feat: vue-seo-layer, the Nuxt half of the SummerCMS SEO plugin

app/middleware/seo.global.ts

@@ -0,0 +1,39 @@
+// Resolves every navigation before its page runs, so a redirect or a 410
+// wins over the page (also for an old address no page matches). The
+// answer is kept in shared state for the head plugin; on hydration the
+// server's answer comes with the payload and nothing is fetched again.
+
+import { seoRedirectTarget, seoResolvableUrl, seoXRobotsTag } from '../utils/seo.ts'
+
+export default defineNuxtRouteMiddleware(async (to) => {
+  const nuxtApp = useNuxtApp()
+  const config = useRuntimeConfig()
+  if ((config.public.seo as { enabled?: boolean } | undefined)?.enabled === false) return
+  if (import.meta.client && nuxtApp.isHydrating && nuxtApp.payload.serverRendered) return
+
+  const state = useSeoAnswer()
+  const url = seoResolvableUrl(to.fullPath)
+  if (!url) {
+    state.value = null
+    return
+  }
+  const answer = await resolveSeo(url)
+  state.value = answer
+  if (!answer) return
+
+  if (import.meta.server) {
+    const header = seoXRobotsTag(answer)
+    if (header) useResponseHeader('X-Robots-Tag').value = header
+  }
+
+  // On the server a redirect only sends Location (as external), so the
+  // target page is not rendered and its headers stay off the redirect.
+  const target = seoRedirectTarget(answer)
+  if (target) {
+    if (import.meta.server) useResponseHeader('X-Robots-Tag').value = undefined
+    return navigateTo(target.to, { redirectCode: target.code, external: target.external || import.meta.server })
+  }
+  if (answer.action === 'gone') {
+    return abortNavigation(createError({ statusCode: 410, statusMessage: 'Gone', fatal: true }))
+  }
+})