fix(gatsby-plugin-sitemap): use pathPrefix when building sitemap index (#12852)

## Description

The Gatsby sitemap plugin can be configured to split up all of the sites URLs into smaller sitemaps, all being referenced from a main "index" sitemap. Unfortunately, this index sitemap doesn't take the `pathPrefix` option into account and will never link to the correct "child" sitemaps.

As `sitemap.js` doesn't have an option for `pathPrefix`, this change appends the `pathPrefix` to the site's `hostname` when more than one sitemap needs to be created.

Author: bennetthardwick

packages/gatsby-plugin-sitemap/src/__tests__/__snapshots__/gatsby-node.js.snap

@@ -15,10 +15,11 @@ exports[`Test plugin sitemap default settings work properly 1`] = `
 </urlset>"
 `;
 
-exports[`Test plugin sitemap sitemap index set sitempa size and urls are less than it. 1`] = `
+exports[`Test plugin sitemap sitemap index set sitemap size and urls are less than it. 1`] = `
 "<?xml version=\\"1.0\\" encoding=\\"UTF-8\\"?>
 <urlset xmlns=\\"http://www.sitemaps.org/schemas/sitemap/0.9\\" xmlns:news=\\"http://www.google.com/schemas/sitemap-news/0.9\\" xmlns:xhtml=\\"http://www.w3.org/1999/xhtml\\" xmlns:mobile=\\"http://www.google.com/schemas/sitemap-mobile/1.0\\" xmlns:image=\\"http://www.google.com/schemas/sitemap-image/1.1\\" xmlns:video=\\"http://www.google.com/schemas/sitemap-video/1.1\\">
 <url> <loc>http://dummy.url/page-1</loc> <changefreq>daily</changefreq> <priority>0.7</priority> </url>
 <url> <loc>http://dummy.url/page-2</loc> <changefreq>daily</changefreq> <priority>0.7</priority> </url>
 </urlset>"
 `;
+

packages/gatsby-plugin-sitemap/src/__tests__/gatsby-node.js

@@ -5,6 +5,7 @@ const path = require(`path`)
 const { onPostBuild } = require(`../gatsby-node`)
 const internals = require(`../internals`)
 const pathPrefix = ``
+const sitemap = require(`sitemap`)
 
 describe(`Test plugin sitemap`, async () => {
   it(`default settings work properly`, async () => {
@@ -167,7 +168,7 @@ describe(`Test plugin sitemap`, async () => {
       expect(originalFile).toEqual(path.join(`public`, `sitemap-index.xml`))
       expect(newFile).toEqual(path.join(`public`, `sitemap.xml`))
     })
-    it(`set sitempa size and urls are less than it.`, async () => {
+    it(`set sitemap size and urls are less than it.`, async () => {
       const options = {
         sitemapSize: 100,
       }
@@ -176,5 +177,16 @@ describe(`Test plugin sitemap`, async () => {
       expect(filePath).toEqual(path.join(`public`, `sitemap.xml`))
       expect(contents).toMatchSnapshot()
     })
+    it(`should include path prefix when creating creating index sitemap`, async () => {
+      const sitemapSpy = jest.spyOn(sitemap, `createSitemapIndex`)
+      const options = {
+        sitemapSize: 1,
+      }
+      const prefix = `/test`
+      await onPostBuild({ graphql, pathPrefix: prefix }, options)
+      expect(sitemapSpy.mock.calls[0][0].hostname).toEqual(
+        `${queryResult.data.site.siteMetadata.siteUrl}${prefix}`
+      )
+    })
   })
 })

packages/gatsby-plugin-sitemap/src/gatsby-node.js

@@ -52,7 +52,9 @@ exports.onPostBuild = async ({ graphql, pathPrefix }, pluginOptions) => {
     )
     const sitemapIndexOptions = {
       ...rest,
-      hostname: hostname || withoutTrailingSlash(siteUrl),
+      hostname:
+        (hostname || withoutTrailingSlash(siteUrl)) +
+        withoutTrailingSlash(pathPrefix || ``),
       targetFolder: publicPath,
       urls,
       callback: error => {