ssh: Speed up RPCs by using #:nodelay.

Partly fixes <https://bugs.gnu.org/41702>.

* guix/ssh.scm (open-ssh-session): Enable #:nodelay.
* m4/guix.m4 (GUIX_CHECK_GUILE_SSH): Add feature check for this new parameter.
* doc/guix.texi (Requirements): Adjust.

Co-authored-by: Ludovic Courtès <ludo@gnu.org>

doc/guix.texi

@@ -791,11 +791,11 @@ The following dependencies are optional:
 
 @itemize
 @item
-@c Note: We need at least 0.12.0 for 'userauth-gssapi!'.
+@c Note: We need at least 0.13.0 for #:nodelay.
 Support for build offloading (@pxref{Daemon Offload Setup}) and
 @command{guix copy} (@pxref{Invoking guix copy}) depends on
 @uref{https://github.com/artyom-poptsov/guile-ssh, Guile-SSH},
-version 0.12.0 or later.
+version 0.13.0 or later.
 
 @item
 When @url{https://www.nongnu.org/lzip/lzlib.html, lzlib} is available, lzlib

guix/ssh.scm

@@ -129,7 +129,11 @@ Throw an error on failure."
                                ;; We need lightweight compression when
                                ;; exchanging full archives.
                                #:compression compression
-                               #:compression-level 3)))
+                               #:compression-level 3
+
+                               ;; Speed up RPCs by creating sockets with
+                               ;; TCP_NODELAY.
+                               #:nodelay #t)))
 
     ;; Honor ~/.ssh/config.
     (session-parse-config! session)

m4/guix.m4

@@ -142,14 +142,16 @@ dnl GUIX_CHECK_GUILE_SSH
 dnl
 dnl Check whether a recent-enough Guile-SSH is available.
 AC_DEFUN([GUIX_CHECK_GUILE_SSH], [
-  dnl Check whether 'userauth-gssapi!' (introduced in 0.12.0) is present.
+  dnl Check whether '#:nodelay' paramater to 'make-session' (introduced in
+  dnl 0.13.0) is present.
   AC_CACHE_CHECK([whether Guile-SSH is available and recent enough],
     [guix_cv_have_recent_guile_ssh],
     [GUILE_CHECK([retval],
       [(and (@ (ssh channel) channel-send-eof)
             (@ (ssh popen) open-remote-pipe)
             (@ (ssh dist node) node-eval)
-            (@ (ssh auth) userauth-gssapi!))])
+            (@ (ssh auth) userauth-gssapi!)
+            ((@ (ssh session) make-session) #:nodelay #t))])
      if test "$retval" = 0; then
        guix_cv_have_recent_guile_ssh="yes"
      else