cmount, mount, mountlib: make --read-only reject modify operations

Normally mount/cmount use `-o ro` to get the kernel to mark the fs as
read only.  However this is ignored by WinFsp, so in addition if
`--read-only` is in effect then return EROFS ("Read only File System")
from all methods which attempt to modify something.

cmd/cmount/fs.go

@@ -44,6 +44,9 @@ func NewFS(f fs.Fs) *FS {
 	if noChecksum {
 		fsys.FS.NoChecksum()
 	}
+	if readOnly {
+		fsys.FS.ReadOnly()
+	}
 	return fsys
 }
 
@@ -659,6 +662,8 @@ func translateError(err error) (errc int) {
 			return -fuse.ESPIPE
 		case mountlib.EBADF:
 			return -fuse.EBADF
+		case mountlib.EROFS:
+			return -fuse.EROFS
 		}
 	}
 	fs.Errorf(nil, "IO error: %v", err)

cmd/mount/fs.go

@@ -5,6 +5,8 @@
 package mount
 
 import (
+	"syscall"
+
 	"bazil.org/fuse"
 	fusefs "bazil.org/fuse/fs"
 	"github.com/ncw/rclone/cmd/mountlib"
@@ -34,6 +36,9 @@ func NewFS(f fs.Fs) *FS {
 	if noChecksum {
 		fsys.FS.NoChecksum()
 	}
+	if readOnly {
+		fsys.FS.ReadOnly()
+	}
 	return fsys
 }
 
@@ -75,12 +80,20 @@ func translateError(err error) error {
 	cause := errors.Cause(err)
 	if mErr, ok := cause.(mountlib.Error); ok {
 		switch mErr {
+		case mountlib.OK:
+			return nil
 		case mountlib.ENOENT:
 			return fuse.ENOENT
 		case mountlib.ENOTEMPTY:
-			return fuse.EEXIST // return fuse.ENOTEMPTY - doesn't exist though so use EEXIST
+			return fuse.Errno(syscall.ENOTEMPTY)
 		case mountlib.EEXIST:
 			return fuse.EEXIST
+		case mountlib.ESPIPE:
+			return fuse.Errno(syscall.ESPIPE)
+		case mountlib.EBADF:
+			return fuse.Errno(syscall.EBADF)
+		case mountlib.EROFS:
+			return fuse.Errno(syscall.EROFS)
 		}
 	}
 	return err

cmd/mountlib/dir.go

@@ -242,6 +242,9 @@ func (d *Dir) ModTime() time.Time {
 
 // SetModTime sets the modTime for this dir
 func (d *Dir) SetModTime(modTime time.Time) error {
+	if d.fsys.readOnly {
+		return EROFS
+	}
 	d.mu.Lock()
 	defer d.mu.Unlock()
 	d.modTime = modTime
@@ -312,6 +315,9 @@ func (d *Dir) ReadDirAll() (items []*DirEntry, err error) {
 
 // Create makes a new file
 func (d *Dir) Create(name string) (*File, *WriteFileHandle, error) {
+	if d.fsys.readOnly {
+		return nil, nil, EROFS
+	}
 	path := path.Join(d.path, name)
 	// fs.Debugf(path, "Dir.Create")
 	src := newCreateInfo(d.f, path)
@@ -328,6 +334,9 @@ func (d *Dir) Create(name string) (*File, *WriteFileHandle, error) {
 
 // Mkdir creates a new directory
 func (d *Dir) Mkdir(name string) (*Dir, error) {
+	if d.fsys.readOnly {
+		return nil, EROFS
+	}
 	path := path.Join(d.path, name)
 	// fs.Debugf(path, "Dir.Mkdir")
 	err := d.f.Mkdir(path)
@@ -349,6 +358,9 @@ func (d *Dir) Mkdir(name string) (*Dir, error) {
 // the receiver, which must be a directory.  The entry to be removed
 // may correspond to a file (unlink) or to a directory (rmdir).
 func (d *Dir) Remove(name string) error {
+	if d.fsys.readOnly {
+		return EROFS
+	}
 	path := path.Join(d.path, name)
 	// fs.Debugf(path, "Dir.Remove")
 	item, err := d.lookupNode(name)
@@ -393,6 +405,9 @@ func (d *Dir) Remove(name string) error {
 
 // Rename the file
 func (d *Dir) Rename(oldName, newName string, destDir *Dir) error {
+	if d.fsys.readOnly {
+		return EROFS
+	}
 	oldPath := path.Join(d.path, oldName)
 	newPath := path.Join(destDir.path, newName)
 	// fs.Debugf(oldPath, "Dir.Rename to %q", newPath)

cmd/mountlib/errors.go

@@ -7,6 +7,8 @@ import "fmt"
 // Error describes low level errors in a cross platform way
 type Error byte
 
+// NB if changing errors translateError in cmd/mount/fs.go, cmd/cmount/fs.go
+
 // Low level errors
 const (
 	OK Error = iota
@@ -15,6 +17,7 @@ const (
 	EEXIST
 	ESPIPE
 	EBADF
+	EROFS
 )
 
 var errorNames = []string{
@@ -24,6 +27,7 @@ var errorNames = []string{
 	EEXIST:    "File exists",
 	ESPIPE:    "Illegal seek",
 	EBADF:     "Bad file descriptor",
+	EROFS:     "Read only file system",
 }
 
 // Error renders the error as a string

cmd/mountlib/file.go

@@ -93,6 +93,9 @@ func (f *File) Attr(noModTime bool) (modTime time.Time, Size, Blocks uint64, err
 
 // SetModTime sets the modtime for the file
 func (f *File) SetModTime(modTime time.Time) error {
+	if f.d.fsys.readOnly {
+		return EROFS
+	}
 	f.mu.Lock()
 	defer f.mu.Unlock()
 
@@ -188,6 +191,9 @@ func (f *File) OpenRead() (fh *ReadFileHandle, err error) {
 
 // OpenWrite open the file for write
 func (f *File) OpenWrite() (fh *WriteFileHandle, err error) {
+	if f.d.fsys.readOnly {
+		return nil, EROFS
+	}
 	// if o is nil it isn't valid yet
 	o, err := f.waitForValidObject()
 	if err != nil {

cmd/mountlib/fs.go

@@ -39,6 +39,7 @@ type FS struct {
 	root       *Dir
 	noSeek     bool // don't allow seeking if set
 	noChecksum bool // don't check checksums if set
+	readOnly   bool // if set FS is read only
 }
 
 // NewFS creates a new filing system and root directory
@@ -66,6 +67,13 @@ func (fsys *FS) NoChecksum() *FS {
 	return fsys
 }
 
+// ReadOnly sets the fs into read only mode, returning EROFS for any
+// write operations.
+func (fsys *FS) ReadOnly() *FS {
+	fsys.readOnly = true
+	return fsys
+}
+
 // Root returns the root node
 func (fsys *FS) Root() (*Dir, error) {
 	// fs.Debugf(fsys.f, "Root()")