nixos/mysql: reformat, move logical steps into variables

define commands like "waiting for the mysql socket to appear" or "setup
initial databases" in a let expression, so the main control flow becomes
more readable.

nixos/modules/services/databases/mysql.nix

@@ -318,13 +318,11 @@ in
           pkgs.nettools
         ];
 
-        preStart =
-          ''
+        preStart = ''
           if ! test -e ${cfg.dataDir}/mysql; then
             ${mysql}/bin/mysql_install_db --defaults-file=/etc/my.cnf ${installOptions}
             touch /tmp/mysql_init
           fi
-
         '';
 
         serviceConfig = {
@@ -336,8 +334,9 @@ in
           ExecStart = "${mysql}/bin/mysqld --defaults-file=/etc/my.cnf ${mysqldOptions} $_WSREP_NEW_CLUSTER $_WSREP_START_POSITION";
         };
 
-        postStart = ''
-          ${lib.optionalString (!hasNotify) ''
+        postStart =
+          let
+            cmdWatchForMysqlSocket = ''
               # Wait until the MySQL server is available for use
               count=0
               while [ ! -e /run/mysqld/mysqld.sock ]
@@ -352,12 +351,8 @@ in
                   count=$((count++))
                   sleep 1
               done
-          ''}
-
-            if [ -f /tmp/mysql_init ]
-            then
-                ${concatMapStrings (database:
-                  ''
+            '';
+            cmdInitialDatabases = concatMapStrings (database: ''
               # Create initial databases
               if ! test -e "${cfg.dataDir}/${database.name}"; then
                   echo "Creating initial database: ${database.name}"
@@ -378,8 +373,13 @@ in
                     ''}
                   ) | ${mysql}/bin/mysql -u root -N
               fi
-                  '') cfg.initialDatabases}
+            '') cfg.initialDatabases;
+          in
 
+          lib.optionalString (!hasNotify) cmdWatchForMysqlSocket + ''
+            if [ -f /tmp/mysql_init ]
+            then
+                ${cmdInitialDatabases}
                 ${optionalString (cfg.replication.role == "master")
                   ''
                     # Set up the replication master