Message-ID: <535674B4.2040706@2ndquadrant.com>
Date: Tue, 22 Apr 2014 15:55:00 +0200
From: Petr Jelinek <petr@2ndquadrant.com>
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To: PostgreSQL-development <pgsql-hackers@postgresql.org>
Subject: shm_mq inconsistent behavior of SHM_MQ_DETACHED
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Hi,

I was playing with shm_mq and found a little odd behavior with detaching 
after sending messages.

Following sequence behaves as expected (receiver gets 2 messages):
P1 -> set_sender
P1 -> attach
P2 -> set_receiver
P2 -> attach
P1 -> send
P2 -> receive
P1 -> send
P1 -> detach
P2 -> receive
P2 -> detach

But if I do first receive after detach like in this sequence:
P1 -> set_sender
P1 -> attach
P2 -> set_receiver
P2 -> attach
P1 -> send
P1 -> send
P1 -> detach
P2 -> receive

I get SHM_MQ_DETACHED on the receiver even though there are messages in 
the ring buffer.

The reason for this behavior is that mqh_counterparty_attached is only 
set by shm_mq_receive. This does not seem to be consistent - I would 
either expect to get SHM_MQ_DETACHED always when other party has 
detached or always get all remaining messages that are in queue (and I 
would strongly prefer the latter).

Maybe the shm_mq_get_bytes_written should be used to determine if there 
is something left for us to read in the receiver if we hit the 
!mqh_counterparty_attached code path with detached sender?


-- 
  Petr Jelinek                  http://www.2ndQuadrant.com/
  PostgreSQL Development, 24x7 Support, Training & Services


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