Dynamically append path alias (aaa/bbb.html = aaa/bbb = aaa/bbb/) int… (#783)

* Dynamically append path alias (aaa/bbb.html = aaa/bbb = aaa/bbb/) into redirects during gatsby onCreateNode callback API to avoid 404

* Consider more cases:
(1) markdown without redirects
(2) duplicated permalink for /docs/pure-render-mixin.html, rename the unused one

* Test permalink with ending `.html` rather then containing them

content/docs/reference-pure-render-mixin.md

@@ -3,7 +3,7 @@ id: pure-render-mixin
 title: PureRenderMixin
 layout: docs
 category: Reference
-permalink: docs/pure-render-mixin.html
+permalink: docs/pure-render-mixin-old.html
 ---
 
 > Note

gatsby/onCreateNode.js

@@ -9,6 +9,20 @@
 // Parse date information out of blog post filename.
 const BLOG_POST_FILENAME_REGEX = /([0-9]+)\-([0-9]+)\-([0-9]+)\-(.+)\.md$/;
 
+function buildRedirectString(permalink, redirect_from) {
+  if (!permalink || !permalink.endsWith('.html')) {
+    return redirect_from ? JSON.stringify(redirect_from) : '';
+  }
+
+  let basePath = permalink.slice(0, -'.html'.length);
+  let redirects = [basePath, basePath + '/'];
+  if (Array.isArray(redirect_from)) {
+    redirects = redirects.concat(redirect_from);
+  }
+
+  return JSON.stringify(redirects);
+}
+
 // Add custom fields to MarkdownRemark nodes.
 module.exports = exports.onCreateNode = ({node, boundActionCreators, getNode}) => {
   const {createNodeField} = boundActionCreators;
@@ -67,8 +81,9 @@ module.exports = exports.onCreateNode = ({node, boundActionCreators, getNode}) =
       createNodeField({
         node,
         name: 'redirect',
-        value: redirect_from ? JSON.stringify(redirect_from) : '',
+        value: buildRedirectString(permalink, redirect_from),
       });
+
       return;
   }
 };