Any ideas how the gnomevfs API exposes the seekable property, I can't
find a method. Opening the file with OPEN_RANDOM causes gnomevfs to
open the stream without an error but then it continues to read random
memory...

Got the follwoing reply from Alexander Larsson when asking about this
on gnome-vfs list:

Hmmm. This sounds strange. gnome_vfs_seek() should return 
GNOME_VFS_ERROR_NOT_SUPPORTED if you seek on a handle where its not 
supported. For local file-uris I don't see how it can read random
memory, gnome_vfs_seek() just calls lseek().

However, there is no "seekable" property you can read. Is there even a 
posix API we could use to implement this?

lseek returns -1 when an error occured, you can check errno to find
out that it was a pipe/socket or fifo (ESPIPE) you were seeking on; I
would suggest to return _NOT_SUPPORTED in that case.

Let me quote the code (which has been this way since a long time):

	if (LSEEK (file_handle->fd, offset, lseek_whence) == -1) {
		if (errno == ESPIPE)
			return GNOME_VFS_ERROR_NOT_SUPPORTED;
		else
			return gnome_vfs_result_from_errno ();
	}

	return GNOME_VFS_OK;

Is this not working?

alex: it appears to work fine.  I'm using _seek(,CURRENT,0) for the
test, which should be a no-op, but hopefully it won't be "optimized"
in the future.

Latest check-in adds a read-only boolean parameter called "seekable".