[pgpool-hackers: 3782] Re: 073.pg_terminate_backend sometimes fails

[Tatsuo Ishii]

> Hi Ishii-San
> 
> I have tried like a dozen times but never got the failure even once.
> 
> Looking at the log snippet you shared  it appears that this failure
> is a random timing-based and it is not really a problem with the
> handling of pg_terminate_backend but somehow it is related to
> missing connection_count_down() call.
> 
> I suppose connection_count_down() is not called in one of the code
> path that gets executed when we get this "write on backend failed"
> warning
> i.e.
> .WARNING:  write on backend 0 failed with error :"Success"
> DETAIL:  while trying to write data from offset: 0 wlen: 5
> 
> The reason I suspect that is, I never saw this above warning message in
> pg_terminate_backend test case logs, but it is present in the log snippet
> you shared above so I suspect if the backend gets terminated when
> pgpool process is about to write to the backend socket then in that code
> patch somehow we don't execute the connection_count_down() function
> that leads to this regression test failure.

I agree that the error is caused by timing but probably the reasoning
is different. I think what is happing is:

1. $PSQL test -p $PGPORT -c "SELECT pg_sleep(10);" &
   connection_count_up is called

(sleep 2)

2. $PSQL test -p $PGPORT -c "SELECT pg_terminate_backend($PID)"
   connection_count_up is called
   connection_count_down is called

3. Check count_up and count_down.

If the machine is slow, 10 seconds has passed since 1 (thus
connection_count_down is called) and test succeeds. Probably my
machine is too fast to execute 3 before 10 seconds passed since 1, and
the test failed.

I inserted "sleep 5" between 2 and 3, and now the test succeeds. Patch
attached. What do you think?

Best regards,
--
Tatsuo Ishii
SRA OSS, Inc. Japan
English: http://www.sraoss.co.jp/index_en.php
Japanese:http://www.sraoss.co.jp
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