FIX: Restore process for dump taken with `pg_dump` 10.3+.

* Since we can no longer restore into a different schema,
  we will move tables in the public schema into the backup schema
  first before restoring the dump file which goes into the public
  schema. The downside to this approach is that we will increase
  the downtime experienced during the restore process. Downtime
  would equal the duration of restoring the dump file.

lib/backup_restore/restorer.rb

@@ -39,23 +39,41 @@ module BackupRestore
       validate_metadata
 
       extract_dump
-      restore_dump
+      dumped_by_version = Gem::Version.new(get_dumped_by_version)
 
-      ### READ-ONLY / START ###
-      enable_readonly_mode
+      if dumped_by_version >= Gem::Version.new("10.3") ||
+         dumped_by_version >= Gem::Version.new("9.5.12")
 
-      pause_sidekiq
-      wait_for_sidekiq
+        enable_readonly_mode
 
-      switch_schema!
+        pause_sidekiq
+        wait_for_sidekiq
 
-      migrate_database
-      reconnect_database
-      reload_site_settings
-      clear_emoji_cache
+        BackupRestore.move_tables_between_schemas("public", "backup")
+        restore_dump
+        migrate_database
+        reconnect_database
 
-      disable_readonly_mode
-      ### READ-ONLY / END ###
+        reload_site_settings
+        clear_emoji_cache
+
+        disable_readonly_mode
+      else
+        restore_dump
+        enable_readonly_mode
+
+        pause_sidekiq
+        wait_for_sidekiq
+
+        switch_schema!
+
+        migrate_database
+        reconnect_database
+        reload_site_settings
+        clear_emoji_cache
+
+        disable_readonly_mode
+      end
 
       extract_uploads
     rescue SystemExit
@@ -238,6 +256,16 @@ module BackupRestore
       end
     end
 
+    def get_dumped_by_version
+      output = Discourse::Utils.execute_command(
+        File.extname(@dump_filename) == '.gz' ? 'zgrep' : 'grep',
+        '-m1', @dump_filename, '-e', "pg_dump",
+        failure_message: "Failed to check version of pg_dump used to generate the dump file"
+      )
+
+      output.match(/version (\d+(\.\d+)?)/)[1]
+    end
+
     def restore_dump_command
       if File.extname(@dump_filename) == '.gz'
         "gzip -d < #{@dump_filename} | #{sed_command} | #{psql_command} 2>&1"