Re: [HACKERS] Which signal to use for CANCEL from postmaster to backend?

From: Massimo Dal Zotto <dz(at)cs(dot)unitn(dot)it>
To: hackers(at)postgreSQL(dot)org (PostgreSQL Hackers)
Cc: tgl(at)sss(dot)pgh(dot)pa(dot)us (Tom Lane)
Subject: Re: [HACKERS] Which signal to use for CANCEL from postmaster to backend?
Date: 1998-07-09 21:01:12
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> Bruce Momjian <maillist(at)candle(dot)pha(dot)pa(dot)us> writes:
> >> All three of SIGINT, SIGHUP, SIGTERM currently do the same thing in a
> >> backend, so it looks like our best choice is to redefine one of those
> >> as the cancel request signal. Any preference?
> > I like SIGQUIT. Looks to be unused. SIGINT is used too much from the
> > command line, and SIGTERM used too much from scripts (the default kill
> > arg.)
> I've been thinking about it a little more, and on the whole I like using
> SIGINT the best. Here's my reasoning:
> 1. SIGINT is not normally generated for non-interactive processes,
> so for the ordinary case of a backend running under the postmaster
> it wouldn't be generated by accident. (As you point out, the default
> signal from kill(1) is SIGTERM not SIGINT.)
> 2. The only case where it *would* be easy to direct SIGINT to a backend
> is in the case of a backend hand-invoked from the command line. In this
> case I think that having control-C do a Cancel rather than kill the
> process is a fine idea --- it is exactly parallel to the behavior that
> psql now offers. People will probably get used enough to psql's
> behavior that having an interactive backend work differently would be
> a bad idea.
> 3. SIGQUIT normally generates a coredump, and is one of the few
> non-error signals that do so. If we use it for the cancel signal we
> will eliminate the easiest, most standard way of externally forcing a
> backend to coredump. That seems like a loss of a useful debug tool.
> Also, in all Unix applications that I'm familiar with, SIGQUIT is a
> "more severe" signal than SIGINT --- one expects that an app may trap
> SIGINT and return keyboard control, but SIGQUIT is supposed to kill it.
> Inverting the severity of SIGINT and SIGQUIT for a pgsql backend doesn't
> sound like a good plan.
> So I think we should leave SIGTERM and SIGQUIT alone, and redefine
> SIGINT to perform a cancel.
> BTW, I realized that SIGHUP is not really free, it's what is used to
> return control from elog(ERROR) to the main loop. The code probably
> should refer to it as SIGHUP not signal "1".
> regards, tom lane
> PS: I've got these changes coded up, and am about to start testing.

These are the signals I'm using in my own current version of pgsql.

The main changes to the old implementation are SIGQUIT instead of
SIGHUP to handle warns, SIGHUP to reread pg_options and redirection
to all backends of SIGHUP, SIGINT, SIGTERM, SIGUSR1 and SIGUSR2.
In this way some of the signals sent to the postmaster can be sent
automatically to all the backends. To shut down postgres one needs
only to send a SIGTERM to postmaster and it will stop automatically
all the backends. This new signal handling mechanism is also used
to prevent SI cache table overflows: when a backend detects the SI
table full at 70% it simply sends a signal to the postmaster which
will wake up all idle backends and make them flush the cache.

The use of SIGINT for the new functionality seems ok to me.
Given your considerations I would also use SIGCHLD instead of SIGQUIT
to handle warn from elog.

signal postmaster backend

SIGHUP kill(*,sighup) read_pg_options
SIGINT kill(*,sigint), die die
SIGCHLD reaper -
SIGTTIN ignored -
SIGTTOU ignored -
SIGQUIT die handle_warn
SIGTERM kill(*,sigterm), kill(*,9), die die
SIGCONT dumpstatus -
SIGPIPE ignored die
SIGFPE - FloatExceptionHandler
SIGTSTP - ignored (is_alive ?)
SIGUSR1 kill(*,sigusr1), die quickdie
SIGUSR2 kill(*,sigusr2) Async_NotifyHandler
(also SI buffer full)
Massimo Dal Zotto

| Massimo Dal Zotto e-mail: dz(at)cs(dot)unitn(dot)it |
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