local: Fix creating data dirs

internal/backend/local/local.go

@@ -57,11 +57,14 @@ func Open(cfg Config) (*Local, error) {
 	// if data dir exists, make sure that all subdirs also exist
 	datadir := be.Dirname(restic.Handle{Type: restic.DataFile})
 	if dirExists(datadir) {
+		debug.Log("datadir %v exists", datadir)
 		for _, d := range be.Paths() {
-			if _, err := filepath.Rel(datadir, d); err != nil {
+			if !fs.HasPathPrefix(datadir, d) {
+				debug.Log("%v is not subdir of datadir %v", d, datadir)
 				continue
 			}
 
+			debug.Log("MkdirAll %v", d)
 			err := fs.MkdirAll(d, backend.Modes.Dir)
 			if err != nil {
 				return nil, errors.Wrap(err, "MkdirAll")

internal/fs/path_prefix.go

@@ -0,0 +1,41 @@
+package fs
+
+import (
+	"path/filepath"
+)
+
+// HasPathPrefix returns true if p is a subdir of (or a file within) base. It
+// assumes a file system which is case sensitive. For relative paths, false is
+// returned.
+func HasPathPrefix(base, p string) bool {
+	if filepath.VolumeName(base) != filepath.VolumeName(p) {
+		return false
+	}
+
+	if !filepath.IsAbs(base) || !filepath.IsAbs(p) {
+		return false
+	}
+
+	base = filepath.Clean(base)
+	p = filepath.Clean(p)
+
+	if base == p {
+		return true
+	}
+
+	for {
+		dir := filepath.Dir(p)
+
+		if base == dir {
+			return true
+		}
+
+		if p == dir {
+			break
+		}
+
+		p = dir
+	}
+
+	return false
+}

internal/fs/path_prefix_test.go

@@ -0,0 +1,52 @@
+package fs
+
+import (
+	"path/filepath"
+	"runtime"
+	"testing"
+)
+
+func fromSlashAbs(p string) string {
+	if runtime.GOOS == "windows" {
+		if len(p) > 0 && p[0] == '/' {
+			p = "c:" + p
+		}
+	}
+
+	return filepath.FromSlash(p)
+}
+
+func TestHasPathPrefix(t *testing.T) {
+	var tests = []struct {
+		base, p string
+		result  bool
+	}{
+		{"", "", false},
+		{"/", "", false},
+		{"/", "x", false},
+		{"x", "/", false},
+		{"/", "/x", true},
+		{"/x", "/y", false},
+		{"/home/user/foo", "/home", false},
+		{"/home/user/foo/", "/home", false},
+		{"/home/user/foo", "/home/", false},
+		{"/home/user/foo/", "/home/", false},
+		{"/home/user/foo", "/home/user/foo/bar", true},
+		{"/home/user/foo", "/home/user/foo/bar/baz/x/y/z", true},
+		{"/home/user/foo", "/home/user/foobar", false},
+		{"/home/user/Foo", "/home/user/foo/bar/baz", false},
+		{"/home/user/foo", "/home/user/Foo/bar/baz", false},
+	}
+
+	for _, test := range tests {
+		t.Run("", func(t *testing.T) {
+			base := fromSlashAbs(test.base)
+			p := fromSlashAbs(test.p)
+			result := HasPathPrefix(base, p)
+			if result != test.result {
+				t.Fatalf("wrong result for HasPathPrefix(%q, %q): want %v, got %v",
+					base, p, test.result, result)
+			}
+		})
+	}
+}