go/zodb: Allow to open a storage in "direct" mode - without local cache

zodb/go provides generic cache (see 7233b4c0 "zodb/go: In-RAM client
cache") primarily in order for prefetch to work. However if we need to
benchmark a storage with loading some objects several times, this cache
can hide the actual time it takes for an object to load.

For such use cases add NoCache open option so that opening does not
create a cache and always conveys load operations directly to storage
driver. The option will be used by zwrk tool (see next patch).

go/zodb/open.go

@@ -32,6 +32,7 @@ import (
 // OpenOptions describes options for OpenStorage
 type OpenOptions struct {
 	ReadOnly bool // whether to open storage as read-only
+	NoCache  bool // don't use cache for read/write operations
 }
 
 // DriverOpener is a function to open a storage driver
@@ -69,6 +70,7 @@ func OpenStorage(ctx context.Context, storageURL string, opt *OpenOptions) (ISto
 
 	// XXX commonly handle some options from url -> opt?
 	// (e.g. ?readonly=1 -> opt.ReadOnly=true + remove ?readonly=1 from URL)
+	// ----//---- nocache
 
 	opener, ok := driverRegistry[u.Scheme]
 	if !ok {
@@ -80,12 +82,16 @@ func OpenStorage(ctx context.Context, storageURL string, opt *OpenOptions) (ISto
 		return nil, err
 	}
 
-	return &storage{
-		IStorageDriver: storDriver,
-
+	var cache *Cache
+	if !opt.NoCache {
 		// small cache so that prefetch can work for loading
 		// XXX 512K hardcoded (= ~ 128 · 4K-entries)
-		l1cache: NewCache(storDriver, 128 * 4*1024),
+		cache = NewCache(storDriver, 128 * 4*1024)
+	}
+
+	return &storage{
+		IStorageDriver: storDriver,
+		l1cache:        cache,
 	}, nil
 }
 
@@ -97,7 +103,7 @@ func OpenStorage(ctx context.Context, storageURL string, opt *OpenOptions) (ISto
 // and other storage-independed higher-level functionality.
 type storage struct {
 	IStorageDriver
-	l1cache *Cache
+	l1cache *Cache // can be =nil, if opened with NoCache
 }
 
 
@@ -106,9 +112,15 @@ type storage struct {
 func (s *storage) Load(ctx context.Context, xid Xid) (*mem.Buf, Tid, error) {
 	// XXX here: offload xid validation from cache and driver ?
 	// XXX here: offload wrapping err -> OpError{"load", err} ?
-	return s.l1cache.Load(ctx, xid)
+	if s.l1cache != nil {
+		return s.l1cache.Load(ctx, xid)
+	} else {
+		return s.IStorageDriver.Load(ctx, xid)
+	}
 }
 
 func (s *storage) Prefetch(ctx context.Context, xid Xid) {
-	s.l1cache.Prefetch(ctx, xid)
+	if s.l1cache != nil {
+		s.l1cache.Prefetch(ctx, xid)
+	}
 }

go/zodb/zodb.go

@@ -179,6 +179,7 @@ type IStorage interface {
 	// started, to complete.
 	//
 	// Prefetch does not return any error.
+	// Prefetch is noop if storage was opened with NoCache option.
 	Prefetch(ctx context.Context, xid Xid)
 }