Fix NextUrl trailing slash normalize for data route (#41311)

x-ref: [slack
thread](https://vercel.slack.com/archives/C045FKE5P51/p1665074590321179)

## Bug

- [x] Related issues linked using `fixes #number`
- [x] Integration tests added
- [ ] Errors have a helpful link attached, see `contributing.md`

packages/next/shared/lib/router/utils/format-next-pathname-info.ts

@@ -17,6 +17,10 @@ export function formatNextPathnameInfo(info: ExtendedInfo) {
     info.ignorePrefix
   )
 
+  if (info.buildId || !info.trailingSlash) {
+    pathname = removeTrailingSlash(pathname)
+  }
+
   if (info.buildId) {
     pathname = addPathSuffix(
       addPathPrefix(pathname, `/_next/data/${info.buildId}`),
@@ -25,8 +29,8 @@ export function formatNextPathnameInfo(info: ExtendedInfo) {
   }
 
   pathname = addPathPrefix(pathname, info.basePath)
-  return info.trailingSlash
-    ? !info.buildId && !pathname.endsWith('/')
+  return !info.buildId && info.trailingSlash
+    ? !pathname.endsWith('/')
       ? addPathSuffix(pathname, '/')
       : pathname
     : removeTrailingSlash(pathname)

test/unit/web-runtime/next-url.test.ts

@@ -263,6 +263,39 @@ it('correctly parses a prefetch url', async () => {
   )
 })
 
+it('correctly handles trailing slash in _next/data', async () => {
+  const url = new NextURL('/abc/', 'http://127.0.0.1:3000')
+  url.buildId = '1234'
+
+  expect(url.pathname).toEqual('/abc/')
+  expect(url.locale).toEqual('')
+  expect(String(url)).toEqual('http://localhost:3000/_next/data/1234/abc.json')
+})
+
+it('correctly handles trailing slash in _next/data with config', async () => {
+  const url = new NextURL('/abc/', 'http://127.0.0.1:3000', {
+    nextConfig: { trailingSlash: true },
+  })
+  url.buildId = '1234'
+
+  expect(url.pathname).toEqual('/abc/')
+  expect(url.locale).toEqual('')
+  expect(String(url)).toEqual('http://localhost:3000/_next/data/1234/abc.json')
+})
+
+it('correctly handles trailing slash in _next/data with basePath', async () => {
+  const url = new NextURL('/docs/abc/', 'http://127.0.0.1:3000', {
+    nextConfig: { basePath: '/docs', trailingSlash: true },
+  })
+  url.buildId = '1234'
+
+  expect(url.pathname).toEqual('/abc/')
+  expect(url.locale).toEqual('')
+  expect(String(url)).toEqual(
+    'http://localhost:3000/docs/_next/data/1234/abc.json'
+  )
+})
+
 it('correctly parses a prefetch index url', async () => {
   const url = new NextURL(
     '/_next/data/development/index.json',