Parse `bytes` as str in Golang.

[In Go, strings can contain arbitrary bytes](https://go.dev/blog/strings)
(similar to the ones in Python 2), and there is no explicit `bytes`
class.
Thus, all the code that deals with arbitrary data in Go is currently
using the string type.

However, the [ogórek package](https://github.com/kisielk/og-rek) used
to load Python pickles in Go, decodes Python 3 `bytes` objects as a
special string type `ogórek.Bytes`, incompatible with APIs expecting
normal strings.
This causes errors such as the following:
```
--- FAIL: TestΔFtail (1.31s)
panic: @03f97a3b982b7bcc: get blkdata from obj 0000000000000002: ZBlk0(0000000000000002): loadBlkData: wendelin.bigfile.file_zodb.ZBlk0(0000000000000002): activate: pysetstate: expect str; got ogórek.Bytes [recovered]
        panic: file:///tmp/TestΔFtail493253177/001/1.fs: @03f97a3b982b7bcc: get blktab: @03f97a3b982b7bcc: get blkdata from obj 0000000000000002: ZBlk0(0000000000000002): loadBlkData: wendelin.bigfile.file_zodb.ZBlk0(0000000000000002): activate: pysetstate: expect str; got ogórek.Bytes [recovered]
        panic: file:///tmp/TestΔFtail493253177/001/1.fs: @03f97a3b982b7bcc: get blktab: @03f97a3b982b7bcc: get blkdata from obj 0000000000000002: ZBlk0(0000000000000002): loadBlkData: wendelin.bigfile.file_zodb.ZBlk0(0000000000000002): activate: pysetstate: expect str; got ogórek.Bytes
```

In order to fix that, we have copied the [`Xstrbytes` function from NEO](
https://lab.nexedi.com/kirr/neo/-/blob/f7776fc1689b0d62e582b132ecc017ab72dc3b23/go/zodb/internal/pickletools/pickletools.go#L49-64)
in the `pycompat.go` file, which accepts either a string or an
`ogórek.Bytes` object and returns the corresponding string.

We have used that function whenever a `bytes` object could be present.

wcfs/internal/pycompat/pycompat.go

@@ -3,7 +3,9 @@
 package pycompat
 
 import (
+	"fmt"
 	"math/big"
+	pickle "github.com/kisielk/og-rek"
 )
 
 // Int64 tries to convert unpickled Python value to int64.
@@ -12,14 +14,31 @@ import (
 //
 // XXX + support for float?
 func Int64(xv interface{}) (v int64, ok bool) {
-        switch v := xv.(type) {
-        case int64:
-                return v, true
-        case *big.Int:
-                if v.IsInt64() {
-                        return v.Int64(), true
-                }
-        }
-
-        return 0, false
+  switch v := xv.(type) {
+  case int64:
+    return v, true
+  case *big.Int:
+    if v.IsInt64() {
+      return v.Int64(), true
+    }
+  }
+
+  return 0, false
 }
+
+// Xstrbytes verifies and extacts str|bytes from unpickled value.
+func Xstrbytes(x interface{}) (string, error) {
+	var s string
+	switch x := x.(type) {
+	default:
+		return "", fmt.Errorf("expect str|bytes; got %T", x)
+
+	case string:
+		s = x
+
+	case pickle.Bytes:
+		s = string(x)
+	}
+
+	return s, nil
+}
\ No newline at end of file

wcfs/internal/xbtree/xbtreetest/treeenv.go

@@ -31,6 +31,7 @@ import (
 	"lab.nexedi.com/kirr/neo/go/zodb"
 
 	"lab.nexedi.com/nexedi/wendelin.core/wcfs/internal/xbtree/blib"
+	"lab.nexedi.com/nexedi/wendelin.core/wcfs/internal/pycompat"
 	"lab.nexedi.com/nexedi/wendelin.core/wcfs/internal/xzodb"
 )
 
@@ -302,8 +303,8 @@ func xGetBlkTab(db *zodb.DB, at zodb.Tid) map[zodb.Oid]ZBlkInfo {
 	defer zblkdir.PDeactivate()
 
 	for xname, xzblk := range zblkdir.Data {
-		name, ok := xname.(string)
-		if !ok {
+		name, err := pycompat.Xstrbytes(xname)
+		if err != nil {
 			exc.Raisef("root['treegen/values']: key [%q]: expected str, got %T", xname, xname)
 		}
 

wcfs/internal/zdata/zblk.go

@@ -97,8 +97,8 @@ func (zb *zBlk0State) PyGetState() interface{} {
 
 // PySetState implements zodb.PyStateful.
 func (zb *zBlk0State) PySetState(pystate interface{}) error {
-	blkdata, ok := pystate.(string)
-	if !ok {
+	blkdata, err := pycompat.Xstrbytes(pystate)
+	if err != nil {
 		return fmt.Errorf("expect str; got %s", xzodb.TypeOf(pystate))
 	}
 
@@ -143,8 +143,8 @@ func (zd *zDataState) PyGetState() interface{} {
 
 // PySetState implements zodb.PyStateful.
 func (zd *zDataState) PySetState(pystate interface{}) error {
-	data, ok := pystate.(string)
-	if !ok {
+	data, err := pycompat.Xstrbytes(pystate)
+	if err != nil {
 		return fmt.Errorf("expect str; got %s", xzodb.TypeOf(pystate))
 	}