Decouple FileStore from *os.File for object-store backends

The FileStore interface returned concrete *os.File from Open/Create/
OpenFile, which no non-filesystem backend (e.g. S3/R2) can produce, and
many file-library hot paths bypassed the interface entirely with direct
os.* / filepath.Walk calls.

Widen the interface to return io.ReadCloser / io.WriteCloser and add
ReadDir, ReadLink, and Walk so directory traversal no longer escapes the
abstraction. Route every file-library call site (fork writers, upload/
download handlers, GetFileNameList, CalcTotalSize/CalcItemCount, the set-
file-info folder rename) through the injected FileStore, and add a
WithFileStore option. OSFileStore keeps byte-identical behavior.

DownloadHandler now nil-guards the optional resource-fork reader instead
of relying on *os.File's nil-receiver tolerance, so a backend returning an
untyped-nil reader does not panic.

4 files changed, 6 insertions, 5 deletions

diff --git a/internal/mobius/handlers_files.go b/internal/mobius/handlers_files.go
index 4052206..cbdf61e 100644
--- a/internal/mobius/handlers_files.go
+++ b/internal/mobius/handlers_files.go
@@ -144,7 +144,7 @@ func HandleSetFileInfo(cc *hotline.ClientConn, t *hotline.Transaction) (res []ho
 			if !cc.Authorize(hotline.AccessRenameFolder) {
 				return cc.NewErrReply(t, ErrMsgNotAllowedRenameFolders)
 			}
-			err = os.Rename(fullFilePath, fullNewFilePath)
+			err = cc.Server.FS.Rename(fullFilePath, fullNewFilePath)
 			if os.IsNotExist(err) {
 				return cc.NewErrReply(t, fmt.Sprintf(ErrMsgCannotRenameFolderNotFound, string(fileName)))
 			}
@@ -382,7 +382,7 @@ func HandleGetFileNameList(cc *hotline.ClientConn, t *hotline.Transaction) (res
 		return cc.NewErrReply(t, ErrMsgNotAllowedViewDropBoxes)
 	}
 
-	fileNames, err := hotline.GetFileNameList(fullPath, cc.Server.Config.IgnoreFiles, cc.TextEncoder(), cc.Logger)
+	fileNames, err := hotline.GetFileNameList(cc.Server.FS, fullPath, cc.Server.Config.IgnoreFiles, cc.TextEncoder(), cc.Logger)
 	if err != nil {
 		cc.Logger.Error("error getting file name list", "err", err)
 		return cc.NewErrReply(t, "Cannot get file list.")
diff --git a/internal/mobius/handlers_files_test.go b/internal/mobius/handlers_files_test.go
index a2073a9..fd022bb 100644
--- a/internal/mobius/handlers_files_test.go
+++ b/internal/mobius/handlers_files_test.go
@@ -748,7 +748,7 @@ func TestHandleGetFileNameList(t *testing.T) {
 							return filepath.Join(path, "/test/config/Files/getFileNameListTestDir")
 						}(),
 					},
-					Server: &hotline.Server{TextDecoder: charmap.Macintosh.NewDecoder(), TextEncoder: charmap.Macintosh.NewEncoder()},
+					Server: &hotline.Server{FS: &hotline.OSFileStore{}, TextDecoder: charmap.Macintosh.NewDecoder(), TextEncoder: charmap.Macintosh.NewEncoder()},
 				},
 				t: hotline.NewTransaction(
 					hotline.TranGetFileNameList, [2]byte{0, 1},
diff --git a/internal/mobius/handlers_transfers.go b/internal/mobius/handlers_transfers.go
index 315135e..c5eab8c 100644
--- a/internal/mobius/handlers_transfers.go
+++ b/internal/mobius/handlers_transfers.go
@@ -117,12 +117,12 @@ func HandleDownloadFolder(cc *hotline.ClientConn, t *hotline.Transaction) (res [
 		return cc.NewErrReply(t, ErrMsgFileNotFound)
 	}
 
-	transferSize, err := hotline.CalcTotalSize(fullFilePath)
+	transferSize, err := hotline.CalcTotalSize(cc.Server.FS, fullFilePath)
 	if err != nil {
 		cc.Logger.Error("download folder: calc total size", "err", err)
 		return cc.NewErrReply(t, ErrMsgDownloadFolder)
 	}
-	itemCount, err := hotline.CalcItemCount(fullFilePath)
+	itemCount, err := hotline.CalcItemCount(cc.Server.FS, fullFilePath)
 	if err != nil {
 		cc.Logger.Error("download folder: calc item count", "err", err)
 		return cc.NewErrReply(t, ErrMsgDownloadFolder)
diff --git a/internal/mobius/handlers_transfers_test.go b/internal/mobius/handlers_transfers_test.go
index 28c2768..4e4ba0f 100644
--- a/internal/mobius/handlers_transfers_test.go
+++ b/internal/mobius/handlers_transfers_test.go
@@ -535,6 +535,7 @@ func TestHandleDownloadFolder_withAccess(t *testing.T) {
 			}(),
 		},
 		Server: &hotline.Server{
+			FS:              &hotline.OSFileStore{},
 			TextDecoder:     charmap.Macintosh.NewDecoder(),
 			TextEncoder:     charmap.Macintosh.NewEncoder(),
 			FileTransferMgr: hotline.NewMemFileTransferMgr(),