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Commit8939020

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Run the postmaster's signal handlers without SA_RESTART.
The postmaster keeps signals blocked everywhere except while waitingfor something to happen in ServerLoop(). The code expects that theselect(2) will be cancelled with EINTR if an interrupt occurs; withoutthat, followup actions that should be performed by ServerLoop() itselfwill be delayed. However, some platforms interpret the SA_RESTARTsignal flag as meaning that they should restart rather than cancelthe select(2). Worse yet, some of them restart it with the originaltimeout delay, meaning that a steady stream of signal interrupts canprevent ServerLoop() from iterating at all if there are no incomingconnection requests.Observable symptoms of this, on an affected platform such as HPUX 10,include extremely slow parallel query startup (possibly as much as30 seconds) and failure to update timestamps on the postmaster's socketsand lockfiles when no new connections arrive for a long time.We can fix this by running the postmaster's signal handlers withoutSA_RESTART. That would be quite a scary change if the range of codewhere signals are accepted weren't so tiny, but as it is, it seemssafe enough. (Note that postmaster children do, and must, reset allthe handlers before unblocking signals; so this change should notaffect any child process.)There is talk of rewriting the postmaster to use a WaitEventSet andnot do signal response work in signal handlers, at which point it mightbe appropriate to revert this patch. But that's not happening beforev11 at the earliest.Back-patch to 9.6. The problem exists much further back, but theworst symptom arises only in connection with parallel query, so itdoes not seem worth taking any portability risks in older branches.Discussion:https://postgr.es/m/9205.1492833041@sss.pgh.pa.us
1 parentcbc2270 commit8939020

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3 files changed

+56
-9
lines changed

3 files changed

+56
-9
lines changed

‎src/backend/postmaster/postmaster.c

Lines changed: 21 additions & 8 deletions
Original file line numberDiff line numberDiff line change
@@ -611,6 +611,15 @@ PostmasterMain(int argc, char *argv[])
611611
/*
612612
* Set up signal handlers for the postmaster process.
613613
*
614+
* In the postmaster, we want to install non-ignored handlers *without*
615+
* SA_RESTART. This is because they'll be blocked at all times except
616+
* when ServerLoop is waiting for something to happen, and during that
617+
* window, we want signals to exit the select(2) wait so that ServerLoop
618+
* can respond if anything interesting happened. On some platforms,
619+
* signals marked SA_RESTART would not cause the select() wait to end.
620+
* Child processes will generally want SA_RESTART, but we expect them to
621+
* set up their own handlers before unblocking signals.
622+
*
614623
* CAUTION: when changing this list, check for side-effects on the signal
615624
* handling setup of child processes. See tcop/postgres.c,
616625
* bootstrap/bootstrap.c, postmaster/bgwriter.c, postmaster/walwriter.c,
@@ -621,16 +630,20 @@ PostmasterMain(int argc, char *argv[])
621630
pqinitmask();
622631
PG_SETMASK(&BlockSig);
623632

624-
pqsignal(SIGHUP,SIGHUP_handler);/* reread config file and have
625-
* children do same */
626-
pqsignal(SIGINT,pmdie);/* send SIGTERM and shut down */
627-
pqsignal(SIGQUIT,pmdie);/* send SIGQUIT and die */
628-
pqsignal(SIGTERM,pmdie);/* wait for children and shut down */
633+
pqsignal_no_restart(SIGHUP,SIGHUP_handler);/* reread config file
634+
* and have children do
635+
* same */
636+
pqsignal_no_restart(SIGINT,pmdie);/* send SIGTERM and shut down */
637+
pqsignal_no_restart(SIGQUIT,pmdie);/* send SIGQUIT and die */
638+
pqsignal_no_restart(SIGTERM,pmdie);/* wait for children and shut
639+
* down */
629640
pqsignal(SIGALRM,SIG_IGN);/* ignored */
630641
pqsignal(SIGPIPE,SIG_IGN);/* ignored */
631-
pqsignal(SIGUSR1,sigusr1_handler);/* message from child process */
632-
pqsignal(SIGUSR2,dummy_handler);/* unused, reserve for children */
633-
pqsignal(SIGCHLD,reaper);/* handle child termination */
642+
pqsignal_no_restart(SIGUSR1,sigusr1_handler);/* message from child
643+
* process */
644+
pqsignal_no_restart(SIGUSR2,dummy_handler);/* unused, reserve for
645+
* children */
646+
pqsignal_no_restart(SIGCHLD,reaper);/* handle child termination */
634647
pqsignal(SIGTTIN,SIG_IGN);/* ignored */
635648
pqsignal(SIGTTOU,SIG_IGN);/* ignored */
636649
/* ignore SIGXFSZ, so that ulimit violations work like disk full */

‎src/include/port.h

Lines changed: 5 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -469,6 +469,11 @@ extern intpg_mkdir_p(char *path, int omode);
469469
/* port/pqsignal.c */
470470
typedefvoid (*pqsigfunc) (intsigno);
471471
externpqsigfuncpqsignal(intsigno,pqsigfuncfunc);
472+
#ifndefWIN32
473+
externpqsigfuncpqsignal_no_restart(intsigno,pqsigfuncfunc);
474+
#else
475+
#definepqsignal_no_restart(signo,func) pqsignal(signo, func)
476+
#endif
472477

473478
/* port/quotes.c */
474479
externchar*escape_single_quotes_ascii(constchar*src);

‎src/port/pqsignal.c

Lines changed: 30 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -32,7 +32,7 @@
3232
#if !defined(WIN32)|| defined(FRONTEND)
3333

3434
/*
35-
* Set up a signal handler for signal "signo"
35+
* Set up a signal handler, with SA_RESTART, for signal "signo"
3636
*
3737
* Returns the previous handler.
3838
*/
@@ -58,4 +58,33 @@ pqsignal(int signo, pqsigfunc func)
5858
#endif
5959
}
6060

61+
/*
62+
* Set up a signal handler, without SA_RESTART, for signal "signo"
63+
*
64+
* Returns the previous handler.
65+
*
66+
* On Windows, this would be identical to pqsignal(), so don't bother.
67+
*/
68+
#ifndefWIN32
69+
70+
pqsigfunc
71+
pqsignal_no_restart(intsigno,pqsigfuncfunc)
72+
{
73+
structsigactionact,
74+
oact;
75+
76+
act.sa_handler=func;
77+
sigemptyset(&act.sa_mask);
78+
act.sa_flags=0;
79+
#ifdefSA_NOCLDSTOP
80+
if (signo==SIGCHLD)
81+
act.sa_flags |=SA_NOCLDSTOP;
82+
#endif
83+
if (sigaction(signo,&act,&oact)<0)
84+
returnSIG_ERR;
85+
returnoact.sa_handler;
86+
}
87+
88+
#endif/* !WIN32 */
89+
6190
#endif/* !defined(WIN32) || defined(FRONTEND) */

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