haiku/docs/user/drivers/fs_modules.dox

/*!
\page fs_modules File System Modules

To support a particular file system (FS), a kernel module implementing a special
interface (\c file_system_module_info defined in \c <fs_interface.h>) has to be
provided. As for any other module the
\c std_ops() hook is invoked with \c B_MODULE_INIT directly after the FS module
has been loaded by the kernel, and with \c B_MODULE_UNINIT before it is
unloaded, thus providing a simple mechanism for one-time module initializations.
The same module is used for accessing any volume of that FS type.


\section objects File System Objects

There are several types of objects a FS module has to deal with directly or
indirectly:

- A \em volume is an instance of a file system. For a disk-based file
  system it corresponds to a disk, partition, or disk image file. When
  mounting a volume the virtual file system layer (VFS) assigns a unique number
  (ID, of type \c mount_id aka \c dev_t) to it and a handle (type
  \c fs_volume, in fact \c void*) provided by
  the file system. Whenever the FS requests a volume-related service from the
  kernel, it has to pass the volume ID, and whenever the VFS asks the FS to
  perform an operation, it supplies the handle. Normally the handle is a pointer
  to a data structure the FS allocates to associate data with the volume.

- A \em node is contained by a volume. It can be of type file, directory, or
  symbolic link (symlink). Just as volumes nodes are associated with an ID
  (type \c vnode_id aka ino_t) and, if in use, also with a handle
  (type \c fs_vnode, in fact \c void*).
  Unlike the volume ID the node ID is defined by the FS. It often
  has a meaning to the FS, e.g. file systems using inodes might choose the
  inode number corresponding to the node. As long as the volume is mounted and
  the node is known to the VFS, its node ID must not change. The node handle is
  again a pointer to a data structure allocated by the FS.
  
- A \em vnode (VFS node) is the VFS representation of a node. A volume may
  contain a great number of nodes, but at a time only a few are represented by
  vnodes, usually only those that are currently in use (sometimes a few more).

- An \em entry (directory entry) belongs to a directory, has a name, and refers
  to a node. It is important to understand the difference between entries and
  nodes: A node doesn't have a name, only the entries that refer to it have.
  If a FS supports to have more than one entry refer to a single node, it is
  also said to support "hard links". It is possible that no entry refers
  to a node. This happens when a node (e.g. a file) is still open, but the last
  entry referring to it has been removed (the node will be deleted when the
  it is closed). While entries are to be understood as independent entities,
  the FS interface does not use IDs or handles to refer to them; it always uses
  directory and entry name pairs to do that.

- An \em attribute is a named and typed data container belonging to a node. A
  node may have any number of attributes; they are organized in a (virtual or
  actually existing) attribute directory, through which one can iterate.

- An \em index is supposed to provide fast searching capabilities for attributes
  with a certain name. A volume's index directory allows for iterating through
  the indices.

- A \em query is a fully virtual object for searching for entries via an
  expression matching entry name, node size, node modification date, and/or
  node attributes. The mechanism of retrieving the entries found by a query
  is similar to that for reading a directory contents. A query can be live
  in which case the creator of the query is notified by the FS whenever an
  entry no longer matches the query expression or starts matching.


\section concepts Generic Concepts

A FS module has to (or can) provide quite a lot of hook functions. There are
a few concepts that apply to several groups of them:

- <em>Opening, Closing, and Cookies</em>: Many FS objects can be opened and
  closed, namely nodes in general, directories, attribute directories,
  attributes, the index directory, and queries. In each case there are three
  hook functions: <tt>open*()</tt>, <tt>close*()</tt>, and
  <tt>free*_cookie()</tt>. The <tt>open*()</tt> hook is passed all that is
  needed to identify the object to be opened and, in some cases, additional
  parameters e.g. specifying a particular opening mode. The implementation
  is required to return a cookie (type \c fs_cookie, in fact \c void*), usually
  a pointer to a data structure the FS allocates. In some cases (e.g. when an
  iteration state is associated with the cookie) a new cookie must be allocated
  for each instance of opening the object. The cookie is passed to all hooks
  that operate on a thusly opened object. The <tt>close*()</tt> hook is invoked
  to signal that the cookie is to be closed. At this point the cookie might
  still be in use. Blocking FS hooks (e.g. blocking read/write operations)
  using the same cookie have to be unblocked. When the cookie stops being in
  use the <tt>free*_cookie()</tt> hook is called; it has to free the cookie.

- <em>Entry Iteration</em>: For the FS objects serving as containers for other
  objects, i.e. directories, attribute directories, the index directory,
  and queries, the cookie mechanism is used for a stateful iteration through
  the contained objects. The <tt>read_*()</tt> hook reads the next one or more
  entries into a <tt>struct dirent</tt> buffer. The <tt>rewind_*()</tt> hook
  resets the iteration state to the first entry.

- <em>Stat Information</em>: In case of nodes, attributes, and indices
  detailed information about an object are requested via a <tt>read*_stat()</tt>
  hook and must be written into a <tt>struct stat</tt> buffer.


\section vnodes VNodes

A vnode is the VFS representation of a node. As soon as an access to a node
is requested, the VFS creates a corresponding vnode. The requesting entity
gets a reference to the vnode for the time it works with the vnode and
releases the reference when done. When the last reference to a vnode has been
surrendered, the vnode is unused and the VFS can decide to destroy it (usually
it is cached for a while longer).

When the VFS creates a vnode, it invokes the FS's
\link file_system_module_info::get_vnode get_vnode() \endlink
hook to let it create the respective node handle (unless the FS requests the
creation of the vnode explicitely by calling publish_vnode()). That's the only
hook that specifies a node by ID; to all other node-related hooks the node
handle is passed. When the VFS deletes the vnode, it invokes the FS's
\link file_system_module_info::put_vnode put_vnode() \endlink
hook or, if the node was marked removed,
\link file_system_module_info::remove_vnode remove_vnode() \endlink.

There are only four FS hooks through which the VFS gains knowledge of the
existence of a node. The first one is the
\link file_system_module_info::mount mount() \endlink
hook. It is supposed to call \c publish_vnode() for the root node of the volume
and return its ID. The second one is the
\link file_system_module_info::lookup lookup() \endlink
hook. Given a node handle of a directory and an entry name, it is supposed to
call \c get_vnode() for the node the entry refers to and return the node ID.
The remaining two hooks,
\link file_system_module_info::read_dir read_dir() \endlink
and
\link file_system_module_info::read_query read_query() \endlink,
both return entries in a <tt>struct dirent</tt> structure, which also contains
the ID of the node the entry refers to.


\section mandatory_hooks Mandatory Hooks

Which hooks a FS module should provide mainly depends on what functionality
it features. E.g. a FS without support for attribute, indices, and/or queries
can omit the respective hooks (i.e. set them to \c NULL in the module
structure). Some hooks are mandatory, though. A minimal read-only FS module
must implement:

- \link file_system_module_info::mount mount() \endlink and
  \link file_system_module_info::unmount unmount() \endlink:
  Mounting and unmounting a volume is required for pretty obvious reasons.

- \link file_system_module_info::lookup lookup() \endlink:
  The VFS uses this hook to resolve path names. It is probably one of the
  most frequently invoked hooks.

- \link file_system_module_info::get_vnode get_vnode() \endlink and
  \link file_system_module_info::put_vnode put_vnode() \endlink:
  Create respectively destroy the FS's private node handle when
  the VFS creates/deletes the vnode for a particular node.

- \link file_system_module_info::read_stat read_stat() \endlink:
  Return a <tt>struct stat</tt> info for the given node, consisting of the
  type and size of the node, its owner and access permissions, as well as
  certain access times.

- \link file_system_module_info::open open() \endlink,
  \link file_system_module_info::close close() \endlink, and
  \link file_system_module_info::free_cookie free_cookie() \endlink:
  Open and close a node as explained in \ref concepts.

- \link file_system_module_info::read read() \endlink:
  Read data from an opened node (file). Even if the FS does not feature files,
  the hook has to be present anyway; it should return an error in this case.

- \link file_system_module_info::open_dir open_dir() \endlink,
  \link file_system_module_info::close_dir close_dir() \endlink, and
  \link file_system_module_info::free_dir_cookie free_dir_cookie() \endlink:
  Open and close a directory for entry iteration as explained in \ref concepts.

- \link file_system_module_info::read_dir read_dir() \endlink and
  \link file_system_module_info::rewind_dir rewind_dir() \endlink:
  Read the next entry/entries from a directory, respectively reset the iterator
  to the first entry, as explained in \ref concepts.

Although not strictly mandatory, a FS should additionally implement the
following hooks:

- \link file_system_module_info::read_fs_info read_fs_info() \endlink:
  Return general information about the volume, e.g. total and free size, and
  what special features (attributes, MIME types, queries) the volume/FS
  supports.

- \link file_system_module_info::read_symlink read_symlink() \endlink:
  Read the value of a symbolic link. Needed only, if the FS and volume support
  symbolic links at all. If absent symbolic links stored on the volume won't
  be interpreted.

- \link file_system_module_info::access access() \endlink:
  Return whether the current user has the given access permissions for a node.
  If the hook is absent the user is considerd to have all permissions.
*/