[bug#74510] [PATCH] doc: Document the (possible) need for network-online for NFS.

[bug#74510] [PATCH] doc: Document the (possible) need for network-online for NFS.

[Tomas Volf]

Depending on networking is not enough in some setups, so a language clarifying
that and an example of network-online service.

* doc/guix.texi (File Systems): Document the possible need for network-online.

Change-Id: I8abe07cc9d6dc61f28eeea7ffa785eb8c9e8fd09
---
 doc/guix.texi | 28 +++++++++++++++++++++++++++-
 1 file changed, 27 insertions(+), 1 deletion(-)

diff --git a/doc/guix.texi b/doc/guix.texi
index 26488b41c8..861e78e6d2 100644
--- a/doc/guix.texi
+++ b/doc/guix.texi
@@ -18018,7 +18018,33 @@ File Systems
 met before mounting the file system.
 
 As an example, an NFS file system would typically have a requirement for
-@code{networking}.
+@code{networking}.  However be aware that depending on @code{networking}
+might not be sufficient in some setups and a variant of custom
+@code{network-online} service might be necessary.  Example:
+
+@lisp
+(simple-service 'network-online shepherd-root-service-type
+  (list
+   (shepherd-service
+    (requirement '(networking))
+    (provision '(network-online))
+    (documentation "Wait for the network to come up.")
+    (start #~(lambda _
+               (let* ((cmd
+                       "set -eux
+c=0
+while ! /run/setuid-programs/ping -qc1 -W1 example.org; do
+	sleep 1
+	[ \"$((c += 1))\" -lt 30 ] || exit 1  # Limit the wait time
+done
+")
+                    (status (system cmd)))
+               (= 0 (status:exit-val status)))))
+   ;; Ordering for one-shot? services does not currently work.
+   ;; https://issues.guix.gnu.org/74284
+   ;; (one-shot? #t)
+   )))
+@end lisp
 
 Typically, file systems are mounted before most other Shepherd services
 are started.  However, file systems with a non-empty
-- 
2.46.0