go/zodb/fs1: Accept both FS21 and FS30 magics

FileStorage/py2 saves data with FS21 magic, while FileStorage/py3 with
FS30 magic:

https://github.com/zopefoundation/ZODB/blob/5.8.1/src/ZODB/_compat.py#L25-L77

Up till now FileStorage/go was accepting only FS21 and so with
py3-generated data it was leading to e.g. the following failure:

    === RUN   TestEmptyDB/py3_pickle3
        filestorage_test.go:90: testdata/py3_pickle3/empty.fs: invalid fs1 magic "FS30"

-> Fix it by accepting both py2 and py3 FileStorage magics.

We do accept them both without any other change because FileStorage
format, even when it comes with those two different magic, is really the
same.

go/zodb/storage/fs1/filestorage_test.go

@@ -87,9 +87,6 @@ func TestEmptyDB(t *testing.T) {
 	ztestdataReg.RunWithEach(t, _TestEmptyDB)
 }
 func _TestEmptyDB(t *testing.T, z *ZTestData) {
-	if z.Kind == "py3_pickle3" {
-		t.Skip("xfail")
-	}
 	fs, _ := xfsopen(t, z.Path("empty.fs"))
 	defer exc.XRun(fs.Close)
 
@@ -100,9 +97,6 @@ func TestLoad(t *testing.T) {
 	ztestdataReg.RunWithEach(t, _TestLoad)
 }
 func _TestLoad(t *testing.T, z *ZTestData) {
-	if z.Kind == "py3_pickle3" {
-		t.Skip("xfail")
-	}
 	fs, _ := xfsopen(t, z.Path("1.fs"))
 	defer exc.XRun(fs.Close)
 
@@ -247,9 +241,6 @@ func TestIterate(t *testing.T) {
 	ztestdataReg.RunWithEach(t, _TestIterate)
 }
 func _TestIterate(t *testing.T, z *ZTestData) {
-	if z.Kind == "py3_pickle3" {
-		t.Skip("xfail")
-	}
 	zz := z.Misc
 
 	fs, _ := xfsopen(t, z.Path("1.fs"))
@@ -292,9 +283,6 @@ func BenchmarkIterate(b *testing.B) {
 	ztestdataReg.BenchWithEach(b, _BenchmarkIterate)
 }
 func _BenchmarkIterate(b *testing.B, z *ZTestData) {
-	if z.Kind == "py3_pickle3" {
-		b.Skip("xfail")
-	}
 	fs, _ := xfsopen(b, z.Path("1.fs"))
 	defer exc.XRun(fs.Close)
 
@@ -347,9 +335,6 @@ func TestOpenRecovery(t *testing.T) {
 	ztestdataReg.RunWithEach(t, _TestOpenRecovery)
 }
 func _TestOpenRecovery(t *testing.T, z *ZTestData) {
-	if z.Kind == "py3_pickle3" {
-		t.Skip("xfail")
-	}
 	X := exc.Raiseif
 	zz := z.Misc
 	main, err := ioutil.ReadFile(z.Path("1.fs")); X(err)
@@ -428,9 +413,6 @@ func TestLoadWhiteout(t *testing.T) {
 	ztestdataReg.RunWithEach(t, _TestLoadWhiteout)
 }
 func _TestLoadWhiteout(t *testing.T, z *ZTestData) {
-	if z.Kind == "py3_pickle3" {
-		t.Skip("xfail")
-	}
 	fs, _ := xfsopen(t, z.Path("whiteout.fs"))
 	defer exc.XRun(fs.Close)
 

go/zodb/storage/fs1/format.go

-// Copyright (C) 2017-2021  Nexedi SA and Contributors.
+// Copyright (C) 2017-2024  Nexedi SA and Contributors.
 //                          Kirill Smelkov <kirr@nexedi.com>
 //
 // This program is free software: you can Use, Study, Modify and Redistribute
@@ -68,7 +68,8 @@ type DataHeader struct {
 }
 
 const (
-	Magic = "FS21" // every FileStorage file starts with this
+	Magic21 = "FS21" // FileStorage file produced by Python2 starts with this
+	Magic30	= "FS30" // ----//---- by Python3
 
 	// on-disk sizes
 	FileHeaderSize    = 4
@@ -153,8 +154,12 @@ func (fh *FileHeader) Load(r io.ReaderAt) error {
 	if err != nil {
 		return fmt.Errorf("%sread magic: %s", ioprefix(r), err)
 	}
-	if string(fh.Magic[:]) != Magic {
+	switch string(fh.Magic[:]) {
+	default:
 		return fmt.Errorf("%sinvalid fs1 magic %q", ioprefix(r), fh.Magic)
+
+	case Magic21, Magic30:
+		// ok
 	}
 
 	return nil