[plugin-terser] The worker approach to running Terser breaks many valid Terser options

[justinfagnani]

• Rollup Plugin Name: plugin-terser
• Rollup Plugin Version: 0.4.4
• Rollup Version: 4.24.0
• Operating System (or Browser): Linux
• Node Version: v18.20.3
• Link to reproduction (⚠️ read below): https://stackblitz.com/edit/rollup-repro-qx7xkl?file=package.json

Expected Behavior

All valid Terser options are supported. For this reproduction, the Terser nth_identifier option works.

Actual Behavior

An exception during build because only some of the code for the nth_identifier option is serialized. Other things seem to break from serializing the options, like nameCache updates.

Additional Information

The offending line that shows up in stack traces is here:

plugins/packages/terser/src/worker.ts

Line 39 in 4a2358a

const options = eval2(`(${data.options})`);

But, the technique of taking options in one realm and serializing them to another, like a worker, is just not generally workable. Not all Terser options will be serializable. Functions are an obvious case, but I've had other cases of objects with prototypes, etc., breaking too.

[midnight-wonderer]

Wow, that's how nameCache doesn't work.

It seems to be officially supported according to the type definition.

plugins/packages/terser/src/type.ts

Line 33 in 92daef0

nameCache?: Options['nameCache'];

I thought there was something wrong with my setup.
Do you have a workaround?

[midnight-wonderer]

Found one!

Basically, nameCache is supported. However, the issue lies in the parallel worker execution.
On each successful terser execution, the nameCache output gets merged into the central cache object.

The problem arises when the same name appears on multiple files that are being processed in parallel. Each mangles independently, sending out conflicting output cache.

The workaround is setting maxWorkers to 1.

[justinfagnani]

That workaround won't work for nth_identifier unfortunately, since it can be a function and functions can't be reliably serialized.