From: Tom Lane <tgl@sss.pgh.pa.us>
To: vignesh C <vignesh21@gmail.com>
cc: PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Subject: Re: Printing backtrace of postgres processes
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Comments: In-reply-to vignesh C <vignesh21@gmail.com>
	message dated "Sun, 22 Nov 2020 08:36:25 +0530"
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Date: Sun, 22 Nov 2020 01:25:08 -0500
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vignesh C <vignesh21@gmail.com> writes:
> The idea here is to implement & expose pg_print_callstack function,
> internally what this function does is, the connected backend will send
> SIGUSR1 signal by setting PMSIGNAL_BACKTRACE_EMIT to the postmaster
> process. Postmaster process will send a SIGUSR1 signal to the process
> by setting PROCSIG_BACKTRACE_PRINT if the process has access to
> ProcSignal. As syslogger process & Stats process don't have access to
> ProcSignal, multiplexing with SIGUSR1 is not possible for these
> processes, hence SIGUSR2 signal will be sent for these processes. Once
> the process receives this signal it will log the backtrace of the
> process.

Surely this is *utterly* unsafe.  You can't do that sort of stuff in
a signal handler.

It might be all right to set a flag that would cause the next
CHECK_FOR_INTERRUPTS to print a backtrace, but I'm not sure
how useful that really is.

The proposed postmaster.c addition seems quite useless, as there
is exactly one stack trace it could ever log.

I would like to see some discussion of the security implications
of such a feature, as well.  ("There aren't any" is the wrong
answer.)

			regards, tom lane