gnu: Add machine type for deployment specifications.

* gnu/machine.scm: New file.
* gnu/machine/ssh.scm: New file.
* gnu/local.mk (GNU_SYSTEM_MODULES): Add it.

gnu/machine.scm

@@ -0,0 +1,107 @@
+;;; GNU Guix --- Functional package management for GNU
+;;; Copyright © 2019 David Thompson <davet@gnu.org>
+;;; Copyright © 2019 Jakob L. Kreuze <zerodaysfordays@sdf.lonestar.org>
+;;;
+;;; This file is part of GNU Guix.
+;;;
+;;; GNU Guix is free software; you can redistribute it and/or modify it
+;;; under the terms of the GNU General Public License as published by
+;;; the Free Software Foundation; either version 3 of the License, or (at
+;;; your option) any later version.
+;;;
+;;; GNU Guix is distributed in the hope that it will be useful, but
+;;; WITHOUT ANY WARRANTY; without even the implied warranty of
+;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;;; GNU General Public License for more details.
+;;;
+;;; You should have received a copy of the GNU General Public License
+;;; along with GNU Guix.  If not, see <http://www.gnu.org/licenses/>.
+
+(define-module (gnu machine)
+  #:use-module (gnu system)
+  #:use-module (guix derivations)
+  #:use-module (guix monads)
+  #:use-module (guix records)
+  #:use-module (guix store)
+  #:use-module ((guix utils) #:select (source-properties->location))
+  #:export (environment-type
+            environment-type?
+            environment-type-name
+            environment-type-description
+            environment-type-location
+
+            machine
+            machine?
+            this-machine
+
+            machine-system
+            machine-environment
+            machine-configuration
+            machine-display-name
+
+            deploy-machine
+            machine-remote-eval))
+
+;;; Commentary:
+;;;
+;;; This module provides the types used to declare individual machines in a
+;;; heterogeneous Guix deployment. The interface allows users of specify system
+;;; configurations and the means by which resources should be provisioned on a
+;;; per-host basis.
+;;;
+;;; Code:
+
+
+;;;
+;;; Declarations for resources that can be provisioned.
+;;;
+
+(define-record-type* <environment-type> environment-type
+  make-environment-type
+  environment-type?
+
+  ;; Interface to the environment type's deployment code. Each procedure
+  ;; should take the same arguments as the top-level procedure of this file
+  ;; that shares the same name. For example, 'machine-remote-eval' should be
+  ;; of the form '(machine-remote-eval machine exp)'.
+  (machine-remote-eval environment-type-machine-remote-eval) ; procedure
+  (deploy-machine      environment-type-deploy-machine)      ; procedure
+
+  ;; Metadata.
+  (name        environment-type-name)       ; symbol
+  (description environment-type-description ; string
+               (default #f))
+  (location    environment-type-location    ; <location>
+               (default (and=> (current-source-location)
+                               source-properties->location))
+               (innate)))
+
+
+;;;
+;;; Declarations for machines in a deployment.
+;;;
+
+(define-record-type* <machine> machine
+  make-machine
+  machine?
+  this-machine
+  (system        machine-system)       ; <operating-system>
+  (environment   machine-environment)  ; symbol
+  (configuration machine-configuration ; configuration object
+                 (default #f)))        ; specific to environment
+
+(define (machine-display-name machine)
+  "Return the host-name identifying MACHINE."
+  (operating-system-host-name (machine-system machine)))
+
+(define (machine-remote-eval machine exp)
+  "Evaluate EXP, a gexp, on MACHINE. Ensure that all the elements EXP refers to
+are built and deployed to MACHINE beforehand."
+  (let ((environment (machine-environment machine)))
+    ((environment-type-machine-remote-eval environment) machine exp)))
+
+(define (deploy-machine machine)
+  "Monadic procedure transferring the new system's OS closure to the remote
+MACHINE, activating it on MACHINE and switching MACHINE to the new generation."
+  (let ((environment (machine-environment machine)))
+    ((environment-type-deploy-machine environment) machine)))