Allow a single docs folder to be consumed by 2 docs plugin instances (use-case: normal vs embed/iframe docs)

[alexandernst]

Have you read the Contributing Guidelines on issues?

• I have read the Contributing Guidelines on issues.

Prerequisites

• I'm using the latest version of Docusaurus.
• I have tried the npm run clear or yarn clear command.
• I have tried rm -rf node_modules yarn.lock package-lock.json and re-installing packages.
• I have tried creating a repro with https://new.docusaurus.io.
• I have read the console error message carefully (if applicable).

Description

If I add the docs plugin to the list of plugins (multi-instance) and I point it to the same folder that the preset is already pointing to (docs by default), the build process stops working and I see the following errors:

SyntaxError: /code/src/......../index.mdx: Identifier 'React' has already been declared. (46:184)

I believe the bug might be caused by the way each mdx file is being processed. Because a mdx file is (basically) a JSX file, import React from 'react'; is being prepended at the top of each file. But since I have two instances of the docs plugin pointing to the same folder, the import statement is being prepended twice (hence the error).

I made a minimal repro codesandbox.

Reproducible demo

https://codesandbox.io/s/trusting-rosalind-sc4s15

Steps to reproduce

1. Open the link
2. Open a new tty and type npm run build
3. The build will fail.
4. Comment the plugins section of the docusaurus.config.js
5. Run npm run build again.
6. The build will succeed.

Expected behavior

I should be able to point multiple instances of the docs plugin to the same folder.

Actual behavior

The build crashes.

Your environment

• Public source code: N/A
• Public site URL: N/A
• Docusaurus version used: 2.0.0-beta20
• Environment name and version (e.g. Chrome 89, Node.js 16.4): N/A
• Operating system and version (e.g. Ubuntu 20.04.2 LTS): N/A

Self-service

• I'd be willing to fix this bug myself.

[Josh-Cena]

Interesting. I get why it fails, but I don't understand what you are trying to achieve—what's the use-case? We do expect plugins to work in isolation and their folder paths do not overlap.

[alexandernst]

I'd like to be able to render the same documentation with different layouts on different URLs. Eg. foo.com/docs/intro should render the navbar, the sidebar, the docs itself, etc..., while foo.com/embedded/intro should render just the intro document, without any other elements (no navbar, no sidebar, no header, no footer, etc...). My use-case is being able to embed a particular document inside an iframe.

[Josh-Cena]

Ah, I see. I don't really think you need multiple plugins to do this, but I don't have a better solution on top of my head right now. Because you have the same set of data and metadata, it should be doable within the same plugin, just passing it to two themes.

[alexandernst]

How could I do that?

[Josh-Cena]

I don't really know. I don't think it's possible with today's setup, which is why I'm still leaving it open :) But it's definitely legitimate and we'll figure out a way to do it. Sorry if you think it's going to be a quick little bug fix—it's probably more complicated than we have thought.

@slorber will be back tomorrow, and maybe he can have some opinions to offer as well.

[alexandernst]

@slorber Hi! Did you have a chance to look into this proposal?

[slorber]

No, I am reviewing existing PRs for now