On Mon, 02 Aug 2021, Martin Steigerwald wrote:
> Hi Neil!
>
> Wow, this is a bit overwhelming for me. However, I got a very specific
> question for userspace developers in order to probably provide valuable
> input to the KDE Baloo desktop search developers:
>
> NeilBrown - 02.08.21, 06:18:29 CEST:
> > The "obvious" choice for a replacement is the file handle provided by
> > name_to_handle_at() (falling back to st_ino if name_to_handle_at isn't
> > supported by the filesystem). This returns an extensible opaque
> > byte-array. It is *already* more reliable than st_ino. Comparing
> > st_ino is only a reliable way to check if two files are the same if
> > you have both of them open. If you don't, then one of the files
> > might have been deleted and the inode number reused for the other. A
> > filehandle contains a generation number which protects against this.
> >
> > So I think we need to strongly encourage user-space to start using
> > name_to_handle_at() whenever there is a need to test if two things are
> > the same.
>
> How could that work for Baloo's use case to see whether a file it
> encounters is already in its database or whether it is a new file.
>
> Would Baloo compare the whole file handle or just certain fields or make a
> hash of the filehandle or what ever? Could you, in pseudo code or
> something, describe the approach you'd suggest. I'd then share it on:
Yes, the whole filehandle.
struct file_handle {
unsigned int handle_bytes; /* Size of f_handle [in, out] */
int handle_type; /* Handle type [out] */
unsigned char f_handle[0]; /* File identifier (sized by
caller) [out] */
};
i.e. compare handle_type, handle_bytes, and handle_bytes worth of
f_handle.
This file_handle is local to the filesytem. Two different filesystems
can use the same filehandle for different files. So the identity of the
filesystem need to be combined with the file_handle.
>
> Bug 438434 - Baloo appears to be indexing twice the number of files than
> are actually in my home directory
>
> https://bugs.kde.org/438434
This bug wouldn't be address by using the filehandle. Using a
filehandle allows you to compare two files within a single filesystem.
This bug is about comparing two filesystems either side of a reboot, to
see if they are the same.
As has already been mentioned in that bug, statfs().f_fsid is the best
solution (unless comparing the mount point is satisfactory).
NeilBrown
