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.

4 files changed, 48 insertions, 33 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);
 
diff --git a/src/rpc/blockchain.cpp b/src/rpc/blockchain.cpp
index 98c2deb4bf..97983cea5a 100644
--- a/src/rpc/blockchain.cpp
+++ b/src/rpc/blockchain.cpp
@@ -862,7 +862,6 @@ static RPCHelpMan gettxoutsetinfo()
 
     const CBlockIndex* pindex{nullptr};
     const CoinStatsHashType hash_type{request.params[0].isNull() ? CoinStatsHashType::HASH_SERIALIZED : ParseHashType(request.params[0].get_str())};
-    CCoinsStats stats{};
     bool index_requested = request.params[2].isNull() || request.params[2].get_bool();
 
     NodeContext& node = EnsureAnyNodeContext(request.context);
@@ -903,7 +902,9 @@ static RPCHelpMan gettxoutsetinfo()
         }
     }
 
-    if (GetUTXOStats(coins_view, *blockman, stats, hash_type, node.rpc_interruption_point, pindex, index_requested)) {
+    const std::optional<CCoinsStats> maybe_stats = GetUTXOStats(coins_view, *blockman, hash_type, node.rpc_interruption_point, pindex, index_requested);
+    if (maybe_stats.has_value()) {
+        const CCoinsStats& stats = maybe_stats.value();
         ret.pushKV("height", (int64_t)stats.nHeight);
         ret.pushKV("bestblock", stats.hashBlock.GetHex());
         ret.pushKV("txouts", (int64_t)stats.nTransactionOutputs);
@@ -923,9 +924,12 @@ static RPCHelpMan gettxoutsetinfo()
             ret.pushKV("total_unspendable_amount", ValueFromAmount(stats.total_unspendable_amount));
 
             CCoinsStats prev_stats{};
-
             if (pindex->nHeight > 0) {
-                GetUTXOStats(coins_view, *blockman, prev_stats, hash_type, node.rpc_interruption_point, pindex->pprev, index_requested);
+                const std::optional<CCoinsStats> maybe_prev_stats = GetUTXOStats(coins_view, *blockman, hash_type, node.rpc_interruption_point, pindex->pprev, index_requested);
+                if (!maybe_prev_stats) {
+                    throw JSONRPCError(RPC_INTERNAL_ERROR, "Unable to read UTXO set");
+                }
+                prev_stats = maybe_prev_stats.value();
             }
 
             UniValue block_info(UniValue::VOBJ);
@@ -2285,7 +2289,7 @@ UniValue CreateUTXOSnapshot(
     const fs::path& temppath)
 {
     std::unique_ptr<CCoinsViewCursor> pcursor;
-    CCoinsStats stats{};
+    std::optional<CCoinsStats> maybe_stats;
     const CBlockIndex* tip;
 
     {
@@ -2305,19 +2309,20 @@ UniValue CreateUTXOSnapshot(
 
         chainstate.ForceFlushStateToDisk();
 
-        if (!GetUTXOStats(&chainstate.CoinsDB(), chainstate.m_blockman, stats, CoinStatsHashType::HASH_SERIALIZED, node.rpc_interruption_point)) {
+        maybe_stats = GetUTXOStats(&chainstate.CoinsDB(), chainstate.m_blockman, CoinStatsHashType::HASH_SERIALIZED, node.rpc_interruption_point);
+        if (!maybe_stats) {
             throw JSONRPCError(RPC_INTERNAL_ERROR, "Unable to read UTXO set");
         }
 
         pcursor = chainstate.CoinsDB().Cursor();
-        tip = CHECK_NONFATAL(chainstate.m_blockman.LookupBlockIndex(stats.hashBlock));
+        tip = CHECK_NONFATAL(chainstate.m_blockman.LookupBlockIndex(maybe_stats->hashBlock));
     }
 
     LOG_TIME_SECONDS(strprintf("writing UTXO snapshot at height %s (%s) to file %s (via %s)",
         tip->nHeight, tip->GetBlockHash().ToString(),
         fs::PathToString(path), fs::PathToString(temppath)));
 
-    SnapshotMetadata metadata{tip->GetBlockHash(), stats.coins_count, tip->nChainTx};
+    SnapshotMetadata metadata{tip->GetBlockHash(), maybe_stats->coins_count, tip->nChainTx};
 
     afile << metadata;
 
@@ -2339,11 +2344,11 @@ UniValue CreateUTXOSnapshot(
     afile.fclose();
 
     UniValue result(UniValue::VOBJ);
-    result.pushKV("coins_written", stats.coins_count);
+    result.pushKV("coins_written", maybe_stats->coins_count);
     result.pushKV("base_hash", tip->GetBlockHash().ToString());
     result.pushKV("base_height", tip->nHeight);
     result.pushKV("path", path.u8string());
-    result.pushKV("txoutset_hash", stats.hashSerialized.ToString());
+    result.pushKV("txoutset_hash", maybe_stats->hashSerialized.ToString());
     // Cast required because univalue doesn't have serialization specified for
     // `unsigned int`, nChainTx's type.
     result.pushKV("nchaintx", uint64_t{tip->nChainTx});
diff --git a/src/validation.cpp b/src/validation.cpp
index 5fdf0398df..40c2618db9 100644
--- a/src/validation.cpp
+++ b/src/validation.cpp
@@ -5095,22 +5095,22 @@ bool ChainstateManager::PopulateAndValidateSnapshot(
 
     assert(coins_cache.GetBestBlock() == base_blockhash);
 
-    CCoinsStats stats{};
     auto breakpoint_fnc = [] { /* TODO insert breakpoint here? */ };
 
     // As above, okay to immediately release cs_main here since no other context knows
     // about the snapshot_chainstate.
     CCoinsViewDB* snapshot_coinsdb = WITH_LOCK(::cs_main, return &snapshot_chainstate.CoinsDB());
 
-    if (!GetUTXOStats(snapshot_coinsdb, m_blockman, stats, CoinStatsHashType::HASH_SERIALIZED, breakpoint_fnc)) {
+    const std::optional<CCoinsStats> maybe_stats = GetUTXOStats(snapshot_coinsdb, m_blockman, CoinStatsHashType::HASH_SERIALIZED, breakpoint_fnc);
+    if (!maybe_stats.has_value()) {
         LogPrintf("[snapshot] failed to generate coins stats\n");
         return false;
     }
 
     // Assert that the deserialized chainstate contents match the expected assumeutxo value.
-    if (AssumeutxoHash{stats.hashSerialized} != au_data.hash_serialized) {
+    if (AssumeutxoHash{maybe_stats->hashSerialized} != au_data.hash_serialized) {
         LogPrintf("[snapshot] bad snapshot content hash: expected %s, got %s\n",
-            au_data.hash_serialized.ToString(), stats.hashSerialized.ToString());
+            au_data.hash_serialized.ToString(), maybe_stats->hashSerialized.ToString());
         return false;
     }