Order chainstate init more logically.

* Order chainstate init more logically - first all of the
  blocktree-related loading, then coinsdb, then
  pcoinsTip/chainActive. Only create objects as needed.

* More clearly document exactly what is and isn't called in
  -reindex and -reindex-chainstate both with comments noting
  calls as no-ops and by adding if guards.

* Move LoadGenesisBlock further down in init. This is a more logical
  location for it, as it is after all of the blockindex-related
  loading and checking, but before any of the UTXO-related loading
  and checking.

* Move all of the VerifyDB()-related stuff into a -reindex +
  -reindex-chainstate if guard. It couldn't do anything useful
  as chainActive.Tip() would be null at this point anyway.

src/init.cpp

@@ -1391,23 +1391,19 @@ bool AppInitMain(boost::thread_group& threadGroup, CScheduler& scheduler)
                 delete pblocktree;
 
                 pblocktree = new CBlockTreeDB(nBlockTreeDBCache, false, fReindex);
-                pcoinsdbview = new CCoinsViewDB(nCoinDBCache, false, fReindex || fReindexChainState);
-                pcoinscatcher = new CCoinsViewErrorCatcher(pcoinsdbview);
 
                 if (fReindex) {
                     pblocktree->WriteReindexing(true);
                     //If we're reindexing in prune mode, wipe away unusable block files and all undo data files
                     if (fPruneMode)
                         CleanupBlockRevFiles();
-                } else {
-                    // If necessary, upgrade from older database format.
-                    if (!pcoinsdbview->Upgrade()) {
-                        strLoadError = _("Error upgrading chainstate database");
-                        break;
-                    }
                 }
+
                 if (fRequestShutdown) break;
 
+                // LoadBlockIndex will load fTxIndex from the db, or set it if
+                // we're reindexing. It will also load fHavePruned if we've
+                // ever removed a block file from disk.
                 if (!LoadBlockIndex(chainparams)) {
                     strLoadError = _("Error loading block database");
                     break;
@@ -1418,12 +1414,6 @@ bool AppInitMain(boost::thread_group& threadGroup, CScheduler& scheduler)
                 if (!mapBlockIndex.empty() && mapBlockIndex.count(chainparams.GetConsensus().hashGenesisBlock) == 0)
                     return InitError(_("Incorrect or no genesis block found. Wrong datadir for network?"));
 
-                // Initialize the block index (no-op if non-empty database was already loaded)
-                if (!fReindex && !LoadGenesisBlock(chainparams)) {
-                    strLoadError = _("Error initializing block database");
-                    break;
-                }
-
                 // Check for changed -txindex state
                 if (fTxIndex != GetBoolArg("-txindex", DEFAULT_TXINDEX)) {
                     strLoadError = _("You need to rebuild the database using -reindex-chainstate to change -txindex");
@@ -1437,10 +1427,34 @@ bool AppInitMain(boost::thread_group& threadGroup, CScheduler& scheduler)
                     break;
                 }
 
+                // At this point blocktree args are consistent with what's on disk.
+                // If we're not mid-reindex (based on disk + args), add a genesis block on disk.
+                // This is called again in ThreadImport in the reindex completes.
+                if (!fReindex && !LoadGenesisBlock(chainparams)) {
+                    strLoadError = _("Error initializing block database");
+                    break;
+                }
+
+                // At this point we're either in reindex or we've loaded a useful
+                // block tree into mapBlockIndex!
+
+                pcoinsdbview = new CCoinsViewDB(nCoinDBCache, false, fReindex || fReindexChainState);
+                pcoinscatcher = new CCoinsViewErrorCatcher(pcoinsdbview);
+
+                // If necessary, upgrade from older database format.
+                // This is a no-op if we cleared the coinsviewdb with -reindex or -reindex-chainstate
+                if (!pcoinsdbview->Upgrade()) {
+                    strLoadError = _("Error upgrading chainstate database");
+                    break;
+                }
+
+                // ReplayBlocks is a no-op if we cleared the coinsviewdb with -reindex or -reindex-chainstate
                 if (!ReplayBlocks(chainparams, pcoinsdbview)) {
                     strLoadError = _("Unable to replay blocks. You will need to rebuild the database using -reindex-chainstate.");
                     break;
                 }
+
+                // The on-disk coinsdb is now in a good state, create the cache
                 pcoinsTip = new CCoinsViewCache(pcoinscatcher);
 
                 if (!fReindex && !fReindexChainState) {
@@ -1463,6 +1477,7 @@ bool AppInitMain(boost::thread_group& threadGroup, CScheduler& scheduler)
                     }
                 }
 
+                if (!fReindex && !fReindexChainState) {
                     uiInterface.InitMessage(_("Verifying blocks..."));
                     if (fHavePruned && GetArg("-checkblocks", DEFAULT_CHECKBLOCKS) > MIN_BLOCKS_TO_KEEP) {
                         LogPrintf("Prune: pruned datadir may not have more than %d blocks; only checking available blocks",
@@ -1486,6 +1501,7 @@ bool AppInitMain(boost::thread_group& threadGroup, CScheduler& scheduler)
                         strLoadError = _("Corrupted block database detected");
                         break;
                     }
+                }
             } catch (const std::exception& e) {
                 LogPrintf("%s\n", e.what());
                 strLoadError = _("Error opening block database");