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.