machine: ssh: Open a single SSH session per machine.

Previously, any call to 'managed-host-remote-eval' and similar would
open a new SSH session to the host.  With this change, an SSH session is
opened once, cached, and then reused by all subsequent calls to
'machine-ssh-session'.

* gnu/machine/ssh.scm (<machine-ssh-configuration>): Add
'this-machine-ssh-configuration'.
[session]: Mark as thunked and change default value to an
'open-machine-ssh-session*' call.
(open-machine-ssh-session, open-machine-ssh-session*): New procedures.
(machine-ssh-session): Replace inline code by call to
'open-machine-ssh-session'.

1 files changed, 29 insertions, 15 deletions

diff --git a/gnu/machine/ssh.scm b/gnu/machine/ssh.scm
index ecd02e336c1..22688f46f4b 100644
--- a/gnu/machine/ssh.scm
+++ b/gnu/machine/ssh.scm
@@ -1,6 +1,6 @@
 ;;; GNU Guix --- Functional package management for GNU
 ;;; Copyright © 2019 Jakob L. Kreuze <zerodaysfordays@sdf.org>
-;;; Copyright © 2020, 2021 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2020-2022 Ludovic Courtès <ludo@gnu.org>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -26,6 +26,7 @@
   #:use-module (gnu system uuid)
   #:use-module ((gnu services) #:select (sexp->system-provenance))
   #:use-module (guix diagnostics)
+  #:use-module (guix memoization)
   #:use-module (guix gexp)
   #:use-module (guix i18n)
   #:use-module (guix modules)
@@ -83,6 +84,7 @@
 (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? ; boolean
@@ -98,29 +100,41 @@
   (identity       machine-ssh-configuration-identity       ; path to a private key
                   (default #f))
   (session        machine-ssh-configuration-session        ; session
-                  (default #f))
+                  (thunked)
+                  (default
+                    ;; By default, open the session once and cache it.
+                    (open-machine-ssh-session* this-machine-ssh-configuration)))
   (host-key       machine-ssh-configuration-host-key       ; #f | string
                   (default #f)))
 
+(define (open-machine-ssh-session config)
+  "Open an SSH session for CONFIG, a <machine-ssh-configuration> record."
+  (let ((host-name (machine-ssh-configuration-host-name config))
+        (user (machine-ssh-configuration-user config))
+        (port (machine-ssh-configuration-port config))
+        (identity (machine-ssh-configuration-identity config))
+        (host-key (machine-ssh-configuration-host-key config)))
+    (unless host-key
+      (warning (G_ "<machine-ssh-configuration> without a 'host-key' \
+is deprecated~%")))
+    (open-ssh-session host-name
+                      #:user user
+                      #:port port
+                      #:identity identity
+                      #:host-key host-key)))
+
+(define open-machine-ssh-session*
+  (mlambdaq (config)
+    "Memoizing variant of 'open-machine-ssh-session'."
+    (open-machine-ssh-session config)))
+
 (define (machine-ssh-session machine)
   "Return the SSH session that was given in MACHINE's configuration, or create
 one from the configuration's parameters if one was not provided."
   (maybe-raise-unsupported-configuration-error machine)
   (let ((config (machine-configuration machine)))
     (or (machine-ssh-configuration-session config)
-        (let ((host-name (machine-ssh-configuration-host-name config))
-              (user (machine-ssh-configuration-user config))
-              (port (machine-ssh-configuration-port config))
-              (identity (machine-ssh-configuration-identity config))
-              (host-key (machine-ssh-configuration-host-key config)))
-          (unless host-key
-            (warning (G_ "<machine-ssh-configuration> without a 'host-key' \
-is deprecated~%")))
-          (open-ssh-session host-name
-                            #:user user
-                            #:port port
-                            #:identity identity
-                            #:host-key host-key)))))
+        (open-machine-ssh-session config))))
 
 
 ;;;