go/zodb/zeo: Factor-out verifying empty database into xtesting.DrvTestEmptyDB

And use it to verify other storages.

go/internal/xtesting/xtesting.go

@@ -136,6 +136,17 @@ func ZPyCommitRaw(zurl string, at zodb.Tid, objv ...ZRawObject) (_ zodb.Tid, err
 
 // ---- tests for storage drivers ----
 
+// DrvTestEmptyDB verifies that zdrv works correctly on empty database.
+func DrvTestEmptyDB(t *testing.T, zdrv zodb.IStorageDriver) {
+	X := FatalIf(t)
+	ctx := context.Background()
+	head, err := zdrv.Sync(ctx); X(err)
+	headOk := zodb.Tid(0)
+	if head != headOk {
+		t.Errorf("head=%s  ; expected %s", head, headOk)
+	}
+}
+
 // state of an object in the database for some particular revision
 type objState struct {
 	tid  zodb.Tid

go/zodb/storage/fs1/filestorage_test.go

@@ -72,6 +72,13 @@ func xfsopenopt(t testing.TB, path string, opt *zodb.DriverOptions) (*FileStorag
 	return fs, at0
 }
 
+func TestEmptyDB(t *testing.T) {
+	fs, _ := xfsopen(t, "testdata/empty.fs")
+	defer exc.XRun(fs.Close)
+
+	xtesting.DrvTestEmptyDB(t, fs)
+}
+
 func TestLoad(t *testing.T) {
 	fs, _ := xfsopen(t, "testdata/1.fs")
 	defer exc.XRun(fs.Close)

go/zodb/storage/zeo/zeo_test.go

@@ -244,13 +244,7 @@ func TestHandshake(t *testing.T) {
 // verify that connection to ZEO server with empty DB works ok.
 func TestEmptyDB(t *testing.T) {
 	withZEO(t, func(t *testing.T, zsrv ZEOSrv, z *zeo) {
-		X := xtesting.FatalIf(t)
-		ctx := context.Background()
-		head, err := z.Sync(ctx); X(err)
-		headOk := zodb.Tid(0)
-		if head != headOk {
-			t.Errorf("head=%s  ; expected %s", head, headOk)
-		}
+		xtesting.DrvTestEmptyDB(t, z)
 	})
 }