Summary

Implementation:

• Do not lock the artifact-dir for check builds + fix uplifting #16307
• Implement fine grain locking for build-dir #16155

Known issues

• Only non-artifact-producing commands (like check) can run in parallel

Unresolved Issues

• Stabilization of Re-organize build-dir by package + hash, rather than artifact type #15010
• Do we need rlimit checks and dynamic rlimits? Implement fine grain locking for build-dir #16155 (comment)
• If there is shared build unit (same ID) with a dirty fingerprint when two checks run, one will block until the other has finished (all units, not just the shared unit) and will rebuild that unit
• Blocking prevents other units from being built on that thread
• Proper handling of blocking message (Implement fine grain locking for build-dir #16155 (comment))
  • We get the blocking message when taking the initial shared lock, but we get no message when taking the exclusive lock right before compiling.

Future Extensions

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Original issue:

Right now whenever a build happens we lock the entirety of the target directory for the whole build, but it may be possible for us to have a more granular locking strategy which allows multiple instances of Cargo to proceed in parallel instead of serializing them.