More user-friendly error message if UTXO DB runs ahead of block DB

This gives LoadChainTip a return value - allowing it to indicate that
the UTXO DB ran ahead of the block DB. This just provides a nicer
error message instead of the previous mysterious
assert(!setBlockIndexCandidates.empty()) error.

This also calls ActivateBestChain in case we just loaded the genesis
block in LoadChainTip, avoiding relying on the ActivateBestChain
in ThreadImport before continuing init process.

src/init.cpp

@@ -1636,7 +1636,17 @@ bool AppInitMain()
                     break;
                 }
                 pcoinsTip = new CCoinsViewCache(pcoinscatcher);
-                LoadChainTip(chainparams);
+
+                // !TODO: after enabling reindex-chainstate
+                // if (!fReindex && !fReindexChainState) {
+                if (!fReindex) {
+                    // LoadChainTip sets chainActive based on pcoinsTip's best block
+                    if (!LoadChainTip(chainparams)) {
+                        strLoadError = _("Error initializing block database");
+                        break;
+                    }
+                    assert(chainActive.Tip() != NULL);
+                }
 
                 // Populate list of invalid/fraudulent outpoints that are banned from the chain
                 invalid_out::LoadOutpoints();

src/validation.cpp

@@ -3517,14 +3517,25 @@ bool static LoadBlockIndexDB(std::string& strError)
     return true;
 }
 
-void LoadChainTip(const CChainParams& chainparams)
+bool LoadChainTip(const CChainParams& chainparams)
 {
-    if (chainActive.Tip() && chainActive.Tip()->GetBlockHash() == pcoinsTip->GetBestBlock()) return;
+    if (chainActive.Tip() && chainActive.Tip()->GetBlockHash() == pcoinsTip->GetBestBlock()) return true;
+
+    if (pcoinsTip->GetBestBlock().IsNull() && mapBlockIndex.size() == 1) {
+        // In case we just added the genesis block, connect it now, so
+        // that we always have a chainActive.Tip() when we return.
+        LogPrintf("%s: Connecting genesis block...\n", __func__);
+        CValidationState state;
+        if (!ActivateBestChain(state)) {
+            return false;
+        }
+    }
 
     // Load pointer to end of best chain
     BlockMap::iterator it = mapBlockIndex.find(pcoinsTip->GetBestBlock());
-    if (it == mapBlockIndex.end())
-        return;
+    if (it == mapBlockIndex.end()) {
+        return false;
+    }
     chainActive.SetTip(it->second);
 
     PruneBlockIndexCandidates();
@@ -3535,6 +3546,7 @@ void LoadChainTip(const CChainParams& chainparams)
             pChainTip->GetBlockHash().GetHex(), pChainTip->nHeight,
             DateTimeStrFormat("%Y-%m-%d %H:%M:%S", pChainTip->GetBlockTime()),
             Checkpoints::GuessVerificationProgress(pChainTip));
+    return true;
 }
 
 CVerifyDB::CVerifyDB()

src/validation.h

@@ -189,7 +189,7 @@ bool LoadGenesisBlock();
  * initializing state if we're running with -reindex. */
 bool LoadBlockIndex(std::string& strError);
 /** Update the chain tip based on database information. */
-void LoadChainTip(const CChainParams& chainparams);
+bool LoadChainTip(const CChainParams& chainparams);
 /** Unload database information */
 void UnloadBlockIndex();
 /** See whether the protocol update is enforced for connected nodes */