When is the MessageContext released?

Hi :
I run a query like "select * from t" and set the break like this:

break exec_simple_query
break MemoryContextDelete
commands
p context->name
c
end

I can see most of the MemoryContext is relased, but never MessageContext,
when will it be released?

/*
* Create the memory context we will use in the main loop.
*
* MessageContext is reset once per iteration of the main loop, ie, upon
* completion of processing of each command message from the client.
*/

I'm hacking the code with the latest commit.
https://github.com/postgres/postgres/commit/414a9d3cf34c7aff1c63533df4c40ebb63bd0840

Thanks!