backup: support specifying volume instead of path on Windows

"C:" (volume name) versus "C:\" (path)

changelog/unreleased/issue-2004

@@ -0,0 +1,19 @@
+Bugfix: Correctly handle passing volume name to `backup` command
+
+On Windows, when the specified backup target only included the volume
+name without a trailing slash, for example, `C:`, then restoring the
+resulting snapshot would result in an error. Note that using `C:\`
+as backup target worked correctly.
+
+Specifying volume names now works correctly.
+
+To restore snapshots created before this bugfix, use the `<snapshot>:<subpath>`
+syntax. For a snapshot with ID `12345678` and a backup of `C:`, the following
+command can be used:
+
+```
+restic restore 12345678:/C/C:./ --target output/folder
+```
+
+https://github.com/restic/restic/issues/2004
+https://github.com/restic/restic/pull/5028

internal/archiver/archiver.go

@@ -715,7 +715,12 @@ func resolveRelativeTargets(filesys fs.FS, targets []string) ([]string, error) {
 	debug.Log("targets before resolving: %v", targets)
 	result := make([]string, 0, len(targets))
 	for _, target := range targets {
-		target = filesys.Clean(target)
+		if target != "" && filesys.VolumeName(target) == target {
+			// special case to allow users to also specify a volume name "C:" instead of a path "C:\"
+			target = target + filesys.Separator()
+		} else {
+			target = filesys.Clean(target)
+		}
 		pc, _ := pathComponents(filesys, target, false)
 		if len(pc) > 0 {
 			result = append(result, target)

internal/archiver/archiver_test.go

@@ -1448,6 +1448,66 @@ func TestArchiverSnapshot(t *testing.T) {
 	}
 }
 
+func TestResolveRelativeTargetsSpecial(t *testing.T) {
+	var tests = []struct {
+		name     string
+		targets  []string
+		expected []string
+		win      bool
+	}{
+		{
+			name:     "basic relative path",
+			targets:  []string{filepath.FromSlash("some/path")},
+			expected: []string{filepath.FromSlash("some/path")},
+		},
+		{
+			name:     "partial relative path",
+			targets:  []string{filepath.FromSlash("../some/path")},
+			expected: []string{filepath.FromSlash("../some/path")},
+		},
+		{
+			name:     "basic absolute path",
+			targets:  []string{filepath.FromSlash("/some/path")},
+			expected: []string{filepath.FromSlash("/some/path")},
+		},
+		{
+			name:     "volume name",
+			targets:  []string{"C:"},
+			expected: []string{"C:\\"},
+			win:      true,
+		},
+		{
+			name:     "volume root path",
+			targets:  []string{"C:\\"},
+			expected: []string{"C:\\"},
+			win:      true,
+		},
+		{
+			name:     "UNC path",
+			targets:  []string{"\\\\server\\volume"},
+			expected: []string{"\\\\server\\volume\\"},
+			win:      true,
+		},
+		{
+			name:     "UNC path with trailing slash",
+			targets:  []string{"\\\\server\\volume\\"},
+			expected: []string{"\\\\server\\volume\\"},
+			win:      true,
+		},
+	}
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			if test.win && runtime.GOOS != "windows" {
+				t.Skip("skip test on unix")
+			}
+
+			targets, err := resolveRelativeTargets(&fs.Local{}, test.targets)
+			rtest.OK(t, err)
+			rtest.Equals(t, test.expected, targets)
+		})
+	}
+}
+
 func TestArchiverSnapshotSelect(t *testing.T) {
 	var tests = []struct {
 		name  string