Fix logical race in rest_getutxos

Calling ActiveHeight() and ActiveTip() subsequently without holding the
::cs_main lock over both calls may result in a height that does not
correspond to the tip due to a race.

Fix this by holding the lock.

Author: MacroFake

src/rest.cpp

@@ -784,14 +784,18 @@ static bool rest_getutxos(const std::any& context, HTTPRequest* req, const std::
     ChainstateManager* maybe_chainman = GetChainman(context, req);
     if (!maybe_chainman) return false;
     ChainstateManager& chainman = *maybe_chainman;
+    decltype(chainman.ActiveHeight()) active_height;
+    uint256 active_hash;
     {
-        auto process_utxos = [&vOutPoints, &outs, &hits](const CCoinsView& view, const CTxMemPool* mempool) {
+        auto process_utxos = [&vOutPoints, &outs, &hits, &active_height, &active_hash, &chainman](const CCoinsView& view, const CTxMemPool* mempool) EXCLUSIVE_LOCKS_REQUIRED(chainman.GetMutex()) {
             for (const COutPoint& vOutPoint : vOutPoints) {
                 Coin coin;
                 bool hit = (!mempool || !mempool->isSpent(vOutPoint)) && view.GetCoin(vOutPoint, coin);
                 hits.push_back(hit);
                 if (hit) outs.emplace_back(std::move(coin));
             }
+            active_height = chainman.ActiveHeight();
+            active_hash = chainman.ActiveTip()->GetBlockHash();
         };
 
         if (fCheckMemPool) {
@@ -819,7 +823,7 @@ static bool rest_getutxos(const std::any& context, HTTPRequest* req, const std::
         // serialize data
         // use exact same output as mentioned in Bip64
         CDataStream ssGetUTXOResponse(SER_NETWORK, PROTOCOL_VERSION);
-        ssGetUTXOResponse << chainman.ActiveChain().Height() << chainman.ActiveChain().Tip()->GetBlockHash() << bitmap << outs;
+        ssGetUTXOResponse << active_height << active_hash << bitmap << outs;
         std::string ssGetUTXOResponseString = ssGetUTXOResponse.str();
 
         req->WriteHeader("Content-Type", "application/octet-stream");
@@ -829,7 +833,7 @@ static bool rest_getutxos(const std::any& context, HTTPRequest* req, const std::
 
     case RESTResponseFormat::HEX: {
         CDataStream ssGetUTXOResponse(SER_NETWORK, PROTOCOL_VERSION);
-        ssGetUTXOResponse << chainman.ActiveChain().Height() << chainman.ActiveChain().Tip()->GetBlockHash() << bitmap << outs;
+        ssGetUTXOResponse << active_height << active_hash << bitmap << outs;
         std::string strHex = HexStr(ssGetUTXOResponse) + "\n";
 
         req->WriteHeader("Content-Type", "text/plain");
@@ -842,8 +846,8 @@ static bool rest_getutxos(const std::any& context, HTTPRequest* req, const std::
 
         // pack in some essentials
         // use more or less the same output as mentioned in Bip64
-        objGetUTXOResponse.pushKV("chainHeight", chainman.ActiveChain().Height());
-        objGetUTXOResponse.pushKV("chaintipHash", chainman.ActiveChain().Tip()->GetBlockHash().GetHex());
+        objGetUTXOResponse.pushKV("chainHeight", active_height);
+        objGetUTXOResponse.pushKV("chaintipHash", active_hash.GetHex());
         objGetUTXOResponse.pushKV("bitmap", bitmapStringRepresentation);
 
         UniValue utxos(UniValue::VARR);