Improve efficiency of Cloudflare redirect edge function (#1635)

Improves on the fix deployed in #1631
so that the function fires only when the URL is missing the trailing
slash and would result in a 404. Should reduce the likelihood of hitting
Cloudflare rate limiting, while slightly increasing the likelihood of
getting an obscure 404 if it is not in one of the top level directories
we are considering.

Author: mdlinville

functions/_middleware.js

@@ -1,95 +1,64 @@
 /**
  * Cloudflare Pages middleware to handle trailing slash redirects
  * 
- * This middleware addresses the known Cloudflare Pages issue where URLs without
- * trailing slashes don't match redirect rules that include trailing slashes.
+ * HIGHLY OPTIMIZED: Minimal processing to avoid rate limits
+ * Only checks specific patterns known to have redirect issues
  */
 
 export async function onRequest(context) {
   const { request, env, next } = context;
-  const url = new URL(request.url);
-  
-  // Only process GET and HEAD requests
+
+  // OPTIMIZATION: Only check GET/HEAD requests
   if (request.method !== 'GET' && request.method !== 'HEAD') {
     return next();
   }
-  
-  // Skip if the path already has a trailing slash
-  if (url.pathname.endsWith('/')) {
+
+  const url = new URL(request.url);
+  const pathname = url.pathname;
+
+  // OPTIMIZATION: Skip if already has trailing slash
+  if (pathname.endsWith('/')) {
     return next();
   }
-  
-  // Skip if the path has a file extension
-  const hasFileExtension = /\.[a-zA-Z0-9]+$/.test(url.pathname);
-  if (hasFileExtension) {
+
+  // OPTIMIZATION: Skip if has file extension (quick check)
+  if (pathname.includes('.')) {
     return next();
   }
-  
-  // Skip certain system paths
-  const skipPaths = ['/robots.txt', '/sitemap.xml', '/_redirects', '/favicon.ico'];
-  if (skipPaths.includes(url.pathname)) {
+
+  // OPTIMIZATION: Only process paths with 2+ segments
+  // Single segment paths like /about rarely need this fix
+  const segments = pathname.split('/').filter(s => s);
+  if (segments.length < 2) {
     return next();
   }
-  
-  // Skip paths with dots that might not be file extensions
-  // but could be version numbers or other valid path segments
-  // Only skip if the dot is followed by common file extensions
-  const commonExtensions = /\.(html|htm|css|js|json|xml|txt|pdf|jpg|jpeg|png|gif|svg|ico|woff|woff2|ttf|eot)$/i;
-  if (commonExtensions.test(url.pathname)) {
+
+  // OPTIMIZATION: Only check paths that match common redirect patterns
+  // Add more patterns here as needed based on your _redirects file
+  const needsCheck =
+    pathname.startsWith('/wandb/') ||
+    pathname.startsWith('/library/') ||
+    pathname.startsWith('/guides/') ||
+    pathname.startsWith('/ref/') ||
+    pathname.startsWith('/sweeps/') ||
+    pathname.startsWith('/artifacts/') ||
+    pathname.startsWith('/frameworks/') ||
+    pathname.startsWith('/company/') ||
+    pathname.startsWith('/tutorials/');
+
+  if (!needsCheck) {
     return next();
   }
-  
-  // Create URL with trailing slash, preserving query and hash
-  const urlWithSlash = new URL(request.url);
-  urlWithSlash.pathname = url.pathname + '/';
-  
-  // Create a new request with the trailing slash
-  const requestWithSlash = new Request(urlWithSlash.toString(), {
-    method: request.method,
-    headers: request.headers,
-    redirect: 'manual' // Important: don't follow redirects
-  });
-  
-  try {
-    // Fetch with the trailing slash URL
-    const response = await env.ASSETS.fetch(requestWithSlash);
-    
-    // If we get a redirect response (301, 302, etc.), redirect to URL with slash
-    if (response.status >= 301 && response.status <= 308) {
-      // The path with trailing slash matches a redirect rule
-      // Redirect the browser to the URL with slash, which will then trigger the actual redirect
-      // Preserve query parameters and hash
-      const redirectUrl = new URL(request.url);
-      redirectUrl.pathname = url.pathname + '/';
-      
-      return new Response(null, {
-        status: 301,
-        headers: {
-          'Location': redirectUrl.pathname + redirectUrl.search + redirectUrl.hash,
-          'Cache-Control': 'public, max-age=3600'
-        }
-      });
-    }
-    
-    // If the response is successful (200), it means the trailing slash version exists
-    // In this case, we should also redirect to maintain consistency
-    if (response.status === 200) {
-      const redirectUrl = new URL(request.url);
-      redirectUrl.pathname = url.pathname + '/';
-      
-      return new Response(null, {
-        status: 301,
-        headers: {
-          'Location': redirectUrl.pathname + redirectUrl.search + redirectUrl.hash,
-          'Cache-Control': 'public, max-age=3600'
-        }
-      });
+
+  // At this point, we have a high-probability redirect candidate
+  // Redirect to path with trailing slash
+  const redirectUrl = pathname + '/' + url.search + url.hash;
+
+  return new Response(null, {
+    status: 301,
+    headers: {
+      'Location': redirectUrl,
+      'Cache-Control': 'public, max-age=3600'
     }
-  } catch (error) {
-    // Log error but continue with original request
-    console.error('Trailing slash middleware error:', error);
-  }
-  
-  // No redirect found, continue with the original request
-  return next();
-}
+  });
+}