archiver: Shortcut for handling empty files

internal/archiver/archiver.go

@@ -363,6 +363,26 @@ func (arch *Archiver) Save(ctx context.Context, snPath, target string, previous
 
 	switch {
 	case fs.IsRegularFile(fi):
+		if fi.Size() == 0 {
+			// Shortcut for empty files. Git uses lots of these, and
+			// some virtual filesystems (notably juicefs; #4257) present
+			// infinitely-sized special files as empty regular files.
+			// We can also save empty files without being able to open them.
+			debug.Log("  %v empty", target)
+
+			node, err := arch.nodeFromFileInfo(snPath, target, fi)
+			if err != nil {
+				return FutureNode{}, false, err
+			}
+			node.Content = restic.IDs{}
+			fn = newFutureNodeWithResult(futureNodeResult{
+				snPath: snPath,
+				target: target,
+				node:   node,
+			})
+			return fn, false, nil
+		}
+
 		debug.Log("  %v regular file", target)
 
 		// check if the file has not changed before performing a fopen operation (more expensive, specially

internal/fs/fs_reader.go

@@ -23,7 +23,6 @@ type Reader struct {
 	// for FileInfo
 	Mode    os.FileMode
 	ModTime time.Time
-	Size    int64
 
 	AllowEmptyFile bool
 
@@ -65,7 +64,6 @@ func (fs *Reader) Open(name string) (f File, err error) {
 func (fs *Reader) fi() os.FileInfo {
 	return fakeFileInfo{
 		name:    fs.Name,
-		size:    fs.Size,
 		mode:    fs.Mode,
 		modtime: fs.ModTime,
 	}
@@ -107,7 +105,6 @@ func (fs *Reader) Lstat(name string) (os.FileInfo, error) {
 	getDirInfo := func(name string) os.FileInfo {
 		fi := fakeFileInfo{
 			name:    fs.Base(name),
-			size:    0,
 			mode:    os.ModeDir | 0755,
 			modtime: time.Now(),
 		}
@@ -292,7 +289,6 @@ func (d fakeDir) Readdir(n int) ([]os.FileInfo, error) {
 // fakeFileInfo implements the bare minimum of os.FileInfo.
 type fakeFileInfo struct {
 	name    string
-	size    int64
 	mode    os.FileMode
 	modtime time.Time
 }
@@ -302,7 +298,8 @@ func (fi fakeFileInfo) Name() string {
 }
 
 func (fi fakeFileInfo) Size() int64 {
-	return fi.size
+	// Fake size to fool the archiver's empty file check.
+	return -1
 }
 
 func (fi fakeFileInfo) Mode() os.FileMode {

internal/fs/fs_reader_test.go

@@ -142,10 +142,6 @@ func verifyDirectoryContentsFI(t testing.TB, fs FS, dir string, want []os.FileIn
 			t.Errorf("entry %d: wrong value for ModTime: want %v, got %v", i, fi1.ModTime(), fi2.ModTime())
 		}
 
-		if fi1.Size() != fi2.Size() {
-			t.Errorf("entry %d: wrong value for Size: want %v, got %v", i, fi1.Size(), fi2.Size())
-		}
-
 		if fi1.Sys() != fi2.Sys() {
 			t.Errorf("entry %d: wrong value for Sys: want %v, got %v", i, fi1.Sys(), fi2.Sys())
 		}
@@ -202,7 +198,6 @@ func TestFSReader(t *testing.T) {
 					mode:    0644,
 					modtime: now,
 					name:    filename,
-					size:    int64(len(data)),
 				}
 				verifyDirectoryContentsFI(t, fs, "/", []os.FileInfo{fi})
 			},
@@ -214,7 +209,6 @@ func TestFSReader(t *testing.T) {
 					mode:    0644,
 					modtime: now,
 					name:    filename,
-					size:    int64(len(data)),
 				}
 				verifyDirectoryContentsFI(t, fs, ".", []os.FileInfo{fi})
 			},
@@ -324,7 +318,6 @@ func TestFSReader(t *testing.T) {
 			ReadCloser: io.NopCloser(bytes.NewReader(data)),
 
 			Mode:    0644,
-			Size:    int64(len(data)),
 			ModTime: now,
 		}
 
@@ -359,7 +352,6 @@ func TestFSReaderDir(t *testing.T) {
 				ReadCloser: io.NopCloser(bytes.NewReader(data)),
 
 				Mode:    0644,
-				Size:    int64(len(data)),
 				ModTime: now,
 			}