go/xzodb: New package that provides zodb-related utilities shared by drivers (draft)

go/internal/xzodb/xzodb.go

+// Copyright (C) 2017-2021  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 xzodb complements package zodb.
+package xzodb
+
+import (
+	"lab.nexedi.com/kirr/neo/go/zodb"
+)
+
+// At2Before converts at to before for ZODB load semantics taking edge cases into account.
+//
+// For most values it is
+//
+//	before = at + 1		; at < ∞
+//
+// but at ∞ (zodb.TidMax) it is just
+//
+//	before = at		; at = ∞
+func At2Before(at zodb.Tid) (before zodb.Tid) {
+	if at < zodb.TidMax {
+		return at + 1
+	} else {
+		// XXX do we need to care here also for at > zodb.TidMax (zodb.Tid is currently unsigned)
+		return zodb.TidMax
+	}
+}
+
+// Before2At is the reverse function to At2Before
+func Before2At(before zodb.Tid) (at zodb.Tid) {
+	if before < zodb.TidMax {
+		// XXX before = 0 ?
+		return before - 1
+	} else {
+		// XXX before > zodb.TidMax (same as in At2Before) ?
+		return zodb.TidMax
+	}
+}

go/zodb/storage/zeo/zeo.go

@@ -32,6 +32,7 @@ import (
 	"lab.nexedi.com/kirr/go123/mem"
 	"lab.nexedi.com/kirr/go123/xerr"
 	"lab.nexedi.com/kirr/go123/xnet"
+	"lab.nexedi.com/kirr/neo/go/internal/xzodb"
 	"lab.nexedi.com/kirr/neo/go/zodb"
 )
 
@@ -95,7 +96,7 @@ func (z *zeo) Load(ctx context.Context, xid zodb.Xid) (buf *mem.Buf, serial zodb
 
 	rpc := z.rpc("loadBefore")
 	enc := z.link.enc
-	xres, err := rpc.call(ctx, enc.Oid(xid.Oid), enc.Tid(xid.At+1)) // XXX at2Before
+	xres, err := rpc.call(ctx, enc.Oid(xid.Oid), enc.Tid(xzodb.At2Before(xid.At)))
 	if err != nil {
 		return nil, 0, err
 	}