fix: make webdav file copy atomic

chfs-ng/internal/api/webdav_test.go

@@ -2,6 +2,7 @@ package api
 
 import (
 	"context"
+	"errors"
 	"net/http/httptest"
 	"os"
 	"path/filepath"
@@ -113,6 +114,64 @@ func TestWebDAVAuthFSMoveMissingSourcePrecedence(t *testing.T) {
 	}
 }
 
+func TestWebDAVStorageFSCopyPreservesExistingDestinationOnSourceReadFailure(t *testing.T) {
+	root := t.TempDir()
+	if err := os.WriteFile(filepath.Join(root, "dst.txt"), []byte("original"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	store := storage.Local{Root: root}
+	if err := os.WriteFile(filepath.Join(root, "src.txt"), []byte("replacement"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	fs := &webdavStorageFS{store: store}
+	if _, err := fs.Copy(context.Background(), "/src.txt", "/dst.txt", &webdav.CopyOptions{}); err != nil {
+		t.Fatalf("copy: %v", err)
+	}
+	got, err := os.ReadFile(filepath.Join(root, "dst.txt"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if string(got) != "replacement" {
+		t.Fatalf("destination content=%q", got)
+	}
+	matches, err := filepath.Glob(filepath.Join(root, "dst.txt.chfs-copy-*"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(matches) != 0 {
+		t.Fatalf("temporary copy files leaked: %v", matches)
+	}
+}
+
+type failingReader struct{}
+
+func (failingReader) Read([]byte) (int, error) { return 0, errors.New("source read failed") }
+
+func TestWebDAVStorageFSCopyFailureDoesNotCorruptDestination(t *testing.T) {
+	root := t.TempDir()
+	store := storage.Local{Root: root}
+	if err := os.WriteFile(filepath.Join(root, "dst.txt"), []byte("original"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	if err := copyFileToDestination(store, failingReader{}, "dst.txt", true); err == nil {
+		t.Fatal("expected source read failure")
+	}
+	got, err := os.ReadFile(filepath.Join(root, "dst.txt"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if string(got) != "original" {
+		t.Fatalf("destination was corrupted: %q", got)
+	}
+	matches, err := filepath.Glob(filepath.Join(root, "dst.txt.chfs-copy-*"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(matches) != 0 {
+		t.Fatalf("temporary copy files leaked: %v", matches)
+	}
+}
+
 func TestWebDAVStorageFSCopyAndMove(t *testing.T) {
 	root1 := t.TempDir()
 	root2 := t.TempDir()