lib/config: Support symlinked root (fixes #4542, fixes #4353)

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/4545
LGTM: imsodin, calmh

lib/config/config_test.go

@@ -10,6 +10,7 @@ import (
 	"bytes"
 	"encoding/json"
 	"fmt"
+	"io/ioutil"
 	"os"
 	"path/filepath"
 	"reflect"
@@ -20,6 +21,7 @@ import (
 
 	"github.com/d4l3k/messagediff"
 	"github.com/syncthing/syncthing/lib/fs"
+	"github.com/syncthing/syncthing/lib/osutil"
 	"github.com/syncthing/syncthing/lib/protocol"
 )
 
@@ -430,6 +432,62 @@ func TestFolderPath(t *testing.T) {
 	}
 }
 
+func TestFolderCheckPath(t *testing.T) {
+	n, err := ioutil.TempDir("", "")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	err = os.MkdirAll(filepath.Join(n, "dir", ".stfolder"), os.FileMode(0777))
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	testcases := []struct {
+		path string
+		err  error
+	}{
+		{
+			path: "",
+			err:  errMarkerMissing,
+		},
+		{
+			path: "does not exist",
+			err:  errPathMissing,
+		},
+		{
+			path: "dir",
+			err:  nil,
+		},
+	}
+
+	err = osutil.DebugSymlinkForTestsOnly(filepath.Join(n, "dir"), filepath.Join(n, "link"))
+	if err == nil {
+		t.Log("running with symlink check")
+		testcases = append(testcases, struct {
+			path string
+			err  error
+		}{
+			path: "link",
+			err:  nil,
+		})
+	} else if runtime.GOOS != "windows" {
+		t.Log("running without symlink check")
+		t.Fatal(err)
+	}
+
+	for _, testcase := range testcases {
+		cfg := FolderConfiguration{
+			Path:       filepath.Join(n, testcase.path),
+			MarkerName: DefaultMarkerName,
+		}
+
+		if err := cfg.CheckPath(); testcase.err != err {
+			t.Errorf("unexpected error in case %s: %s != %s", testcase.path, err, testcase.err)
+		}
+	}
+}
+
 func TestNewSaveLoad(t *testing.T) {
 	path := "testdata/temp.xml"
 	os.Remove(path)

lib/config/folderconfiguration.go

@@ -16,8 +16,9 @@ import (
 )
 
 var (
-	errPathMissing   = errors.New("folder path missing")
-	errMarkerMissing = errors.New("folder marker missing")
+	errPathNotDirectory = errors.New("folder path not a directory")
+	errPathMissing      = errors.New("folder path missing")
+	errMarkerMissing    = errors.New("folder marker missing")
 )
 
 const DefaultMarkerName = ".stfolder"
@@ -124,12 +125,28 @@ func (f *FolderConfiguration) CreateMarker() error {
 // CheckPath returns nil if the folder root exists and contains the marker file
 func (f *FolderConfiguration) CheckPath() error {
 	fi, err := f.Filesystem().Stat(".")
-	if err != nil || !fi.IsDir() {
+	if err != nil {
+		if !fs.IsNotExist(err) {
+			return err
+		}
 		return errPathMissing
 	}
 
+	// Users might have the root directory as a symlink or reparse point.
+	// Furthermore, OneDrive bullcrap uses a magic reparse point to the cloudz...
+	// Yet it's impossible for this to happen, as filesystem adds a trailing
+	// path separator to the root, so even if you point the filesystem at a file
+	// Stat ends up calling stat on C:\dir\file\ which, fails with "is not a directory"
+	// in the error check above, and we don't even get to here.
+	if !fi.IsDir() && !fi.IsSymlink() {
+		return errPathNotDirectory
+	}
+
 	_, err = f.Filesystem().Stat(f.MarkerName)
 	if err != nil {
+		if !fs.IsNotExist(err) {
+			return err
+		}
 		return errMarkerMissing
 	}