Merge bitcoin/bitcoin#29462: [fuzz] Avoid partial negative result

9dae3b970a7a82e8d9f3f755048d427da78c49da [fuzz] Avoid partial negative result (Murch)

Pull request description:

  May address the problem reported by maflcko in https://github.com/bitcoin/bitcoin/pull/27877#pullrequestreview-1890304914.

  For some values, `MAX_MONEY - max_spendable - max_output_groups` could result in a partial negative value. By putting the addition of `group_pos.size()` first, all partial results in this line will be strictly positive.

  I opened this as a draft, since I was unable to reproduce the issue, so I’m waiting for confirmation whether this in fact mitigates the problem.

ACKs for top commit:
  maflcko:
    ACK 9dae3b970a7a82e8d9f3f755048d427da78c49da
  sipa:
    utACK 9dae3b970a7a82e8d9f3f755048d427da78c49da
  achow101:
    ACK 9dae3b970a7a82e8d9f3f755048d427da78c49da
  brunoerg:
    crACK 9dae3b970a7a82e8d9f3f755048d427da78c49da

Tree-SHA512: 744b4706268d8dfd77538b99492ecf3cf77d229095f9bcd416a412131336830e2f134f2b2846c79abd3d193426f97c1f71eeaf68b16ab00e76318d57ee3673c7

1 files changed, 1 insertions, 1 deletions

diff --git a/src/wallet/test/fuzz/coinselection.cpp b/src/wallet/test/fuzz/coinselection.cpp
index 3ffeecdf34..297432de9e 100644
--- a/src/wallet/test/fuzz/coinselection.cpp
+++ b/src/wallet/test/fuzz/coinselection.cpp
@@ -158,7 +158,7 @@ FUZZ_TARGET(coin_grinder_is_optimal)
         // Only make UTXOs with positive effective value
         const CAmount input_fee = coin_params.m_effective_feerate.GetFee(n_input_bytes);
         // Ensure that each UTXO has at least an effective value of 1 sat
-        const CAmount eff_value{fuzzed_data_provider.ConsumeIntegralInRange<CAmount>(1, MAX_MONEY - max_spendable - max_output_groups + group_pos.size())};
+        const CAmount eff_value{fuzzed_data_provider.ConsumeIntegralInRange<CAmount>(1, MAX_MONEY + group_pos.size() - max_spendable - max_output_groups)};
         const CAmount amount{eff_value + input_fee};
         std::vector<COutput> temp_utxo_pool;