go/zodb: Start of FileStorage support

Start implementing FileStorage support by adding code to load/decode
FileStorage records and way to iterate a FileStorage.

Tests will come in a later patch together with ZODB-level loading
support.

go/zodb/storage/doc.go

+// Copyright (C) 2017  Nexedi SA and Contributors.
+//                     Kirill Smelkov <kirr@nexedi.com>
+//
+// This program is free software: you can Use, Study, Modify and Redistribute
+// it under the terms of the GNU General Public License version 3, or (at your
+// option) any later version, as published by the Free Software Foundation.
+//
+// You can also Link and Combine this program with other software covered by
+// the terms of any of the Free Software licenses or any of the Open Source
+// Initiative approved licenses and Convey the resulting work. Corresponding
+// source of such a combination shall include the source code for all other
+// software used.
+//
+// This program is distributed WITHOUT ANY WARRANTY; without even the implied
+// warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+//
+// See COPYING file for full licensing terms.
+// See https://www.nexedi.com/licensing for rationale and options.
+
+// Package storage provides common infrastructure related to ZODB storages.
+package storage

go/zodb/storage/fs1/util.go

+// Copyright (C) 2017  Nexedi SA and Contributors.
+//                     Kirill Smelkov <kirr@nexedi.com>
+//
+// This program is free software: you can Use, Study, Modify and Redistribute
+// it under the terms of the GNU General Public License version 3, or (at your
+// option) any later version, as published by the Free Software Foundation.
+//
+// You can also Link and Combine this program with other software covered by
+// the terms of any of the Free Software licenses or any of the Open Source
+// Initiative approved licenses and Convey the resulting work. Corresponding
+// source of such a combination shall include the source code for all other
+// software used.
+//
+// This program is distributed WITHOUT ANY WARRANTY; without even the implied
+// warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+//
+// See COPYING file for full licensing terms.
+// See https://www.nexedi.com/licensing for rationale and options.
+
+package fs1
+
+import (
+	"io"
+	"os"
+
+	"lab.nexedi.com/kirr/go123/xbufio"
+)
+
+// noEOF returns err, but changes io.EOF -> io.ErrUnexpectedEOF
+func noEOF(err error) error {
+	if err == io.EOF {
+		err = io.ErrUnexpectedEOF
+	}
+	return err
+}
+
+// okEOF returns err, but changes io.EOF -> nil
+func okEOF(err error) error {
+	if err == io.EOF {
+		err = nil
+	}
+	return err
+}
+
+// record reading routines work on abstract file-like interfaces.
+// when they report e.g. decoding error, if reader has name, e.g. as os.File
+// does, this name is included into error as prefix.
+
+// named represents something having a name - ex. os.File
+type named interface {
+	Name() string
+}
+
+// ioname returns f's name if it has one.
+// if not - "" is returned.
+func ioname(f interface{}) string {
+	if fn, ok := f.(named); ok {
+		return fn.Name()
+	}
+	return ""
+}
+
+// ioprefix returns "<name>: " for f if it has name.
+// if not - "" is returned.
+func ioprefix(f interface{}) string {
+	n := ioname(f)
+	if n != "" {
+		return n + ": "
+	}
+	return ""
+}
+
+
+// seqReadAt wraps os.File with xbufio.SeqReaderAt for its .ReadAt method.
+// in particular f's name is preserved.
+func seqReadAt(f *os.File) *seqFileReaderAt {
+	fseq := xbufio.NewSeqReaderAt(f)
+	return &seqFileReaderAt{fseq, f}
+}
+
+type seqFileReaderAt struct {
+	*xbufio.SeqReaderAt
+
+	// keep os.File still exposed for .Name()
+	// XXX better xbufio.SeqReaderAt expose underlying reader?
+	*os.File
+}
+
+func (f *seqFileReaderAt) ReadAt(p []byte, off int64) (int, error) {
+	// route ReadAt to wrapper
+	return f.SeqReaderAt.ReadAt(p, off)
+}