services: posgresql: Add option to specify UID/GID for postgres user.

Add 'createAccount?', 'uid' and 'gid' to <postgresql-configuation>.

Unlike other system daemons, the PostgreSQL data directory is typically
meant to persist across 'guix system reconfigure' and once created, you
don't want it's UID or GID to change anymore.

Furthermore, if you want to place the data directory on a network share
and use NFSv4 with idmap, then the 'postgres' user must exist when the
'rpc.idmapd' daemon is launched; prior to mounting the share.  And it
needs to be possible to mount the share without configuring PostgreSQL.

With NFSv3, the UID and GID typically needs to match those on the
server.

The added options allow for both of these scenarios:

You can either create the user in (operating-system (users)) completely
independently of the 'postgresql-service-type' (for instance to get your
NFS setup working first prior to configuring your databases) - or "pin"
it's UID / GID values.

* gnu/services/databases.scm (<postgresql-configuration>)[create-account?]
[uid, gid]: New fields.
(%postgresql-accounts): Remove.
(create-postgresql-account): New procedure.
(postgresql-service-type)[extensions]: Use it.
* doc/guix.texi (Database Services): Update accordingly.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>

gnu/services/databases.scm

@@ -180,17 +180,30 @@ host	all	all	::1/128 	md5"))
   (data-directory     postgresql-configuration-data-directory
                       (default "/var/lib/postgresql/data"))
   (extension-packages postgresql-configuration-extension-packages
-                      (default '())))
+                      (default '()))
+  (create-account?    postgresql-configuration-create-account?
+                      (default #t))
+  (uid                postgresql-configuration-uid
+                      (default #f))
+  (gid                postgresql-configuration-gid
+                      (default #f)))
 
-(define %postgresql-accounts
-  (list (user-group (name "postgres") (system? #t))
-        (user-account
-         (name "postgres")
-         (group "postgres")
-         (system? #t)
-         (comment "PostgreSQL server user")
-         (home-directory "/var/empty")
-         (shell (file-append shadow "/sbin/nologin")))))
+(define (create-postgresql-account config)
+  (match-record config <postgresql-configuration>
+    (create-account? uid gid)
+    (if (not create-account?) '()
+        (list (user-group
+               (name "postgres")
+               (id gid)
+               (system? #t))
+              (user-account
+               (name "postgres")
+               (group "postgres")
+               (system? #t)
+               (uid uid)
+               (comment "PostgreSQL server user")
+               (home-directory "/var/empty")
+               (shell (file-append shadow "/sbin/nologin")))))))
 
 (define (final-postgresql postgresql extension-packages)
   (if (null? extension-packages)
@@ -327,7 +340,7 @@ host	all	all	::1/128 	md5"))
           (service-extension activation-service-type
                              postgresql-activation)
           (service-extension account-service-type
-                             (const %postgresql-accounts))
+                             create-postgresql-account)
           (service-extension
            profile-service-type
            (compose list postgresql-configuration-postgresql))))