chain: Revalidate blocks for new deployments.

This adds functionality to unmark blocks that were previously marked
failed if their median time is after the earliest possible start time of
newly detected consensus rules.  This ensures clients that did not
update prior to new rules activating are able to automatically recover
under the new rules without having to download the entire chain again.

blockchain/chainio.go

@@ -1244,7 +1244,15 @@ func (b *BlockChain) createChainState() error {
 // constructs the block index into the provided index parameter.  It is not safe
 // for concurrent access as it is only intended to be used during initialization
 // and database migration.
-func loadBlockIndex(dbTx database.Tx, genesisHash *chainhash.Hash, index *blockIndex) error {
+//
+// If a non-zero start time for new consensus rules is provided, it will unmark
+// blocks that were previously marked failed if their median time is after that
+// time.  This ensures clients that did not update prior to new rules activating
+// are able to automatically recover under the new rules without having to
+// download the entire chain again.
+func loadBlockIndex(dbTx database.Tx, genesisHash *chainhash.Hash,
+	index *blockIndex, newConsensusRulesStartTime uint64) error {
+
 	// Determine how many blocks will be loaded into the index in order to
 	// allocate the right amount as a single alloc versus a whole bunch of
 	// little ones to reduce pressure on the GC.
@@ -1297,11 +1305,30 @@ func loadBlockIndex(dbTx database.Tx, genesisHash *chainhash.Hash, index *blockI
 			}
 		}
 
-		// Initialize the block node, connect it, and add it to the block
-		// index.
+		// Initialize the block node.
 		node := &blockNodes[i]
 		initBlockNode(node, header, parent)
+
+		// Set the status based on the database entry.
+		//
+		// If new consensus rules were detected, unmark blocks that were
+		// previously marked failed if their median time is after the earliest
+		// possible start time of the new consensus rules.  This ensures clients
+		// that did not update prior to new rules activating are able to
+		// automatically recover under the new rules without having to download
+		// the entire chain again.
 		node.status = entry.status
+		if newConsensusRulesStartTime != 0 &&
+			(node.status.KnownValidateFailed() ||
+				node.status.KnownInvalidAncestor()) {
+
+			medianTime := node.CalcPastMedianTime()
+			if uint64(medianTime.Unix()) >= newConsensusRulesStartTime {
+				node.status &^= statusValidateFailed | statusInvalidAncestor
+			}
+		}
+
+		// Connect the block node and add it to the block index.
 		node.isFullyLinked = parent == nil || index.canValidate(parent)
 		node.votes = entry.voteInfo
 		index.addNodeFromDB(node)
@@ -1488,6 +1515,7 @@ func (b *BlockChain) initChainState(ctx context.Context,
 
 	// Attempt to load the chain state and block index from the database.
 	var tip *blockNode
+	var newRulesStartTime uint64
 	err = b.db.View(func(dbTx database.Tx) error {
 		// Fetch the stored best chain state from the database.
 		state, err := dbFetchBestState(dbTx)
@@ -1498,9 +1526,11 @@ func (b *BlockChain) initChainState(ctx context.Context,
 		log.Infof("Loading block index...")
 		bidxStart := time.Now()
 
-		// Load all of the block index entries from the database and
-		// construct the block index.
-		err = loadBlockIndex(dbTx, &b.chainParams.GenesisHash, b.index)
+		// Load all of the block index entries from the database and construct
+		// the block index.
+		newRulesStartTime := newDeploymentsStartTime(dbTx, b.chainParams)
+		err = loadBlockIndex(dbTx, &b.chainParams.GenesisHash, b.index,
+			newRulesStartTime)
 		if err != nil {
 			return err
 		}
@@ -1588,6 +1618,18 @@ func (b *BlockChain) initChainState(ctx context.Context,
 		return err
 	}
 
+	// Flush the block index if new consensus rules were detected since blocks
+	// may have been unmarked as failing validation when loading the block
+	// index.  This is done before updating the deployment version to ensure
+	// that any updated block index records are saved to the database before
+	// persisting the updated deployment version.
+	if newRulesStartTime != 0 {
+		err = b.flushBlockIndex()
+		if err != nil {
+			return err
+		}
+	}
+
 	// Update the deployment version as needed.
 	err = b.db.Update(func(dbTx database.Tx) error {
 		return updateDeploymentVersion(dbTx, b.chainParams)