Re: Segmentation fault in libpq

Hi,

comments inline ...

On 06/29/2017 03:08 PM, Merlin Moncure wrote:
> On Thu, Jun 29, 2017 at 4:01 AM, Michal Novotny
> <michal(dot)novotny(at)greycortex(dot)com> wrote:
>> Hi all,
>>
>> we've developed an application using libpq to access a table in the PgSQL
>> database but we're sometimes experiencing segmentation fault on
>> resetPQExpBuffer() function of libpq called from PQexecParams() with
>> prepared query.
>>
>> PostgreSQL version is 9.6.3 and the backtrace is:
>>
>> Core was generated by `/usr/ti/bin/status-monitor2 -m
>> /usr/lib64/status-monitor2/modules'.
>> Program terminated with signal 11, Segmentation fault.
>> #0 resetPQExpBuffer (str=str(at)entry=0x9f4a28) at pqexpbuffer.c:152
>> 152 str->data[0] = '\0';
>>
>> Thread 1 (Thread 0x7fdf68de3840 (LWP 3525)):
>> #0 resetPQExpBuffer (str=str(at)entry=0x9f4a28) at pqexpbuffer.c:152
>> No locals.
>> #1 0x00007fdf66e0333d in PQsendQueryStart (conn=conn(at)entry=0x9f46d0) at
>> fe-exec.c:1371
>> No locals.
>> #2 0x00007fdf66e044b9 in PQsendQueryParams (conn=conn(at)entry=0x9f46d0,
>> command=command(at)entry=0x409a98 "SELECT min, hour, day, month, dow, sensor,
>> module, params, priority, rt_due FROM sm.cron WHERE sensor = $1 ORDER BY
>> priority DESC", nParams=nParams(at)entry=1, paramTypes=paramTypes(at)entry=0x0,
>> paramValues=paramValues(at)entry=0xa2b7b0, paramLengths=paramLengths(at)entry=0x0,
>> paramFormats=paramFormats(at)entry=0x0, resultFormat=resultFormat(at)entry=0) at
>> fe-exec.c:1192
>> No locals.
>> #3 0x00007fdf66e0552b in PQexecParams (conn=0x9f46d0, command=0x409a98
>> "SELECT min, hour, day, month, dow, sensor, module, params, priority, rt_due
>> FROM sm.cron WHERE sensor = $1 ORDER BY priority DESC", nParams=1,
>> paramTypes=0x0, paramValues=0xa2b7b0, paramLengths=0x0, paramFormats=0x0,
>> resultFormat=0) at fe-exec.c:1871
>> No locals.
>>
>> Unfortunately we didn't have more information from the crash, at least for
>> now.
>>
>> Is this a known issue and can you help me with this one?
> Is your application written in C? We would need to completely rule
> out your code (say, by double freeing result or something else nasty)
> before assuming problem was withing libpq itself, particularly in this
> area of the code. How reproducible is the problem?
>
> merlin

The application is written in plain C. The issue is it happens just
sometimes - sometimes it happens and sometimes it doesn't. Once it
happens it causes the application crash but as it's systemd unit with
Restart=on-failure flag it's automatically being restarted.

What's being done is:
1) Ensure connection already exists and create a new one if it doesn't
exist yet
2) Run PQexecParams() with specified $params that has $params_cnt elements:

res = PQexecParams(conn, prepared_query, params_cnt, NULL, (const char
**)params, NULL, NULL, 0);

3) Check for result and report error and exit if "PQresultStatus(res) !=
PGRES_TUPLES_OK"
4) Do some processing with the result
5) Clear result using PQclear()

It usually works fine but sometimes it's crashing and I don't know how
to investigate further.

Could you please help me based on information provided above?

Thanks,
Michal

--
Michal Novotny
System Development Lead
michal(dot)novotny(at)greycortex(dot)com

GREYCORTEX s.r.o.
Purkynova 127, 61200 Brno
Czech Republic
www.greycortex.com