fs: Handle absolute pathname for --stdin-filename

Return valid directory info from Lstat() for parent directories of the
specified filename. Previously only "/" and "." were valid directories.

Also set directory mode as this is checked by archiver.

Closes restic#2063

changelog/unreleased/issue-2063

@@ -0,0 +1,6 @@
+Bugfix: Allow absolute path for filename when backing up from stdin
+
+When backing up from stdin, handle directory path for `--stdin-filename`.
+This can be used to specify the full path for the backed-up file.
+
+https://github.com/restic/restic/issues/2063

internal/fs/fs_reader.go

@@ -103,17 +103,32 @@ func (fs *Reader) Stat(name string) (os.FileInfo, error) {
 // describes the symbolic link.  Lstat makes no attempt to follow the link.
 // If there is an error, it will be of type *PathError.
 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(),
+		}
+		return fi
+	}
+
 	switch name {
 	case fs.Name:
 		return fs.fi(), nil
 	case "/", ".":
-		fi := fakeFileInfo{
-			name:    name,
-			size:    0,
-			mode:    0755,
-			modtime: time.Now(),
+		return getDirInfo(name), nil
+	}
+
+	dir := fs.Dir(fs.Name)
+	for {
+		if dir == "/" || dir == "." {
+			break
+		}
+		if name == dir {
+			return getDirInfo(name), nil
 		}
-		return fi, nil
+		dir = fs.Dir(dir)
 	}
 
 	return nil, os.ErrNotExist

internal/fs/fs_reader_test.go

@@ -270,7 +270,7 @@ func TestFSReader(t *testing.T) {
 					t.Fatal(err)
 				}
 
-				checkFileInfo(t, fi, "/", time.Time{}, 0755, false)
+				checkFileInfo(t, fi, "/", time.Time{}, os.ModeDir|0755, true)
 			},
 		},
 		{
@@ -281,7 +281,16 @@ func TestFSReader(t *testing.T) {
 					t.Fatal(err)
 				}
 
-				checkFileInfo(t, fi, ".", time.Time{}, 0755, false)
+				checkFileInfo(t, fi, ".", time.Time{}, os.ModeDir|0755, true)
+			},
+		},
+		{
+			name: "dir/Lstat-error-not-exist",
+			f: func(t *testing.T, fs FS) {
+				_, err := fs.Lstat("other")
+				if err != os.ErrNotExist {
+					t.Fatal(err)
+				}
 			},
 		},
 		{
@@ -292,7 +301,7 @@ func TestFSReader(t *testing.T) {
 					t.Fatal(err)
 				}
 
-				checkFileInfo(t, fi, "/", time.Time{}, 0755, false)
+				checkFileInfo(t, fi, "/", time.Time{}, os.ModeDir|0755, true)
 			},
 		},
 		{
@@ -303,7 +312,7 @@ func TestFSReader(t *testing.T) {
 					t.Fatal(err)
 				}
 
-				checkFileInfo(t, fi, ".", time.Time{}, 0755, false)
+				checkFileInfo(t, fi, ".", time.Time{}, os.ModeDir|0755, true)
 			},
 		},
 	}
@@ -324,6 +333,54 @@ func TestFSReader(t *testing.T) {
 	}
 }
 
+func TestFSReaderDir(t *testing.T) {
+	data := test.Random(55, 1<<18+588)
+	now := time.Now()
+
+	var tests = []struct {
+		name     string
+		filename string
+	}{
+		{
+			name:     "Lstat-absolute",
+			filename: "/path/to/foobar",
+		},
+		{
+			name:     "Lstat-relative",
+			filename: "path/to/foobar",
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			fs := &Reader{
+				Name:       test.filename,
+				ReadCloser: ioutil.NopCloser(bytes.NewReader(data)),
+
+				Mode:    0644,
+				Size:    int64(len(data)),
+				ModTime: now,
+			}
+
+			dir := path.Dir(fs.Name)
+			for {
+				if dir == "/" || dir == "." {
+					break
+				}
+
+				fi, err := fs.Lstat(dir)
+				if err != nil {
+					t.Fatal(err)
+				}
+
+				checkFileInfo(t, fi, dir, time.Time{}, os.ModeDir|0755, true)
+
+				dir = path.Dir(dir)
+			}
+		})
+	}
+}
+
 func TestFSReaderMinFileSize(t *testing.T) {
 	var tests = []struct {
 		name        string