James Henstridge

One of the changes in the Gnome 2.9 development series is the removal of most of the Bonobo code from Nautilus, which results in a speed boost due to lower complexity and less IPC overhead. This had the effect of breaking existing bonobo based context menus, property pages and views. The first two can be converted to the Nautilus extension interface, but the second has no equivalent in the new code (partly because Nautilus is concentrating on being a file manager these days rather than a universal component shell like it was in the early days).

Two of the casualties of the change were gnome-control-center‘s font and theme code, and nautilus-media. Since I wrote the font browser code in gnome-control-center, I updated it to work again. It isn’t clear whether nautilus-media will be updated, since the view was a major component of it, and most of the remaining functionality is provided by totem.

Context Menus

If you are looking at updating a Nautilus context menu to use the new extension interface, fontilus-context-menu.c is a pretty good example to model your code on.

One of the big differences is the way Nautilus extensions are loaded compared to the old context menu API. With the old API, you would provide a Bonobo component and set a number of properties in the bonobo-activation server file listing a menu label, the list of mime types the context menu applies to, what URI schemes it supports and whether it supports multiple files. Nautilus could then do a single bonobo-activation query to find out what context menu items correspond to the current selection, and add them to the menu. If the user selected one of the items, the corresponding component would be activated, and an event sent to its Bonobo::EventListener interface.

In contrast, Nautilus extensions are initialised on Nautilus startup. They indicate that they provide context menu items by implementing the NautilusMenuProvider interface. When the user brings up the context menu, the get_file_items method will be called on all extensions that implement that interface. A list of NautilusFileInfo objects is passed in, and the method returns a list of NautilusMenuItem objects. Also, Nautilus extensions are run in-process while Bonobo components could be written for in-process or out of process use.

One of the benefits of this system is the added control of when to display a menu item, and what to use as the label. If you want to only display your context menu item when 42 text/html files and one image/png file are selected you can. However it does mean that each new extension causes some code to be run before popping up a context menu. I have no idea how this compares time wise to the time taken for the previous bonobo-activation query though.

Property Pages

The interface for property pages is quite similar to the context menu interface. As with context menus, you have an imperative NautilusPropertyPageProvider::get_pages interface rather than a declaritive interface based on activation properties. This has the benefit that you can simply not provide the page when the properties in question are not available for the file (with the old setup, you’d end up providing a properties page stating that there is nothing to display).

The other interesting parts of the extension interface is the NautilusInfoProvider interface that lets you attach extra information to files, such as extra emblems or custom attributes, and NautilusColumnProvider, which lets you provide additional columns for the list view that map to custom file attributes. One example of this is nautilus-vcs, which can show revision numbers for files in CVS working copies and adds emblems indicating the file state.

Of course, there are downsides to the extension interface too — since extensions are always in process, they can crash Nautilus or leak memory. However, it was already possible for Bonobo based extensions to do this if they were designed as in-process components and badly written …

Another issue is that language bindings might find it more difficult to support the extension interface where the language runtime would have to cooperate with Nautilus, compared to out of process Bonobo components where they have more control. I guess we’ll see what happens.

Drive Mount Applet

The new drive mount applet is now checked into the HEAD branch of gnome-applets, so will be in Gnome 2.10. There are a few things left to do, such as making it possible to open the file manager as well as unmounting/ejecting it. I did up a screenshot showing what it looks like as an applet.

Libtool

Finally managed to reproduce a particular libtool bug that people have reported on and off. It does show why some people decide that .la files are evil, since it doesn’t occur when people delete those files …

A reduced test case can be found here. The problem occurs when you have multiple copies of a library in your linker library search path with associated .la files. In the test case, there are the following libraries:

• libfoo.so and libfoo.la in the directory /A. This is the library we want to link to.
• libfoo.so and libfoo.la in the directory /B. We don’t want to link to this one, because it is old.
• libbar.so and libbar.la in the directory /B.

Let’s say I then try to link an app that needs libbar and the new version of libfoo, and happen to use the following link line:

libtool --mode=link gcc -o main main.c -lbar -L/A -L/B -lfoo

In the absense of libtool, this would result in us linking against /B/libbar.so and /A/libfoo.so (since /A comes before /B in the search path).

However, libtool ends up doing something quite different. When it sees -lbar, it notices that there is a libbar.la in /B, expands that argument to the full path name of the actual library (/B/libbar.so), and prepends /B to the library search path. This means that when it gets round to processing -lfoo, it finds /B/libfoo.la instead of /A/libfoo.la, and links to the wrong library.

If this sounds like an obscure bug, note that it also happens if we replace /B with /usr/lib. In this case, we don’t even need the -L/usr/lib argument. So the following command results in linking with /usr/lib/libfoo.so instead of /A/libfoo.so:

libtool --mode=link gcc -o main main.c -lbar -L/A -lfoo

This sort of situation is quite common when trying to build some software into a separate prefix that is also provided by the OS, when you are relying on a few libraries installed in /usr/lib with .la files.

After putting together the test case I tested it out in the latest development release (1.9f), and it appears that the problem has been fixed. Given that the libtool developers are so close to a 2.0 release, I don’t know whether they would bother putting out another 1.5.x release to fix the problem.

So if you do run into the problem, some possible solutions are:

1. Upgrade to libtool-1.9f. I’m not sure how good an idea this is if you are producing tarballs, since they will be packaged with the development release too.
2. Remove all the .la files in /usr/lib. Some distributors seem to take this route (eg. Ximian/Novell and Red Hat).

I started to look at bringing the drive mount applet from gnome-applts up to scratch, since it hasn’t really had much work done on it other than porting to the 2.x development platform.

The applet is a classic example of Gnome 1.x user interface complexity. The applet shows a button that can be clicked to mount or unmount a particular mount point. For this simple functionality, it provides the following preferences:

• The mount directory
• The interval at which to check the mounted state
• Which of a set of custom images to use to show the mounted state (eg. floppy drive, cdrom drive, etc).
• The ability to specify custom mounted/unmounted images.
• Whether to eject the disk after unmounting.
• Whether to use a second method for checking the mounted state which might work better with automounters.

The applet also had a few problems, such as not being able to unmount a disk if there was a trash directory on it (since fam would have a dnotify watch active on that directory).

By using some of the newer gnome-vfs APIs, I think it should be possible to remove all the preferences:

1. The GnomeVFSVolumeManager API can be used to get a list of attached drives and volumes, and receive notification when volumes are mounted or new drives are connected or disconnected. This allows one applet to display the status of all user (un)mountable drives/volumes in one applet.
2. By using the gnome-vfs APIs to unmount or eject a volume, a volume_pre_unmount signal to interested applications before attempting to unmount it. When Nautilus receives this signal, it drops its FAM watches on the trash directory for that volume, and closes all associated windows. This means that Nautilus/FAM won’t cause a “volume is busy” error (although some other apps might hold files opne on the volume).
3. The gnome-vfs APIs tell us what the volume type is. This way we can automatically do an unmount+eject for cdrom, zip and jaz drives like Nautilus does rather than having to provide a preference.
4. Gnome-vfs also tells us what icon name should be used for a particular drive or volume. This way the applet can just pull the icons from the current icon theme rather than having to maintain separate ones.

My code is at the point where it produces nice screenshots, and has the above features. The screenshot shows it as a separate window, but adding the applet wrapper stuff isn’t too difficult (it is easier to test as a standalone app). It is less than half the size of the old applet too, which is promising.

Federal Election

Looks like we are going to have at least another three years with The Rodent. It also looks like they will have a majority in the senate, which will reduce the senate’s effectiveness as a house of review.

We might not have John Howard for the entire term though, since he is of retirement age. NineMSN seems to think that Peter Costello is already the leader.

It also looks like The Democrats senators up for reelection got completely wiped out, with much of their support going to The Greens.

Gnome-VFS

Robert Collins wrote an interesting critique on the gnome-vfs API. I don’t agree with all the points, and there are some reasons why the API isn’t as elegant as it could be. Below are some responses.

Initialisation

One thing that gnome_vfs_init() does is to call g_thread_init(). Before this function is called, the locking APIs in glib are no-ops. You really want this function called early on if the app is going to use threads, otherwise you will end up with inconsistencies (eg. a lock() call might be a no-op, but the unlock() call might not be if g_thread_init() is called in between).

The other issue is that gnome_vfs_init() can fail. If it is called automatically, then any function that might invoke the initialisation routine now has a new failure mode. I don’t know whether this is a real problem or not though.

Calling Style – Inconsistent Ordering

One big difference between the out parameters in gnome_vfs_open() and gnome_vfs_read() is that the first function is essentially a constructor for a file handle, while the second is a method for a file handle that fills in a provided buffer.

I’ll agree that the calling conventions are not as nice as they could be though. If they were being designed today, I suspect that they would look more like this:

GnomeVFSHandle  *gnome_vfs_open (const gchar     *text_uri,
                                 GnomeVFSOpenMode open_mode,
                                 GError         **error);
GnomeVFSFileSize gnome_vfs_read (GnomeVFSHandle  *handle,
                                 gpointer         buffer,
                                 GnomeVFSFileSize bytes,
                                 GError         **error);

Unfortunately, the GError API was not developped til the 2.0 series, while these parts of the gnome-vfs API persist from the 1.x days.

Calling Style – Inconsistent Method Naming

I agree that the gnome_vfs_truncate() function name is inconsistent. My guess as to why they chose gnome_vfs_truncate and gnome_vfs_truncate_handle() was to match the underlying truncate() and ftruncate() C library calls. This was probably a case of balancing consistent APIs with ease of transition from libc APIs to gnome-vfs APIs.

Returning data to pointers

I agree that the existing calling convention is not as nice as it could be. As I said earlier, it would probably have been designed to use GError if it was being developed today.

The GError API has a number of benefits over errno style ones, including:

1. Automatically threadsafe. The place where the error is reported is on the stack. A global variable is a problem for multi threaded apps on systems without Linux 2.6 style thread local storage (you need to do tricks like making errno into a function that returns the appropriate variable for the current thread).
2. In Robert’s example, the actual error information is looked up from a file handle. What do you do if there is no file handle involved in the function call? Also, wouldn’t gnome_vfs_open() return a NULL file handle on error?
3. For the cases where there is a file handle to look up the error info on, what happens if two threads are working with the file handle at the same time?
4. GError is consistent with other Gnome APIs

The GError API also makes it easy to pass error data up a number of call frames similar to exceptions. If your function has a GError argument, you can simply pass that same error object to other functions when you call them. If those functions fail on an error, simply return immediately, and the caller can handle the error.

Streams Interface

There actually is a stream interface available in one of the libraries both GTK and gnome-vfs depend on: GIOChannel. I guess it would be nice if gnome-vfs provided a GIOChannel implementation for VFS file handles. The main thing that would be needed here would be the io_create_watch() implementation, which would probably require exposing a file descriptor to poll on (this could probably be implemented using a pipe pretty easily).

Doing this as GObject interfaces isn’t really an option, since GIOChannel is implemented in libglib which is below libgobject, and gnome-vfs file handles aren’t GObjects. I know that at one point Ian was planning to change the various handles to GObjects, but this didn’t happen. It would probably be possible to do this kind of change while only requiring changes to VFS methods, so it can’t be completely ruled out.

Directory Interface

You can asynchronously load a directory listing using gnome_vfs_async_load_directory(). I don’t blame you for missing it — the organisation of the APIs in the various headers is a bit confusing.

Language Bindings

There are a lot of things a language binding can do to make gnome-vfs nicer to use. Some of these things include:

• If the language provides exceptions, convert error GnomeVFSResult‘s to exceptions and change the calling conventions to something more sane.
• If the language allows for runtime type checking or multiple dispatch, don’t wrap the gnome_vfs_foo() and gnome_vfs_foo_uri() functions separately. Instead, just check if a string or a GnomeVFSURI was passed in and do the right thing.
• If the language has a standard file handle interface or convention, try to implement it in the binding.

The Python bindings do some of these things, and definitely make things easier to use.