go/zodb: Split Loader & co. interfaces from IStorage

There are situations when we want to work with, or emulate, only subset
of whole IStorage functionality, with one already established example
being Cache. For this reason split IStorage interface into finer-grained
ones. For now:

	- Loader
	- Prefetcher
	- Iterator
	- Committer
	- Notifier

go/zodb/cache.go

@@ -38,7 +38,7 @@ import (
 // Cache provides RAM caching layer that can be used over a storage.
 type Cache struct {
 	loader interface {
-		StorLoader
+		Loader
 		URL() string
 	}
 
@@ -111,12 +111,6 @@ type revCacheEntry struct {
 	waitBufRef int32
 }
 
-// StorLoader represents loading part of a storage.
-// XXX -> zodb.IStorageLoader (or zodb.Loader ?) ?
-type StorLoader interface {
-	Load(ctx context.Context, xid Xid) (buf *mem.Buf, serial Tid, err error)
-}
-
 // lock order: Cache.mu   > oidCacheEntry
 //             Cache.gcMu > oidCacheEntry
 
@@ -124,7 +118,7 @@ type StorLoader interface {
 // NewCache creates new cache backed up by loader.
 //
 // The cache will use not more than ~ sizeMax bytes of RAM for cached data.
-func NewCache(loader interface { StorLoader; URL() string }, sizeMax int) *Cache {
+func NewCache(loader interface { Loader; URL() string }, sizeMax int) *Cache {
 	c := &Cache{
 		loader:   loader,
 		entryMap: make(map[Oid]*oidCacheEntry),

go/zodb/cache_test.go

@@ -636,7 +636,7 @@ func (c *Checker) assertEq(a, b interface{}) {
 
 // ----------------------------------------
 
-// noopStorage is dummy StorLoader which for any oid/xid always returns 1-byte data.
+// noopStorage is dummy Loader which for any oid/xid always returns 1-byte data.
 type noopStorage struct{}
 var noopData = []byte{0}
 
@@ -648,15 +648,15 @@ func (s *noopStorage) Load(_ context.Context, xid Xid) (buf *mem.Buf, serial Tid
 	return mkbuf(noopData), 1, nil
 }
 
-// benchLoad serially benchmarks a StorLoader - either storage directly or a cache on top of it.
+// benchLoad serially benchmarks a Loader - either storage directly or a cache on top of it.
 //
 // oid accessed are [0, worksize)
-func benchLoad(b *testing.B, l StorLoader, worksize int) {
+func benchLoad(b *testing.B, l Loader, worksize int) {
 	benchLoadN(b, b.N, l, worksize)
 }
 
 // worker for benchLoad, with n overridding b.N
-func benchLoadN(b *testing.B, n int, l StorLoader, worksize int) {
+func benchLoadN(b *testing.B, n int, l Loader, worksize int) {
 	ctx := context.Background()
 
 	xid := Xid{At: 1, Oid: 0}
@@ -723,7 +723,7 @@ func BenchmarkCacheHit(b *testing.B) {
 // ---- parallel benchmarks (many requests to 1 cache) ----
 
 // benchLoadPar is like benchLoad but issues loads in parallel
-func benchLoadPar(b *testing.B, l StorLoader, worksize int) {
+func benchLoadPar(b *testing.B, l Loader, worksize int) {
 	ctx := context.Background()
 	np := runtime.GOMAXPROCS(0)
 	p := uint64(0)
@@ -776,7 +776,7 @@ func BenchmarkCacheHitPar(b *testing.B) {
 // XXX this benchmark part will probably go away
 
 // benchLoadProc is like benchLoad but works with PB, not B
-func benchLoadProc(pb *testing.PB, l StorLoader, worksize int) error {
+func benchLoadProc(pb *testing.PB, l Loader, worksize int) error {
 	ctx := context.Background()
 
 	xid := Xid{At: 1, Oid: 0}

go/zodb/zodb.go

@@ -212,6 +212,11 @@ func (e *OpError) Cause() error {
 type IStorage interface {
 	IStorageDriver
 
+	Prefetcher
+}
+
+// Prefetcher provides functionality to prefetch objects.
+type Prefetcher interface {
 	// Prefetch prefetches object addressed by xid.
 	//
 	// If data is not yet in cache loading for it is started in the background.
@@ -236,6 +241,12 @@ type IStorageDriver interface {
 	// If no transactions have been committed yet, LastTid returns 0.
 	LastTid(ctx context.Context) (Tid, error)
 
+	Loader
+	Iterator
+}
+
+// Loader provides functionality to load objects.
+type Loader interface {
 	// Load loads object data addressed by xid from database.
 	//
 	// Returned are:
@@ -277,7 +288,10 @@ type IStorageDriver interface {
 	// cache without serial_next returned from Load. For this reason in ZODB/go
 	// Load specification comes without specifying serial_next return.
 	Load(ctx context.Context, xid Xid) (buf *mem.Buf, serial Tid, err error)
+}
 
+// Committer provides functionality to commit transactions.
+type Committer interface {
 	// TODO: write mode
 
 	// Store(ctx, oid Oid, serial Tid, data []byte, txn ITransaction) error
@@ -287,12 +301,19 @@ type IStorageDriver interface {
 	// TpcVote(txn)
 	// TpcFinish(txn, callback)
 	// TpcAbort(txn)
+}
 
+
+// Notifier allows to be notified of database changes made by other clients.
+type Notifier interface {
 	// TODO: invalidation channel (notify about changes made to DB not by us from outside)
+}
 
 
 	// TODO: History(ctx, oid, size=1)
 
+// Iterator provides functionality to iterate through storage transactions sequentially.
+type Iterator interface {
 	// Iterate creates iterator to iterate storage in [tidMin, tidMax] range.
 	//
 	// Iterate does not return any error. If there was error when setting