xattr: return EOPNOTSUPP for unsupported attributes

mv is unhappy when we return EPERM when it tries to set
system.posix_acl_access:

   mv: preserving permissions for ‘b/x’: Operation not permitted

Now we return EOPNOTSUPP like tmpfs does and mv seems happy.

internal/fusefrontend/xattr.go

@@ -48,13 +48,15 @@ func (fs *FS) GetXAttr(path string, attr string, context *fuse.Context) ([]byte,
 	return data, fuse.OK
 }
 
+const _EOPNOTSUPP = fuse.Status(syscall.EOPNOTSUPP)
+
 // SetXAttr implements pathfs.Filesystem.
 func (fs *FS) SetXAttr(path string, attr string, data []byte, flags int, context *fuse.Context) fuse.Status {
 	if fs.isFiltered(path) {
 		return fuse.EPERM
 	}
 	if disallowedXAttrName(attr) {
-		return fuse.EPERM
+		return _EOPNOTSUPP
 	}
 
 	flags = filterXattrSetFlags(flags)
@@ -74,7 +76,7 @@ func (fs *FS) RemoveXAttr(path string, attr string, context *fuse.Context) fuse.
 		return fuse.EPERM
 	}
 	if disallowedXAttrName(attr) {
-		return fuse.EPERM
+		return _EOPNOTSUPP
 	}
 	cPath, err := fs.getBackingPath(path)
 	if err != nil {

tests/defaults/main_test.go

@@ -193,3 +193,42 @@ func TestWrite0200File(t *testing.T) {
 		t.Fatal("wrong restored permissions")
 	}
 }
+
+// When xattr support was introduced, mv threw warnings like these:
+//   mv: preserving permissions for ‘b/x’: Operation not permitted
+// because we returned EPERM when it tried to set system.posix_acl_access.
+// Now we return EOPNOTSUPP and mv is happy.
+func TestMvWarnings(t *testing.T) {
+	fn := test_helpers.TmpDir + "/TestMvWarnings"
+	err := ioutil.WriteFile(fn, nil, 0600)
+	if err != nil {
+		t.Fatalf("creating file failed: %v", err)
+	}
+	cmd := exec.Command("mv", fn, test_helpers.DefaultPlainDir)
+	out, err := cmd.CombinedOutput()
+	if err != nil {
+		t.Log(string(out))
+		t.Fatal(err)
+	}
+	if len(out) != 0 {
+		t.Fatalf("Got warnings from mv:\n%s", string(out))
+	}
+}
+
+// See TestCpWarnings.
+func TestCpWarnings(t *testing.T) {
+	fn := test_helpers.TmpDir + "/TestCpWarnings"
+	err := ioutil.WriteFile(fn, []byte("foo"), 0600)
+	if err != nil {
+		t.Fatalf("creating file failed: %v", err)
+	}
+	cmd := exec.Command("cp", "-a", fn, test_helpers.DefaultPlainDir)
+	out, err := cmd.CombinedOutput()
+	if err != nil {
+		t.Log(string(out))
+		t.Fatal(err)
+	}
+	if len(out) != 0 {
+		t.Fatalf("Got warnings from cp -a:\n%s", string(out))
+	}
+}