machine: Automatically authorize the coordinator's signing key.

* guix/ssh.scm (remote-authorize-signing-key): New variable.
* gnu/machine/ssh.scm (deploy-managed-host): Authorize coordinator's
signing key before any invocations of 'remote-eval'.
(deploy-managed-host): Display an error if a signing key does not exist.
* doc/guix.texi (Invoking guix deploy): Remove section describing manual
signing key authorization.
(Invoking guix deploy): Add section describing the 'authorize?' field.

doc/guix.texi

@@ -25586,6 +25586,9 @@ with an @code{environment} of @code{managed-host-environment-type}.
 @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{authorize?} (default: @code{#t})
+If true, the coordinator's signing key will be added to the remote's ACL
+keyring.
 @item @code{port} (default: @code{22})
 @item @code{user} (default: @code{"root"})
 @item @code{identity} (default: @code{#f})

gnu/machine/ssh.scm

@@ -28,13 +28,16 @@ (define-module (gnu machine ssh)
   #:use-module (guix i18n)
   #:use-module (guix modules)
   #:use-module (guix monads)
+  #:use-module (guix pki)
   #:use-module (guix records)
   #:use-module (guix remote)
   #:use-module (guix scripts system reconfigure)
   #:use-module (guix ssh)
   #:use-module (guix store)
   #:use-module (guix utils)
+  #:use-module (gcrypt pk-crypto)
   #:use-module (ice-9 match)
+  #:use-module (ice-9 textual-ports)
   #:use-module (srfi srfi-1)
   #:use-module (srfi srfi-19)
   #:use-module (srfi srfi-26)
@@ -48,6 +51,7 @@ (define-module (gnu machine ssh)
 
             machine-ssh-configuration-host-name
             machine-ssh-configuration-build-locally?
+            machine-ssh-configuration-authorize?
             machine-ssh-configuration-port
             machine-ssh-configuration-user
             machine-ssh-configuration-session))
@@ -70,17 +74,19 @@ (define-record-type* <machine-ssh-configuration> machine-ssh-configuration
   make-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?
+  (host-name      machine-ssh-configuration-host-name)     ; string
+  (system         machine-ssh-configuration-system)        ; string
+  (build-locally? machine-ssh-configuration-build-locally? ; boolean
                   (default #t))
-  (port           machine-ssh-configuration-port       ; integer
+  (authorize?     machine-ssh-configuration-authorize?     ; boolean
+                  (default #t))
+  (port           machine-ssh-configuration-port           ; integer
                   (default 22))
-  (user           machine-ssh-configuration-user       ; string
+  (user           machine-ssh-configuration-user           ; string
                   (default "root"))
-  (identity       machine-ssh-configuration-identity   ; path to a private key
+  (identity       machine-ssh-configuration-identity       ; path to a private key
                   (default #f))
-  (session        machine-ssh-configuration-session    ; session
+  (session        machine-ssh-configuration-session        ; session
                   (default #f)))
 
 (define (machine-ssh-session machine)
@@ -359,6 +365,19 @@ (define (deploy-managed-host machine)
   "Internal implementation of 'deploy-machine' for MACHINE instances with an
 environment type of 'managed-host."
   (maybe-raise-unsupported-configuration-error machine)
+  (when (machine-ssh-configuration-authorize?
+         (machine-configuration machine))
+    (unless (file-exists? %public-key-file)
+      (raise (condition
+              (&message
+               (message (format #f (G_ "no signing key '~a'. \
+have you run 'guix archive --generate-key?'")
+                                %public-key-file))))))
+    (remote-authorize-signing-key (call-with-input-file %public-key-file
+                                    (lambda (port)
+                                      (string->canonical-sexp
+                                       (get-string-all port))))
+                                  (machine-ssh-session machine)))
   (mlet %store-monad ((_ (check-deployment-sanity machine))
                       (boot-parameters (machine-boot-parameters machine)))
     (let* ((os (machine-operating-system machine))

guix/ssh.scm

@@ -21,6 +21,7 @@ (define-module (guix ssh)
   #:use-module (guix inferior)
   #:use-module (guix i18n)
   #:use-module ((guix utils) #:select (&fix-hint))
+  #:use-module (gcrypt pk-crypto)
   #:use-module (ssh session)
   #:use-module (ssh auth)
   #:use-module (ssh key)
@@ -40,6 +41,7 @@ (define-module (guix ssh)
             remote-daemon-channel
             connect-to-remote-daemon
             remote-system
+            remote-authorize-signing-key
             send-files
             retrieve-files
             retrieve-files*
@@ -300,6 +302,27 @@ (define (remote-system session)
   (inferior-remote-eval '(begin (use-modules (guix utils)) (%current-system))
                         session))
 
+(define (remote-authorize-signing-key key session)
+  "Send KEY, a canonical sexp containing a public key, over SESSION and add it
+to the system ACL file if it has not yet been authorized."
+  (inferior-remote-eval
+   `(begin
+      (use-modules (guix build utils)
+                   (guix pki)
+                   (guix utils)
+                   (gcrypt pk-crypto)
+                   (srfi srfi-26))
+
+      (define acl (current-acl))
+      (define key (string->canonical-sexp ,(canonical-sexp->string key)))
+
+      (unless (authorized-key? key)
+        (let ((acl (public-keys->acl (cons key (acl->public-keys acl)))))
+          (mkdir-p (dirname %acl-file))
+          (with-atomic-file-output %acl-file
+            (cut write-acl acl <>)))))
+   session))
+
 (define* (send-files local files remote
                      #:key
                      recursive?