Detect chain reorgs that happen during a rescan.

While here, switch rescan method to use hashes, not heights, to refer
to starting and ending blocks.

Closes #151.

ok @davecgh

rpcwebsocket.go

@@ -18,6 +18,7 @@ import (
 	"time"
 
 	"code.google.com/p/go.crypto/ripemd160"
+	"github.com/conformal/btcdb"
 	"github.com/conformal/btcjson"
 	"github.com/conformal/btcscript"
 	"github.com/conformal/btcutil"
@@ -1460,6 +1461,13 @@ type rescanKeys struct {
 	unspent             map[btcwire.OutPoint]struct{}
 }
 
+// ErrRescanReorg defines the error that is returned when an unrecoverable
+// reorganize is detected during a rescan.
+var ErrRescanReorg = btcjson.Error{
+	Code:    btcjson.ErrDatabase.Code,
+	Message: "Reorganize",
+}
+
 // rescanBlock rescans all transactions in a single block.  This is a helper
 // function for handleRescan.
 func rescanBlock(wsc *wsClient, lookups *rescanKeys, blk *btcutil.Block) {
@@ -1593,8 +1601,63 @@ func rescanBlock(wsc *wsClient, lookups *rescanKeys, blk *btcutil.Block) {
 	}
 }
 
+// recoverFromReorg attempts to recover from a detected reorganize during a
+// rescan.  It fetches a new range of block shas from the database and
+// verifies that the new range of blocks is on the same fork as a previous
+// range of blocks.  If this condition does not hold true, the JSON-RPC error
+// for an unrecoverable reorganize is returned.
+func recoverFromReorg(db btcdb.Db, minBlock, maxBlock int64,
+	lastBlock *btcutil.Block) ([]btcwire.ShaHash, *btcjson.Error) {
+
+	hashList, err := db.FetchHeightRange(minBlock, maxBlock)
+	if err != nil {
+		rpcsLog.Errorf("Error looking up block range: %v", err)
+		return nil, &btcjson.ErrDatabase
+	}
+	if lastBlock == nil || len(hashList) == 0 {
+		return hashList, nil
+	}
+	blk, err := db.FetchBlockBySha(&hashList[0])
+	if err != nil {
+		rpcsLog.Errorf("Error looking up possibly reorged block: %v",
+			err)
+		return nil, &btcjson.ErrDatabase
+	}
+	jsonErr := descendantBlock(lastBlock, blk)
+	if jsonErr != nil {
+		return nil, jsonErr
+	}
+	return hashList, nil
+}
+
+// descendantBlock returns the appropiate JSON-RPC error if a current block
+// 'cur' fetched during a reorganize is not a direct child of the parent block
+// 'prev'.
+func descendantBlock(prev, cur *btcutil.Block) *btcjson.Error {
+	curSha := &cur.MsgBlock().Header.PrevBlock
+	prevSha, err := prev.Sha()
+	if err != nil {
+		rpcsLog.Errorf("Unknown problem creating block sha: %v", err)
+		return &btcjson.ErrInternal
+	}
+	if !prevSha.IsEqual(curSha) {
+		rpcsLog.Errorf("Stopping rescan for reorged block %v "+
+			"(replaced by block %v)", prevSha, curSha)
+		return &ErrRescanReorg
+	}
+	return nil
+}
+
 // handleRescan implements the rescan command extension for websocket
 // connections.
+//
+// NOTE: This does not smartly handle reorgs, and fixing requires database
+// changes (for safe, concurrent access to full block ranges, and support
+// for other chains than the best chain).  It will, however, detect whether
+// a reorg removed a block that was previously processed, and result in the
+// handler erroring.  Clients must handle this by finding a block still in
+// the chain (perhaps from a rescanprogress notification) to resume their
+// rescan.
 func handleRescan(wsc *wsClient, icmd btcjson.Cmd) (interface{}, *btcjson.Error) {
 	cmd, ok := icmd.(*btcws.RescanCmd)
 	if !ok {
@@ -1678,9 +1741,30 @@ func handleRescan(wsc *wsClient, icmd btcjson.Cmd) (interface{}, *btcjson.Error)
 		lookups.unspent[*outpoint] = struct{}{}
 	}
 
-	minBlock := int64(cmd.BeginBlock)
-	maxBlock := int64(cmd.EndBlock)
-	lastBlock := int64(-1) // -1 indicates no blocks scanned
+	db := wsc.server.server.db
+
+	minBlockSha, err := btcwire.NewShaHashFromStr(cmd.BeginBlock)
+	if err != nil {
+		return nil, &btcjson.ErrDecodeHexString
+	}
+	minBlock, err := db.FetchBlockHeightBySha(minBlockSha)
+	if err != nil {
+		return nil, &btcjson.ErrBlockNotFound
+	}
+
+	maxBlock := btcdb.AllShas
+	if cmd.EndBlock != "" {
+		maxBlockSha, err := btcwire.NewShaHashFromStr(cmd.EndBlock)
+		if err != nil {
+			return nil, &btcjson.ErrDecodeHexString
+		}
+		maxBlock, err = db.FetchBlockHeightBySha(maxBlockSha)
+		if err != nil {
+			return nil, &btcjson.ErrBlockNotFound
+		}
+	}
+
+	var lastBlock *btcutil.Block
 
 	// A ticker is created to wait at least 10 seconds before notifying the
 	// websocket client of the current progress completed by the rescan.
@@ -1689,7 +1773,7 @@ func handleRescan(wsc *wsClient, icmd btcjson.Cmd) (interface{}, *btcjson.Error)
 
 	// FetchHeightRange may not return a complete list of block shas for
 	// the given range, so fetch range as many times as necessary.
-	db := wsc.server.server.db
+fetchRange:
 	for minBlock < maxBlock {
 		hashList, err := db.FetchHeightRange(minBlock, maxBlock)
 		if err != nil {
@@ -1700,11 +1784,56 @@ func handleRescan(wsc *wsClient, icmd btcjson.Cmd) (interface{}, *btcjson.Error)
 			break
 		}
 
+	loopHashList:
 		for i := range hashList {
 			blk, err := db.FetchBlockBySha(&hashList[i])
 			if err != nil {
-				rpcsLog.Errorf("Error looking up block sha: %v", err)
-				return nil, &btcjson.ErrDatabase
+				// Only handle reorgs if a block could not be
+				// found for the hash.
+				if err != btcdb.BlockShaMissing {
+					rpcsLog.Errorf("Error looking up "+
+						"block: %v", err)
+					return nil, &btcjson.ErrDatabase
+				}
+
+				// If an absolute max block was specified, don't
+				// attempt to handle the reorg.
+				if maxBlock != btcdb.AllShas {
+					rpcsLog.Errorf("Stopping rescan for "+
+						"reorged block %v",
+						cmd.EndBlock)
+					return nil, &ErrRescanReorg
+				}
+
+				// If the lookup for the previously valid block
+				// hash failed, there may have been a reorg.
+				// Fetch a new range of block hashes and verify
+				// that the previously processed block (if there
+				// was any) still exists in the database.  If it
+				// doesn't, we error.
+				//
+				// A goto is used to branch executation back to
+				// before the range was evaluated, as it must be
+				// reevaluated for the new hashList.
+				minBlock += int64(i)
+				var jsonErr *btcjson.Error
+				hashList, jsonErr = recoverFromReorg(db, minBlock,
+					maxBlock, lastBlock)
+				if jsonErr != nil {
+					return nil, jsonErr
+				}
+				if len(hashList) == 0 {
+					break fetchRange
+				}
+				goto loopHashList
+			}
+			if i == 0 && lastBlock != nil {
+				// Ensure the new hashList is on the same fork
+				// as the last block from the old hashList.
+				jsonErr := descendantBlock(lastBlock, blk)
+				if jsonErr != nil {
+					return nil, jsonErr
+				}
 			}
 
 			// A select statement is used to stop rescans if the
@@ -1716,6 +1845,7 @@ func handleRescan(wsc *wsClient, icmd btcjson.Cmd) (interface{}, *btcjson.Error)
 				return nil, nil
 			default:
 				rescanBlock(wsc, &lookups, blk)
+				lastBlock = blk
 			}
 
 			// Periodically notify the client of the progress
@@ -1727,7 +1857,9 @@ func handleRescan(wsc *wsClient, icmd btcjson.Cmd) (interface{}, *btcjson.Error)
 				continue
 			}
 
-			n := btcws.NewRescanProgressNtfn(int32(blk.Height()))
+			n := btcws.NewRescanProgressNtfn(hashList[i].String(),
+				int32(blk.Height()),
+				blk.MsgBlock().Header.Timestamp.Unix())
 			mn, err := n.MarshalJSON()
 			if err != nil {
 				rpcsLog.Errorf("Failed to marshal rescan "+
@@ -1744,7 +1876,6 @@ func handleRescan(wsc *wsClient, icmd btcjson.Cmd) (interface{}, *btcjson.Error)
 		}
 
 		minBlock += int64(len(hashList))
-		lastBlock = minBlock - 1
 	}
 
 	// Notify websocket client of the finished rescan.  Due to how btcd
@@ -1754,7 +1885,14 @@ func handleRescan(wsc *wsClient, icmd btcjson.Cmd) (interface{}, *btcjson.Error)
 	// received before the rescan RPC returns.  Therefore, another method
 	// is needed to safely inform clients that all rescan notifiations have
 	// been sent.
-	n := btcws.NewRescanFinishedNtfn(int32(lastBlock))
+	blkSha, err := lastBlock.Sha()
+	if err != nil {
+		rpcsLog.Errorf("Unknown problem creating block sha: %v", err)
+		return nil, &btcjson.ErrInternal
+	}
+	n := btcws.NewRescanFinishedNtfn(blkSha.String(),
+		int32(lastBlock.Height()),
+		lastBlock.MsgBlock().Header.Timestamp.Unix())
 	if mn, err := n.MarshalJSON(); err != nil {
 		rpcsLog.Errorf("Failed to marshal rescan finished "+
 			"notification: %v", err)