Better behavior when BucketListDB account cache disabled (#4656)

# Description

This improves behavior when the account cache is disabled in
BucketListDB. Previously, `BUCKETLIST_DB_MEMORY_FOR_CACHING == 0` was
still valid, but we would still create a random eviction cache object
that had no capacity. This change makes is such that we don't construct
the cache at all in this case. I've also added an explicit test.

# Checklist
- [x] Reviewed the
[contributing](https://github.com/stellar/stellar-core/blob/master/CONTRIBUTING.md#submitting-changes)
document
- [x] Rebased on top of master (no merge commits)
- [x] Ran `clang-format` v8.0.0 (via `make format` or the Visual Studio
extension)
- [x] Compiles
- [x] 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)

Author: SirTyson

src/bucket/LiveBucketIndex.cpp

@@ -103,16 +103,16 @@ LiveBucketIndex::maybeInitializeCache(size_t totalBucketListAccountsSizeBytes,
         mDiskIndex->getBucketEntryCounters().entryTypeCounts.at(
             LedgerEntryTypeAndDurability::ACCOUNT);
 
-    // Nothing to cache
-    if (accountsInThisBucket == 0)
-    {
-        return;
-    }
-
     // Convert from MB to bytes, max size for entire BucketList cache
     auto maxBucketListBytesToCache =
         cfg.BUCKETLIST_DB_MEMORY_FOR_CACHING * 1024 * 1024;
 
+    // Nothing to cache. or cache is disabled
+    if (accountsInThisBucket == 0 || maxBucketListBytesToCache == 0)
+    {
+        return;
+    }
+
     std::unique_lock<std::shared_mutex> lock(mCacheMutex);
     if (totalBucketListAccountsSizeBytes < maxBucketListBytesToCache)
     {

src/bucket/test/BucketIndexTests.cpp

@@ -637,6 +637,31 @@ TEST_CASE("key-value lookup", "[bucket][bucketindex]")
 
 TEST_CASE("bl cache", "[bucket][bucketindex]")
 {
+    SECTION("disable cache")
+    {
+        Config cfg(getTestConfig());
+        cfg.BUCKETLIST_DB_INDEX_CUTOFF = 0;
+        cfg.BUCKETLIST_DB_MEMORY_FOR_CACHING = 0;
+
+        auto test = BucketIndexTest(cfg);
+        test.buildGeneralTest(/*isCacheTest=*/true);
+        test.run();
+
+        auto& liveBL = test.getBM().getLiveBucketList();
+        for (auto i = 0; i < LiveBucketList::kNumLevels; ++i)
+        {
+            auto level = liveBL.getLevel(i);
+            REQUIRE(level.getCurr()->getMaxCacheSize() == 0);
+            REQUIRE(level.getSnap()->getMaxCacheSize() == 0);
+        }
+
+        // We shouldn't meter anything when cache is disabled
+        auto& hitMeter = test.getBM().getCacheHitMeter();
+        auto& missMeter = test.getBM().getCacheMissMeter();
+        REQUIRE(hitMeter.count() == 0);
+        REQUIRE(missMeter.count() == 0);
+    }
+
     auto runCacheTest = [](size_t cacheSizeMb, auto checkCacheSize,
                            double expectedHitRate) {
         // Use disk index for all levels so each bucket has a cache