.

go/zodb/storage/fs1/filestorage.go

@@ -87,6 +87,7 @@ type FileStorage struct {
 	index	*Index	// oid -> data record position in transaction which last changed oid
 
 	// transaction headers for min/max transactions committed
+	// (both with .Len=0 & .Tid=0 if database is empty)
 	// XXX keep loaded with LoadNoStrings ?
 	txnhMin	TxnHeader
 	txnhMax TxnHeader
@@ -107,13 +108,11 @@ func (fs *FileStorage) URL() string {
 
 
 func (fs *FileStorage) LastTid(_ context.Context) (zodb.Tid, error) {
-	// XXX check we have transactions at all - what to return if not?
 	// XXX must be under lock
-	return fs.txnhMax.Tid, nil
+	return fs.txnhMax.Tid, nil // txnhMax.Tid = 0, if empty
 }
 
 func (fs *FileStorage) LastOid(_ context.Context) (zodb.Oid, error) {
-	// XXX check we have objects at all - what to return if not?
 	// XXX must be under lock
 	// XXX what if an oid was deleted?
 	lastOid, _ := fs.index.Last() // returns zero-value, if empty
@@ -240,14 +239,11 @@ const (
 func (zi *zIter) NextTxn(_ context.Context) (*zodb.TxnInfo, zodb.IDataIterator, error) {
 	switch {
 	case zi.zFlags & zIterEOF != 0:
-		//println("already eof")
 		return nil, nil, io.EOF
 
-	// XXX needed?
 	case zi.zFlags & zIterPreloaded != 0:
 		// first element is already there - preloaded by who initialized TxnIter
 		zi.zFlags &= ^zIterPreloaded
-		//fmt.Println("preloaded:", zi.Txnh.Tid)
 
 	default:
 		err := zi.iter.NextTxn(LoadAll)
@@ -257,10 +253,8 @@ func (zi *zIter) NextTxn(_ context.Context) (*zodb.TxnInfo, zodb.IDataIterator,
 		}
 	}
 
-	// XXX how to make sure last good txnh is preserved?
 	if (zi.iter.Dir == IterForward && zi.iter.Txnh.Tid > zi.tidStop) ||
 	   (zi.iter.Dir == IterBackward && zi.iter.Txnh.Tid < zi.tidStop) {
-		//println("-> EOF")
 		zi.zFlags |= zIterEOF
 		return nil, nil, io.EOF
 	}
@@ -313,29 +307,26 @@ func (e *iterStartError) NextTxn(_ context.Context) (*zodb.TxnInfo, zodb.IDataIt
 }
 
 
-// findTxnRecord finds smallest transaction record with txn.tid >= tid	XXX or <= ?
-// if there is no such transaction returned TxnHeader will be invalid (.Pos = 0) and error = nil
-// error != nil only on IO error
-// XXX ^^^ text
+// findTxnRecord finds transaction record with min(txn.tid): txn.tid >= tid
+//
+// if there is no such transaction returned error will be EOF.
 func (fs *FileStorage) findTxnRecord(r io.ReaderAt, tid zodb.Tid) (TxnHeader, error) {
-	//fmt.Printf("findTxn %v\n", tid)
-
 	// XXX read snapshot under lock
 
+	// check for empty database
 	if fs.txnhMin.Len == 0 {
 		// empty database - no such record
-		return TxnHeader{}, nil
+		return TxnHeader{}, io.EOF
 	}
 
-	// NOTE cloning to unalias strings memory
+	// now we know the database is not empty and thus txnh min & max are valid
+	// clone them to unalias strings memory
 	var tmin, tmax TxnHeader
 	tmin.CloneFrom(&fs.txnhMin)
 	tmax.CloneFrom(&fs.txnhMax)
 
-	// now we know the database is not empty and thus tmin & tmax are valid
-
 	if tmax.Tid < tid {
-		return TxnHeader{}, nil	// no such record
+		return TxnHeader{}, io.EOF // no such record
 	}
 	if tmin.Tid >= tid {
 		return tmin, nil	// tmin satisfies
@@ -344,7 +335,6 @@ func (fs *FileStorage) findTxnRecord(r io.ReaderAt, tid zodb.Tid) (TxnHeader, er
 	// now we know tid ∈ (tmin, tmax]
 	// iterate and scan either from tmin or tmax, depending which way it is
 	// likely closer, to searched tid.
-	// when iterating use IO optimized for sequential access
 	iter := &Iter{R: r}
 
 	if (tid - tmin.Tid) < (tmax.Tid - tid) {
@@ -393,25 +383,21 @@ func (fs *FileStorage) findTxnRecord(r io.ReaderAt, tid zodb.Tid) (TxnHeader, er
 
 // Iterate creates zodb-level iterator for tidMin..tidMax range
 func (fs *FileStorage) Iterate(tidMin, tidMax zodb.Tid) zodb.ITxnIterator {
-	//fmt.Printf("iterate %v..%v\n", tidMin, tidMax)
-
 	// when iterating use IO optimized for sequential access
-	// XXX -> IterateRaw ?
 	fsSeq := xbufio.NewSeqReaderAt(fs.file)
 	ziter := &zIter{iter: Iter{R: fsSeq}}
 	iter := &ziter.iter
 
 	// find first txn : txn.tid >= tidMin
 	txnh, err := fs.findTxnRecord(fsSeq, tidMin)
-	if err != nil {
-		return &iterStartError{err}	// XXX err ctx
-	}
-	if txnh.Pos == 0 {	// XXX -> txnh.Valid() ?
+	switch {
+	case err == io.EOF:
 		ziter.zFlags |= zIterEOF        // empty
 		return ziter
-	}
 
-	//fmt.Printf("tidRange: %v..%v -> found %v @%v\n", tidMin, tidMax, txnh.Tid, txnh.Pos)
+	case err != nil:
+		return &iterStartError{err}
+	}
 
 	// setup iter from what findTxnRecord found
 	iter.Txnh = txnh