Call RewindBlockIndex even if we're about to run -reindex-chainstate

RewindBlockIndex works over both chainActive - disconnecting blocks
from the tip that need witness verification - and mapBlockIndex -
requiring redownload of blocks missing witness data.

It should never have been the case that the second half is skipped
if we're about to run -reindex-chainstate.

src/init.cpp

@@ -1452,7 +1452,10 @@ bool AppInitMain(boost::thread_group& threadGroup, CScheduler& scheduler)
                     assert(chainActive.Tip() != NULL);
                 }
 
-                if (!fReindex && chainActive.Tip() != NULL) {
+                if (!fReindex) {
+                    // Note that RewindBlockIndex MUST run even if we're about to -reindex-chainstate.
+                    // It both disconnects blocks based on chainActive, and drops block data in
+                    // mapBlockIndex based on lack of available witness data.
                     uiInterface.InitMessage(_("Rewinding blocks..."));
                     if (!RewindBlockIndex(chainparams)) {
                         strLoadError = _("Unable to rewind the database to a pre-fork state. You will need to redownload the blockchain");

src/validation.cpp

@@ -3763,6 +3763,8 @@ bool RewindBlockIndex(const CChainParams& params)
 {
     LOCK(cs_main);
 
+    // Note that during -reindex-chainstate we are called with an empty chainActive!
+
     int nHeight = 1;
     while (nHeight <= chainActive.Height()) {
         if (IsWitnessEnabled(chainActive[nHeight - 1], params.GetConsensus()) && !(chainActive[nHeight]->nStatus & BLOCK_OPT_WITNESS)) {
@@ -3832,13 +3834,20 @@ bool RewindBlockIndex(const CChainParams& params)
         }
     }
 
+    if (chainActive.Tip() != NULL) {
+        // We can't prune block index candidates based on our tip if we have
+        // no tip due to chainActive being empty!
         PruneBlockIndexCandidates();
 
         CheckBlockIndex(params.GetConsensus());
 
+        // FlushStateToDisk can possibly read chainActive. Be conservative
+        // and skip it here, we're about to -reindex-chainstate anyway, so
+        // it'll get called a bunch real soon.
         if (!FlushStateToDisk(params, state, FLUSH_STATE_ALWAYS)) {
             return false;
         }
+    }
 
     return true;
 }