go/zodb: Decouple driver-specific options from OpenOptions

OpenOptions is high-level options clients give to ZODB when they want to
open a high-level IStorage, and not all of those options apply to drivers
- for example NoCache is meaningless since cache is provided by ZODB
infrastructure, not a driver.

On the other hand we are going to introduce driver-specific options,
that either low-lever users, or ZODB infrastructure itself will use when
opening drivers.

For this reasons decouple driver-specific options from OpenOptions into
DriverOptions.

go/zodb/open.go

-// Copyright (C) 2017-2018  Nexedi SA and Contributors.
+// Copyright (C) 2017-2019  Nexedi SA and Contributors.
 //                          Kirill Smelkov <kirr@nexedi.com>
 //
 // This program is free software: you can Use, Study, Modify and Redistribute
@@ -35,8 +35,13 @@ type OpenOptions struct {
 	NoCache  bool // don't use cache for read/write operations; prefetch will be noop
 }
 
+// DriverOptions describes options for DriverOpener.
+type DriverOptions struct {
+	ReadOnly bool // whether to open storage as read-only
+}
+
 // DriverOpener is a function to open a storage driver.
-type DriverOpener func (ctx context.Context, u *url.URL, opt *OpenOptions) (IStorageDriver, error)
+type DriverOpener func (ctx context.Context, u *url.URL, opt *DriverOptions) (IStorageDriver, error)
 
 // {} scheme -> DriverOpener
 var driverRegistry = map[string]DriverOpener{}
@@ -77,7 +82,11 @@ func OpenStorage(ctx context.Context, storageURL string, opt *OpenOptions) (ISto
 		return nil, fmt.Errorf("zodb: URL scheme \"%s://\" not supported", u.Scheme)
 	}
 
-	storDriver, err := opener(ctx, u, opt)
+	drvOpt := &DriverOptions{
+		ReadOnly: opt.ReadOnly,
+	}
+
+	storDriver, err := opener(ctx, u, drvOpt)
 	if err != nil {
 		return nil, err
 	}

go/zodb/storage/fs1/url.go

-// Copyright (C) 2017  Nexedi SA and Contributors.
+// Copyright (C) 2017-2019  Nexedi SA and Contributors.
 //                          Kirill Smelkov <kirr@nexedi.com>
 //
 // This program is free software: you can Use, Study, Modify and Redistribute
@@ -28,7 +28,7 @@ import (
 	"lab.nexedi.com/kirr/neo/go/zodb"
 )
 
-func openByURL(ctx context.Context, u *url.URL, opt *zodb.OpenOptions) (zodb.IStorageDriver, error) {
+func openByURL(ctx context.Context, u *url.URL, opt *zodb.DriverOptions) (zodb.IStorageDriver, error) {
 	// TODO handle query
 	// XXX u.Path is not always raw path - recheck and fix
 	path := u.Host + u.Path

go/zodb/storage/zeo/zeo.go

-// Copyright (C) 2018  Nexedi SA and Contributors.
+// Copyright (C) 2018-2019  Nexedi SA and Contributors.
 //                          Kirill Smelkov <kirr@nexedi.com>
 //
 // This program is free software: you can Use, Study, Modify and Redistribute
@@ -280,7 +280,7 @@ func (r rpc) ereplyf(format string, argv ...interface{}) *errorUnexpectedReply {
 
 // ---- open ----
 
-func openByURL(ctx context.Context, u *url.URL, opt *zodb.OpenOptions) (_ zodb.IStorageDriver, err error) {
+func openByURL(ctx context.Context, u *url.URL, opt *zodb.DriverOptions) (_ zodb.IStorageDriver, err error) {
 	url := u.String()
 	defer xerr.Contextf(&err, "open %s:", url)