[pgpool-general: 6134] kind mismatch among backends with multiple clients and prepared statements

[Peter Bauer]

Hello all,

i hope you can help me with this, thx in advance.

The scenario is:
- a pgpool (version 3.7.4) instance using two backends, one of the 
backends on the same host
- two clients (PostgreSQL 9.6.2), heavily using transactions and 
prepared statements

procedure to reproduce the problem:
1. let the two clients crash so there is no cleanup of the prepared 
statements (DEALLOCATE or DISCARD ALL) done by the clients
2. both clients are started again and add/execute their prepared 
statements again

the following error happens for both clients at some point after a lot 
of statements were already executed successfully:

Client A:
2018-06-20 07:41:49: pid 30748: LOG:  DB node id: 1 backend pid: 10873 
statement: BEGIN
2018-06-20 07:41:49: pid 30748: WARNING:  packet kind of backend 1 ['A'] 
does not match with master/majority nodes packet kind ['C']
2018-06-20 07:41:49: pid 30748: FATAL:  failed to read kind from backend
2018-06-20 07:41:49: pid 30748: DETAIL:  kind mismatch among backends. 
Possible last query was: "BEGIN" kind details are: 0[C] 1[A]
2018-06-20 07:41:49: pid 30748: HINT:  check data consistency among db nodes

Client B:
2018-06-20 07:41:50: pid 30746: LOG:  statement: BEGIN
2018-06-20 07:41:50: pid 30746: LOG:  DB node id: 0 backend pid: 5060 
statement: BEGIN
2018-06-20 07:41:50: pid 30746: LOG:  DB node id: 1 backend pid: 10894 
statement: BEGIN
2018-06-20 07:41:50: pid 30746: WARNING:  packet kind of backend 1 ['A'] 
does not match with master/majority nodes packet kind ['C']
2018-06-20 07:41:50: pid 30746: FATAL:  failed to read kind from backend
2018-06-20 07:41:50: pid 30746: DETAIL:  kind mismatch among backends. 
Possible last query was: "BEGIN" kind details are: 0[C] 1[A]
2018-06-20 07:41:50: pid 30746: HINT:  check data consistency among db nodes

I attached the pgpool logs and the PostgreSQL log of Client A.

I found some bug reports regarding pgpool and prepared statements 
resulting in this error messages so i thought this may be another one.
Could this be the reason for the issue or should i look somewhere else?

thx,
Peter

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