Remove usage of reduce in maxReadQuotaForKey, simplify accounting dat…

…a structure (#4419)

# Description

Remove usage of `std::reduce` from `LedgerKeyMeter::maxReadQuotaForKey`

# Checklist
- [ ] Reviewed the
[contributing](https://github.com/stellar/stellar-core/blob/master/CONTRIBUTING.md#submitting-changes)
document
- [ ] Rebased on top of master (no merge commits)
- [ ] Ran `clang-format` v8.0.0 (via `make format` or the Visual Studio
extension)
- [ ] Compiles
- [ ] Ran all tests
- [ ] If change impacts performance, include supporting evidence per the
[performance
document](https://github.com/stellar/stellar-core/blob/master/performance-eval/performance-eval.md)

src/ledger/LedgerTxn.cpp

@@ -182,10 +182,10 @@ LedgerKeyMeter::canLoad(LedgerKey const& key, size_t entrySizeBytes) const
 void
 LedgerKeyMeter::addTxn(SorobanResources const& resources)
 {
-    mTxReadBytes.push_back(resources.readBytes);
-    auto txId = mTxReadBytes.size() - 1;
+    TxReadBytesPtr txReadBytesPtr =
+        std::make_shared<uint32_t>(resources.readBytes);
     auto addKeyToTxnMap = [&](auto const& key) {
-        mLedgerKeyToTxs[key].emplace_back(txId);
+        mLedgerKeyToTxReadBytes[key].emplace_back(txReadBytesPtr);
     };
     std::for_each(resources.footprint.readOnly.begin(),
                   resources.footprint.readOnly.end(), addKeyToTxnMap);
@@ -197,8 +197,8 @@ void
 LedgerKeyMeter::updateReadQuotasForKey(LedgerKey const& key,
                                        size_t entrySizeBytes)
 {
-    auto iter = mLedgerKeyToTxs.find(key);
-    if (iter == mLedgerKeyToTxs.end())
+    auto iter = mLedgerKeyToTxReadBytes.find(key);
+    if (iter == mLedgerKeyToTxReadBytes.end())
     {
         // Key does not belong to the footprint of any transaction.
         // Ensure this is not a soroban key as they should always be metered.
@@ -208,17 +208,16 @@ LedgerKeyMeter::updateReadQuotasForKey(LedgerKey const& key,
     }
     // Update the read quota for every transaction containing this key.
     bool exceedsQuotaForAllTxns = true;
-    for (auto txn : iter->second)
+    for (TxReadBytesPtr txReadBytesPtr : iter->second)
     {
-        auto& quota = mTxReadBytes.at(txn);
-        if (quota < entrySizeBytes)
+        if (*txReadBytesPtr < entrySizeBytes)
         {
-            quota = 0;
+            *txReadBytesPtr = 0;
         }
         else
         {
             exceedsQuotaForAllTxns = false;
-            quota -= entrySizeBytes;
+            *txReadBytesPtr -= entrySizeBytes;
         }
     }
     if (exceedsQuotaForAllTxns)
@@ -230,8 +229,8 @@ LedgerKeyMeter::updateReadQuotasForKey(LedgerKey const& key,
 uint32_t
 LedgerKeyMeter::maxReadQuotaForKey(LedgerKey const& key) const
 {
-    auto iter = mLedgerKeyToTxs.find(key);
-    if (iter == mLedgerKeyToTxs.end())
+    auto iter = mLedgerKeyToTxReadBytes.find(key);
+    if (iter == mLedgerKeyToTxReadBytes.end())
     {
         // Key does not belong to the footprint of any transaction,
         // therefore it is not quota-limited.
@@ -240,10 +239,9 @@ LedgerKeyMeter::maxReadQuotaForKey(LedgerKey const& key) const
                       key.type() != CONTRACT_DATA);
         return std::numeric_limits<uint32_t>::max();
     }
-    return std::reduce(iter->second.begin(), iter->second.end(), 0,
-                       [&](uint32_t maxReadQuota, size_t const txn) {
-                           return std::max(maxReadQuota, mTxReadBytes.at(txn));
-                       });
+    return **std::max_element(
+        iter->second.begin(), iter->second.end(),
+        [&](TxReadBytesPtr a, TxReadBytesPtr b) { return *a < *b; });
 }
 
 bool

src/ledger/LedgerTxn.h

@@ -350,14 +350,11 @@ class LedgerKeyMeter : public NonMovableOrCopyable
   private:
     // Returns the maximum read quota across all transactions with this key.
     uint32_t maxReadQuotaForKey(LedgerKey const& key) const;
-
-    // Stores the read quota for each transaction.
-    std::vector<uint32_t> mTxReadBytes{};
-    // Stores the keys that each transaction will read. i.e.
-    // mLedgerKeyToTxs[key] is a vector containing  the indices of
-    // mTxReadBytes corresponding to the transactions which have
-    // the key in their footprint.
-    UnorderedMap<LedgerKey, std::vector<size_t>> mLedgerKeyToTxs{};
+    using TxReadBytesPtr = std::shared_ptr<uint32_t>;
+    // Stores a mapping from keys to a vector of pointers to the read bytes
+    // of the transactions that will read the keys.
+    UnorderedMap<LedgerKey, std::vector<TxReadBytesPtr>>
+        mLedgerKeyToTxReadBytes{};
     // Stores the keys that were not loaded due to insufficient read quota.
     LedgerKeySet mNotLoadedKeys{};
 };