Mock prover `assert_satisfied_par` always fails with "cell not assigned"

[nulltea]

If a circuit builder has at least one relation between cells (e.g. addition, etc), the MockProver::assert_satisfied_par() fails with the errors bellow. The assert_satisfied() method works without an issue.

Total 1 fixed cells
Total range check advice cells to lookup per phase: [0, 0, 0]
error: cell not assigned

  Cell layout in region 'BaseCircuitBuilder generated circuit':
    | Offset | A0 |
    +--------+----+
    |    1   |  X | <--{ X marks the spot! 🦜
    |    2   | x1 |
    |    3   | x2 |
    |    4   | x3 |

  Gate '1 column a + b * c = out' (applied at offset 1) queries these cells.

error: cell not assigned

  Cell layout in region 'BaseCircuitBuilder generated circuit':
    | Offset | A0 |
    +--------+----+
    |    1   | x0 |
    |    2   |  X | <--{ X marks the spot! 🦜
    |    3   | x2 |
    |    4   | x3 |

  Gate '1 column a + b * c = out' (applied at offset 1) queries these cells.

thread 'sync_step_circuit::tests::test_step_circuit' panicked at /Users/timofey/.cargo/registry/src/index.crates.io-6f17d22bba15001f/halo2-axiom-0.4.2/src/dev.rs:1571:13:
circuit was not satisfied
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
test sync_step_circuit::tests::test_step_circuit ... FAILED

[jonathanpwang]

Good catch! We have not been using any of the parallel assignment APIs, so we didn't catch this. The reason is likely because we turned off all offset calculations in the Layouter so when you parallel assign it will just assign different regions all overlapping without offsets.

I am not sure how to fix this without needing to call assign_region twice everywhere -- but it's been on my mind as something to try to resolve.

[nulltea]

Yes, it's not a critical one, but it's good to have an issue for visibly in case someone will encounter this.