python-apsw-doc 3.7.6.3-r1-1build1 / usr / share / doc / python-apsw / html / _sources / vfs.txt

.. Automatically generated by code2rst.py
   code2rst.py src/vfs.c doc/vfs.rst
   Edit src/vfs.c not this file!

.. currentmodule:: apsw

.. _vfs:

Virtual File System (VFS)
*************************

SQLite 3.6 has new `VFS functionality
<http://sqlite.org/c3ref/vfs.html>`_ which defines the interface
between the SQLite core and the underlying operating system. The
majority of the functionality deals with files. APSW exposes this
functionality letting you provide your own routines. You can also
*inherit* from an existing vfs making it easy to augment or override
specific routines. For example you could obfuscate your database by
XORing the data implemented by augmenting the read and write
methods. The method names are exactly the same as SQLite uses making
it easier to read the SQLite documentation, trouble tickets, web
searches or mailing lists. The SQLite convention results in names like
xAccess, xCurrentTime and xWrite.

You specify which VFS to use as a parameter to the :class:`Connection`
constructor.

.. code-block:: python

  db=apsw.Connection("file", vfs="myvfs")

The easiest way to get started is to make a :class:`VFS` derived class
that inherits from the default vfs.  Then override methods you want to
change behaviour of.  If you want to just change how file operations
are done then you have to override :meth:`VFS.xOpen` to return a file
instance that has your overridden :class:`VFSFile` methods.  The
:ref:`example <example-vfs>` demonstrates obfuscating the database
file contents.

.. note::

  All strings supplied and returned to :class:`VFS`/:class:`VFSFile`
  routines are treated as Unicode.

Exceptions and errors
=====================

To return an error from any routine you should raise an exception. The
exception will be translated into the appropriate SQLite error code
for SQLite. To return a specific SQLite error code use
:meth:`exceptionfor`.  If the exception does not map to any specific
error code then :const:`SQLITE_ERROR` which corresponds to
:exc:`SQLError` is returned to SQLite.

The SQLite code that deals with VFS errors behaves in varying
ways. Some routines have no way to return an error (eg `xDlOpen
<http://www.sqlite.org/c3ref/vfs.html>`_ just returns zero/NULL on
being unable to load a library, `xSleep
<http://www.sqlite.org/c3ref/vfs.html>`_ has no error return
parameter), others have any error values ignored (eg
:cvstrac:`xCurrentTime <3394>`), others are unified (eg almost any
error in xWrite will be returned to the user as disk full
error). Sometimes errors are ignored as they are harmless such as when
a journal can't be deleted after a commit (the journal is marked as
obsolete before being deleted).  Simple operations such as opening a
database can result in many different VFS function calls such as hot
journals being detected, locking, and read/writes for
playback/rollback.

To avoid confusion with exceptions being raised in the VFS and
exceptions from normal code to open Connections or execute SQL
queries, VFS exceptions are not raised in the normal way. (If they
were, only one could be raised and it would obscure whatever
exceptions the :class:`Connection` open or SQL query execute wanted to
raise.)  Instead the :meth:`VFS.excepthook` or
:meth:`VFSFile.excepthook` method is called with a tuple of exception
type, exception value and exception traceback. The default
implementation of ``excepthook`` calls ``sys.excepthook()`` which
under Python 2 shows the stack trace and under Python 3 merely prints
the exception value. (If ``sys.excepthook`` fails then
``PyErr_Display()`` is called.)

In normal VFS usage there will be no exceptions raised, or specific
expected ones which APSW clears after noting them and returning the
appropriate value back to SQLite. The exception hooking behaviour
helps you find issues in your code or unexpected behaviour of the
external environment. Remember that :ref:`augmented stack traces
<augmentedstacktraces>` are available which significantly increase
detail about the exceptions.

As an example, lets say you have a divide by zero error in your xWrite
routine. The table below shows what happens with time going down and
across.

+----------------------------------------------+--------------------------------+---------------------------------------------+
| Python Query Code                            | SQLite and APSW C code         | Python VFS code                             |
+==============================================+================================+=============================================+
| ``cursor.execute("update table set foo=3")`` |                                |                                             |
+----------------------------------------------+--------------------------------+---------------------------------------------+
|                                              | SQLite starts executing query  |                                             |
+----------------------------------------------+--------------------------------+---------------------------------------------+
|                                              |                                | Your VFS routines are called                |
+----------------------------------------------+--------------------------------+---------------------------------------------+
|                                              |                                | Your xWrite divides by zero                 |
+----------------------------------------------+--------------------------------+---------------------------------------------+
|                                              |                                | :meth:`VFSFile.excepthook` is called with   |
|                                              |                                | ZeroDivision exception                      |
+----------------------------------------------+--------------------------------+---------------------------------------------+
|                                              | :const:`SQLITE_ERROR` (closest |                                             |
|                                              | matching SQLite error code) is |                                             |
|                                              | returned to SQLite by APSW     |                                             |
+----------------------------------------------+--------------------------------+---------------------------------------------+
|                                              | SQLite error handling and      | More VFS routines are called.  Any          |
|                                              | recovery operates which calls  | exceptions in these routines will result in |
|                                              | more VFS routines.             | :meth:`VFSFile.excepthook` being called with|
|                                              |                                | them.                                       |
+----------------------------------------------+--------------------------------+---------------------------------------------+
|                                              | SQLite returns                 |                                             |
|                                              | :const:`SQLITE_FULL` to APSW   |                                             |
+----------------------------------------------+--------------------------------+---------------------------------------------+
| APSW returns :class:`apsw.FullError`         |                                |                                             |
+----------------------------------------------+--------------------------------+---------------------------------------------+

VFS class
=========

.. index:: sqlite3_vfs_register, sqlite3_vfs_find

.. class:: VFS(name[, base=None, makedefault=False, maxpathname=1024])

    Provides operating system access.  You can get an overview in the
    `SQLite documentation <http://sqlite.org/c3ref/vfs.html>`_.  To
    create a VFS your Python class must inherit from :class:`VFS`.

    :param name: The name to register this vfs under.  If the name
                 already exists then this vfs will replace the prior one of the
                 same name.  Use :meth:`apsw.vfsnames` to get a list of
                 registered vfs names.
    :param base: If you would like to inherit behaviour from an
                 already registered vfs then give their name.  To inherit from the
                 default vfs, use a zero length string ``""`` as the name.
    :param makedefault: If true then this vfs will be registered as
      the default, and will be used by any opens that don't specify a
      vfs.
    :param maxpathname: The maximum length of database name in bytes
      when represented in UTF-8.  If a pathname is passed in longer than
      this value then SQLite will not :cvstrac:`be able to open it <3373>`.

    :raises ValueError: If *base* is not :const:`None` and the named vfs is not
      currently registered.

    Calls:
      * :sqliteapi:`sqlite3_vfs_register <vfs_find>`
      * :sqliteapi:`sqlite3_vfs_find <vfs_find>`

.. method:: VFS.excepthook(etype, evalue, etraceback)

    Called when there has been an exception in a :class:`VFS` routine.
    The default implementation calls ``sys.excepthook`` and if that
    fails then ``PyErr_Display``.  The three arguments correspond to
    what ``sys.exc_info()`` would return.

    :param etype: The exception type
    :param evalue: The exception  value
    :param etraceback: The exception traceback.  Note this
      includes all frames all the way up to the thread being started.

.. index:: sqlite3_vfs_unregister

.. method:: VFS.unregister()

   Unregisters the VFS making it unavailable to future database
   opens. You do not need to call this as the VFS is automatically
   unregistered by when the VFS has no more references or open
   datatabases using it. It is however useful to call if you have made
   your VFS be the default and wish to immediately make it be
   unavailable. It is safe to call this routine multiple times.

   Calls: :sqliteapi:`sqlite3_vfs_unregister <vfs_find>`

.. method:: VFS.xAccess(pathname, flags) -> bool

    SQLite wants to check access permissions.  Return True or False
    accordingly.

    :param pathname: File or directory to check
    :param flags: One of the `access flags <http://sqlite.org/c3ref/c_access_exists.html>`_

.. method:: VFS.xCurrentTime()  -> float

  Return the `Julian Day Number
  <http://en.wikipedia.org/wiki/Julian_day>`_ as a floating point
  number where the integer portion is the day and the fractional part
  is the time. Do not adjust for timezone (ie use `UTC
  <http://en.wikipedia.org/wiki/Universal_Time>`_). Although SQLite
  allows for an error return, that is :cvstrac:`ignored <3394>`.

.. method:: VFS.xDelete(filename, syncdir)

    Delete the named file.

    :param filename: File to delete

    :param syncdir: If True then the directory should be synced
      ensuring that the file deletion has been recorded on the disk
      platters.  ie if there was an immediate power failure after this
      call returns, on a reboot the file would still be deleted.

.. method:: VFS.xDlClose(handle)

    Close and unload the library corresponding to the handle you
    returned from :meth:`~VFS.xDlOpen`.  You can use ctypes to do
    this::

      def xDlClose(handle):
         # Note leading underscore in _ctypes
         _ctypes.dlclose(handle)       # Linux/Mac/Unix
         _ctypes.FreeLibrary(handle)   # Windows

.. method:: VFS.xDlError() -> string

    Return an error string describing the last error of
    :meth:`~VFS.xDlOpen` or :meth:`~VFS.xDlSym` (ie they returned
    zero/NULL). If you do not supply this routine then SQLite provides
    a generic message. To implement this method, catch exceptions in
    :meth:`~VFS.xDlOpen` or :meth:`~VFS.xDlSym`, turn them into
    strings, save them, and return them in this routine. Note that the
    message may be truncated to 255 characters - see
    :cvstrac:`3305`. If you have an error in this routine or return
    None then SQLite's generic message will be used.

.. method:: VFS.xDlOpen(filename) -> number

   Load the shared library. You should return a number which will be
   treated as a void pointer at the C level. On error you should
   return 0 (NULL). The number is passed as is to
   :meth:`~VFS.xDlSym`/:meth:`~VFS.xDlClose` so it can represent
   anything that is convenient for you (eg an index into an
   array). You can use ctypes to load a library::

     def xDlOpen(name):
        return ctypes.cdll.LoadLibrary(name)._handle

.. method:: VFS.xDlSym(handle, symbol) -> address

    Returns the address of the named symbol which will be called by
    SQLite. On error you should return 0 (NULL). You can use ctypes::

      def xDlSym(ptr, name):
         return _ctypes.dlsym (ptr, name)  # Linux/Unix/Mac etc (note leading underscore)
         return ctypes.win32.kernel32.GetProcAddress (ptr, name)  # Windows

    :param handle: The value returned from an earlier :meth:`~VFS.xDlOpen` call
    :param symbol: A string
    :rtype: An int/long with the symbol address

.. method:: VFS.xFullPathname(name) -> string

  Return the absolute pathname for name.  You can use ``os.path.abspath`` to do this.

.. method:: VFS.xGetLastError() -> string

   This method is to return text describing the last error that
   happened in this thread. If not implemented SQLite's more generic
   message is used. However the method is :cvstrac:`never called
   <3337>` by SQLite.

.. method:: VFS.xGetSystemCall(name) -> int

    Returns a pointer for the current method implementing the named
    system call.  Return None if the call does not exist.

.. method:: VFS.xNextSystemCall(name) -> String or None

    This method is repeatedly called to iterate over all of the system
    calls in the vfs.  When called with None you should return the
    name of the first system call.  In subsequent calls return the
    name after the one passed in.  If name is the last system call
    then return None.

    .. note::

      Because of internal SQLite implementation semantics memory will
      be leaked on each call to this function.  Consequently you
      should build up the list of call names once rather than
      repeatedly doing it.

.. method:: VFS.xOpen(name, flags) -> VFSFile or similar object

    This method should return a new file object based on name.  You
    can return a :class:`VFSFile` from a completely different VFS.

    :param name: File to open.  Note that *name* may be :const:`None` in which
        case you should open a temporary file with a name of your
        choosing.

    :param flags: A list of two integers ``[inputflags,
      outputflags]``.  Each integer is one or more of the `open flags
      <http://www.sqlite.org/c3ref/c_open_create.html>`_ binary orred
      together.  The ``inputflags`` tells you what SQLite wants.  For
      example :const:`SQLITE_OPEN_DELETEONCLOSE` means the file should
      be automatically deleted when closed.  The ``outputflags``
      describes how you actually did open the file.  For example if you
      opened it read only then :const:`SQLITE_OPEN_READONLY` should be
      set.

.. method:: VFS.xRandomness(numbytes) -> bytes

  This method is called once when SQLite needs to seed the random
  number generator. It is called on the default VFS only. It is not
  called again, even across :meth:`apsw.shutdown` calls.  You can
  return less than the number of bytes requested including None. If
  you return more then the surplus is ignored.

  :rtype: (Python 2) string, buffer (Python 3) bytes, buffer

.. method:: VFS.xSetSystemCall(name, pointer) -> bool

    Change a system call used by the VFS.  This is useful for testing
    and some other scenarios such as sandboxing.

    :param name: The string name of the system call

    :param pointer: A pointer provided as an int/long.  There is no
      reference counting or other memory tracking of the pointer.  If
      you provide one you need to ensure it is around for the lifetime
      of this and any other related VFS.

    Raise an exception to return an error.  If the system call does
    not exist then raise :exc:`NotFoundError`.

    :returns: True if the system call was set.  False if the system
      call is not known.

.. method:: VFS.xSleep(microseconds) -> integer

    Pause exection of the thread for at least the specified number of
    microseconds (millionths of a second).  This routine is typically called from the busy handler.

    :returns: How many microseconds you actually requested the
      operating system to sleep for. For example if your operating
      system sleep call only takes seconds then you would have to have
      rounded the microseconds number up to the nearest second and
      should return that rounded up value.

VFSFile class
=============

.. class:: VFSFile(vfs, name, flags)

    Wraps access to a file.  You only need to derive from this class
    if you want the file object returned from :meth:`VFS.xOpen` to
    inherit from an existing VFS implementation.

    .. note::

       All file sizes and offsets are 64 bit quantities even on 32 bit
       operating systems.

    :param vfs: The vfs you want to inherit behaviour from.  You can
       use an empty string ``""`` to inherit from the default vfs.
    :param name: The name of the file being opened.
    :param flags: A two list ``[inflags, outflags]`` as detailed in :meth:`VFS.xOpen`.

    :raises ValueError: If the named VFS is not registered.

    .. note::

      If the VFS that you inherit from supports :ref:`write ahead
      logging <wal>` then your :class:`VFSFile` will also support the
      xShm methods necessary to implement wal.

    .. seealso::

      :meth:`VFS.xOpen`

.. method:: VFSFile.excepthook(etype, evalue, etraceback)

    Called when there has been an exception in a :class:`VFSFile`
    routine.  The default implementation calls ``sys.excepthook`` and
    if that fails then ``PyErr_Display``.  The three arguments
    correspond to what ``sys.exc_info()`` would return.

    :param etype: The exception type
    :param evalue: The exception  value
    :param etraceback: The exception traceback.  Note this
      includes all frames all the way up to the thread being started.

.. method:: VFSFile.xCheckReservedLock()

  Returns True if any database connection (in this or another process)
  has a lock other than `SQLITE_LOCK_NONE or SQLITE_LOCK_SHARED
  <http://sqlite.org/c3ref/c_lock_exclusive.html>`_.

.. method:: VFSFile.xClose()

  Close the database. Note that even if you return an error you should
  still close the file.  It is safe to call this method mutliple
  times.

.. method:: VFSFile.xDeviceCharacteristics() -> int

  Return `I/O capabilities
  <http://sqlite.org/c3ref/c_iocap_atomic.html>`_ (bitwise or of
  appropriate values). If you do not implement the function or have an
  error then 0 (the SQLite default) is returned.

.. method:: VFSFile.xFileControl(op, ptr) -> bool

   Receives `file control
   <http://sqlite.org/c3ref/file_control.html>`_ request typically
   issued by :meth:`Connection.filecontrol`.  See
   :meth:`Connection.filecontrol` for an example of how to pass a
   Python object to this routine.

   :param op: A numeric code.  Codes below 100 are reserved for SQLite
     internal use.
   :param ptr: An integer corresponding to a pointer at the C level.

   :returns: A boolean indicating if the op was understood

   As of SQLite 3.6.10, this method is called by SQLite if you have
   inherited from an underlying VFSFile.  Consequently ensure you pass
   any unrecognised codes through to your super class.  For example::

            def xFileControl(self, op, ptr):
                if op==1027:
                    process_quick(ptr)
                elif op==1028:
                    obj=ctypes.py_object.from_address(ptr).value
                else:
                    # this ensures superclass implementation is called
                    return super(MyFile, self).xFileControl(op, ptr)
		# we understood the op
	        return True

.. method:: VFSFile.xFileSize() -> int

  Return the size of the file in bytes.  Remember that file sizes are
  64 bit quantities even on 32 bit operating systems.

.. method:: VFSFile.xLock(level)

  Increase the lock to the level specified which is one of the
  `SQLITE_LOCK <http://sqlite.org/c3ref/c_lock_exclusive.html>`_
  family of constants. If you can't increase the lock level because
  someone else has locked it, then raise :exc:`BusyError`.

.. method:: VFSFile.xRead(amount, offset) -> bytes

    Read the specified *amount* of data starting at *offset*. You
    should make every effort to read all the data requested, or return
    an error. If you have the file open for non-blocking I/O or if
    signals happen then it is possible for the underlying operating
    system to do a partial read. You will need to request the
    remaining data. Except for empty files SQLite considers short
    reads to be a fatal error.

    :param amount: Number of bytes to read
    :param offset: Where to start reading. This number may be 64 bit once the database is larger than 2GB.

    :rtype: (Python 2) string, buffer.  (Python 3) bytes, buffer

.. method:: VFSFile.xSectorSize() -> int

    Return the native underlying sector size. SQLite uses the value
    returned in determining the default database page size. If you do
    not implement the function or have an error then 512 (the SQLite
    default) is returned.

.. method:: VFSFile.xSync(flags)

  Ensure data is on the disk platters (ie could survive a power
  failure immediately after the call returns) with the `sync flags
  <http://sqlite.org/c3ref/c_sync_dataonly.html>`_ detailing what
  needs to be synced.  You can sync more than what is requested.

.. method:: VFSFile.xTruncate(newsize)

  Set the file length to *newsize* (which may be more or less than the
  current length).

.. method:: VFSFile.xUnlock(level)

    Decrease the lock to the level specified which is one of the
    `SQLITE_LOCK <http://sqlite.org/c3ref/c_lock_exclusive.html>`_
    family of constants.

.. method:: VFSFile.xWrite(data, offset)

  Write the *data* starting at absolute *offset*. You must write all the data
  requested, or return an error. If you have the file open for
  non-blocking I/O or if signals happen then it is possible for the
  underlying operating system to do a partial write. You will need to
  write the remaining data.

  :param offset: Where to start writing. This number may be 64 bit once the database is larger than 2GB.
  :param data: (Python 2) string, (Python 3) bytes