Refactor PostgreSQL logic

What does this Merge Request do and why?

This MR aims to refactor and improve the PostgreSQL logic to make upcoming relating MRs less complex.

Merge Request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise Issue to follow-up.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.

lib/gdk/postgresql.rb

@@ -10,22 +10,23 @@ module GDK
       target_version.canonical_segments[0]
     end
 
+    def initialize(config = GDK.config)
+      @config = config
+    end
+
     def psql_cmd(args)
       pg_cmd(args, database: default_database).flatten
     end
 
     def current_data_dir
-      @current_data_dir ||= begin
-        config = GDK::Config.new
-        File.join(config.postgresql.dir, 'data')
-      end
+      @current_data_dir ||= postgresql_config.data_dir
     end
 
     def current_version
       @current_version ||= begin
         raise "PG_VERSION not found in #{pg_version_file}. Is PostgreSQL initialized?" unless installed?
 
-        version = File.read(pg_version_file).to_f
+        version = pg_version_file.read.to_f
 
         # After PostgreSQL 9.6, PG_VERSION uses a single integer (10, 11, 12, etc.)
         version >= 10 ? version.to_i : version
@@ -33,7 +34,7 @@ module GDK
     end
 
     def installed?
-      File.exist?(pg_version_file)
+      pg_version_file.exist?
     end
 
     def ready?
@@ -55,7 +56,7 @@ module GDK
     end
 
     def use_tcp?
-      !config.host.start_with?('/')
+      !postgresql_config.host.start_with?('/')
     end
 
     def upgrade_needed?(target_version = self.class.target_version_major)
@@ -82,26 +83,24 @@ module GDK
 
     private
 
-    def config
-      @config ||= GDK.config.postgresql
-    end
+    attr_reader :config
 
-    def host
-      config.dir.to_s
+    def base_postgresql_config
+      @base_postgresql_config ||= config.postgresql
     end
 
-    def port
-      config.port.to_s
+    def postgresql_config
+      @postgresql_config ||= base_postgresql_config
     end
 
     def pg_version_file
-      @pg_version_file ||= File.join(current_data_dir, 'PG_VERSION')
+      @pg_version_file ||= current_data_dir.join('PG_VERSION')
     end
 
     def pg_cmd(*args, program: 'psql', database: nil, command: nil)
-      cmd = [bin_dir.join(program).to_s]
-      cmd << "--host=#{host}"
-      cmd << "--port=#{port}"
+      cmd = [base_postgresql_config.bin_dir.join(program).to_s]
+      cmd << "--host=#{postgresql_config.dir}"
+      cmd << "--port=#{postgresql_config.port}"
       cmd << "--dbname=#{database}" if database
       cmd << "--command=#{command}" if command
 
@@ -111,9 +110,5 @@ module GDK
     def default_database
       'gitlabhq_development'
     end
-
-    def bin_dir
-      config.bin_dir
-    end
   end
 end