This PR is related to #17268.
It adds a mempool stress test which makes a really big complicated tx graph, and then, similar to mempool_eviction test, trims the size.
The test setup is to make 100 original transactions with Rand(10)+2 outputs each.
Then, 800 times:
we create a new transaction with Rand(10) + 1 parents that are randomly sampled from all existing transactions (with unspent outputs). From each such parent, we then select Rand(remaining outputs) +1 50% of the time, or 1 outputs 50% of the time.
Then, we trim the size to 3/4. Then we trim it to just a single transaction.
This creates, hopefully, a big bundle of transactions with lots of complex structure, that should really put a strain on the mempool graph algorithms.
This ends up testing both the descendant and ancestor tracking.
I don’t love that the test is “unstable”. That is, in order to compare this test to another, you really can’t modify any of the internal state because it will have a different order of invocations of the deterministic randomness. However, it certainly suffices for comparing branches.