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Google Drive and GNOME — what is a volatile path?

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TL;DR: If you are using any GFile API that can create a new file or directory, then please take care of “volatile” paths. Look at standard::is-volatile, g_file_output_stream_query_info and g_file_query_info. Read further if you don’t trust me, or if you develop file management software.

Like other cloud storages, Drive is database-based. Every file or folder is identified by an opaque, server generated, immutable, blob (say “0B9J_DkziBDRFTj1TRWLPam9kbnc”), and has a human-readable title that can be displayed in the user interface (say “Summer Vacation”). This is unlike POSIX filesystems where a file is identified by its path and, barring encoding issues, the basename of that path is its name. ie., the name of the file “/home/alice/Summer\ Vacation” is “Summer Vacation”. Sounds like an innocuous distinction at first, but this is at the heart of the issue.

Let’s look at a few operations to get a feel for this.



mv /home/alice/foo /home/alice/bar

We need the paths to carry out the operation and once successfully finished, the file’s path changes. Since files are identified by their paths, the operation would fail if there existed another file called “bar” in the same location.

Google Drive:

set_title (id_of_foo, bar)

We only need the identifier (or ID) and the new title for the operation and once successfully finished, the title changes but the ID remains the same. Since IDs are unique and immutable, you can have two files with the same title in the same location. That’s a bit strange, you might think, but not so much.

Creating a New File


touch /home/alice/foo

If you want a file named “foo” in a certain location, barring encoding issues, you ask for a path that has “foo” appended to the path of the location. It will work as long as there isn’t another “foo” in the same location.

Google Drive:

create_file (id_of_parent_folder, foo) → id_of_new_file

Notice that we cannot specify the ID of the new file. We provide a title and the server generates the ID for it. Again, there is no restriction on having two files with the same title in the same location. This is where it starts getting weird.

GIO and GVfs

The file handling APIs in GIO are based on the POSIX model, with some room to accommodate minor diversions. They are ill-equipped to deal with a case like this.

As a quick aside, both MTP and PTP are somewhat similar in this regard. Both use IDs to refer to objects (think of them as files) stored on the phone or camera, and we have backends implementing them in GVfs. These backends construct POSIX-like paths using the human-readable titles of the files, and have an internal cache mapping IDs to paths and vice-versa. Easy enough, but there are two important differences:

  • MTP allows no parallelism. You can only perform one operation at a time — a new one won’t start until the earlier has finished. So, the GVfs backend can assume complete control over the storage. Google Drive, on the other hand, is massively parallel. You might be modifying it via GVfs, via a web browser, from a different computer, or someone might be sharing something to you from yet another computer. It is better to rely on the server generated IDs to identify files as much as possible, and let the server worry about keeping the data consistent.
  • MTP and PTP are usually backed by a traditional filesystem on the device. eg., FAT12, FAT16, FAT32, exFAT or ext3. Therefore the issue of duplicate titles doesn’t arise.

So, how do we deal with this?

Assuming that we are only doing read-only operations, the URIs used by GVfs’ Google Drive backend look like:


The backend has its own scheme, the account is mentioned, and the path is made up of identifiers. Quite easy. The server doesn’t care about having a POSIX-like path, but we create one to fit into GIO’s POSIX-like world view.

These IDs are decidedly ugly and unreadable, so we use the standard::display-name attribute to tag them with the titles. If you run g_file_query_info on one of these URIs, you will get the ID as the standard::name and the title as the standard::display-name. Thus, the user won’t encounter any ugliness unless she deliberately went looking for it.

It gets complicated when we go beyond read-only access.

Let’s look at creating new files again. Since the GIO and GVfs APIs are designed around the POSIX model, a GVfs backend receives the request as:

create ("/id1/id2/some-title")

Even though the application thinks that it has created a new file named “some-title” inside “/id1/id2” that can be accessed by the path “/id1/id2/some-title”, the new file’s identifier is not “some-title”. It is whatever the server generated for us, say “id3”. If the application has to carry on the illusion of accessing the file as “/id1/id2/some-title”, then the backend has to map this “fake” path back to “/id1/id2/id3”; and this has to happen somewhat recursively because if we are copying a directory, then multiple elements in the file path can be “fake”. eg., “/id1/id2/title-of-subdir/title-of-file”, and so on.

We call these volatile paths. These are identified by the standard::is-volatile attribute and you can use standard::symlink-target to find the actual ID that it maps to. It is recommended that applications map volatile paths to the real IDs as soon as possible because the mapping can break if a parallel operation changes the title of the file.

In the specific case where you have created a new file and writing to it through a GFileOutputStream, then it is better to use g_file_output_stream_query_info instead of g_file_query_info. The output stream knows exactly which file it is writing to, and doesn’t have to do the mapping based on the volatile path, which could fail if the contents of the Drive changed out of band.

For working examples, see Nautilus.

Written by Debarshi Ray

13 September, 2015 at 01:21

Google Drive and GNOME — 6 years later

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GNOME 3.18 is going to be another milestone in our journey to bring various online services closer to the desktop, but this is one that took us 6 years to reach. You can now access your Google Drive through your favourite GNOME applications and the usual GIO APIs

Thibault Saunier started working on this way back in 2009 as a Google Summer of Code project. I picked it up a year ago, and after a few hiccups along the way and some head scratching — more on that later — it is finally here for you to enjoy. You can either wait for distributions like Fedora Workstation 23 to ship GNOME 3.18, or try the 3.17.92 release candidate tarballs that are coming out next week.



Thanks to Alexander Larsson, Philip Withnall and Ondrej Holy for the endless discussions, code reviews and testing for the last one year.

Written by Debarshi Ray

10 September, 2015 at 11:22

Transparent terminals are back in Fedora

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Two years ago, after the release of GNOME 3.6, background transparency was removed from gnome-terminal. Over the years several users expressed their fondness for this feature, leading to much drama, tricky workarounds, and rosy promises.

Anyway, this is now back in Fedora 20 and the GNOME 3.12 COPR. Rawhide will follow soon.

I must point out that this is a downstream patch carried by Fedora, which has been rejected by the upstream gnome-terminal maintainer. If you want, you can ask your distributor to include it. Versions of the patch applicable to different GNOME branches can be found in this Git tree. You will also need a vte that has the fixes for bug 730023 and bug 729884 depending on the branch that you are using.

Be aware that this has exposed a bug in Adwaita where it is not drawing the background of the menubar when transparency is turned on. We are looking into it and hoping to fix it soon.

Written by Debarshi Ray

15 May, 2014 at 16:45

Posted in Fedora, GNOME, Terminal

GNOME Documents: come join us

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GNOME 3.12 is out and it is time to look forward to the next release. Lots of new features to implement and bugs to fix. I spent some time with Allan Day preparing a list of bugs for Documents that are waiting for someone to attack them. The list is here. Pick one that interests you and attach your patch to the bug.

This is part of an experiment that Allan started. We will keep adding new bugs to the list as they come in, and other modules might join the experiment in future.

So, fire up your text editors and see you in bugzilla!

Written by Debarshi Ray

30 March, 2014 at 13:38

Posted in Documents, GNOME

Documents and Photos: a content application update

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We have continued our work towards having a nice set of core applications for finding and selecting the user’s content. Documents, which is the eldest, received a round of thorough bug squashing for GNOME 3.12. It is a much more mature and well-behaved application now than it was six months ago, mainly due to the work of our QA teams – both upstream and downstream. Photos received a set of nice new features and is well on its way towards fulfilling the role it was designed for.


Álvaro Peña added support for browsing Facebook photos based on his libgfbgraph library.



This was something that was sorely lacking in the previous releases. With the increase in the number of online sources, the inability to be able to filter the content based on some parameters was badly felt. But not any more.

User help

The documentation team has paid a lot of attention to the end-user help for these two applications. Older pages have been refreshed and newer ones written, and I can easily say that GNOME 3.12 will be one of the best in terms of documentation coverage.


Written by Debarshi Ray

19 March, 2014 at 08:35

Flickring backgrounds

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Flickr integration was introduced in GNOME 3.10 when we added support for browsing Flickr content in Photos. In GNOME 3.12 we have taken this one step ahead. The Background Settings panel will now let you choose one of your Flickr pictures as your desktop’s background.

The panel only shows the 50 most recent pictures from each account to avoid overloading it with hundreds of items. If you are looking for that gorgeous photograph that you took a few summers ago, then you might want to use Photos instead. It has better search capability and is meant to handle larger data sets.


Written by Debarshi Ray

18 March, 2014 at 00:40

Robbed. Camera gone.

with 3 comments

We got robbed in Barcelona the day before yesterday. It happened a little after 23:00 hr on the Passeig de Colom. We could have lost more, but in the end it was just my Canon EOS 60D with the EF-S 18-135mm f/3.5-5.6 IS lens attached to it.

We filed a report at the police station on Carrer Nou de la Rambla with the serial numbers of the devices. We believe we have identified one of the two robbers — the one who had confronted us from the front. I must say that the police were friendly and helpful, and they had people who spoke good English.

But that is not the point of this blog post.

Thanks to my friend, Arjun, we figured out that the EXIF data has the serial number of the camera and the lens embedded in it; and if, like me, you upload your photos to Flickr, you can easily see all the metadata even if you do not have any fancy photography software at hand. This makes me wonder if it is possible for websites like Flickr or Facebook or Imgur to flag uploads originating from a tainted device. Client-side programs could do this too. I guess, this needs some kind of stolen cameras and lenses database, which could be tricky. Does something like this exist?

Anyway, I will leave the details of my stolen camera and lens here to leave a trail in the sands of the Internet.

  • Serial number: 1881125429
  • Internal serial number: WB0778966
  • Lens serial number: 0000124725

Written by Debarshi Ray

15 December, 2013 at 13:37


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