non-existent entries should not be metered (#4428)

# Description

Resolves stellar/stellar-core-internal#282

Remove optimization that checks whether there is quota remaining for a
ledger key before attempting to load the entry from the bucketlist. This
ensures that non-existent entries are not metered and result in a
`nullptr` entry in the cache. This will prevent point loads when writing
newly created entries post-contract invocation.

src/bucket/BucketSnapshot.cpp

@@ -101,24 +101,6 @@ BucketSnapshot::loadKeysWithLimits(std::set<LedgerKey, LedgerEntryIdCmp>& keys,
     auto indexIter = index.begin();
     while (currKeyIt != keys.end() && indexIter != index.end())
     {
-        if (lkMeter)
-        {
-            auto keySize = xdr::xdr_size(*currKeyIt);
-            if (!lkMeter->canLoad(*currKeyIt, keySize))
-            {
-                // If the transactions containing this key have a remaining
-                // quota less than the size of the key, we cannot load the
-                // entry, as xdr_size(key) <= xdr_size(entry). Here we consume
-                // keySize bytes from the quotas of transactions containing the
-                // key so that they will have zero remaining quota and
-                // additional entries belonging to only those same transactions
-                // will not be loaded even if they would fit in the remaining
-                // quota before this update.
-                lkMeter->updateReadQuotasForKey(*currKeyIt, keySize);
-                currKeyIt = keys.erase(currKeyIt);
-                continue;
-            }
-        }
         auto [offOp, newIndexIter] = index.scan(indexIter, *currKeyIt);
         indexIter = newIndexIter;
         if (offOp)

src/ledger/test/LedgerTxnTests.cpp

@@ -2975,6 +2975,19 @@ TEST_CASE("LedgerTxnRoot prefetch soroban entries", "[ledgertxn]")
         expectedSuccessKeys.emplace(LedgerEntryKey(classicEntry));
         checkPrefetch(expectedSuccessKeys);
     }
+    SECTION("non existent entries should not affect quota")
+    {
+        // Prefetch an entry which has not been added to the database.
+        auto nonExistentEntry =
+            LedgerTestUtils::generateValidLedgerEntryOfType(CONTRACT_DATA);
+        // Both should succeed, as the non-existent entry is not metered and
+        // should result in a null entry in the cache.
+        addTxn(false /* not enough */, {contractDataEntry, nonExistentEntry});
+        std::set<LedgerKey> expectedSuccessKeys{
+            LedgerEntryKey(contractDataEntry),
+            LedgerEntryKey(nonExistentEntry)};
+        checkPrefetch(expectedSuccessKeys);
+    }
 }
 
 TEST_CASE("LedgerKeyMeter tests")