use decache to unload plugin modules

[akosyakov]

What it does

fix #5756: use decache to unload plugin modules:

• don't touch exports!
• traverse only modules loaded by the plugin, not the entire require cache

How to test

You can verify that modules' exports are not messed up in anyway by:

• installing emmet (https://registry.npmjs.org/@theia/vscode-builtin-emmet/-/vscode-builtin-emmet-0.1.0.tgz) extension and using it
• installing IBM blockchain extension, see Fix plugin loading to cope with modules that have immutable exports #5520 (comment)

I don't know how one can verify that there is no memory leak, because UI does not allow to load the same plugin twice in the vanilla Theia and decaching is not required. cc @benoitf

I wonder should decaching belong to this repo at all since it is not used or it should be moved to a corresponding client? cc @svenefftinge

Review checklist

• as an author, I have thoroughly tested my changes and carefully reviewed following the review guidelines

Reminder for reviewers

• as a reviewer, I agree to behave in accordance with the review guidelines

[akosyakov]

@marcdumais-work they are both of MIT license, unfortunately callsite does not have Git repo, but it is MIT in the package.json

[kittaakos]

Here you go: https://github.com/tj/callsite

[marcdumais-work]

Both dependencies look good: MIT licenses and no production dependencies of their own (so no potential that something with inappropriate license would be pulled).

[akosyakov]

That's also bad that PluginHostRPC does not belong to DI context. No way for clients to work it around.

[akosyakov]

@AlexTugarev please review, it is critical to resolve, some vs code extensions don't work

[AlexTugarev]

@akosyakov, how to proceed with @benoitf's use case from #5756 (comment) ? how to test that path?

[AlexTugarev]

@akosyakov is this supposed to fix all errors of the emmet extension? Error: Cannot find module 'vscode-emmet-helper' is still present.

[akosyakov]

@AlexTugarev most important that proposals for emmet extension should not work, let me check whether an error will be gone if we completely remove modules unload

I don't know how to proceed with memory leak. There is not issues in Theia. It is Che issue and should be fixed in Che. We cannot test core code against concrete products. It was already mentioned once.

[benoitf]

@akosyakov well, if a plug-in can be stopped/unloaded without restarting browser and that there is a memory leak it's still a theia issue.

[akosyakov]

@benoitf Is it possible right now? Could you verify that it does not leak? Could you provide tests?

[benoitf]

I can't but I'm fine to check later (after merged) in upcoming versions and provide fixes if required

[akosyakov]

@benoitf thank you, then we proceed with fixing emmet and try to preserve unloading if it is possible

[akosyakov]

Sorry for confusion, I've found a real root cause, it is not related to unloading. built-in extensions don't contain node_modules because of npm packaging. It worked for me because i've installed emmet extension from source code at the same time when I've removed a line which deleting exports what got me to wrong conclusions.