V8's developer guide[1] recommends running un-trusted code in separate processes. So each worker job would need to fork.
[1] https://github.com/v8/v8/wiki/Untrusted-code-mitigations#san...
[1] https://github.com/v8/v8/wiki/Untrusted-code-mitigations#san...
i.e. the Chrome team, post-Spectre, are assuming that any value in a process' memory is readable by any code executing within that process.