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

From: Bruce Momjian <maillist(at)candle(dot)pha(dot)pa(dot)us>
To: tgl(at)sss(dot)pgh(dot)pa(dot)us (Tom Lane)
Cc: hackers(at)postgreSQL(dot)org
Subject: Re: [HACKERS] Which signal to use for CANCEL from postmaster to backend?
Date: 1998-07-07 22:01:27
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers
> 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.

All very good points.  SIGINT is good.

> 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".

Patch applied.

> 			regards, tom lane
> PS: I've got these changes coded up, and am about to start testing.


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