fix: Do not use nHeight when trying to identify the very first/initial snapshot (#5538)

## Issue being fixed or feature implemented

`-1` will only mean `not initialized` from now

Should fix crashes like
https://gitlab.com/dashpay/dash/-/jobs/4893359925
This fix applied on top of #5525
https://gitlab.com/UdjinM6/dash/-/pipelines/972154075

## What was done?
Introduce and use `m_initial_snapshot_index` instead of re-using
`nHeight`. Added a couple of asserts to make sure:
1. we never create mn lists with `nHeight` set to `-1` _explicitly_ (but
it's ok for ctor with no params to do so)
2. we never set `nHeight` to `-1` for an existing mn list
3. we never try to get a height for a non-initialized list
4. `GetListForBlockInternal` never returns non-initialized mn lists

## How Has This Been Tested?
Run tests, run regtest/testnet wallets.

## Breaking Changes
We never stored snapshots with `nHeight == -1`, should be no breaking
changes I think.

## Checklist:
- [x] I have performed a self-review of my own code
- [ ] I have commented my code, particularly in hard-to-understand areas
- [ ] I have added or updated relevant unit/integration/functional/e2e
tests
- [ ] I have made corresponding changes to the documentation
- [x] I have assigned this pull request to a milestone _(for repository
code-owners and collaborators only)_

src/evo/deterministicmns.cpp

@@ -607,17 +607,13 @@ bool CDeterministicMNManager::ProcessBlock(const CBlock& block, const CBlockInde
             return true;
         }
 
-        if (newList.GetHeight() == -1) {
-            newList.SetHeight(nHeight);
-        }
-
         newList.SetBlockHash(pindex->GetBlockHash());
 
         oldList = GetListForBlockInternal(pindex->pprev);
         diff = oldList.BuildDiff(newList);
 
         m_evoDb.Write(std::make_pair(DB_LIST_DIFF, newList.GetBlockHash()), diff);
-        if ((nHeight % DISK_SNAPSHOT_PERIOD) == 0 || oldList.GetHeight() == -1) {
+        if ((nHeight % DISK_SNAPSHOT_PERIOD) == 0 || pindex->pprev == m_initial_snapshot_index) {
             m_evoDb.Write(std::make_pair(DB_LIST_SNAPSHOT, newList.GetBlockHash()), newList);
             mnListsCache.emplace(newList.GetBlockHash(), newList);
             LogPrintf("CDeterministicMNManager::%s -- Wrote snapshot. nHeight=%d, mapCurMNs.allMNsCount=%d\n",
@@ -1051,8 +1047,11 @@ CDeterministicMNList CDeterministicMNManager::GetListForBlockInternal(const CBlo
         CDeterministicMNListDiff diff;
         if (!m_evoDb.Read(std::make_pair(DB_LIST_DIFF, pindex->GetBlockHash()), diff)) {
             // no snapshot and no diff on disk means that it's the initial snapshot
-            snapshot = CDeterministicMNList(pindex->GetBlockHash(), -1, 0);
+            m_initial_snapshot_index = pindex;
+            snapshot = CDeterministicMNList(pindex->GetBlockHash(), pindex->nHeight, 0);
             mnListsCache.emplace(pindex->GetBlockHash(), snapshot);
+            LogPrintf("CDeterministicMNManager::%s -- initial snapshot. blockHash=%s nHeight=%d\n",
+                    __func__, snapshot.GetBlockHash().ToString(), snapshot.GetHeight());
             break;
         }
 
@@ -1088,6 +1087,7 @@ CDeterministicMNList CDeterministicMNManager::GetListForBlockInternal(const CBlo
         }
     }
 
+    assert(snapshot.GetHeight() != -1);
     return snapshot;
 }
 

src/evo/deterministicmns.h

@@ -187,6 +187,7 @@ public:
         nHeight(_height),
         nTotalRegisteredCount(_totalRegisteredCount)
     {
+        assert(nHeight >= 0);
     }
 
     template <typename Stream, typename Operation>
@@ -299,10 +300,12 @@ public:
     }
     [[nodiscard]] int GetHeight() const
     {
+        assert(nHeight >= 0);
         return nHeight;
     }
     void SetHeight(int _height)
     {
+        assert(_height >= 0);
         nHeight = _height;
     }
     [[nodiscard]] uint32_t GetTotalRegisteredCount() const
@@ -586,6 +589,7 @@ private:
     std::unordered_map<uint256, CDeterministicMNList, StaticSaltedHasher> mnListsCache GUARDED_BY(cs);
     std::unordered_map<uint256, CDeterministicMNListDiff, StaticSaltedHasher> mnListDiffsCache GUARDED_BY(cs);
     const CBlockIndex* tipIndex GUARDED_BY(cs) {nullptr};
+    const CBlockIndex* m_initial_snapshot_index GUARDED_BY(cs) {nullptr};
 
 public:
     explicit CDeterministicMNManager(CEvoDB& evoDb, CConnman& _connman) :