Better logic to determine if an upgrade is needed

Reuse the "best block" logic to figure out if an upgrade is needed. Also
use it to ensure that older nodes are unable to start after the upgrade
was performed.

Author: codablock

src/evo/deterministicmns.cpp

@@ -20,7 +20,6 @@
 
 static const std::string DB_LIST_SNAPSHOT = "dmn_S";
 static const std::string DB_LIST_DIFF = "dmn_D";
-static const std::string DB_DMN_MGR_UPGRADED = "dmn_upgraded";
 
 CDeterministicMNManager* deterministicMNManager;
 
@@ -1046,13 +1045,27 @@ void CDeterministicMNManager::UpgradeDBIfNeeded()
 {
     LOCK(cs_main);
 
-    if (evoDb.GetRawDB().Exists(std::string(DB_DMN_MGR_UPGRADED))) {
+    if (chainActive.Tip() == nullptr) {
         return;
     }
 
+    if (evoDb.GetRawDB().Exists(EVODB_BEST_BLOCK)) {
+        return;
+    }
+
+    // Removing the old EVODB_BEST_BLOCK value early results in older version to crash immediately, even if the upgrade
+    // process is cancelled in-between. But if the new version sees that the old EVODB_BEST_BLOCK is already removed,
+    // then we must assume that the upgrade process was already running before but was interrupted.
+    if (chainActive.Height() > 1 && !evoDb.GetRawDB().Exists(std::string("b_b"))) {
+        LogPrintf("CDeterministicMNManager::%s -- ERROR, upgrade process was interrupted and can't be continued. You need to reindex now.\n", __func__);
+    }
+    evoDb.GetRawDB().Erase(std::string("b_b"));
+
     if (chainActive.Height() < Params().GetConsensus().DIP0003Height) {
         // not reached DIP3 height yet, so no upgrade needed
-        evoDb.GetRawDB().Write(std::string(DB_DMN_MGR_UPGRADED), (uint8_t)1);
+        auto dbTx = evoDb.BeginTransaction();
+        evoDb.WriteBestBlock(chainActive.Tip()->GetBlockHash());
+        dbTx->Commit();
         return;
     }
 
@@ -1083,6 +1096,10 @@ void CDeterministicMNManager::UpgradeDBIfNeeded()
 
     LogPrintf("CDeterministicMNManager::%s -- done upgrading\n", __func__);
 
-    evoDb.GetRawDB().Write(std::string(DB_DMN_MGR_UPGRADED), (uint8_t)1);
+    // Writing EVODB_BEST_BLOCK (which is b_b2 now) marks the DB as upgraded
+    auto dbTx = evoDb.BeginTransaction();
+    evoDb.WriteBestBlock(chainActive.Tip()->GetBlockHash());
+    dbTx->Commit();
+
     evoDb.GetRawDB().CompactFull();
 }

src/evo/evodb.h

@@ -9,7 +9,9 @@
 #include "sync.h"
 #include "uint256.h"
 
-static const std::string EVODB_BEST_BLOCK = "b_b";
+// "b_b" was used in the initial version of deterministic MN storage
+// "b_b2" was used after compact diffs were introduced
+static const std::string EVODB_BEST_BLOCK = "b_b2";
 
 class CEvoDB
 {