[pgpool-hackers: 1950] Problem with log_line_prefix()

Tatsuo Ishii ishii at sraoss.co.jp
Sun Dec 25 08:59:55 JST 2016


If I hit CTRL-C key while executing the regression test, sometimes a
orphan pgpool child remains. Here's a stack trace of the process:

t-ishii at localhost: gdb temp/installed/bin/pgpool 3794
GNU gdb (Ubuntu 7.7.1-0ubuntu5~14.04.2) 7.7.1
Loaded symbols for /lib/x86_64-linux-gnu/libnss_files.so.2
__lll_lock_wait_private () at ../nptl/sysdeps/unix/sysv/linux/x86_64/lowlevellock.S:95
95	../nptl/sysdeps/unix/sysv/linux/x86_64/lowlevellock.S: そのようなファイルやディレクトリはありません.
(gdb) bt
#0  __lll_lock_wait_private () at ../nptl/sysdeps/unix/sysv/linux/x86_64/lowlevellock.S:95
#1  0x00007f67fe20ccba in _L_lock_12808 () from /lib/x86_64-linux-gnu/libc.so.6
#2  0x00007f67fe20a6b5 in __GI___libc_malloc (bytes=15) at malloc.c:2887
#3  0x00007f67fe21072a in __GI___strdup (s=0x7f67fe305dd8 "/etc/localtime") at strdup.c:42
#4  0x00007f67fe239f51 in tzset_internal (always=<optimized out>, explicit=explicit at entry=1)
    at tzset.c:444
#5  0x00007f67fe23a913 in __tz_convert (timer=timer at entry=0x7ffce1c1b7f8, 
    use_localtime=use_localtime at entry=1, tp=tp at entry=0x7f67fe54bde0 <_tmbuf>) at tzset.c:632
#6  0x00007f67fe2387d1 in __GI_localtime (t=t at entry=0x7ffce1c1b7f8) at localtime.c:42
#7  0x000000000045627b in log_line_prefix (buf=buf at entry=0x7ffce1c1b8d0, line_prefix=<optimized out>, 
    edata=<optimized out>) at ../../src/utils/error/elog.c:2059
#8  0x000000000045894d in send_message_to_server_log (edata=0x753320 <errordata>)
    at ../../src/utils/error/elog.c:2084
#9  EmitErrorReport () at ../../src/utils/error/elog.c:1129
#10 0x0000000000456d8e in errfinish (dummy=<optimized out>) at ../../src/utils/error/elog.c:434
#11 0x0000000000421f57 in die (sig=2) at protocol/child.c:925
#12 <signal handler called>
#13 _int_malloc (av=0x7f67fe546760 <main_arena>, bytes=4176) at malloc.c:3302
#14 0x00007f67fe20a6c0 in __GI___libc_malloc (bytes=4176) at malloc.c:2891
#15 0x000000000045561d in AllocSetAlloc (context=0xf90000, size=4128) at ../../src/utils/mmgr/aset.c:674
#16 0x000000000042cd1e in pool_init_cp () at protocol/pool_connection_pool.c:70
#17 0x0000000000423024 in do_child (fds=fds at entry=0xfa35c0) at protocol/child.c:194
#18 0x00000000004086f5 in fork_a_child (fds=0xfa35c0, id=11) at main/pgpool_main.c:723
#19 0x00000000004090cc in reaper () at main/pgpool_main.c:2427
#20 0x000000000040d869 in PgpoolMain (discard_status=discard_status at entry=1 '\001', 
    clear_memcache_oidmaps=clear_memcache_oidmaps at entry=0 '\000') at main/pgpool_main.c:457
#21 0x0000000000406eb3 in main (argc=<optimized out>, argv=<optimized out>) at main/main.c:305

As you can see, the process was stuck while calling malloc().

pool_init_cp() was interrupted by SIGINT (because I hit the key), and
the signal handler die() was called. In die(), elog() was
called. elog() calls log_line_prefix(), which calls
localtime(). localtime() calls malloc(), which seems to try to obtain
a lock, then it causes a dead lock since pool_init_cp() was
interrupted while calling malloc().

log_line_prefix() was copied from PostgreSQL but I see subtle
differences from the PostgreSQL's original log_line_prefix().:

pg_strftime(strfbuf, sizeof(strfbuf),
	"%Y-%m-%d %H:%M:%S %Z",
	pg_localtime(&stamp_time, log_timezone));

	strftime(strbuf, 128, "%Y-%m-%d %H:%M:%S", localtime(&now));

Apparently the call stuck shows that the hang happened inside
localtime() because it calls malloc(). On the other hand,
pg_localtime() seems to avoid calling localtime() by using their home
made version of localtime(), which does *not* call malloc(). I'm not
sure if it's a intention but it seems it'd better to follow
PostgreSQL's way to avoid the hang problem.

What do you think, Usama?

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

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