machine: Allow non-root users to deploy.

* doc/guix.texi (Invoking guix deploy): Add section describing
prerequisites for deploying as a non-root user.
* guix/remote.scm (remote-pipe-for-gexp): New optional 'become-command'
argument.
(%remote-eval): New optional 'become-command' argument.
(remote-eval): New 'become-command' keyword argument.
* guix/ssh.scm (remote-inferior): New optional 'become-command'
argument.
(inferior-remote-eval): New optional 'become-command' argument.
(remote-authorize-signing-key): New optional 'become-command' argument.
* gnu/machine/ssh.scm (machine-become-command): New variable.
(managed-host-remote-eval): Invoke 'remote-eval' with the
'#:become-command' keyword.
(deploy-managed-host): Invoke 'remote-authorize-signing-key' with the
'#:become-command' keyword.

doc/guix.texi

@@ -25514,6 +25514,7 @@ evaluates to.  As an example, @var{file} might contain a definition like this:
        (environment managed-host-environment-type)
        (configuration (machine-ssh-configuration
                        (host-name "localhost")
+                       (user "alice")
                        (identity "./id_rsa")
                        (port 2222)))))
 @end example
@@ -25546,6 +25547,15 @@ accepts store items it receives from the coordinator:
 # guix archive --authorize < coordinator-public-key.txt
 @end example
 
+@code{user}, in this example, specifies the name of the user account to log in
+as to perform the deployment.  Its default value is @code{root}, but root
+login over SSH may be forbidden in some cases.  To work around this,
+@command{guix deploy} can log in as an unprivileged user and employ
+@code{sudo} to escalate privileges.  This will only work if @code{sudo} is
+currently installed on the remote and can be invoked non-interactively as
+@code{user}.  That is: the line in @code{sudoers} granting @code{user} the
+ability to use @code{sudo} must contain the @code{NOPASSWD} tag.
+
 @deftp {Data Type} machine
 This is the data type representing a single machine in a heterogeneous Guix
 deployment.

gnu/machine/ssh.scm

@@ -101,6 +101,14 @@ one from the configuration's parameters if one was not provided."
 ;;; Remote evaluation.
 ;;;
 
+(define (machine-become-command machine)
+  "Return as a list of strings the program and arguments necessary to run a
+shell command with escalated privileges for MACHINE's configuration."
+  (if (string= "root" (machine-ssh-configuration-user
+                       (machine-configuration machine)))
+      '()
+      '("/run/setuid-programs/sudo" "-n" "--")))
+
 (define (managed-host-remote-eval machine exp)
   "Internal implementation of 'machine-remote-eval' for MACHINE instances with
 an environment type of 'managed-host."

guix/remote.scm

@@ -27,6 +27,8 @@
   #:use-module (guix utils)
   #:use-module (ssh popen)
   #:use-module (srfi srfi-1)
+  #:use-module (srfi srfi-34)
+  #:use-module (srfi srfi-35)
   #:use-module (ice-9 match)
   #:export (remote-eval))
 
@@ -41,29 +43,41 @@
 ;;;
 ;;; Code:
 
-(define (remote-pipe-for-gexp lowered session)
+(define* (remote-pipe-for-gexp lowered session #:optional become-command)
-  "Return a remote pipe for the given SESSION to evaluate LOWERED."
+  "Return a remote pipe for the given SESSION to evaluate LOWERED.  If
+BECOME-COMMAND is given, use that to invoke the remote Guile REPL."
   (define shell-quote
     (compose object->string object->string))
 
-  (apply open-remote-pipe* session OPEN_READ
+  (define repl-command
-         (string-append (derivation-input-output-path
+    (append (or become-command '())
-                         (lowered-gexp-guile lowered))
+            (list
-                        "/bin/guile")
+             (string-append (derivation-input-output-path
-         "--no-auto-compile"
+                             (lowered-gexp-guile lowered))
-         (append (append-map (lambda (directory)
+                            "/bin/guile")
-                               `("-L" ,directory))
+             "--no-auto-compile")
-                             (lowered-gexp-load-path lowered))
+            (append-map (lambda (directory)
-                 (append-map (lambda (directory)
+                          `("-L" ,directory))
-                               `("-C" ,directory))
+                        (lowered-gexp-load-path lowered))
-                             (lowered-gexp-load-path lowered))
+            (append-map (lambda (directory)
-                 `("-c"
+                          `("-C" ,directory))
-                   ,(shell-quote (lowered-gexp-sexp lowered))))))
+                        (lowered-gexp-load-path lowered))
+            `("-c"
+              ,(shell-quote (lowered-gexp-sexp lowered)))))
 
-(define (%remote-eval lowered session)
+  (let ((pipe (apply open-remote-pipe* session OPEN_READ repl-command)))
+    (when (eof-object? (peek-char pipe))
+      (raise (condition
+              (&message
+               (message (format #f (G_ "failed to run '~{~a~^ ~}'")
+                                repl-command))))))
+    pipe))
+
+(define* (%remote-eval lowered session #:optional become-command)
   "Evaluate LOWERED, a lowered gexp, in SESSION.  This assumes that all the
-prerequisites of EXP are already available on the host at SESSION."
+prerequisites of EXP are already available on the host at SESSION.  If
-  (let* ((pipe   (remote-pipe-for-gexp lowered session))
+BECOME-COMMAND is given, use that to invoke the remote Guile REPL."
+  (let* ((pipe   (remote-pipe-for-gexp lowered session become-command))
          (result (read-repl-response pipe)))
     (close-port pipe)
     result))
@@ -92,7 +106,8 @@ result to the current output port using the (guix repl) protocol."
                       (build-locally? #t)
                       (system (%current-system))
                       (module-path %load-path)
-                      (socket-name "/var/guix/daemon-socket/socket"))
+                      (socket-name "/var/guix/daemon-socket/socket")
+                      (become-command #f))
   "Evaluate EXP, a gexp, on the host at SESSION, an SSH session.  Ensure that
 all the elements EXP refers to are built and deployed to SESSION beforehand.
 When BUILD-LOCALLY? is true, said dependencies are built locally and sent to
@@ -119,7 +134,7 @@ remote store."
             (built-derivations inputs)
             ((store-lift send-files) to-send remote #:recursive? #t)
             (return (close-connection remote))
-            (return (%remote-eval lowered session))))
+            (return (%remote-eval lowered session become-command))))
         (let ((to-send (append (map (compose derivation-file-name
                                              derivation-input-derivation)
                                     inputs)
@@ -128,4 +143,4 @@ remote store."
             ((store-lift send-files) to-send remote #:recursive? #t)
             (return (build-derivations remote inputs))
             (return (close-connection remote))
-            (return (%remote-eval lowered session)))))))
+            (return (%remote-eval lowered session become-command)))))))

guix/ssh.scm

@@ -98,16 +98,27 @@ specifies; otherwise use them.  Throw an error on failure."
                 (message (format #f (G_ "SSH connection to '~a' failed: ~a~%")
                                  host (get-error session))))))))))
 
-(define (remote-inferior session)
+(define* (remote-inferior session #:optional become-command)
-  "Return a remote inferior for the given SESSION."
+  "Return a remote inferior for the given SESSION.  If BECOME-COMMAND is
-  (let ((pipe (open-remote-pipe* session OPEN_BOTH
+given, use that to invoke the remote Guile REPL."
-                                 "guix" "repl" "-t" "machine")))
+  (let* ((repl-command (append (or become-command '())
+                               '("guix" "repl" "-t" "machine")))
+         (pipe (apply open-remote-pipe* session OPEN_BOTH repl-command)))
+    ;; XXX: 'channel-get-exit-status' would be better here, but hangs if the
+    ;; process does succeed. This doesn't reflect the documentation, so it's
+    ;; possible that it's a bug in guile-ssh.
+    (when (eof-object? (peek-char pipe))
+      (raise (condition
+              (&message
+               (message (format #f (G_ "failed to run '~{~a~^ ~}'")
+                                repl-command))))))
     (port->inferior pipe)))
 
-(define (inferior-remote-eval exp session)
+(define* (inferior-remote-eval exp session #:optional become-command)
   "Evaluate EXP in a new inferior running in SESSION, and close the inferior
-right away."
+right away.  If BECOME-COMMAND is given, use that to invoke the remote Guile
-  (let ((inferior (remote-inferior session)))
+REPL."
+  (let ((inferior (remote-inferior session become-command)))
     (dynamic-wind
       (const #t)
       (lambda ()