feat: add logging to understand impact of trailing slash 308s (#2743)

Author: orinokai

Diff for: src/run/handlers/server.ts

@@ -108,7 +108,7 @@ export default async (request: Request) => {
 
     await adjustDateHeader({ headers: response.headers, request, span, tracer, requestContext })
 
-    setCacheControlHeaders(response, request, requestContext)
+    setCacheControlHeaders(response, request, requestContext, nextConfig)
     setCacheTagsHeaders(response.headers, requestContext)
     setVaryHeaders(response.headers, request, nextConfig)
     setCacheStatusHeader(response.headers)

Diff for: src/run/headers.ts

@@ -3,7 +3,7 @@ import type { NextConfigComplete } from 'next/dist/server/config-shared.js'
 
 import { encodeBlobKey } from '../shared/blobkey.js'
 
-import type { RequestContext } from './handlers/request-context.cjs'
+import { getLogger, RequestContext } from './handlers/request-context.cjs'
 import type { RuntimeTracer } from './handlers/tracer.cjs'
 import { getRegionalBlobStore } from './regional-blob-store.cjs'
 
@@ -216,6 +216,7 @@ export const setCacheControlHeaders = (
   { headers, status }: Response,
   request: Request,
   requestContext: RequestContext,
+  nextConfig: NextConfigComplete,
 ) => {
   if (
     typeof requestContext.routeHandlerRevalidate !== 'undefined' &&
@@ -234,6 +235,13 @@ export const setCacheControlHeaders = (
     return
   }
 
+  // temporary diagnostic to evaluate number of trailing slash redirects
+  if (status === 308 && request.url.endsWith('/') !== nextConfig.trailingSlash) {
+    getLogger()
+      .withFields({ trailingSlash: nextConfig.trailingSlash })
+      .log('NetlifyHeadersHandler.trailingSlashRedirect')
+  }
+
   if (status === 404 && request.url.endsWith('.php')) {
     // temporary CDN Cache Control handling for bot probes on PHP files
     // https://linear.app/netlify/issue/FRB-1344/prevent-excessive-ssr-invocations-due-to-404-routes