lib/osutil: Replace IsDir with TraversesSymlink (fixes #3839)

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3883
LGTM: calmh

lib/osutil/traversessymlink_test.go

@@ -0,0 +1,77 @@
+// Copyright (C) 2016 The Syncthing Authors.
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this file,
+// You can obtain one at http://mozilla.org/MPL/2.0/.
+
+package osutil_test
+
+import (
+	"os"
+	"testing"
+
+	"github.com/syncthing/syncthing/lib/osutil"
+	"github.com/syncthing/syncthing/lib/symlinks"
+)
+
+func TestTraversesSymlink(t *testing.T) {
+	if !symlinks.Supported {
+		t.Skip("pointless test")
+		return
+	}
+
+	os.RemoveAll("testdata")
+	defer os.RemoveAll("testdata")
+	os.MkdirAll("testdata/a/b/c", 0755)
+	symlinks.Create("testdata/a/l", "b", symlinks.TargetDirectory)
+
+	// a/l -> b, so a/l/c should resolve by normal stat
+	info, err := osutil.Lstat("testdata/a/l/c")
+	if err != nil {
+		t.Fatal("unexpected error", err)
+	}
+	if !info.IsDir() {
+		t.Fatal("error in setup, a/l/c should be a directory")
+	}
+
+	cases := []struct {
+		name      string
+		traverses bool
+	}{
+		// Exist
+		{".", false},
+		{"a", false},
+		{"a/b", false},
+		{"a/b/c", false},
+		// Don't exist
+		{"x", false},
+		{"a/x", false},
+		{"a/b/x", false},
+		{"a/x/c", false},
+		// Symlink or behind symlink
+		{"a/l", true},
+		{"a/l/c", true},
+		// Non-existing behind a symlink
+		{"a/l/x", true},
+	}
+
+	for _, tc := range cases {
+		if res := osutil.TraversesSymlink("testdata", tc.name); tc.traverses == (res == nil) {
+			t.Errorf("TraversesSymlink(%q) = %v, should be %v", tc.name, res, tc.traverses)
+		}
+	}
+}
+
+var traversesSymlinkResult error
+
+func BenchmarkTraversesSymlink(b *testing.B) {
+	os.RemoveAll("testdata")
+	defer os.RemoveAll("testdata")
+	os.MkdirAll("testdata/a/b/c", 0755)
+
+	for i := 0; i < b.N; i++ {
+		traversesSymlinkResult = osutil.TraversesSymlink("testdata", "a/b/c")
+	}
+
+	b.ReportAllocs()
+}