simplify the master<->slave first synchronisation

shared/db/postgres.nix

@@ -62,24 +62,12 @@ in
         };
     };
     # This preStart script sync the slaves to the master
-    # systemd.services.<name>.preStart has a mergeable type
-    systemd.services.postgresql.preStart = lib.mkIf (builtins.elem myName mapping.db.slaves) ''
-        if test -e ${cfg.dataDir}/.first_startup; then
+    # systemd.services.<name>.preStart has a mergeable type, 
+    # this code will be prepend to the default one
+    systemd.services.postgresql.preStart = lib.mkIf (builtins.elem myName mapping.db.slaves) (lib.mkBefore ''
+        if ! test -e ${cfg.dataDir}/PG_VERSION; then
             echo "Setting up PostgreSQL slave replication..."
             
-            # This is a sl that's defined by the default preStart script
-            # We need an empty dataDir for the pg_basebackup
-            # And there is no easy ways that I'm aware of
-            # to get the hash of that file without recomputing it
-            PSQL_CONF_PATH="${cfg.dataDir}/postgresql.conf"
-            PSQL_CONF_TARGET=$(readlink "$PSQL_CONF_PATH")
-
-            # Remove data dir
-            if [ -d "${cfg.dataDir}" ]; then
-                echo "Deleting postgres data dir: ${cfg.dataDir}"
-                rm -rf "${cfg.dataDir}"
-            fi
-            
             # Perform base backup from master
             echo "Starting base backup from master: ${masterIP}"
             ${cfg.package}/bin/pg_basebackup \
@@ -91,12 +79,9 @@ in
                 -Xs \
                 -R
 
-            # Symlink back the psql configFile
-            ln -sf "$PSQL_CONF_TARGET" "$PSQL_CONF_PATH"
-
             echo "PostgreSQL slave setup completed successfully"
         else
             echo "PostgreSQL standby already configured (standby.signal exists), skipping base backup"
         fi
-    '';
+    '');
 }