test: add HaveInputs call-path unit tests

Add unit tests covering `CCoinsViewCache::HaveInputs()`.

The tests document that `HaveInputs()` consults the cache first and that a cache miss pulls from the backing view via `GetCoin()`.

Co-authored-by: Novo <eunovo9@gmail.com>

src/test/coins_tests.cpp

@@ -1059,6 +1059,69 @@ BOOST_FIXTURE_TEST_CASE(ccoins_flush_behavior, FlushTest)
     }
 }
 
+BOOST_AUTO_TEST_CASE(ccoins_cache_behavior)
+{
+    class CCoinsViewSpy final : public CCoinsView
+    {
+    public:
+        const COutPoint expected;
+        mutable size_t getcoin_calls{0};
+        bool allow_getcoin{true};
+
+        explicit CCoinsViewSpy(const COutPoint& out) : expected{out} {}
+
+        std::optional<Coin> GetCoin(const COutPoint& out) const override
+        {
+            BOOST_CHECK(out == expected);
+            if (!allow_getcoin) {
+                BOOST_FAIL("Base GetCoin should not be called when input is cached");
+                return std::nullopt;
+            }
+            ++getcoin_calls;
+            return Coin{CTxOut{1, CScript{}}, 1, false};
+        }
+
+        bool HaveCoin(const COutPoint& out) const override
+        {
+            BOOST_CHECK(out == expected);
+            BOOST_FAIL("Base HaveCoin should never be called");
+            return false;
+        }
+    };
+
+    const COutPoint prevout{Txid::FromUint256(m_rng.rand256()), 0};
+    CCoinsViewSpy base{prevout};
+    CCoinsViewCache cache{&base};
+
+    CMutableTransaction mtx;
+    mtx.vin.emplace_back(prevout);
+    const CTransaction tx{mtx};
+    BOOST_CHECK(!tx.IsCoinBase());
+
+    BOOST_CHECK(cache.HaveInputs(tx));
+    BOOST_CHECK_EQUAL(base.getcoin_calls, 1);
+
+    base.allow_getcoin = false;
+    BOOST_CHECK(cache.HaveInputs(tx));
+    BOOST_CHECK(cache.GetCoin(prevout));
+    BOOST_CHECK(!cache.AccessCoin(prevout).IsSpent());
+    BOOST_CHECK(cache.HaveCoin(prevout));
+    BOOST_CHECK_EQUAL(base.getcoin_calls, 1);
+
+    const COutPoint prefilled_prevout{Txid::FromUint256(m_rng.rand256()), 0};
+    cache.AddCoin(prefilled_prevout, Coin{CTxOut{1, CScript{}}, 1, false}, /*possible_overwrite=*/false);
+    BOOST_CHECK(cache.HaveCoinInCache(prefilled_prevout));
+
+    CMutableTransaction prefilled_mtx;
+    prefilled_mtx.vin.emplace_back(prefilled_prevout);
+    const CTransaction prefilled_tx{prefilled_mtx};
+    BOOST_CHECK(!prefilled_tx.IsCoinBase());
+    BOOST_CHECK(cache.HaveInputs(prefilled_tx));
+
+    BOOST_CHECK(cache.SpendCoin(prevout));
+    BOOST_CHECK(!cache.HaveCoinInCache(prevout));
+}
+
 BOOST_AUTO_TEST_CASE(coins_resource_is_used)
 {
     CCoinsMapMemoryResource resource;