Add rewind logic to deal with post-fork software updates

Includes logic for dealing with pruning by Suhas Daftuar.

src/chain.h

@@ -144,6 +144,8 @@ enum BlockStatus: uint32_t {
     BLOCK_FAILED_VALID       =   32, //! stage after last reached validness failed
     BLOCK_FAILED_CHILD       =   64, //! descends from failed block
     BLOCK_FAILED_MASK        =   BLOCK_FAILED_VALID | BLOCK_FAILED_CHILD,
+
+    BLOCK_OPT_WITNESS       =   128, //! block data in blk*.data was received with a witness-enforcing client
 };
 
 /** The block chain is a tree shaped structure starting with the

src/init.cpp

@@ -1284,6 +1284,14 @@ bool AppInit2(boost::thread_group& threadGroup, CScheduler& scheduler)
                     break;
                 }
 
+                if (!fReindex) {
+                    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");
+                        break;
+                    }
+                }
+
                 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",

src/main.cpp

@@ -658,6 +658,9 @@ CBlockIndex* FindForkInGlobalIndex(const CChain& chain, const CBlockLocator& loc
             CBlockIndex* pindex = (*mi).second;
             if (chain.Contains(pindex))
                 return pindex;
+            if (pindex->GetAncestor(chain.Height()) == chain.Tip()) {
+                return chain.Tip();
+            }
         }
     }
     return chain.Genesis();
@@ -2777,7 +2780,7 @@ void static UpdateTip(CBlockIndex *pindexNew, const CChainParams& chainParams) {
 }
 
 /** Disconnect chainActive's tip. You probably want to call mempool.removeForReorg and manually re-limit mempool size after this, with cs_main held. */
-bool static DisconnectTip(CValidationState& state, const CChainParams& chainparams)
+bool static DisconnectTip(CValidationState& state, const CChainParams& chainparams, bool fBare = false)
 {
     CBlockIndex *pindexDelete = chainActive.Tip();
     assert(pindexDelete);
@@ -2797,6 +2800,8 @@ bool static DisconnectTip(CValidationState& state, const CChainParams& chainpara
     // Write the chain state to disk, if necessary.
     if (!FlushStateToDisk(state, FLUSH_STATE_IF_NEEDED))
         return false;
+
+    if (!fBare) {
         // Resurrect mempool transactions from the disconnected block.
         std::vector<uint256> vHashUpdate;
         BOOST_FOREACH(const CTransaction &tx, block.vtx) {
@@ -2815,6 +2820,8 @@ bool static DisconnectTip(CValidationState& state, const CChainParams& chainpara
         // UpdateTransactionsFromBlock finds descendants of any transactions in this
         // block that were added back and cleans up the mempool state.
         mempool.UpdateTransactionsFromBlock(vHashUpdate);
+    }
+
     // Update chainActive and related variables.
     UpdateTip(pindexDelete->pprev, chainparams);
     // Let wallets know transactions went from 1-confirmed to
@@ -3266,6 +3273,9 @@ bool ReceivedBlockTransactions(const CBlock &block, CValidationState& state, CBl
     pindexNew->nDataPos = pos.nPos;
     pindexNew->nUndoPos = 0;
     pindexNew->nStatus |= BLOCK_HAVE_DATA;
+    if (IsWitnessEnabled(pindexNew->pprev, Params().GetConsensus())) {
+        pindexNew->nStatus |= BLOCK_OPT_WITNESS;
+    }
     pindexNew->RaiseValidity(BLOCK_VALID_TRANSACTIONS);
     setDirtyBlockIndex.insert(pindexNew);
 
@@ -4214,6 +4224,90 @@ bool CVerifyDB::VerifyDB(const CChainParams& chainparams, CCoinsView *coinsview,
     return true;
 }
 
+bool RewindBlockIndex(const CChainParams& params)
+{
+    LOCK(cs_main);
+
+    int nHeight = 1;
+    while (nHeight <= chainActive.Height()) {
+        if (IsWitnessEnabled(chainActive[nHeight - 1], params.GetConsensus()) && !(chainActive[nHeight]->nStatus & BLOCK_OPT_WITNESS)) {
+            break;
+        }
+        nHeight++;
+    }
+
+    // nHeight is now the height of the first insufficiently-validated block, or tipheight + 1
+    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, true)) {
+            return error("RewindBlockIndex: unable to disconnect block at height %i", pindex->nHeight);
+        }
+        // Occasionally flush state to disk.
+        if (!FlushStateToDisk(state, FLUSH_STATE_PERIODIC))
+            return false;
+    }
+
+    // Reduce validity flag and have-data flags.
+    // We do this after actual disconnecting, otherwise we'll end up writing the lack of data
+    // 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;
+
+        // 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.
+            pindexIter->nStatus &= ~(BLOCK_HAVE_DATA | BLOCK_HAVE_UNDO);
+            // Remove storage location.
+            pindexIter->nFile = 0;
+            pindexIter->nDataPos = 0;
+            pindexIter->nUndoPos = 0;
+            // Remove various other things
+            pindexIter->nTx = 0;
+            pindexIter->nChainTx = 0;
+            pindexIter->nSequenceId = 0;
+            // Make sure it gets written.
+            setDirtyBlockIndex.insert(pindexIter);
+            // Update indexes
+            setBlockIndexCandidates.erase(pindexIter);
+            std::pair<std::multimap<CBlockIndex*, CBlockIndex*>::iterator, std::multimap<CBlockIndex*, CBlockIndex*>::iterator> ret = mapBlocksUnlinked.equal_range(pindexIter->pprev);
+            while (ret.first != ret.second) {
+                if (ret.first->second == pindexIter) {
+                    mapBlocksUnlinked.erase(ret.first++);
+                } else {
+                    ++ret.first;
+                }
+            }
+        } else if (pindexIter->IsValid(BLOCK_VALID_TRANSACTIONS) && pindexIter->nChainTx) {
+            setBlockIndexCandidates.insert(pindexIter);
+        }
+    }
+
+    PruneBlockIndexCandidates();
+
+    CheckBlockIndex(params.GetConsensus());
+
+    if (!FlushStateToDisk(state, FLUSH_STATE_ALWAYS)) {
+        return false;
+    }
+
+    return true;
+}
+
 void UnloadBlockIndex()
 {
     LOCK(cs_main);

src/main.h

@@ -459,6 +459,9 @@ bool TestBlockValidity(CValidationState& state, const CChainParams& chainparams,
 /** Check whether witness commitments are required for block. */
 bool IsWitnessEnabled(const CBlockIndex* pindexPrev, const Consensus::Params& params);
 
+/** When there are blocks in the active chain with missing data, rewind the chainstate and remove them from the block index */
+bool RewindBlockIndex(const CChainParams& params);
+
 /** Update uncommitted block structures (currently: only the witness nonce). This is safe for submitted blocks. */
 void UpdateUncommittedBlockStructures(CBlock& block, const CBlockIndex* pindexPrev, const Consensus::Params& consensusParams);