fix(core): docusaurus serve answers 400 to .. segments that reach the fake root name

packages/docusaurus/src/commands/serve/__tests__/requestHandler.test.ts

@@ -813,15 +813,15 @@ describe('createServeRequestHandler', () => {
       ).resolves.toBe('500 Internal Server Error');
     });
 
-    // TODO fix: answer 400, like other ".." segments going above the root
-    it('redirects ".." segments that reach the name of the fake root', async () => {
+    // Like other ".." segments going above the root
+    it('answers 400 to ".." segments that reach the name of the fake root', async () => {
       await expect(
         serveRequest(
           {baseUrl: '/', trailingSlash: undefined},
           '/../__docusaurus_root__/file.html',
           {asIs: true},
         ),
-      ).resolves.toBe('301 /file → 200 file.html');
+      ).resolves.toBe('400 Bad Request');
     });
 
     // Browsers send the baseUrl percent-encoded: "/caf%C3%A9/"

packages/docusaurus/src/commands/serve/requestHandler.ts

@@ -159,7 +159,20 @@ async function getRedirectLocation(
 // Like parseURLPath(), but undefined when ".." segments go above the root
 // It also keeps "//host/path" as a path instead of parsing another host
 function parseRootURLPath(url: string): URLPath | undefined {
-  const fakeRoot = '/__docusaurus_root__';
+  // ".." segments going above the root can reach the name of a fake root,
+  // like "/../__docusaurus_root__/file", but not the names of 2 fake roots
+  const urlPath = parseFakeRootURLPath(url, '/__docusaurus_root__');
+  const otherUrlPath = parseFakeRootURLPath(url, '/__docusaurus_other_root__');
+  if (urlPath === undefined || otherUrlPath === undefined) {
+    return undefined;
+  }
+  return urlPath;
+}
+
+function parseFakeRootURLPath(
+  url: string,
+  fakeRoot: string,
+): URLPath | undefined {
   const {pathname, search} = parseURLPath(`.${url}`, `${fakeRoot}/`);
   if (!pathname.startsWith(`${fakeRoot}/`)) {
     return undefined;