[gatsby-plugin-mdx] Remark plugins using mutateSource seems to be ignored

[Strajk]

Description

Remark plugins are able both to transform AST and mutate original source

exports.mutateSource = ({ markdownNode }) => {
  markdownNode.internal.content = markdownNode.internal.content.replace("foo", "bar")
}

exports.default = ({ markdownAST }) => {
  visit(markdownAST, ["heading"], (node) => { … })
  return markdownAST
}

Remark plugins in gatsby-plugin-mdx seem to ignore the mutateSource hook and only run AST transformation.
(or more precisely, it ignores plugins without default export && doesn't check for mutateSource on plugins)

• There seems to be some code prepared to handle this, but it's commented out
https://github.com/gatsbyjs/gatsby/blob/master/packages/gatsby-plugin-mdx/utils/gen-mdx.js#L22
(Commented out by @ChristopherBiscardi so tagging, sry if not appropriate)

• Filtering out seems to be done here
https://github.com/gatsbyjs/gatsby/blob/master/packages/gatsby-plugin-mdx/utils/get-source-plugins-as-remark-plugins.js#L50

• There's an issue for this in old repo
ChristopherBiscardi/gatsby-mdx#118
The proposed workaround in the last comment is to rewrite the plugin to use AST transformation instead of mutateSource, so not a real solution

Steps to reproduce

https://github.com/Strajk/jungle/tree/master/gatsby-replace-md-with-md

There are two local plugins:

• ./plugins/remark-ast – add ⭐ at the end of headings
• ./plugins/remark-mutate-source – replaces "dog" with dog emoji in html tag

Project uses both transformer-remark & plugin-mdx for easier reproducibility.
Both are configured to use both plugins https://github.com/Strajk/jungle/blob/master/gatsby-replace-md-with-md/gatsby-config.js

There are two sample posts to test the functionality

• ./content/blog/hello-world-md/index.md
• ./content/blog/hello-world-mdx/index.mdx

Expected result

Both posts (md & mdx) should be transformed with both plugins.

Actual result

• MD post is transformed with both plugins
• MDX post is transformed with just ./plugins/remark-ast plugin

Environment

Gatsby 2.20.25 – Expand for all info

  System:
    OS: macOS 10.15.4
    CPU: (8) x64 Intel(R) Core(TM) i7-4870HQ CPU @ 2.50GHz
    Shell: 5.7.1 - /bin/zsh
  Binaries:
    Node: 13.8.0 - ~/.nvm/versions/node/v13.8.0/bin/node
    Yarn: 1.19.1 - /usr/local/bin/yarn
    npm: 6.13.6 - ~/.nvm/versions/node/v13.8.0/bin/npm
  Languages:
    Python: 2.7.16 - /usr/bin/python
  Browsers:
    Chrome: 81.0.4044.122
    Firefox: 75.0
    Safari: 13.1
  npmPackages:
    gatsby: ^2.20.25 => 2.20.25 
    gatsby-image: ^2.3.4 => 2.3.4 
    gatsby-plugin-feed: ^2.4.3 => 2.4.3 
    gatsby-plugin-google-analytics: ^2.2.4 => 2.2.4 
    gatsby-plugin-manifest: ^2.3.5 => 2.3.5 
    gatsby-plugin-mdx: ^1.1.10 => 1.1.10 
    gatsby-plugin-offline: ^3.1.4 => 3.1.4 
    gatsby-plugin-react-helmet: ^3.2.4 => 3.2.4 
    gatsby-plugin-sharp: ^2.5.6 => 2.5.6 
    gatsby-plugin-typography: ^2.4.3 => 2.4.3 
    gatsby-remark-copy-linked-files: ^2.2.3 => 2.2.3 
    gatsby-remark-images: ^3.2.4 => 3.2.4 
    gatsby-remark-prismjs: ^3.4.3 => 3.4.3 
    gatsby-remark-responsive-iframe: ^2.3.3 => 2.3.3 
    gatsby-remark-smartypants: ^2.2.3 => 2.2.3 
    gatsby-source-filesystem: ^2.2.4 => 2.2.4 
    gatsby-transformer-remark: ^2.7.3 => 2.7.3 
    gatsby-transformer-sharp: ^2.4.6 => 2.4.6 
  npmGlobalPackages:
    gatsby-cli: 2.11.14

Tagging @mathieudutour and @vladar as I noticed they done some (maybe) similar work in #21188
(sry if not appropriate 🙏)

[Strajk]

Real-world use case:

Remark plugin for finding and replacing, something like https://github.com/angeloashmore/gatsby-remark-find-replace, but with support for markdown and HTML in replacing text, eg:

{
  COMPANY: "The Company" // Supported in mention plugin
  NOTE: "This is a note with **markdown**" // Not Supported
  TOP: "Go to <a href="#top" style=''>top</a>" // Not Supported
}

[vladar]

Just to clarify - is this a recent break? Or was it working like this for a long time now?

[mathieudutour]

I don't think it ever worked

[Strajk]

@vladar @mathieudutour even though I don't use Gatsby for a long time, I agree with @mathieudutour that it probably never worked –– that commented out code is there from October 2018

Was not sure if this issue should be Feature request or Bug

[github-actions]

Hiya!

This issue has gone quiet. Spooky quiet. 👻

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[github-actions]

Hey again!

It’s been 30 days since anything happened on this issue, so our friendly neighborhood robot (that’s me!) is going to close it.
Please keep in mind that I’m only a robot, so if I’ve closed this issue in error, I’m HUMAN_EMOTION_SORRY. Please feel free to reopen this issue or create a new one if you need anything else.
As a friendly reminder: the best way to see this issue, or any other, fixed is to open a Pull Request. Check out gatsby.dev/contribute for more information about opening PRs, triaging issues, and contributing!

Thanks again for being part of the Gatsby community! 💪💜

[artt]

@Strajk you ever find a solution to this? Thanks!