flakes: SQLite database is busy

[manveru]

Describe the bug

Running two (or more) builds of the same flake in parallel will result in an error like this:

SQLite database '/home/manveru/.cache/nix/eval-cache-v1/7bfed1c712ac845ecca1eebf57ba6b5c5f4571c0b7a204c10dcbb6ee21ceb5ce.sqlite' is busy

Steps To Reproduce

1. Pick any flake
2. Make it evaluate in parallel (before it is cached)
3. See error

Expected behavior

Just like with normal builds, I'd like Nix to wait for the DB to be unlocked.

nix --version output

nix (Nix) 2.4pre20200622_334e26b

Additional context

There is no good way to catch this error and retry without parsing the output. So instead you have to do an eval first and hope nothing changes in-between that might cause re-evaluation.