Horton

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

Fujii Masao <masao.fujii@oss.nttdata.com>

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From: Fujii Masao <masao.fujii@oss.nttdata.com>
To: Kyotaro Horiguchi <horikyota.ntt@gmail.com>, bharath.rupireddyforpostgres@gmail.com
Cc: magnus@hagander.net, houzj.fnst@cn.fujitsu.com, pgsql-hackers@lists.postgresql.org, m.usama@gmail.com
Date: 2021-03-18T07:16:54Z
Lists: pgsql-hackers 

On 2021/03/17 11:58, Kyotaro Horiguchi wrote:
> The first suggested signature for pg_terminate_backend() with timeout
> was pg_terminate_backend(pid, timeout).  The current signature (pid,
> wait?, timeout) looks redundant.  Maybe the reason for rejecting 0
> astimeout is pg_terminate_backend(pid, true, 0) looks odd but it we
> can wait forever in that case (as other features does).

I'm afraid that "waiting forever" can cause something like deadlock situation,
as follows. We have no mechanism to detect this for now.

1. backend 1 took the lock on the relation A.
2. backend 2 took the lock on the relation B.
3. backend 1 tries to take the lock on the relation B and is waiting for
    the lock to be released.
4. backend 2 accidentally executes pg_wait_for_backend_termination() with
    the pid of backend 1, and then is waiting for backend 1 to be terminated.

Regards,

-- 
Fujii Masao
Advanced Computing Technology Center
Research and Development Headquarters
NTT DATA CORPORATION

Commits

  1. Copy-edit text for the pg_terminate_backend() "timeout" parameter.

  2. Remove pg_wait_for_backend_termination().

  3. Add functions to wait for backend termination

  4. Fix typo in comment.