Check if limit price is respected

[m-lord-renkse]

Enforce the limit price required in the settlement contract: https://github.com/cowprotocol/contracts/blob/main/src/contracts/GPv2Settlement.sol#L368-L371

[fleupold]

I'm quite surprised this PR is needed (checking the limit price sounds like a very basic thing the solver code base should already be doing).

Can you please add more context & explanation on why this is needed? From quickly searching the code I found at least these two places, which already check the limit price, so I'd like to be able to read from the PR description why those aren't sufficient:

1. solvers/src/domain/solver/dex/mod.rs

   Line 171 in 446942d

   let valid = swap.satisfies(order);

2. solvers/src/domain/solution.rs

   Line 185 in 446942d

   if order.sell.amount.checked_mul(buy)? < order.buy.amount.checked_mul(sell)? {

[squadgazzz]

I would really appreciate more context for this PR. It is not clear why this change is required due to the absence of tests. Does the driver do the same validation?

[m-lord-renkse]

@fleupold @squadgazzz we are getting simulations failing here: https://github.com/cowprotocol/contracts/blob/main/src/contracts/GPv2Settlement.sol#L368-L371 for these solvers. Which seems to indicate we aren't doing this check properly.

2. solvers/src/domain/solution.rs

   Line 185 in 446942d

   if order.sell.amount.checked_mul(buy)? < order.buy.amount.checked_mul(sell)? {

I somehow didn't see this. Maybe this math is not 100 % precise and we have some cases which triggers this error in the settlement contract.

[m-lord-renkse]

Does the driver do the same validation?

Exactly the same.

[squadgazzz]

Ok, so if I got it correctly, this is required to reduce the driver's error rate and, instead, just use warnings on the solver's side, right? But does it make sense to keep everything as it is so we can still have signals that there is something wrong with solutions?