From: Daira Hopwood Date: Fri, 29 May 2015 01:26:39 +0000 (+0100) Subject: Editing of Water Dragons section. X-Git-Url: https://git.rkrishnan.org/reliability?a=commitdiff_plain;h=3ddd5818a4685c8c170a8f5fffc4d9800e425a72;p=tahoe-lafs%2Ftahoe-lafs.git Editing of Water Dragons section. Signed-off-by: Daira Hopwood --- diff --git a/docs/proposed/magic-folder/remote-to-local-sync.rst b/docs/proposed/magic-folder/remote-to-local-sync.rst index f7176522..8fd2fe02 100644 --- a/docs/proposed/magic-folder/remote-to-local-sync.rst +++ b/docs/proposed/magic-folder/remote-to-local-sync.rst @@ -836,32 +836,56 @@ client reading a file at the same time cannot cause any inconsistency. Water Dragons: Handling deletion and renames '''''''''''''''''''''''''''''''''''''''''''' -*Deletion* - -deletion of a file is like overwriting it with a "deleted" marker - -[TODO: deletion of a directory?] - -*Renames* - -suppose that a subfolder of the Magic Folder is renamed on one of the -Magic Folder clients. it is not clear how to handle this at all: - -* if the folder is renamed automatically on other clients, then apps that - were using files in that folder may break. The behavior differs between - Windows and Unix: on Windows, it might not be possible to rename the - folder at all if it contains open files, while on Unix, open file handles - will stay open but operations involving the old path will fail. either - way the behaviour is likely to be confusing. - -* for conflict detection, it is unclear whether existing entries in the - magic folder db under the old path should be updated to their new path. - -* another possibility is treat the rename like a copy, i.e. all clients - end up with a copy of the directory under both names. effectively we - treat the move event as a directory creation, and also pretend that there - has been a modification of the directory at the old name by all other - Magic Folder clients. this is the easiest option to implement. +Deletion of a file +~~~~~~~~~~~~~~~~~~ + +When a file is deleted from the filesystem of a Magic Folder client, +the most intuitive behavior is for it also to be deleted under that +name from other clients. To avoid data loss, the other clients should +actually rename their copies to a backup (``*.old``) filename. + +It would not be sufficient for a Magic Folder client that deletes +a file to implement this simply by removing the directory entry from +its DMD. Indeed, the entry may not exist in the client's DMD if it +has never previously changed the file. + +Instead, the client links a zero-length file into its DMD and sets +``deleted: true`` in the directory entry metadata. Other clients +take this as a signal to rename their copies to the backup filename. + +Note that the entry for this zero-length file has a version number as +usual, and later versions may restore the file. + +When a Magic Folder client restarts, we can detect files that had +been downloaded but were deleted while it was not running, because +their paths will have last-downloaded records in the magic folder db +without any corresponding local file. + +Deletion of a directory +~~~~~~~~~~~~~~~~~~~~~~~ + +Local filesystems (unlike a Tahoe-LAFS filesystem) normally cannot +unlink a directory that has any remaining children. Therefore a +Magic Folder client cannot delete local copies of directories in +general, because they will typically contain backup files. This must +be done manually on each client if desired. + +Nevertheless, a Magic Folder client that deletes a directory should +set ``deleted: true`` on the metadata entry for the corresponding +zero-length file. This avoids the directory being recreated after +it has been manually deleted from a client. + +Renaming +~~~~~~~~ + +It is sufficient to handle renaming of a file by treating it as a +deletion and an addition under the new name. + +This also applies to directories, although users may find the +resulting behavior unintuitive: all of the files under the old name +will be renamed to backup filenames, and a new directory structure +created under the new name. We believe this is the best that can be +done without imposing unreasonable implementation complexity. Summary