coinstats: Return purely out-param CCoinsStats

In previous commits in this patchset, we removed all in-param members of
CCoinsStats. Now that that's done, we can modify GetUTXOStats to return
an optional CCoinsStats instead of a status bool. Callers are modified
accordingly.

In rpc/blockchain.cpp, we discover that GetUTXOStats' status bool when
getting UTXO stats for pprev was not checked for error. We fix this as
well.

2 files changed, 29 insertions, 19 deletions

diff --git a/src/node/coinstats.cpp b/src/node/coinstats.cpp
index 13f1041a51..8fc53fe172 100644
--- a/src/node/coinstats.cpp
+++ b/src/node/coinstats.cpp
@@ -9,6 +9,7 @@
 #include <crypto/muhash.h>
 #include <hash.h>
 #include <index/coinstatsindex.h>
+#include <optional>
 #include <serialize.h>
 #include <uint256.h>
 #include <util/overflow.h>
@@ -144,22 +145,31 @@ static bool GetUTXOStats(CCoinsView* view, BlockManager& blockman, CCoinsStats&
     return true;
 }
 
-bool GetUTXOStats(CCoinsView* view, BlockManager& blockman, CCoinsStats& stats, CoinStatsHashType hash_type, const std::function<void()>& interruption_point, const CBlockIndex* pindex, bool index_requested)
+std::optional<CCoinsStats> GetUTXOStats(CCoinsView* view, BlockManager& blockman, CoinStatsHashType hash_type, const std::function<void()>& interruption_point, const CBlockIndex* pindex, bool index_requested)
 {
-    switch (hash_type) {
-    case(CoinStatsHashType::HASH_SERIALIZED): {
-        CHashWriter ss(SER_GETHASH, PROTOCOL_VERSION);
-        return GetUTXOStats(view, blockman, stats, ss, interruption_point, pindex, hash_type, index_requested);
-    }
-    case(CoinStatsHashType::MUHASH): {
-        MuHash3072 muhash;
-        return GetUTXOStats(view, blockman, stats, muhash, interruption_point, pindex, hash_type, index_requested);
-    }
-    case(CoinStatsHashType::NONE): {
-        return GetUTXOStats(view, blockman, stats, nullptr, interruption_point, pindex, hash_type, index_requested);
+    CCoinsStats stats{};
+
+    bool success = [&]() -> bool {
+        switch (hash_type) {
+        case(CoinStatsHashType::HASH_SERIALIZED): {
+            CHashWriter ss(SER_GETHASH, PROTOCOL_VERSION);
+            return GetUTXOStats(view, blockman, stats, ss, interruption_point, pindex, hash_type, index_requested);
+        }
+        case(CoinStatsHashType::MUHASH): {
+            MuHash3072 muhash;
+            return GetUTXOStats(view, blockman, stats, muhash, interruption_point, pindex, hash_type, index_requested);
+        }
+        case(CoinStatsHashType::NONE): {
+            return GetUTXOStats(view, blockman, stats, nullptr, interruption_point, pindex, hash_type, index_requested);
+        }
+        } // no default case, so the compiler can warn about missing cases
+        assert(false);
+    }();
+
+    if (!success) {
+        return std::nullopt;
     }
-    } // no default case, so the compiler can warn about missing cases
-    assert(false);
+    return stats;
 }
 
 // The legacy hash serializes the hashBlock
diff --git a/src/node/coinstats.h b/src/node/coinstats.h
index adc67eb0a6..36cbf19f29 100644
--- a/src/node/coinstats.h
+++ b/src/node/coinstats.h
@@ -71,11 +71,11 @@ struct CCoinsStats {
  *
  * @param[in] index_requested Signals if the coinstatsindex should be used (when available).
  */
-bool GetUTXOStats(CCoinsView* view, node::BlockManager& blockman,
-                  CCoinsStats& stats, CoinStatsHashType hash_type,
-                  const std::function<void()>& interruption_point = {},
-                  const CBlockIndex* pindex = nullptr,
-                  bool index_requested = true);
+std::optional<CCoinsStats> GetUTXOStats(CCoinsView* view, node::BlockManager& blockman,
+                                        CoinStatsHashType hash_type,
+                                        const std::function<void()>& interruption_point = {},
+                                        const CBlockIndex* pindex = nullptr,
+                                        bool index_requested = true);
 
 uint64_t GetBogoSize(const CScript& script_pub_key);