Purpose of this document
This document describes some aspects of GVfs architecture and explains reasons why things are done the way they are. It is intended for developers as a starting point, to help easily find the area to look at. Most information can be read from the sources and I'm also not going to describe all details here.
What it is and what it does
GVfs is a set of libraries and daemons extending the GIO API. In fact it's an extension point, default GIO implementation of non-local services. It's designed to provide information suitable for usage in the GUI, that means in a polite and filtered way.
It provides everything needed for accessing files on remote servers (networking) or devices attached to the physical computer. It also takes care of local storage management.
Requirements, running and usage
The only requirement is a running user d-bus session, typically started with desktop session on login. When using GVfs in an isolated shell (console, ssh login) make sure to start the d-bus session (the dbus-launch command, e.g. export `dbus-launch`). Active GVfs mounts are only available to the specific session, making it work nicely in concurrent environment like multi-seat systems or terminal servers.
Everything is autostarted on-demand on first access through GIO. No need to start any daemon explicitly. Some mounts may require credentials, this is done using standard GMountOperation object.
All daemons are listening to session bus changes and will exit once the bus is torn down. Should this not be true, please file a bug.
See also Running GIO applications in the GIO Reference Manual for common GIO environment variables.
The following environment variables can be used for controlling certain aspects of some services:
DBUS_SESSION_BUS_ADDRESS - if unset, gvfs will refuse to load to prevent spawning private bus
GVFS_DEBUG - if set (no matter the value), daemons will be printing more information on console. This is useful for debugging and reveals parts of internal protocol. You can also toggle the debugging for each backend using SIGUSR2.
GVFS_DEBUG_FUSE - if set (no matter the value), fuse daemon will be printing more information on console.
GVFS_MOUNTABLE_EXTENSION - filename extension of gvfs backend setup files. Default value is ".mount"
GVFS_MOUNTABLE_DIR - a path to look for the backend setup files. Default value is determined by configure script
GVFS_MONITOR_DIR - a path to look for volume monitors setup files (".monitor" suffix). Default value is determined by configure script
GVFS_REMOTE_VOLUME_MONITOR_IGNORE - if set (no matter the value), gvfs volume monitors will not be activated
GVFS_DISABLE_FUSE - if set (no matter the value), FUSE daemon will not be autostarted
GVFS_AFC_DEBUG - if set (no matter the value), the AFC backend will print more debugging messages
GVFS_GPHOTO2_DEBUG - controls amount of debug information printed out by the gphoto2 backend. Allowed values are "all", "data", "debug", "verbose".
GVFS_HTTP_DEBUG - controls amount of debug information printed out by the http backend. Allowed values are "all", "body", "header".
GVFS_SMB_DEBUG - sets libsmbclient debug level. Allowed values are integers from 0 to 10.
GVFS_MTP_DEBUG - sets libmtp debug level. Allowed values are "all", "data", "usb", "ptp".
GVFS_NFS_DEBUG - sets libnfs verbosity. Allowed values are integers from 0 to ?.
Components, the architecture
The project consists of three more or less independent parts. Those parts comprise of one or more daemons. Everything is glued together using d-bus, it's best to illustrate implementation details on the gvfs d-bus interface.
Since GVfs is implemented as GIO extension point, its libraries are loaded in every process. This may be expensive and GVfs may need some time to initialize, etc. That's one of the reasons a client-server architecture was chosen, client libraries (loaded in applications, GIO clients) are basically only proxy modules communicating with the gvfs server side through d-bus.
Volume monitors provide a set of GDrive/GVolume/GMount objects representing physical device or service hierarchy. So called native volume monitors provide access to locally available devices, i.e. those appearing in /dev and mountable by standard POSIX ways (mount, umount). GVfs provides several implementations of those (hal, gdu, udisks2) and only one can be active. This is determined by a priority number, the highest priority monitor is used.
Every volume monitor possesses its own process, communicating with the client side through GProxyVolumeMonitor infrastructure over d-bus (see the note about client-server architecture above). See the /monitor/proxy/dbus-interfaces.xml file for d-bus interface description. Other than usual volume monitor-related signals and methods the interface also wraps GMountOperation implemented by combination of method calls and signals for bi-directional communication. Signals have been chosen to work around possible deadlocks.
Every volume monitor ships a .monitor file containing key-value information and that is used for registration. Registration is done on startup (more specifically on libgioremote-volume-monitor.so load) by going through the .monitor files in /usr/share/gvfs/remote-volume-monitors directory. The .monitor file provides a d-bus service name used for daemon autostart, a d-bus service file should be installed along this.
Other than native volume monitors GVfs ships several others that are related to some kind of service or device, in both cases access is provided usually only through GVfs backends. The goal is to present GVolume objects mountable by user on demand. This applies e.g. to digital cameras or media players connected to the computer.
On the client side, GVolumeMonitor will mix results from all volume monitors together, hiding implementation details. Check output of gvfs-mount -l for illustration.
udisks2 volume monitor
The udisks2 volume monitor is the preferred native volume monitor for the moment. It's the most up-to date and fully featured among others and majority of fixes go there.
David Zeuthen wrote a nice document describing what is exposed to the UI and how to control it: /monitor/udisks2/what-is-shown.txt
Core VFS Daemon
The master process, gvfsd, is basically a manager and message router. Initialized on the client side again as an extension point by loading the libgvfsdbus.so library, the master daemon is autospawned on a first use. The client library (running in GIO application) provides number of interfaces that are proxied to the master daemon or its backends.
All the hard I/O work is done by backends - separate processes for one or more mounts and the master gvfs daemon is just controlling spawning, mount, unmount and informs clients which backend to use for the requested file. Having backends out of process brings robustness (crashed backend is treated as unmounted) and solves some technical difficulties such as threading, locking, forking, usually specific to an underlying library. Separate processes can also link to non-GPL compatible code without violating licenses of the other parts.
Mount tracker, GDaemonVolumeMonitor
Among other things the master daemon is doing on startup, is registering the org.gtk.vfs.MountTracker interface. It keeps track of active mounts, handles mounting, unmounting and provides runtime mount information. On the client side, GDaemonVolumeMonitor for gvfs mounts is available by registering itself at gvfs extension point, tracking changes on the mount tracker. This is also used by the FUSE daemon to detect new mounts.
Spawning and unmounting backends
Mounting is typically initiated by the g_file_mount_enclosing_volume() call, which goes through GDaemonFile, calling the org.gtk.vfs.MountTracker.MountLocation() d-bus method. The mount tracker (master gvfs daemon) then checks whether the location is not mounted and either spawns new process for the particular mount, or, in case of "Mountable", it uses d-bus service autostart feature for the associated d-bus name.
The difference is that Mountable backends are able to handle any request for specified schemes, sharing one process for any target hostname, contrary to standard backends having separate processes for each hostname. This is indicated by .mount files (the /usr/share/gvfs/mounts/ path by default or $GVFS_MOUNTABLE_DIR and $GVFS_MOUNTABLE_EXTENSION for override) carrying backend-specific information. The principle of setup files is nearly identical to .monitor files described in the Volume monitors section.
Backends are typically spawned by the master daemon but can also be started manually, e.g. when debugging. In case of being spawned by the master daemon, a --spawner commandline argument with a value of master daemon d-bus connection name and d-bus path are appended. These are then used by the backend to announce itself (using the org.gtk.vfs.Spawner d-bus interface), awaiting more information. The master daemon calls org.gtk.vfs.Mountable.Mount() method back to the backend with more arguments and a mountspec.
The mountspec is a crucial piece of information required for the mount operation to succeed. When started manually, mountspec must be provided as commandline arguments (in form of key1=value key2=value ...) since the master daemon is not aware about this initiative and can't provide any information. Once mountspec is provided, the backend can start the mount process, creating GVfsJobMount, going through the backend methods. Once that job is finished, the result is sent back to the master daemon by the org.gtk.vfs.MountTracker.RegisterMount() call and only then the mount is treated as mounted. A Mounted signal is emitted by the mount tracker and caught to subscribed GDaemonVolumeMonitor instances on client sides.
Unmounting is easier, the mount tracker watches d-bus name of all registered backends and when it disappears, i.e. exits or crashes, the mount tracker emits an Unmounted signal. Historically this used to be the only way of unmount signaling, letting the backend exit gracefully first. However certain issues with unexpected forking in backends were discovered (trigerred by external library used), preventing the detection to work properly, so org.gtk.vfs.MountTracker.UnregisterMount() d-bus method has been put in use. It's also the preferred way of unmounting, called at the GVfsJobUnmount operation finish.
Some backends don't need credentials or specific mount options, this is the case of general mounts like computer://, network://, trash://, etc. For this the automounting feature has been introduced, backends need to indicate that in their .mount setup files. Technically such backends are not running by default and are only spawned on a first use. Automounted mounts need to be type of Mountable as they can be mounted only once. Automounted mounts are usually hidden from GDaemonVolumeMonitor and neither the mount tracker is propagating their presence.
Most of GFile methods are implemented in GDaemonFile class, on the client side. All methods have common start, doing some validation (mounted location check), creating private connection to the backend and returning GDBusProxy instance for immediate use. This proxy is then used for calling respective org.gtk.vfs.Mount method whose roughly correspond to GFile methods. The first argument of every call represents path inside the mount, i.e. stripped from path elements that should be part of the mount info - mount prefix (think of smb://server/share/path).
On the daemon side, incoming org.gtk.vfs.Mount method calls are directly transformed to new jobs. The job, descendant of the GVfsJobDBus class, takes reference to the incoming d-bus message (GDBusMethodInvocation) and keeps it until the job is finished. Once finished, the result from the job is sent as a method call reply, together with any (optional) data. If job has failed, error is returned and is properly handled by the client side, propagated back to GDaemonFile method caller, usually displayed in UI. For this reason it's crucial to set method call timeouts in a sane way, i.e. for long I/O operations the timeout should be set to infinite.
Jobs are essentially wrappers around backend methods, carrying all arguments from the d-bus method call. Backend methods are those doing actual I/O, for that reason we need to have some kind of control over them. Two types of methods exist in the backend. Methods prefixed with try_ are supposed to be asynchronous and non-blocking as much as possible. Return value of boolean indicates whether the request has been handled or not. If not (or the try_ method is not implemented), the do_ prefixed method is called (or not prefixed version respectively). This call is more heavy-weight and runs in a thread pool. That way the backend could handle multiple requests simultaneously. Daemon thread pool is on daemon (process, see below) basis and the maximum number of threads is controlled by the MAX_JOB_THREADS define during compilation (see Makefile.am).
When talking about backends, let's see how objects are organized. A daemon instance represents the process itself. Daemon can handle one or more mounts (this is actually determined by the master gvfs daemon, the backend daemon infrastructure is generally always able to handle multiple mounts) and maintains list of job sources. Default job source is created for every single mount (GVfsBackend instance), additional job sources are created for each opened file (both for reading and writing, GVfsChannel). Job source (GVfsJobSource) is an interface implemented by GVfsBackend and GVfsChannel.
Every job source maintains a list of actual jobs. Jobs are submitted via the g_vfs_daemon_queue_job() call. Typically every new job created by the org.gtk.vfs.Mount method call handler queues itself once all arguments are collected. More jobs are queued during file transfer, from the GVfsChannel side. This will then go through the try_ and do_ backend methods which effectively starts the operation.
Backends are required to return status of the operation by calling either g_vfs_job_succeed() or g_vfs_job_failed() in case of error. That will indicate the GVfsJob instance to finish. This is also the way the asynchronous try_ methods should complete. Some job classes may provide more methods that are required to be called in order to pass some data to the job (e.g. file infos for GVfsJobEnumerate).
If anything happens to the master gvfs daemon (crashes or is being replaced by another instance) and its d-bus name owner changes, every running backend daemon will go through the list of active job sources and re-registers all mounted backends with the new master daemon. This is a nice way of crash recovery, bringing more robustness.
Cancellation is well integrated by GIO nature and GVfs needs to provide a channel to cancel running operations. This is done by calling the org.gtk.vfs.Daemon.Cancel() method taking serial number as the only argument. This is available on both session bus and private connections.
The job system in backends works by taking reference to the originating d-bus message for later reply. Every d-bus message contains unique serial number and by going through the list of running operations we are able to find the right one to cancel.
Private peer-to-peer connection
For particular GDaemonFile operations a private peer-to-peer d-bus connection is created between client library and the backend directly. This allows us to overcome potential weak point of overloaded session bus daemon, relieve from data marshalling and being somewhat independent to the session bus. This is quite independent implementation and can be easily switched back to use of a session bus. Similarly we provide private sockets for raw data transfers, to maximize throughput.
Initiated from most GDaemonFile methods, the way of setting up private connection is rather complex. On the client side we first need to know backend d-bus ID. By calling the backend org.gtk.vfs.Daemon.GetConnection() method we get a d-bus address. Using this address we create a new d-bus connection that is used for further org.gtk.vfs.Mount calls. Every client maintains a cache of thread-local private connections and tries to reuse them whenever possible.
On the backend side, on GetConnection() request, a socket with unique name is created and GDBus server is started on it. The server then awaits incoming client and registers several interfaces on the connection once client comes. Server is then killed as no more clients are expected, leaving active connection open. When connection is closed, backend cancels all active jobs.
Skeletons, registered paths
Related to private connections, we need to make available several interfaces on them. Everytime a new private connection is opened, several GDBusInterfaceSkeleton objects are registered on it.
Historically GDBus was not able to export single GDBusInterfaceSkeleton over multiple connections and a workaround solution was created. By calling g_vfs_daemon_register_path() interested parties register themselves handing over an object path and a callback that is used for creating new skeleton for the new connection. This way we export org.gtk.vfs.Mount and org.gtk.vfs.Monitor interfaces on the private connection.
Similar approach is taken on the client side for GDaemonFile methods, interested parties register their ability to provide certain service by calling _g_dbus_register_vfs_filter() and their callback is then used to create interface skeletons on every new private connection (by calling _g_dbus_connect_vfs_filters()). This way we export org.gtk.vfs.Enumerator and org.gtk.vfs.MonitorClient interfaces, typically used as a form of callback on events from daemon to clients.
Enumeration (initiated by g_file_enumerate_children()), just like several other GFile methods, needs to be stateful to be able to continuously provide data back to the client. For this, GVfs provides GDaemonFileEnumerator that is used on client side. A new VFS filter (see above) is registered to provide the org.gtk.vfs.Enumerator interface on the connection. That's wrapped by GDaemonFileEnumerator and is used to receive data from the backend. Every GDaemonFileEnumerator instance creates unique ID that is used as an object path for creating interface skeleton on the client side and for GDBusProxy construction on the backend side. It's worth noting that GDaemonFileEnumerator can handle both sync and async ways of enumeration.
GVfsJobEnumerate provides several methods used by backends to send data back to client. Collected infos are sent using the org.gtk.vfs.Enumerator.GotInfo() call as an array and when enumeration is finished, org.gtk.vfs.Enumerator.Done() is called to indicate that to the client side. Additional backend info is added in GVfsJobEnumerate, metadata info is added on the client side if applicable.
Note that any unsuccessful method call will lead to a warning message printed to the console, sometimes messages can be seen when client closes the enumerator before it's fully finished.
Structure-wise monitoring is very similar to enumeration. Client g_file_monitor*() calls will result in GVfsJobCreateMonitor on the backend side which lets the backend create a GVfsMonitor instance and hand it over to the job. This instance wraps around the org.gtk.vfs.Monitor interface, waiting for clients to subscribe and unsubscribe particular files and directories they're interested in monitoring. This is done on the client side, when the GVfsJobCreateMonitor call is finished, client creates a GDaemonFileMonitor instance and subscribes itself to the backend-side monitor. Every subscription takes a reference to the GVfsMonitor instance, every unsubscribe releases it. That way the monitor is automatically destroyed when no one is interested in monitoring anymore. Subscriptions on a particular private d-bus connection are automatically unsubscribed when the connection is closed.
When backend emits a change event, GVfsMonitor calls the org.gtk.vfs.MonitorClient.Changed() method and the client side GDaemonFileMonitor emits an event to the original client.
For GDaemonFile methods that are taking a GMountOperation argument, we need to provide a way how to proxy that to the daemon side. This is done by the g_mount_operation_dbus_wrap() call which wraps the foreign instance on specified d-bus connection. Internally that means creating a org.gtk.vfs.MountOperation interface skeleon on the client side and assigning its d-bus specific data to a newly created GMountSource instance. These are then used by several org.gtk.vfs.Mount methods to reconstruct a GMountSource instance on the daemon side and finally get a GMountOperation instance through g_mount_source_get_operation(). This call creates new local GMountOperation instance catching its signals. When asked for something, the machinery makes a call back to the client and sets the GMountOperation credentials if succeeded.
File copy or move in form of copy-and-delete is supported in GVfs however the open-read-write-close fallback is done automatically by GIO when G_IO_ERROR_NOT_SUPPORTED is returned. To overcome potential backend limitations and to optimize the data flow, two new backend operations have been introduced along standard copy() and move(): push() and pull(). These methods work with one endpoint being a local file, pull() having source file local, push() transfers data from backend to a local file. The standard copy() and move() are supposed to work within the same mount only.
Some backends are not able to provide universal open, read, write, close methods either due to the service nature or due to a limitation of underlying library. The push()/pull() methods are taking care of file transfer completely, i.e. file open, data transfer and file close, with optional progress callback reporting. These methods are only used if implemented by the backend and if one of the endpoint is a local file.
In order to provide progress report during the transfer, the org.gtk.vfs.Progress d-bus interface is set up on the client side of the connection, awaiting incoming Progress() method calls that will then call the progress callback specified in the original GDaemonFile call.
Reading and writing data
Since the open-read-write-close operations are used as a copy fallback by GIO, it's recommended to have these methods implemented to maintain a degree of universality. This is impossible for some backends though (obex, mtp) where full file has to be always trasferred.
Unlike file copy, which handles data blocks internally, the GDaemonFile.read(), GDaemonFile.create() and similar methods are returning GFileInputStream and GFileOutputStream respectively, meaning the data are being transferred outside of GVfs. In order to efficiently transfer data blocks between two processes, unix socket is created and using the GDBus fd-passing feature the client receives an fd for reading/writing. GFileInputStream and GFileOutputStream are implemented by GDaemonFileInputStream and GDaemonFileOutputStream respectively, providing all necessary infrastructure for data handling. This allows us to overcome unnecessary round-trips through the d-bus daemon, achieving almost native speed.
The daemon open job creates a GVfsReadChannel or GVfsWriteChannel respectively, taking pointer returned by the backend as an internal file handle. The GVfsChannel base class is taking care of creating unix socket for data transfer, buffering, simple protocol and queue handling etc. Its descdendants implement more functionality like readahead and are processing the channel protocol, calling particular jobs (GVfsJobRead, GVfsJobWrite, GVfsJobSeekRead, GVfsJobSeekWrite, GVfsJobQueryInfoRead, GVfsJobQueryInfoWrite, GVfsJobCloseRead, GVfsJobCloseWrite) to do the actual work.
Mount spec is a tiny class that helps to identify and match mounts. It carries key-value pairs, e.g. information about URI scheme used, backend type, hostname, port, username, domain etc.
Specifically during mount operation two instances of mount spec are used. The source one carries requested information and is handed over to the backend's GVfsJobMount operation. It is a backend responsibility to process this mount spec, create a new one based on the source one and send it back to the mount tracker for registration. Backend can filter out some information that are not strictly required and only sets those values required for unique mount identification.
Mount spec matching is done by comparing all its values, it must be completely equal. In practice, this also means that e.g. scheme://hostname/ and scheme://username@hostname/ may result in two mounts even if user enters the same username in a credential prompt. There's another limitation in real applications at the moment, requesting mount on scheme://hostname/ wouldn't match with scheme://username@hostname/ at the beginning, when mount tracker is trying to find existing mount since we don't know what username to use at this point. Consequently, when going through the mount process and username is known, we don't have a tool for canceling current mount operation and redirecting ourselves to an active mount.
URI mappers are tiny classes helping to parse URI strings with respect to special parts, like a domain in smb:// or to distinguish between smb-share and smb-server backend type etc. Used mostly when converting URI to a mount spec and back.
Default parser is used when specific URI mapper is not available for the given scheme.
Secret storage integration is actually very basic, two helper functions are available for direct backend use, for storing and retrieving credentials with additional arguments. Also please take the mount spec specifics in account, having two nearly identical records with the difference of having username set is perfectly okay.
This feature also helps debugging significantly, allowing developer to overcome credential prompt when running the backend manually since it's is able to pick credentials up from keyring. Just be sure to pass correct mountspec arguments so that matching is successful.
The package provides convenient FUSE mount for easy POSIX access to active GIO mounts. This is handled by the gvfsd-fuse process, daemon that is autostarted when gvfsd is spawned. The default mount point is /run/user/<UID>/gvfs, usually located on tmpfs, with a fallback to the old location ~/.gvfs when not available. On some systems, the user needs to be in the fuse group to be actually able to mount FUSE filesystems.
Even if the FUSE daemon is not autostarted (by running gvfsd --no-fuse) it can be started manually anytime. The daemon registers itself with the master gvfs daemon by calling org.gtk.vfs.MountTracker.RegisterFuse() d-bus method. When the master gvfs daemon disappears from the session bus, the FUSE daemon is terminated.
The daemon creates flat structure of active GIO mounts at the topmost level, named by internal GMountSpec string representation (in the form of protocol:arg1=val,arg2=val). This string is not guaranteed and may change in the future.
Please note that due to different nature of POSIX and GIO filesystems, not everything is possible in POSIX world and there will always be trade-offs. Moreover not all backends implement every GVfsBackend method, e.g. backend implements file open/read/write/close but not seeking. Some POSIX applications may not function properly due to that, the translation is not 1:1. Things like returning zero file system size and free space just because the actual backend has no native support may make some apps confused refusing to write any data.
There are several security concerns, the first and most visible is denied access to other UIDs including UID 0. This causes troubles to system commands and daemons stat-ing the filesystem (i.e. df or systemd-tmpfiles) with an error printed out. See bug 560658 for details and discussion.
The other security concern is revealing symlinks from backends. Unmodified they may point to the host system in case of absolute target paths and may not be constrained in the mountpoint in any way. For now the fuse daemon dereferences all symlinks and presents them as regular files or directories. Only broken symlinks are still presented as symlinks. See bug 696298 for discussion.
Local path mapping
The g_file_get_path() method as implemented in GDaemonFile can provide local pathname when FUSE mountpoint is registered and connected with the master gvfs daemon. This path can then be easily used for spawning applications with a local path. When application is asked to create new GFile instance using the local FUSE path, it is automatically converted to a native URI. That way we don't need to pass URI strings outside of GIO world.
GVfs provides its own persistent storage for simple key-value pairs associated to a particular file. Typically used for runtime data such as icon positions, emblems, position within the document, notes, etc. Only non-critical data should be stored. Usage wise it's a separate file attribute namespace for easy use with GFileInfo.
All data are stored in so-called metadata database, a set of binary files in ~/.local/share/gvfs-metadata. Every database file carries a journal where most recent changes are stored for a short moment of time before they're written to the master database file. Every file roughly corresponds to a GMount, identified either by mount spec (for GVfs mounts) or by UUID or device name for physical mounts. That brings some flexibility, when a mount is mounted in a different path (automount; usually physical devices), the metadata subsystem is still able to identify the device and match existing database. A fallback to database "root" is in place, taking everything starting with "/".
Retrieving data is done within GDaemonFile (and GDaemonFileEnumerator) methods, directly accessing database files. Storing data is going through separate daemon, gvfsd-metadata, as a single point queueing all requests and not blocking applications. Incoming data are first written in a journal file and then after certain time (60 seconds) or in case of full journal, they're written to the master database file (metatree file). Journal contains all incoming requests chronologically stacked up while the metatree file contains clean key-value pairs with no duplicates.
The process of writeout is called rotation. The daemon first reads existing metatree file and iterates over the journal going from oldest entries to newest ones applying the changes on top of existing values. Data are written in a new metatree file and this file then atomically replaces the old database. Keeping the old database file still open, the "rotated" bit is written in and file is closed. Since it was atomically replaced by new database, it is unlinked and data are destroyed by operating system in ideal case. There still may be clients having the very same file open however, in process of data retrieval. The rotated bit then indicates that the currently open database is old and reopen is necessary.
Special care is taken when the metadata directory resides on NFS (e.g having homedir on NFS). See safe_open() in metadata/metatree.c for details, the code is trying to work around possible stale file handle by linking to a temp file, opening and unlinking again so that we actually don't operate on the file directly, just modifying the same data.
This all works natively for files on GVfs mounts (GDaemonFile), for local file:// scheme the GIO API has been extended to allow GLocalFile go through the default VFS implementation (g_vfs_get_default(), GVfs class) and ask for metadata associated to a local file specifically.
Metadata are tied to the particular file or directory. If it's moved or deleted (using GIO), changes are reflected to the metadata database the same way. It works very similar to xattrs.
There were some thoughts about feeding metadata in a Tracker store for easy indexing, not sure if there are any drivers for that nowadays.
While not a part of the architecture, there's a bunch of commandline tools providing convenient access to GIO resources. Imitating well known POSIX commands, taking URI instead of filenames. They're all using GIO API, nothing GVfs-specific, the name gvfs-* is misleading. In the future, they may be moved in glib source tree.
Integrated test suite is available and covers quite large area. Testing project like GVfs is difficult as you need to provide either services for the client (gvfs backends) or even simulate physical devices (for volume monitors). This is more or less done by using available projects (apache, samba, twisted) or system infrastructure (scsi-debug kernel module, systemd). Special care needs to be held for distro support, taking in account various versions of required dependencies and different (config files) locations.
The test suite is able to run in several modes, the most useful is the in-tree sandboxed mode, which starts its own d-bus session bus and uses the binaries from the source tree. More advanced tests are available when executed as root.
Distributors are encouraged to use and integrate this test suite in their infrastructure and report any oddities and problems upstream. We expect the test suite to improve over time.
For more details please contact Martin Pitt <email@example.com> as the test suite creator.
describe GDaemonFileInputStream and GDaemonFileOutputStream in detail
- icon extensions
describe GVfsChannel machinery
Running applications under root
Starting desktop session under root is generally discouraged and considered as a security risk. However since it's a complete session GVfs will work just fine.
Bigger problem is starting particular applications through su, sudo or any other graphical equivalent. Also applies for tunneled X connections. Please understand that this effectively cuts original user's d-bus session out, error messages printed on console will clearly show that as well. Tunneling d-bus connection can be treated as a security risk and is disabled AFAIK. You won't be able to access any active mounts, they all need to be opened again within the new session.
The right solution would be to provide proxy root backend through PolicyKit.
Writing your own backend
GVfs doesn't provide any public headers and the daemon d-bus protocol is considered private and may change any time (and that actually happened with the GDBus port). Out-of-tree backends are simply not supported, the preferred way is to fork/branch gvfs sources and write your backend on top of it. If you then send resulting patch to upstream developers, they'll be happy to review it for you for potential inclusion.
The best way to start is grabbing an existing backend and studying its contents. The localtest backend is basically a proxy to file:// scheme originally written for testing purposes. Backends are quite independent from the rest of GVfs codebase and you shouldn't be needing any further changes outside the backend.