Properties
==========

Names
-----

The names of properties may not end in the character ``.``, because that has a
special meaning in unevaluated property application specifications.

Properties with ``:APPLY`` subroutines occupy the function cells of symbols,
so except in the case of properties with no ``:APPLY`` subroutine, do not try
to define an ordinary function with the same name as a property.

Working directories
-------------------

Except where specified otherwise in property docstrings, relative paths are
relative to the remote home directory.  ``:LISP`` properties may assume they
will be executed in the remote home directory, and ``:POSIX`` properties may
assume that commands will be executed in the remote home directory, and that
relative paths passed to ``READ-REMOTE-FILE`` and ``WRITE-REMOTE-FILE`` are
relative to the remote home directory.  Use ``WITH-REMOTE-CURRENT-DIRECTORY``
to change the remote working directory in a way which ensures it will get
changed back.

Property subroutines
--------------------

A property is composed of up to five subroutines, which all have the same
lambda list (take the same arguments).  At least one of ``:hostattrs``,
``:apply`` or ``:unapply`` must be present.

``:desc`` subroutines
~~~~~~~~~~~~~~~~~~~~~

Pure function of the property's arguments which returns a description of
applying the property, to be used in stdout by deployments to inform the user
what work is being done.

``:preprocess`` subroutines
~~~~~~~~~~~~~~~~~~~~~~~~~~~

Pure function executed to modify the arguments that will be passed to the
other subroutines; should return a fresh list of the new arguments.  This
subroutine is called on each atomic property application within a property
application specification before the effects of property combinators have been
applied.  That is, it is effectively executed on atomic property applications
in isolation from the property application specifications in which they occur.

``:hostattrs`` subroutines
~~~~~~~~~~~~~~~~~~~~~~~~~~

Executed in the root Lisp to (i) add static informational attributes of hosts
to which this property is applied or is to be applied; and (ii) check that
applying this property makes sense -- e.g. that we're not trying to install a
package using apt(1) on a FreeBSD host.

Can retrieve existing static informational attributes using ``GET-HOSTATTRS``,
or things which wrap ``GET-HOSTATTRS``, such as ``GET-HOSTNAME``.  Should
signal the condition ``INCOMPATIBLE-PROPERTY`` if existing static
informational attributes indicate that the property should not be applied to
this host.  Can use ``PUSH-HOSTATTRS`` and ``REQUIRE-DATA`` to add new entries
to the host's static information atributes.

Other than as described in the previous paragraph, should be a pure function.
In particular, should not examine the actual state of the host.  Essentially a
conversion of the arguments to the property to appropriate static
informational attributes.

``:check`` subroutines
~~~~~~~~~~~~~~~~~~~~~~

Determine whether or not the property is already applied to the host and
return a generalised boolean indicating such.  Whether or not the ``:apply``
and ``:unapply`` subroutines get called depends on this return value.  If
absent, it is always assumed the property is unapplied, i.e., an attempt to
apply the property will always be made.

``:apply`` and ``:unapply`` subroutines
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Apply or unapply the property.  Should return ``:no-change`` if the property
was already applied; any other return value is interpreted as meaning that the
property was not (fully) applied before we ran, but now it is.  (If the
``:check`` function indicated that neither ``:apply`` nor ``:unapply`` should
be run, then this is equivalent to those subroutines returning ``:no-change``.)

The point of having both these return value semantics and the ``:check``
subroutine is that a property might only be able to check whether it made a
change after trying to apply itself -- it might check whether running a
command actually made a change to a particular file, for example.

Errors in attempting to apply a property are indicated by signalling a
``FAILED-CHANGE`` error condition.

``:posix`` vs. ``:lisp`` properties
-----------------------------------

``:posix`` properties should not make any assumptions about what localhost is
-- they may be running in the root Lisp, but they might be running in a Lisp
image running on an intermediary host, or even on the host to be configured.
They should perform I/O only by calling ``RUN``, ``RUNLINES``,
``READ-REMOTE-FILE``, ``WRITE-REMOTE-FILE``, requesting prerequisite data, and
applying or unapplying other ``:posix`` properties.  Otherwise, they should be
pure functions.

``:lisp`` properties, by contrast, may (and should) assume that they are
running in a Lisp image on the host to which they are to be applied, so they
can perform arbitrary I/O in that context.  They can also make use of ``RUN``,
``RUNLINES``, ``READ-REMOTE-FILE`` and ``WRITE-REMOTE-FILE`` if desired.

``:posix`` properties are characterised by the limited set of ways in which
they perform I/O, not by the use of only facilities defined in the Single UNIX
Specification.  Nevertheless, if a ``:posix`` property or function intended to
be called by ``:posix`` properties uses non-POSIX facilities, but it is not
obvious given the stated purpose of the property that it will do this, it is
good to mention the use of non-POSIX facilities in the docstring.  For
examples of this, see ``USER:HAS-LOGIN-SHELL`` and ``USER:PASSWD-FIELD``.
