Make drop_table idempotent so concurrent retention runs don't crash (#3595)

lib/postal/message_db/provisioner.rb

@@ -59,10 +59,12 @@ def create_table(table_name, options)
       end
 
       #
-      # Drop a table
+      # Drop a table. Uses IF EXISTS so that two workers running retention
+      # concurrently do not crash when the second one tries to drop a table
+      # the first has already removed.
       #
       def drop_table(table_name)
-        @database.query("DROP TABLE `#{@database.database_name}`.`#{table_name}`")
+        @database.query("DROP TABLE IF EXISTS `#{@database.database_name}`.`#{table_name}`")
       end
 
       #

spec/lib/postal/message_db/database_spec.rb

@@ -73,5 +73,15 @@
         end.to raise_error(Mysql2::Error)
       end
     end
+
+    describe "#drop_table" do
+      # The retention task's lock can be stolen after a few minutes, so two
+      # workers may call drop_table for the same table. Dropping a table that
+      # has already been removed must not raise (regression for #3595).
+      it "does not raise when the table no longer exists" do
+        expect { database.provisioner.drop_table("raw-does-not-exist") }
+          .not_to raise_error
+      end
+    end
   end
 end