Performance issues with new versions of @babel/parser

[ehmicky]

Some builds are experiencing a big slowdown when we upgrade @babel/parser from 7.16.8 to 7.16.10 (note: there is no 7.16.9 release).

We are currently unable to reproduce this locally. However, @eduardoboucas and I are able to reproduce this in production using the following Netlify site.

The only PR introduced by that release is babel/babel#14130, so it appears to be the reason. Two bugs related to that PR are open, which could be the root cause, but we are not sure yet: babel/babel#14182, babel/babel#14175

From extensive debugging, we are certain that the problem comes from this specific package and this specific version. We also know the problem comes from @babel/parser as a direct dependency of @netlify/zip-it-and-ship-it (see this and that), as opposed to as an indirect dependency through precinct (see this and that) and node-source-walker (see this and that).

The problem only impacts our legacy bundler, not the nft nor esbuild bundlers.

We have made a PR, now released, which pins the version of @babel/parser to 7.16.8 as a way to mitigate the problem: #966

However, this is not a long-term solution because it prevents us from upgrading @babel/parser in the future, which we might need to do for compatibility with the latest JavaScript syntax, or for security fixes. I see several possible long-term solutions there:

1. Keep the old version of @babel/parser until we fully deprecate the legacy bundler.
2. Follow the existing bug reports and new versions of @babel/parser hoping they fix this problem.
3. Find the reason why this new version of @babel/parser is creating an issue with our code in this repository. Only 4 files are using this, so it might be doable, or at least worth some investigation.

@eduardoboucas @Skn0tt @minivan What are your thoughts on this?

[eduardoboucas]

I think we should keep the old version of @babel/parser for the time being, unless we find a strong reason to upgrade (like a bug or a missing feature).

[nicolo-ribaudo]

This might have been fixed in Babel 7.17.0 by babel/babel#14174.