Merge bitcoin/bitcoin#28215: fuzz: fix a couple incorrect assertions in the coins_view target

e417c98 fuzz: coins_view: remove an incorrect assertion (Antoine Poinsot)
c5f6b1d fuzz: coins_view: correct an incorrect assertion (Antoine Poinsot)

Pull request description:

  The `coins_view` fuzz target would assert in two places that the cache is consistent with the backend. But it's never the case (that's the whole point of using a cache).

  The only reason this didn't result in a crash was that we would never actually hit these assertions. I ran into this while introducing a new target with an in-memory `CCoinsViewDB` as the backend view (see bitcoin/bitcoin#28216) which made the code paths with those assertions actually reachable.

ACKs for top commit:
  dergoegge:
    Code review ACK e417c98

Tree-SHA512: 5847bb2744a2f2831dace62d32b79cc491bf54e2af4ce425411d245d566622d9aff816d9be5ec8e830d10851c13f2500bf4f0c004d88b4d7cca1d483ef8960a6

Author: fanquake

src/test/fuzz/coins_view.cpp

@@ -155,14 +155,16 @@ FUZZ_TARGET(coins_view, .init = initialize_coins_view)
         }
         assert((exists_using_access_coin && exists_using_have_coin_in_cache && exists_using_have_coin && exists_using_get_coin) ||
                (!exists_using_access_coin && !exists_using_have_coin_in_cache && !exists_using_have_coin && !exists_using_get_coin));
+        // If HaveCoin on the backend is true, it must also be on the cache if the coin wasn't spent.
         const bool exists_using_have_coin_in_backend = backend_coins_view.HaveCoin(random_out_point);
-        if (exists_using_have_coin_in_backend) {
+        if (!coin_using_access_coin.IsSpent() && exists_using_have_coin_in_backend) {
             assert(exists_using_have_coin);
         }
         Coin coin_using_backend_get_coin;
         if (backend_coins_view.GetCoin(random_out_point, coin_using_backend_get_coin)) {
             assert(exists_using_have_coin_in_backend);
-            assert(coin_using_get_coin == coin_using_backend_get_coin);
+            // Note we can't assert that `coin_using_get_coin == coin_using_backend_get_coin` because the coin in
+            // the cache may have been modified but not yet flushed.
         } else {
             assert(!exists_using_have_coin_in_backend);
         }