fix: support experimentalWarmUp

src/module.ts

@@ -159,6 +159,14 @@ export interface ModuleOptions extends SitemapDefinition {
    * @default true
    */
   sortEntries: boolean
+  /**
+   * Warm up the sitemap route(s) cache when Nitro starts.
+   *
+   * May be implemented by default in a future minor version.
+   *
+   * @experimental
+   */
+  experimentalWarmUp?: boolean
 }
 
 export interface ModuleHooks {
@@ -396,6 +404,9 @@ declare module 'vue-router' {
       nuxt.options.routeRules[`/${config.sitemapName}`] = routeRules
     }
 
+    if (config.experimentalWarmUp)
+      addServerPlugin(resolve('./runtime/plugins/warm-up'))
+
     // @ts-expect-error runtime types
     if (hasNuxtModule('@nuxt/content') && (!!nuxt.options.content?.documentDriven || config.strictNuxtContentPaths)) {
       addServerPlugin(resolve('./runtime/plugins/nuxt-content'))
@@ -452,6 +463,7 @@ declare module 'vue-router' {
       })
       sitemaps.index = {
         sitemapName: 'index',
+        _route: withBase('sitemap_index.xml', nuxt.options.app.baseURL || '/'),
         // TODO better index support
         // @ts-expect-error untyped
         sitemaps: config.sitemaps!.index || [],
@@ -464,6 +476,7 @@ declare module 'vue-router' {
           sitemaps[sitemapName as keyof typeof sitemaps] = defu(
             {
               sitemapName,
+              _route: withBase(`${sitemapName}-sitemap.xml`, nuxt.options.app.baseURL || '/'),
               _hasSourceChunk: typeof definition.urls !== 'undefined' || definition.sources?.length || !!definition.dynamicUrlsApiEndpoint,
             },
             { ...definition, urls: undefined, sources: undefined },
@@ -485,6 +498,7 @@ declare module 'vue-router' {
       // note: we don't need urls for the root sitemap, only child sitemaps
       sitemaps[config.sitemapName] = <SitemapDefinition> {
         sitemapName: config.sitemapName,
+        route: withBase(config.sitemapName, nuxt.options.app.baseURL || '/'), // will contain the xml
         defaults: config.defaults,
         include: config.include,
         exclude: config.exclude,

src/runtime/plugins/warm-up.ts

@@ -0,0 +1,24 @@
+import { defineNitroPlugin } from 'nitropack/dist/runtime/plugin'
+import { withLeadingSlash } from 'ufo'
+import { useRuntimeConfig } from '#imports'
+
+export default defineNitroPlugin((nitroApp) => {
+  const { sitemaps } = useRuntimeConfig()['nuxt-simple-sitemap']
+  const queue: (() => Promise<Response>)[] = []
+  const sitemapsWithRoutes = Object.entries(sitemaps)
+    .filter(([, sitemap]) => sitemap._route)
+  for (const [, sitemap] of sitemapsWithRoutes)
+    queue.push(() => nitroApp.localFetch(withLeadingSlash(sitemap._route), {}))
+
+  // run async
+  setTimeout(() => {
+    // work the queue step by step await the promise from each task, delay 1s after each task ends
+    const next = async () => {
+      if (queue.length === 0)
+        return
+      await queue.shift()!()
+      setTimeout(next, 1000) // arbitrary delay to avoid throttling
+    }
+    next()
+  }, 2500 /* https://github.com/unjs/nitro/pull/1906 */)
+})

src/runtime/types.ts

@@ -47,7 +47,7 @@ export interface AutoI18nConfig {
 
 export interface ModuleRuntimeConfig extends Pick<ModuleOptions, 'sitemapName' | 'excludeAppSources' | 'sortEntries' | 'defaultSitemapsChunkSize' | 'xslColumns' | 'xslTips' | 'debug' | 'discoverImages' | 'autoLastmod' | 'xsl' | 'credits' > {
   version: string
-  sitemaps: { index?: { sitemapName: string; sitemaps: SitemapIndexEntry[] } } & Record<string, Omit<SitemapDefinition, 'urls' | 'sources'> & { _hasSourceChunk?: boolean }>
+  sitemaps: { index?: Pick<SitemapDefinition, 'sitemapName' | '_route'> & { sitemaps: SitemapIndexEntry[] } } & Record<string, Omit<SitemapDefinition, 'urls' | 'sources'> & { _hasSourceChunk?: boolean }>
   autoI18n?: AutoI18nConfig
   isMultiSitemap: boolean
   isI18nMapped: boolean
@@ -102,6 +102,10 @@ export interface SitemapDefinition {
    * @deprecated use `sources`
    */
   dynamicUrlsApiEndpoint?: string | false
+  /**
+   * @internal
+   */
+  _route?: string
 }
 
 export interface SitemapRenderCtx {