feat(v2): Implement plugin creating feed for blog posts (#1916)

* feat(v2): Implement feed for blog posts

Fixes: #1698

Test plan:
- added tests

Ran `yarn build` on website with the following config (and disabled blog
from preset-classic):

```js
[
'@docusaurus/plugin-content-blog',
  {
    path: '../website-1.x/blog',
    feedOptions: {
      copyright: 'Copy',
      type: 'atom',
    },
  },
],
```
which genereted the following feed:
```xml
<?xml version="1.0" encoding="utf-8"?>
<feed xmlns="http://www.w3.org/2005/Atom">
    <id>https://v2.docusaurus.io/blog</id>
    <title>Docusaurus Blog</title>
    <updated>2018-12-14T00:00:00.000Z</updated>
    <generator>https://github.com/jpmonette/feed</generator>
    <link rel="alternate" href="https://v2.docusaurus.io/blog"/>
    <subtitle>Docusaurus Blog</subtitle>
    <icon>https://v2.docusaurus.io/img/docusaurus.ico</icon>
    <rights>Copy</rights>
    <entry>
        <title type="html"><![CDATA[Happy 1st Birthday Slash!]]></title>
        <id>Happy 1st Birthday Slash!</id>
        <link href="https://v2.docusaurus.io/blog/2018/12/14/Happy-First-Birthday-Slash"/>
        <updated>2018-12-14T00:00:00.000Z</updated>
        <summary type="html"><![CDATA[![First Birthday Slash](/img/docusaurus-slash-first-birthday.svg)]]></summary>
    </entry>
    <entry>
        <title type="html"><![CDATA[Towards Docusaurus 2]]></title>
        <id>Towards Docusaurus 2</id>
        <link href="https://v2.docusaurus.io/blog/2018/09/11/Towards-Docusaurus-2"/>
        <updated>2018-09-11T00:00:00.000Z</updated>
        <summary type="html"><![CDATA[Docusaurus was [officially announced](https://docusaurus.io/blog/2017/12/14/introducing-docusaurus) over nine months ago as a way to easily build open source documentation websites. Since then, it has amassed over 8,600 GitHub Stars, and is used by many popular open source projects such as [React Native](https://facebook.github.io/react-native/), [Babel](https://babeljs.io/), [Jest](https://jestjs.io/), [Reason](https://reasonml.github.io/) and [Prettier](https://prettier.io/).]]></summary>
    </entry>
    <entry>
        <title type="html"><![CDATA[How I Converted Profilo to Docusaurus in Under 2 Hours]]></title>
        <id>How I Converted Profilo to Docusaurus in Under 2 Hours</id>
        <link href="https://v2.docusaurus.io/blog/2018/04/30/How-I-Converted-Profilo-To-Docusaurus"/>
        <updated>2018-04-30T00:00:00.000Z</updated>
        <summary type="html"><![CDATA[> _“Joel and I were discussing having a website and how it would have been great to launch with it. So I challenged myself to add Docusaurus support. It took just over an hour and a half. I'm going to send you a PR with the addition so you can take a look and see if you like it. Your workflow for adding docs wouldn't be much different from editing those markdown files.”_]]></summary>
    </entry>
    <entry>
        <title type="html"><![CDATA[Introducing Docusaurus]]></title>
        <id>Introducing Docusaurus</id>
        <link href="https://v2.docusaurus.io/blog/2017/12/14/introducing-docusaurus"/>
        <updated>2017-12-14T00:00:00.000Z</updated>
        <summary type="html"><![CDATA[![Introducing Slash](/img/slash-introducing.svg)]]></summary>
    </entry>
</feed>
```

* new feedOptions type 'all' and use correct path

packages/docusaurus-plugin-content-blog/src/index.ts

@@ -5,16 +5,13 @@
  * LICENSE file in the root directory of this source tree.
  */
 import fs from 'fs-extra';
-import globby from 'globby';
 import _ from 'lodash';
 import path from 'path';
-import {parse, normalizeUrl, docuHash} from '@docusaurus/utils';
+import {normalizeUrl, docuHash} from '@docusaurus/utils';
 
 import {
-  DateLink,
   PluginOptions,
   BlogTags,
-  BlogPost,
   Tag,
   BlogContent,
   BlogItemsToModules,
@@ -24,19 +21,10 @@ import {
   LoadContext,
   PluginContentLoadedActions,
   ConfigureWebpackUtils,
+  Props,
 } from '@docusaurus/types';
 import {Configuration} from 'webpack';
-
-// YYYY-MM-DD-{name}.mdx?
-// prefer named capture, but old node version do not support
-const FILENAME_PATTERN = /^(\d{4}-\d{1,2}-\d{1,2})-?(.*?).mdx?$/;
-
-function toUrl({date, link}: DateLink) {
-  return `${date
-    .toISOString()
-    .substring(0, '2019-01-01'.length)
-    .replace(/-/g, '/')}/${link}`;
-}
+import {generateBlogFeed, generateBlogPosts} from './blogUtils';
 
 const DEFAULT_OPTIONS: PluginOptions = {
   path: 'blog', // Path to data on filesystem, relative to site dir.
@@ -74,69 +62,13 @@ export default function pluginContentBlog(
 
     // Fetches blog contents and returns metadata for the necessary routes.
     async loadContent() {
-      const {postsPerPage, include, routeBasePath} = options;
-      const {siteConfig, siteDir} = context;
-      const blogDir = contentPath;
+      const {postsPerPage, routeBasePath} = options;
 
-      if (!fs.existsSync(blogDir)) {
+      const blogPosts = await generateBlogPosts(contentPath, context, options);
+      if (!blogPosts) {
         return null;
       }
 
-      const {baseUrl = ''} = siteConfig;
-      const blogFiles = await globby(include, {
-        cwd: blogDir,
-      });
-
-      const blogPosts: BlogPost[] = [];
-
-      await Promise.all(
-        blogFiles.map(async (relativeSource: string) => {
-          // Cannot use path.join() as it resolves '../' and removes the '@site'. Let webpack loader resolve it.
-          const source = path.join(blogDir, relativeSource);
-          const aliasedSource = `@site/${path.relative(siteDir, source)}`;
-          const blogFileName = path.basename(relativeSource);
-
-          const fileString = await fs.readFile(source, 'utf-8');
-          const {frontMatter, excerpt} = parse(fileString);
-
-          let date;
-          // extract date and title from filename
-          const match = blogFileName.match(FILENAME_PATTERN);
-          let linkName = blogFileName.replace(/\.mdx?$/, '');
-          if (match) {
-            const [, dateString, name] = match;
-            date = new Date(dateString);
-            linkName = name;
-          }
-          // prefer usedefined date
-          if (frontMatter.date) {
-            date = new Date(frontMatter.date);
-          }
-          // use file create time for blog
-          date = date || (await fs.stat(source)).birthtime;
-          frontMatter.title = frontMatter.title || linkName;
-
-          blogPosts.push({
-            id: frontMatter.id || frontMatter.title,
-            metadata: {
-              permalink: normalizeUrl([
-                baseUrl,
-                routeBasePath,
-                frontMatter.id || toUrl({date, link: linkName}),
-              ]),
-              source: aliasedSource,
-              description: frontMatter.description || excerpt,
-              date,
-              tags: frontMatter.tags,
-              title: frontMatter.title,
-            },
-          });
-        }),
-      );
-      blogPosts.sort(
-        (a, b) => b.metadata.date.getTime() - a.metadata.date.getTime(),
-      );
-
       // Colocate next and prev metadata
       blogPosts.forEach((blogPost, index) => {
         const prevItem = index > 0 ? blogPosts[index - 1] : null;
@@ -160,6 +92,9 @@ export default function pluginContentBlog(
       // Example: `/blog`, `/blog/page/1`, `/blog/page/2`
       const totalCount = blogPosts.length;
       const numberOfPages = Math.ceil(totalCount / postsPerPage);
+      const {
+        siteConfig: {baseUrl = ''},
+      } = context;
       const basePageUrl = normalizeUrl([baseUrl, routeBasePath]);
 
       const blogListPaginated = [];
@@ -442,5 +377,41 @@ export default function pluginContentBlog(
         },
       };
     },
+
+    async postBuild({outDir}: Props) {
+      if (!options.feedOptions) {
+        return;
+      }
+
+      const {
+        feedOptions: {type: feedType},
+      } = options;
+      const feed = await generateBlogFeed(context, options);
+      if (!feed) {
+        return;
+      }
+      let feedTypes = [];
+      if (feedType === 'all') {
+        feedTypes = ['rss', 'atom'];
+      } else {
+        feedTypes.push(feedType);
+      }
+
+      await Promise.all(
+        feedTypes.map(feedType => {
+          const feedPath = path.join(
+            outDir,
+            options.routeBasePath,
+            `${feedType}.xml`,
+          );
+          const feedContent = feedType === 'rss' ? feed.rss2() : feed.atom1();
+          return fs.writeFile(feedPath, feedContent, err => {
+            if (err) {
+              throw new Error(`Generating ${feedType} feed failed: ${err}`);
+            }
+          });
+        }),
+      );
+    },
   };
 }