fix: readme protocol links (#1189)

| [![PR App][icn]][demo] | Fix RM-XYZ |
| :--------------------: | :--------: |

## 🧰 Changes

Fixes sanitization of readme style protocol links `[label](doc:slug)`.

We recently switched our plain markdown parsing to use the latest
version of this package with `{ format: 'md' }`. We want to eventually
remove `@readme/markdown@6`, but it also helped reduce our bundle size.
In the process, we failed to port over our custom sanitization schema.
For the most part, I think this is fine to drop support for old, custom
markdown features. But it does break our custom protocol links, which at
least one enterprise customer was using.

## 🧬 QA & Testing

- [Broken on production][prod].
- [Working in this PR app][demo].

[demo]: https://markdown-pr-PR_NUMBER.herokuapp.com
[prod]: https://SUBDOMAIN.readme.io
[icn]:
https://user-images.githubusercontent.com/886627/160426047-1bee9488-305a-4145-bb2b-09d8b757d38a.svg

Author: kellyjosephprice

__tests__/lib/compile.test.ts

@@ -0,0 +1,12 @@
+import { compile } from '../../index';
+
+describe('compile', () => {
+  describe("{ format: 'md' }", () => {
+    it('returns plain text of markdown components', () => {
+      const md = '[link to doc](doc:getting-started)';
+
+      const tree = compile(md, { format: 'md' });
+      expect(tree).toMatch(/href: "doc:getting-started"/);
+    });
+  });
+});

lib/compile.ts

@@ -2,8 +2,9 @@ import type { CompileOptions } from '@mdx-js/mdx';
 import type { PluggableList } from 'unified';
 
 import { compileSync as mdxCompileSync } from '@mdx-js/mdx';
+import deepmerge from 'deepmerge';
 import rehypeRaw from 'rehype-raw';
-import rehypeSanitize from 'rehype-sanitize';
+import rehypeSanitize, { defaultSchema } from 'rehype-sanitize';
 import remarkFrontmatter from 'remark-frontmatter';
 import remarkGfm from 'remark-gfm';
 
@@ -22,6 +23,10 @@ export type CompileOpts = CompileOptions & {
 
 const { codeTabsTransformer, ...transforms } = defaultTransforms;
 
+const sanitizeSchema = deepmerge(defaultSchema, {
+  protocols: ['doc', 'ref', 'blog', 'changelog', 'page'],
+});
+
 const compile = (
   text: string,
   { components = {}, missingComponents, copyButtons, useTailwind, ...opts }: CompileOpts = {},
@@ -58,7 +63,7 @@ const compile = (
         passThrough: ['mdxjsEsm'],
       },
     ]);
-    rehypePlugins.push(rehypeSanitize);
+    rehypePlugins.push([rehypeSanitize, sanitizeSchema]);
   }
 
   try {

package-lock.json

@@ -30,6 +30,7 @@
     "@readme/variable": "^18.0.0",
     "copy-to-clipboard": "^3.3.2",
     "debug": "^4.3.4",
+    "deepmerge": "^4.3.1",
     "emoji-regex": "^10.2.1",
     "estree-util-value-to-estree": "^3.1.1",
     "gemoji": "^8.1.0",