remote: Build derivations appropriate for the remote's

* gnu/machine/ssh.scm (machine-ssh-configuration): Add 'system' field.
(managed-host-remote-eval): Pass 'system' field to 'remote-eval'.
(machine-check-building-for-appropriate-system): New variable.
(check-deployment-sanity): Add call to
'machine-check-building-for-appropriate-system'.
* doc/guix.texi (Invoking guix deploy): Describe new 'system' field.
* guix/ssh.scm (remote-system): New variable.
* guix/remote.scm (remote-eval): Use result of 'remote-system' when
lowering the G-Expression.
(remote-eval): Add 'system' keyword argument.
(trampoline): Return a <program-file> rather than a <scheme-file>.

doc/guix.texi

@@ -25573,6 +25573,9 @@ with an @code{environment} of @code{managed-host-environment-type}.
 
 @table @asis
 @item @code{host-name}
+@item @code{system}
+The Nix system type describing the architecture of the machine being deployed
+to. This should look something like ``x86_64-linux''.
 @item @code{port} (default: @code{22})
 @item @code{user} (default: @code{"root"})
 @item @code{identity} (default: @code{#f})

gnu/machine/ssh.scm

@@ -36,6 +36,7 @@ (define-module (gnu machine ssh)
   #:use-module (ice-9 match)
   #:use-module (srfi srfi-19)
   #:use-module (srfi srfi-26)
+  #:use-module (srfi srfi-34)
   #:use-module (srfi srfi-35)
   #:export (managed-host-environment-type
 
@@ -68,6 +69,7 @@ (define-record-type* <machine-ssh-configuration> machine-ssh-configuration
   machine-ssh-configuration?
   this-machine-ssh-configuration
   (host-name      machine-ssh-configuration-host-name) ; string
+  (system         machine-ssh-configuration-system)    ; string
   (build-locally? machine-ssh-configuration-build-locally?
                   (default #t))
   (port           machine-ssh-configuration-port       ; integer
@@ -103,10 +105,12 @@ (define (managed-host-remote-eval machine exp)
   "Internal implementation of 'machine-remote-eval' for MACHINE instances with
 an environment type of 'managed-host."
   (maybe-raise-unsupported-configuration-error machine)
+  (let ((config (machine-configuration machine)))
     (remote-eval exp (machine-ssh-session machine)
                  #:build-locally?
-               (machine-ssh-configuration-build-locally?
-                (machine-configuration machine))))
+                 (machine-ssh-configuration-build-locally? config)
+                 #:system
+                 (machine-ssh-configuration-system config))))
 
 
 ;;;
@@ -240,10 +244,29 @@ (define dev
               device)
     (return #t)))
 
+(define (machine-check-building-for-appropriate-system machine)
+  "Raise a '&message' error condition if MACHINE is configured to be built
+locally and the 'system' field does not match the '%current-system' reported
+by MACHINE."
+  (let ((config (machine-configuration machine))
+        (system (remote-system (machine-ssh-session machine))))
+    (when (and (machine-ssh-configuration-build-locally? config)
+               (not (string= system (machine-ssh-configuration-system config))))
+      (raise (condition
+              (&message
+               (message (format #f (G_ "incorrect target system \
+('~a' was given, while the system reports that it is '~a')~%")
+                                (machine-ssh-configuration-system config)
+                                system)))))))
+  (with-monad %store-monad (return #t)))
+
 (define (check-deployment-sanity machine)
   "Raise a '&message' error condition if it is clear that deploying MACHINE's
 'system' declaration would fail."
+  ;; Order is important here -- an incorrect value for 'system' will cause
+  ;; invocations of 'remote-eval' to fail.
   (mbegin %store-monad
+    (machine-check-building-for-appropriate-system machine)
     (machine-check-file-system-availability machine)
     (machine-check-initrd-modules machine)))
 

guix/remote.scm

@@ -24,6 +24,7 @@ (define-module (guix remote)
   #:use-module (guix monads)
   #:use-module (guix modules)
   #:use-module (guix derivations)
+  #:use-module (guix utils)
   #:use-module (ssh popen)
   #:use-module (srfi srfi-1)
   #:use-module (ice-9 match)
@@ -71,7 +72,7 @@ (define (trampoline exp)
   "Return a \"trampoline\" gexp that evaluates EXP and writes the evaluation
 result to the current output port using the (guix repl) protocol."
   (define program
-    (scheme-file "remote-exp.scm" exp))
+    (program-file "remote-exp.scm" exp))
 
   (with-imported-modules (source-module-closure '((guix repl)))
     #~(begin
@@ -89,6 +90,7 @@ (define program
 (define* (remote-eval exp session
                       #:key
                       (build-locally? #t)
+                      (system (%current-system))
                       (module-path %load-path)
                       (socket-name "/var/guix/daemon-socket/socket"))
   "Evaluate EXP, a gexp, on the host at SESSION, an SSH session.  Ensure that
@@ -96,7 +98,9 @@ (define* (remote-eval exp session
 When BUILD-LOCALLY? is true, said dependencies are built locally and sent to
 the remote store afterwards; otherwise, dependencies are built directly on the
 remote store."
-  (mlet %store-monad ((lowered (lower-gexp (trampoline exp)
+  (mlet* %store-monad ((lowered (lower-gexp (trampoline exp)
+                                            #:system system
+                                            #:guile-for-build #f
                                             #:module-path %load-path))
                        (remote -> (connect-to-remote-daemon session
                                                             socket-name)))

guix/ssh.scm

@@ -39,6 +39,7 @@ (define-module (guix ssh)
             remote-inferior
             remote-daemon-channel
             connect-to-remote-daemon
+            remote-system
             send-files
             retrieve-files
             retrieve-files*
@@ -282,6 +283,12 @@ (define export
                              ,(object->string
                                (object->string export))))))
 
+(define (remote-system session)
+  "Return the system type as expected by Nix, usually ARCHITECTURE-KERNEL, of
+the machine on the other end of SESSION."
+  (inferior-remote-eval '(begin (use-modules (guix utils)) (%current-system))
+                        session))
+
 (define* (send-files local files remote
                      #:key
                      recursive?