mount: allow files of unkown size to be read properly

Before this change, files of unknown size (eg Google Docs) would
appear in file listings with 0 size and would only allow 0 bytes to be
read.

This change sets the direct_io flag in the FUSE return which bypasses
the cache for these files.  This means that they can be read properly.

This is compatible with some, but not all applications.

cmd/cmount/fs.go

@@ -299,6 +299,9 @@ func (fsys *FS) Open(path string, flags int) (errc int, fh uint64) {
 		return translateError(err), fhUnset
 	}
 
+	// FIXME add support for unknown length files setting direct_io
+	// See: https://github.com/billziss-gh/cgofuse/issues/38
+
 	return 0, fsys.openHandle(handle)
 }
 

cmd/mount/file.go

@@ -73,6 +73,11 @@ func (f *File) Open(ctx context.Context, req *fuse.OpenRequest, resp *fuse.OpenR
 		return nil, translateError(err)
 	}
 
+	// If size unknown then use direct io to read
+	if handle.Node().DirEntry().Size() < 0 {
+		resp.Flags |= fuse.OpenDirectIO
+	}
+
 	return &FileHandle{handle}, nil
 }
 

docs/content/drive.md

@@ -882,9 +882,12 @@ without downloading them.
 Google docs will transfer correctly with `rclone sync`, `rclone copy`
 etc as rclone knows to ignore the size when doing the transfer.
 
-However an unfortunate consequence of this is that you can't download
-Google docs using `rclone mount` - you will get a 0 sized file.  If
-you try again the doc may gain its correct size and be downloadable.
+However an unfortunate consequence of this is that you may not be able
+to download Google docs using `rclone mount`. If it doesn't work you
+will get a 0 sized file.  If you try again the doc may gain its
+correct size and be downloadable. Whther it will work on not depends
+on the application accessing the mount and the OS you are running -
+experiment to find out if it does work for you!
 
 ### Duplicated files ###
 

docs/content/googlephotos.md

@@ -277,13 +277,14 @@ that when syncing to Google Photos, rclone can only do a file
 existence check.
 
 It is possible to read the size of the media, but this needs an extra
-HTTP HEAD request per media item so is very slow and uses up a lot of
+HTTP HEAD request per media item so is **very slow** and uses up a lot of
 transactions.  This can be enabled with the `--gphotos-read-size`
 option or the `read_size = true` config parameter.
 
-If you want to use the backend with `rclone mount` you will need to
-enable this flag otherwise you will not be able to read media off the
-mount.
+If you want to use the backend with `rclone mount` you may need to
+enable this flag (depending on your OS and application using the
+photos) otherwise you may not be able to read media off the mount.
+You'll need to experiment to see if it works for you without the flag.
 
 ### Albums