Improve RewindBlockIndex when pruning

sdaftuar · sipa · commit 8adb03a24d1c · 2016-05-30T23:00:21.000+02:00
Pruning nodes that upgrade after segwit activation may not HAVE_DATA for all
blocks back to the activation point.  Since pruning nodes won't be serving such
blocks to peers anyway, don't try to rewind past the HAVE_DATA point, as that
would fail and force pruning nodes to redownload the blockchain.
diff --git a/src/main.cpp b/src/main.cpp
@@ -4084,6 +4084,14 @@ bool RewindBlockIndex(const CChainParams& params)
     CValidationState state;
     CBlockIndex* pindex = chainActive.Tip();
     while (chainActive.Height() >= nHeight) {
+        if (fPruneMode && !(chainActive.Tip()->nStatus & BLOCK_HAVE_DATA)) {
+            // If pruning, don't try rewinding past the HAVE_DATA point;
+            // since older blocks can't be served anyway, there's
+            // no need to walk further, and trying to DisconnectTip()
+            // will fail (and require a needless reindex/redownload
+            // of the blockchain).
+            break;
+        }
         if (!DisconnectTip(state, params.GetConsensus(), true)) {
             return error("RewindBlockIndex: unable to disconnect block at height %i", pindex->nHeight);
         }
@@ -4097,7 +4105,13 @@ bool RewindBlockIndex(const CChainParams& params)
     // to disk before writing the chainstate, resulting in a failure to continue if interrupted.
     for (BlockMap::iterator it = mapBlockIndex.begin(); it != mapBlockIndex.end(); it++) {
         CBlockIndex* pindexIter = it->second;
-        if (IsWitnessEnabled(pindexIter->pprev, params.GetConsensus()) && !(pindexIter->nStatus & BLOCK_OPT_WITNESS)) {
+
+        // Note: If we encounter an insufficiently validated block that
+        // is on chainActive, it must be because we are a pruning node, and
+        // this block or some successor doesn't HAVE_DATA, so we were unable to
+        // rewind all the way.  Blocks remaining on chainActive at this point
+        // must not have their validity reduced.
+        if (IsWitnessEnabled(pindexIter->pprev, params.GetConsensus()) && !(pindexIter->nStatus & BLOCK_OPT_WITNESS) && !chainActive.Contains(pindexIter)) {
             // Reduce validity
             pindexIter->nStatus = std::min<unsigned int>(pindexIter->nStatus & BLOCK_VALID_MASK, BLOCK_VALID_TREE) | (pindexIter->nStatus & ~BLOCK_VALID_MASK);
             // Remove have-data flags.
