[Libmtp-discuss] My GSoC Proposal: MTP Collection rewrite with emphasis on Android device support (for Amarok)

[Linus Walleij]

2013/4/29 Matěj Laitl <matej at laitl.cz>:

> Which leads to a question: I don't see a method in libmtp to "atomically
> replace MTP file identified by this id with these contents". What's the
> recommended approach here? Upload a new file (perhaps with exactly same name,
> which seems to be allowed) and then remove the old file?

This is from README:


API is obscure - I want plain files!
------------------------------------

PTP/MTP devices does not actually contain "files", they contain
objects. These objects have file names, but that is actually
just a name tag on the object.

Folders/directories aren't really such entities: they are just
objects too, albeit objects that can act as parent to other
objects. They are called "associations" and are created in atomic
fashion and even though there is an MTP command to get all the
associations of a certain object, this command is optional
so it is perfectly possible (and most common, actually) to create
devices where the "folders" (which are actually associations) have
no idea whatsoever of what files they are associated as parents to
(i.e. which files they contain). This is very easy for device
manufacturers to implement, all the association (i.e. finding out
which files are in a certain folder) has to be done by the MTP
Initiator / host computer.

Moving a file to a new folder is for example very simple in a
"real" file system. In PTP/MTP devices it is often not even possible,
some devices *may* be able to do that, if they support command
0x1019 "Move Object", but actually the only reliable way of executing
file movement is to upload the file to the host, download it with
the new parent, then delete the old file. We have played with the
idea of implementing this time consuming function as a fallback
in case the device does not support command 0x1019, perhaps one day
we will do that. (Some devices also support command 0x101a
"Copy Object".)

Then the issue that in PTP/MTP it is legal for two files to have
exactly the same path as long as their object IDs differ. A
folder/association can contain two files with the exact same name.
(And on the Creative devices this even works, too, though most devices
implicitly fail at this.) Perhaps one could add some custom hook for
handling that, so they become  /Foo.mp3 and /Foo.mp3(1) or something
similar, but it's really a bit kludgy.

Playlists and albums aren't really files, thinking about
them as files like the hacks in libgphoto2 is really backwards. They are
called associations and are more like a symbolic link that links in a
star-shaped pattern to all the files that are part of the album/playlist.
Some devices (Samsung) thought that was too complicated and have a
different way of storing playlists in an UTF-16 encoded .spl-like file
instead! This is why playlists/albums must have their own structs and
functions.

Plain file access also assumes to be able to write files of an
undetermined size, which is simply not possible in a transactional
file system like PTP/MTP. (See further below.)


> On a related note: is there a hope for at least some (Android?) MTP devices to
> support (and update) attributes like Rating, UseCount, SkipCount,
> LastAccessed, EffectiveRating?

Check out a git of libmtp, go into logs/ and datamine the mtp-detect
logs, and you will find the answer to this question.

Yours,
Linus Walleij