From: Roman Khapov <rkhapov@yandex-team.ru>
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Subject: Additional message in pg_terminate_backend
Message-Id: <064E214D-D86F-4917-A0B6-67EAD6BCB24A@yandex-team.ru>
Date: Sat, 13 Dec 2025 12:44:46 +0500
To: pgsql-hackers@lists.postgresql.org
Archived-At: 
 <https://www.postgresql.org/message-id/064E214D-D86F-4917-A0B6-67EAD6BCB24A%40yandex-team.ru>
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Hi hackers!

Recently I started working on patch for adding additional message from =
admin=20
in pg_terminate_backend for one of our greenplum fork. The main idea is =
that there must be=20
done little investigation every time you see 'FATAL:  terminating =
connection due to administrator command=E2=80=99 to understand the =
reason, especially in cases where connection was terminated by another
user. So it was decided to create some new functions, that allows to =
terminate connection with
additional message.

I did POC patches with the next main ideas:
 - lets add termReasonStr field in every PGPROC, that field can be used =
in ProcessInterrupts()=20
 - implementation of pg_terminate_backend/pg_cancel_backend should be =
accessible from extensions, so lets move it in =
pg_terminate_backend_impl/pg_cancel_backend_impl and add definitions for =
it somewhere
 - write simple extensions, which defines functions like =
pg_terminate_backend_msg, that sets termReasonStr and calls =
pg_terminate_backend_impl

After patch and added extension, it is possible to do smth like:
postgres=3D# select pg_terminate_backend_msg(pg_backend_pid(), 0, =E2=80=99=
the message');
FATAL:  terminating connection due to administrator command: the message

The general question I want to ask: can this patches be useful for =
vanilla PostgreSQL?

If so, there are some questions about patch improvements:
 - maybe the message can be delivered to backend by some other way than =
the field in struct PGPROC?
termReasonStr field consumes some shared memory and are used in rare =
cases
 - names of all new funcs/fields/etc should be changed to some better =
names
 - new file signalfuncs.h seems like too complicated solutions to define =
*_impl functions, it can be done in
some other files?=20

-----
Best regards,
Roman Khapov


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DEFAULT=200,=20reason=20text=20DEFAULT=20'')=0A+RETURNS=20boolean=0A+AS=20=
'MODULE_PATHNAME',=20'pg_terminate_backend_msg'=0A+LANGUAGE=20C=20=
STRICT;=0A+=0A+CREATE=20FUNCTION=20pg_cancel_backend_msg(pid=20integer,=20=
reason=20text=20DEFAULT=20'')=0A+RETURNS=20boolean=0A+AS=20=
'MODULE_PATHNAME',=20'pg_cancel_backend_msg'=0A+LANGUAGE=20C=20STRICT;=0A=
diff=20--git=20a/contrib/pg_term_reason/pg_term_reason.c=20=
b/contrib/pg_term_reason/pg_term_reason.c=0Anew=20file=20mode=20100644=0A=
index=200000000000..5c6ceb00fd=0A---=20/dev/null=0A+++=20=
b/contrib/pg_term_reason/pg_term_reason.c=0A@@=20-0,0=20+1,89=20@@=0A=
+/*-----------------------------------------------------------------------=
--=0A+=20*=0A+=20*=20pg_term_reason.c=0A+=20*=09=20=20Functions=20to=20=
terminate/cancel=20postgres=20backends=20with=20additional=20message=0A+=20=
*=0A+=20*=20IDENTIFICATION=0A+=20*=09=20=20=
contrib/pg_term_reason/pg_term_reason.c=0A+=20*=0A+=20=
*-------------------------------------------------------------------------=
=0A+=20*/=0A+=0A+#include=20"postgres.h"=0A+=0A+#include=20"fmgr.h"=0A=
+#include=20"varatt.h"=0A+#include=20"storage/signalfuncs.h"=0A+#include=20=
"storage/lock.h"=0A+#include=20"storage/procarray.h"=0A+#include=20=
"storage/proc.h"=0A+=0A+PG_MODULE_MAGIC_EXT(=0A+=09=09=09=09=09.name=20=3D=
=20"pg_term_reason",=0A+=09=09=09=09=09.version=20=3D=20PG_VERSION=0A+);=0A=
+=0A+PG_FUNCTION_INFO_V1(pg_terminate_backend_msg);=0A=
+PG_FUNCTION_INFO_V1(pg_cancel_backend_msg);=0A+=0A+static=20void=0A=
+set_reason(int=20pid,=20const=20char=20*msg,=20int=20msglen)=0A+{=0A+=09=
int=09=09=09len;=0A+=09PGPROC=09=09*proc;=0A+=0A+=09if=20(msglen=20<=3D=20=
0)=20{=0A+=09=09return;=0A+=09}=0A+=0A+=09LWLockAcquire(ProcArrayLock,=20=
LW_SHARED);=0A+=0A+=09proc=20=3D=20BackendPidGetProcWithLock(pid);=0A+=0A=
+=09if=20(proc=20!=3D=20NULL)=20{=0A+=09=09len=20=3D=20=
Min(PROC_TERM_REASON_MAX_LEN=20-=201,=20msglen);=0A+=09=09=
strncpy(proc->termReasonStr,=20msg,=20len);=0A+=09}=0A+=0A+=09=
LWLockRelease(ProcArrayLock);=0A+}=0A+=0A+Datum=0A=
+pg_cancel_backend_msg(PG_FUNCTION_ARGS)=0A+{=0A+=09int=09=09=09pid;=0A+=09=
text=09=09*reason;=0A+=09char=09=09*reason_str;=0A+=09int=09=09=09=
reason_len;=0A+=0A+=09pid=20=3D=20PG_GETARG_INT32(0);=0A+=09reason=20=3D=20=
PG_GETARG_TEXT_P(1);=0A+=0A+=09reason_str=20=3D=20VARDATA(reason);=0A+=09=
reason_len=20=3D=20VARSIZE(reason)=20-=20VARHDRSZ;=0A+=0A+=09=
set_reason(pid,=20reason_str,=20reason_len);=0A+=0A+=09return=20=
pg_cancel_backend_impl(pid);=0A+}=0A+=0A+Datum=0A=
+pg_terminate_backend_msg(PG_FUNCTION_ARGS)=0A+{=0A+=09int=09=09=09pid;=0A=
+=09int=09=09=09timeout;=09=09/*=20milliseconds=20*/=0A+=09text=09=09=
*reason;=0A+=09char=09=09*reason_str;=0A+=09int=09=09=09reason_len;=0A+=0A=
+=09pid=20=3D=20PG_GETARG_INT32(0);=0A+=09timeout=20=3D=20=
PG_GETARG_INT64(1);=0A+=09reason=20=3D=20PG_GETARG_TEXT_P(2);=0A+=0A+=09=
reason_str=20=3D=20VARDATA(reason);=0A+=09reason_len=20=3D=20=
VARSIZE(reason)=20-=20VARHDRSZ;=0A+=0A+=09set_reason(pid,=20reason_str,=20=
reason_len);=0A+=0A+=09return=20pg_terminate_backend_impl(pid,=20=
timeout);=0A+}=0Adiff=20--git=20=
a/contrib/pg_term_reason/pg_term_reason.control=20=
b/contrib/pg_term_reason/pg_term_reason.control=0Anew=20file=20mode=20=
100644=0Aindex=200000000000..d3b04459aa=0A---=20/dev/null=0A+++=20=
b/contrib/pg_term_reason/pg_term_reason.control=0A@@=20-0,0=20+1,5=20@@=0A=
+#=20pg_term_reason=20extension=0A+comment=20=3D=20'extension=20to=20=
terminate=20backends=20with=20additional=20reason=20string'=0A=
+default_version=20=3D=20'1.0'=0A+module_pathname=20=3D=20=
'$libdir/pg_term_reason'=0A+relocatable=20=3D=20true=0Adiff=20--git=20=
a/contrib/pg_term_reason/t/001_basic.pl=20=
b/contrib/pg_term_reason/t/001_basic.pl=0Anew=20file=20mode=20100644=0A=
index=200000000000..5d9dadc462=0A---=20/dev/null=0A+++=20=
b/contrib/pg_term_reason/t/001_basic.pl=0A@@=20-0,0=20+1,28=20@@=0A+use=20=
strict;=0A+use=20warnings=20FATAL=20=3D>=20'all';=0A+use=20=
PostgreSQL::Test::Cluster;=0A+use=20PostgreSQL::Test::Utils;=0A+use=20=
Test::More;=0A+=0A+my=20$node=20=3D=20=
PostgreSQL::Test::Cluster->new('primary');=0A+$node->init();=0A=
+$node->start;=0A+=0A+$node->safe_psql('postgres',=20'create=20extension=20=
pg_term_reason;');=0A+=0A+my=20($stdout,=20$stderr);=0A=
+$node->psql('postgres',=0A+=09q[select=20=
pg_terminate_backend_msg(pg_backend_pid(),=200,=20'hello=20from=20tap=20=
tests!');],=0A+=09stdout=20=3D>=20\$stdout,=20stderr=20=3D>=20\$stderr);=0A=
+like($stderr,=20qr/hello=20from=20tap=20tests\!/,=20"expected=20message=20=
to=20be=20passed");=0A+=0A+$stdout=20=3D=20'';=0A+$stderr=20=3D=20'';=0A=
+$node->psql('postgres',=0A+=09q[select=20=
pg_cancel_backend_msg(pg_backend_pid(),=20'hello=20from=20tap=20tests=20=
again!');],=0A+=09stdout=20=3D>=20\$stdout,=20stderr=20=3D>=20\$stderr);=0A=
+like($stderr,=20qr/hello=20from=20tap=20tests=20again\!/,=20"expected=20=
message=20to=20be=20passed");=0A+=0A+$node->stop;=0A+=0A+done_testing();=0A=
--=20=0A2.50.1=20(Apple=20Git-155)=0A=0A=

--Apple-Mail=_FE8A0B23-0BE6-4862-85F8-A4F09E8E948C--