fix: revert trailing slash handling + improve dev base usage

fix #1664

packages/playground/html/index.html

@@ -1,7 +1,7 @@
-<link rel="stylesheet" href="./main.css" />
+<link rel="stylesheet" href="/main.css" />
 
 <!-- comment one -->
 <h1>Hello</h1>
 <!-- comment two -->
 
-<script type="module" src="./main.js"></script>
+<script type="module" src="/main.js"></script>

packages/vite/src/node/server/index.ts

@@ -406,7 +406,12 @@ export async function createServer(
           {
             from: /\/$/,
             to({ parsedUrl }: any) {
-              return parsedUrl.pathname + 'index.html'
+              const rewritten = parsedUrl.pathname + 'index.html'
+              if (fs.existsSync(path.join(root, rewritten))) {
+                return rewritten
+              } else {
+                return `/index.html`
+              }
             }
           }
         ]

packages/vite/src/node/server/middlewares/base.ts

@@ -15,13 +15,24 @@ export function baseMiddleware({
     const path = parsed.pathname || '/'
 
     if (path.startsWith(base)) {
-      // rewrite url to remove base.. this ensures that other middleware does not need to consider base being prepended or not
+      // rewrite url to remove base.. this ensures that other middleware does
+      // not need to consider base being prepended or not
       req.url = url.replace(base, '/')
     } else if (path === '/' || path === '/index.html') {
-      // to prevent confusion, do not allow access at / if we have specified a base path
-      res.statusCode = 404
+      // redirect root visit to based url
+      res.writeHead(302, {
+        Location: base
+      })
       res.end()
       return
+    } else if (req.headers.accept?.includes('text/html')) {
+      // non-based page visit
+      res.statusCode = 404
+      res.end(
+        `The server is configured with a public base URL of ${base} - ` +
+          `did you mean to visit ${base}${url.slice(1)} instead?`
+      )
+      return
     }
 
     next()