {}
+				}
+			},
+			async load(id) {
+				const code = await tryLoadModule(id);
+				if (code) {
+					return { code };
+				}
+			},
+		},
+		{
+			name: 'astro:load-fallback-hmr',
+			enforce: 'pre',
+			handleHotUpdate(context) {
+				// Wrap context.read so it checks our filesystem first.
+				const read = context.read;
+				context.read = async () => {
+					const source = await tryLoadModule(context.file);
+					if (source) return source;
+					return read.call(context);
+				};
+			},
+		},
+	];
+}