Append-only mode fails even when supported by the underlying file system

[earlruby]

Append-only files are used by applications such as Redis. To set a file as append-only on an ext4 fs, use chattr +a file:

root@ubuntu:~# mkdir /test/ext4fs
root@ubuntu:~# cd /test/ext4fs
root@ubuntu:/test/ext4fs# touch append-only-file
root@ubuntu:/test/ext4fs# chattr +a append-only-file
root@ubuntu:/test/ext4fs# echo this >> append-only-file
root@ubuntu:/test/ext4fs# echo is >> append-only-file
root@ubuntu:/test/ext4fs# echo an >> append-only-file
root@ubuntu:/test/ext4fs# echo append only file >> append-only-file

An append-only file cannot be overwritten:

root@ubuntu:/test/ext4fs# echo cannot overwrite > append-only-file
-bash: append-only-file: Operation not permitted

Or deleted:

root@ubuntu:/test/ext4fs# rm append-only-file
rm: cannot remove 'append-only-file': Operation not permitted
root@ubuntu:/test/ext4fs# chattr -a append-only-file
root@ubuntu:/test/ext4fs# rm append-only-file

bindfs does not support append-only mode, even though the underlying file system does:

root@ubuntu:/test/ext4fs# mkdir /test/bindfs
root@ubuntu:/test/ext4fs# bindfs /test/ext4fs /test/bindfs
root@ubuntu:/test/ext4fs# cd /test/bindfs
root@ubuntu:/test/bindfs# touch aof
root@ubuntu:/test/bindfs# chattr +a aof
chattr: Function not implemented while reading flags on aof

Expected behavior is to allow chattr +a file on a bindfs file system if the underlying file system supports it.

[mpartel]

This should now be implemented in the master branch. Please test and let me know if it works.

[earlruby]

I rebuilt using the source code from your master branch but I still see this issue:

root@ubuntu-14:# bindfs -V
bindfs 1.13.1
root@ubuntu-14:~# mkdir -p /test/realfs /test/bindfs1 
root@ubuntu-14:~# bindfs /test/realfs /test/bindfs1
root@ubuntu-14:~# cd /test/bindfs1
root@ubuntu-14:/test/bindfs1# touch aof
root@ubuntu-14:/test/bindfs1# chattr +a aof
chattr: Function not implemented while reading flags on aof

[mpartel]

Sorry, I forgot to mention you need to pass the new flag --enable-ioctl to make it work (new man page entry).

I decided against making it the default behavior because then a root mount would allow users to do any ioctl on any mirrored file as root, and I'm not yet aware of all the implications of that. It may make sense to have a whitelist of ioctls at some point, but even then I'd probably lean towards opt-in.

[earlruby]

That works! Thanks.

root@ubuntu-14:~# bindfs --enable-lock-forwarding --enable-ioctl --multithreaded /test/realfs /test/bindfs1
root@ubuntu-14:~# bindfs --enable-lock-forwarding --enable-ioctl --multithreaded /test/realfs /test/bindfs2
root@ubuntu-14:~# cd /test/bindfs1
root@ubuntu-14:/test/bindfs1# touch aof
root@ubuntu-14:/test/bindfs1# chattr +a aof
root@ubuntu-14:/test/bindfs1# echo XXXX >> aof
root@ubuntu-14:/test/bindfs1# echo YYYY > aof
-bash: aof: Operation not permitted
root@ubuntu-14:/test/bindfs1# echo ZZZZ >> aof
root@ubuntu-14:/test/bindfs1# cat aof
XXXX
ZZZZ

XXXX and ZZZZ both allowed, since they're append operations.