Allows sitemap output on root of public folder (#31130)

Author: pedrolamas

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

@@ -44,4 +44,26 @@ exports[`gatsby-plugin-sitemap SSR API should create a Link if createLinkInHead
 }
 `;
 
+exports[`gatsby-plugin-sitemap SSR API should create a Link in root when output is "/" 1`] = `
+[MockFunction] {
+  "calls": Array [
+    Array [
+      Array [
+        <link
+          href="/sitemap-index.xml"
+          rel="sitemap"
+          type="application/xml"
+        />,
+      ],
+    ],
+  ],
+  "results": Array [
+    Object {
+      "type": "return",
+      "value": undefined,
+    },
+  ],
+}
+`;
+
 exports[`gatsby-plugin-sitemap SSR API should not create Link if createLinkInHead is false 1`] = `[MockFunction]`;

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

@@ -28,6 +28,23 @@ describe(`gatsby-plugin-sitemap SSR API`, () => {
     expect(setHeadComponents).toMatchSnapshot()
     expect(setHeadComponents).toHaveBeenCalledTimes(1)
   })
+  it(`should create a Link in root when output is "/"`, async () => {
+    const pluginOptions = {
+      createLinkInHead: true,
+      output: `/`,
+    }
+    const setHeadComponents = jest.fn()
+
+    await onRenderBody(
+      {
+        setHeadComponents,
+      },
+      pluginOptions
+    )
+
+    expect(setHeadComponents).toMatchSnapshot()
+    expect(setHeadComponents).toHaveBeenCalledTimes(1)
+  })
   it(`should not create Link if createLinkInHead is false`, async () => {
     const pluginOptions = {
       createLinkInHead: false,

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

@@ -1,6 +1,6 @@
 import * as React from "react"
 import { withPrefix as fallbackWithPrefix, withAssetPrefix } from "gatsby"
-import { withoutTrailingSlash } from "./internals"
+import { posix } from "path"
 
 // TODO: Remove for v3 - Fix janky path/asset prefixing
 const withPrefix = withAssetPrefix || fallbackWithPrefix
@@ -17,7 +17,7 @@ exports.onRenderBody = ({ setHeadComponents }, pluginOptions) => {
       key={`gatsby-plugin-sitemap`}
       rel="sitemap"
       type="application/xml"
-      href={withPrefix(withoutTrailingSlash(output) + `/sitemap-index.xml`)}
+      href={withPrefix(posix.join(output, `/sitemap-index.xml`))}
     />,
   ])
 }