Re: A new function to wait for the backend exit after termination

On Wed, Oct 21, 2020 at 6:13 AM Magnus Hagander <magnus@hagander.net> wrote:

> On Wed, Oct 21, 2020 at 3:02 PM Bharath Rupireddy <
> bharath.rupireddyforpostgres@gmail.com> wrote:
>
>> Hi,
>>
>> Currently pg_terminate_backend(), sends SIGTERM to the backend process
>> but doesn't ensure it's exit. There are chances that backends still are
>> running(even after pg_terminate_backend() is called) until the interrupts
>> are processed(using ProcessInterrupts()). This could cause problems
>> especially in testing, for instance in a sql file right after
>> pg_terminate_backend(), if any test case depends on the backend's
>> non-existence[1], but the backend is not terminated. As discussed in [1],
>> we have wait_pid()(see regress.c and sql/dblink.sql), but it's not usable
>> across the system. In [1], we thought it would be better to have functions
>> ensuring the backend's exit on the similar lines of pg_terminate_backend().
>>
>> I propose to have two functions:
>>
>> 1. pg_terminate_backend_and_wait() -- which sends SIGTERM to the backend
>> and wait's until it's exit.
>>
>
> I think it would be nicer to have a pg_terminate_backend(pid, wait=false),
> so a function with a second parameter which defaults to the current
> behaviour of not waiting. And it might be a good idea to also give it a
> timeout parameter?
>

Agreed on the overload, and the timeouts make sense too - with the caller
deciding whether a timeout results in a failure or a false return value.


>
>> 2. pg_wait_backend() -- which waits for a given backend process. Note
>> that this function has to be used carefully after pg_terminate_backend(),
>> if used on a backend that's not ternmited it simply keeps waiting in a loop.
>>
>
> It seems this one also very much would need a timeout value.
>
>
Is there a requirement for waiting to be superuser only?  You are not
affecting any session but your own during the waiting period.

I could imagine, in theory at least, wanting to wait for a backend to go
idle as well as for it disappearing.  Scope creep in terms of this patch's
goal but worth at least considering now.

David J.