transaction: Use Atomic to store/load the status

Transactions save the status of each operation in a status field, however
such field could be written concurrently by various operations, so we
need to be sure that:
 - We always return the status for the current operation
 - We store the status in a atomic way so that other actions won't
   create write races

In general, in a multi-thread operation one should not rely on
Transaction.Error() to get info about the last operation.

transaction.go

@@ -27,6 +27,7 @@ import (
 	"runtime"
 	"runtime/cgo"
 	"strings"
+	"sync/atomic"
 	"unsafe"
 )
 
@@ -129,22 +130,22 @@ func cbPAMConv(s C.int, msg *C.char, c C.uintptr_t) (*C.char, C.int) {
 //
 //nolint:errname
 type Transaction struct {
-	handle *C.pam_handle_t
-	conv   *C.struct_pam_conv
-	status C.int
-	c      cgo.Handle
+	handle     *C.pam_handle_t
+	conv       *C.struct_pam_conv
+	lastStatus atomic.Int32
+	c          cgo.Handle
 }
 
 // transactionFinalizer cleans up the PAM handle and deletes the callback
 // function.
 func transactionFinalizer(t *Transaction) {
-	C.pam_end(t.handle, t.status)
+	C.pam_end(t.handle, C.int(t.lastStatus.Load()))
 	t.c.Delete()
 }
 
 // Allows to call pam functions managing return status
 func (t *Transaction) handlePamStatus(cStatus C.int) error {
-	t.status = cStatus
+	t.lastStatus.Store(int32(cStatus))
 	if cStatus != success {
 		return t
 	}
@@ -212,13 +213,13 @@ func start(service, user string, handler ConversationHandler, confDir string) (*
 		err = t.handlePamStatus(C.pam_start_confdir(s, u, t.conv, c, &t.handle))
 	}
 	if err != nil {
-		return nil, errors.Join(Error(t.status), err)
+		return nil, errors.Join(Error(t.lastStatus.Load()), err)
 	}
 	return t, nil
 }
 
 func (t *Transaction) Error() string {
-	return Error(t.status).Error()
+	return Error(t.lastStatus.Load()).Error()
 }
 
 // Item is a an PAM information type.
@@ -363,8 +364,10 @@ func (t *Transaction) GetEnvList() (map[string]string, error) {
 	env := make(map[string]string)
 	p := C.pam_getenvlist(t.handle)
 	if p == nil {
-		return nil, t.handlePamStatus(C.int(ErrBuf))
+		t.lastStatus.Store(int32(ErrBuf))
+		return nil, t
 	}
+	t.lastStatus.Store(success)
 	for q := p; *q != nil; q = next(q) {
 		chunks := strings.SplitN(C.GoString(*q), "=", 2)
 		if len(chunks) == 2 {