services: nscd: Cause PID 1 to drop nscd database mappings on shutdown.

Partly fixes guix/guix#4269.

Fixes a bug whereby shepherd (PID 1) could retain memory mappings for
/var/run/nscd/dbXXX, which are created by glibc’s NSS from database file
descriptors sent by nscd.  Those mappings could then prevent
‘root-file-system’ from re-mounting the root file system as read-write.
This change causes PID 1 to drop these mappings.

PID 1 typically calls libc database functions such as ‘getgr’ when dealing
with AF_UNIX endpoints for socket-activated services, to look up the socket’s
owner and group.  This is where the bug would manifest.

The regression may have been introduced by
85ac164c41, which caused nscd to handle the
password and group databases.

* gnu/services/base.scm (nscd-shepherd-service): In ‘stop’ procedure, call
‘getpw’, ‘getgr’, and ‘getaddrinfo’.
* gnu/tests/base.scm (run-root-unmount-test) <"open libc NSS database">: New
test.
(%test-root-unmount): Add #:imported-modules.

Change-Id: I197cc8c82165c631f857415898137412ce9bd439
Reported-by: Rutherther <rutherther@ditigal.xyz>
Signed-off-by: Ludovic Courtès <ludo@gnu.org>
Merges: #4828

gnu/tests/base.scm

@@ -827,6 +827,18 @@ in a loop.  See <http://bugs.gnu.org/26931>.")
                                                      ,witness-size))))))
                                marionette))
 
+            ;; Cause PID 1 to create a mapping to nscd's database files.
+            ;; Those mappings used to prevent 'root-file-system' to remount
+            ;; read-only on shutdown.  See
+            ;; <https://codeberg.org/guix/guix/issues/4269>.
+            (test-equal "open libc NSS database"
+              "root"
+              (marionette-eval '(begin
+                                  (use-modules (gnu services herd))
+                                  (start-service 'nscd) ;wait for nscd
+                                  (eval-there '(passwd:name (getgr "root"))))
+                               marionette))
+
             ;; Halt the system.
             (marionette-eval '(system* "/run/current-system/profile/sbin/halt")
                              marionette)
@@ -891,7 +903,10 @@ in a loop.  See <http://bugs.gnu.org/26931>.")
     "Make sure the root file system is cleanly unmounted when the system is
 halted.")
    (value
-    (let ((os (marionette-operating-system %simple-os)))
+    (let ((os (marionette-operating-system
+               %simple-os
+               #:imported-modules '((gnu services herd)
+                                    (guix combinators)))))
       (run-root-unmount-test os)))))