archiver: Shortcut for handling empty files

Regular files that are empty according to stat are now not opened for
reading their contents. Such files are quite common (in my homedir, at
least) and we can save multiple system calls this way. On a network
filesystem, that can mean round trips. Also, we can back up empty files
that we cannot open for reading. Finally, fixes restic#4257.

Existing tests cover this case. fs.Reader now no longer has a meaningful
Size. Nothing depended on that.

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,
 			}