always retain buckets referenced by last closed ledger

the in memory state (tracked by BucketManager) may change as merges happen,
this may cause buckets referenced by the LCL HAS to not be the same

src/bucket/BucketManagerImpl.cpp

@@ -6,6 +6,7 @@
 #include "bucket/BucketList.h"
 #include "crypto/Hex.h"
 #include "history/HistoryManager.h"
+#include "ledger/LedgerManager.h"
 #include "main/Application.h"
 #include "main/Config.h"
 #include "overlay/StellarXDR.h"
@@ -237,28 +238,55 @@ BucketManagerImpl::getBucketByHash(uint256 const& hash)
 std::set<Hash>
 BucketManagerImpl::getReferencedBuckets() const
 {
-    auto referenced = std::set<Hash>{};
+    std::set<Hash> referenced;
+    // retain current bucket list
     for (uint32_t i = 0; i < BucketList::kNumLevels; ++i)
     {
         auto const& level = mBucketList.getLevel(i);
-        referenced.insert(level.getCurr()->getHash());
-        referenced.insert(level.getSnap()->getHash());
+        auto rit = referenced.insert(level.getCurr()->getHash());
+        if (rit.second)
+        {
+            CLOG(TRACE, "Bucket")
+                << binToHex(*rit.first) << " referenced by bucket list";
+        }
+        rit = referenced.insert(level.getSnap()->getHash());
+        if (rit.second)
+        {
+            CLOG(TRACE, "Bucket")
+                << binToHex(*rit.first) << " referenced by bucket list";
+        }
         for (auto const& h : level.getNext().getHashes())
         {
-            referenced.insert(hexToBin256(h));
+            rit = referenced.insert(hexToBin256(h));
+            if (rit.second)
+            {
+                CLOG(TRACE, "Bucket") << h << " referenced by bucket list";
+            }
+        }
+    }
+    // retain any bucket referenced by the last closed ledger as recorded in the
+    // database (as merge complete, the bucket list drifts from that state)
+    auto lclHas = mApp.getLedgerManager().getLastClosedLedgerHAS();
+    auto lclBuckets = lclHas.allBuckets();
+    for (auto const& h : lclBuckets)
+    {
+        auto rit = referenced.insert(hexToBin256(h));
+        if (rit.second)
+        {
+            CLOG(TRACE, "Bucket") << h << " referenced by LCL";
         }
     }
 
-    // Implicitly retain any buckets that are referenced by a state in
-    // the publish queue.
+    // retain buckets that are referenced by a state in the publish queue.
     auto pub = mApp.getHistoryManager().getBucketsReferencedByPublishQueue();
     {
         for (auto const& h : pub)
         {
-            CLOG(DEBUG, "Bucket")
-                << "BucketManager::forgetUnreferencedBuckets: " << h
-                << " referenced by publish queue";
-            referenced.insert(hexToBin256(h));
+            auto rit = referenced.insert(hexToBin256(h));
+            if (rit.second)
+            {
+                CLOG(TRACE, "Bucket") << h << " referenced by publish queue";
+            }
         }
     }