Introduce pg_signal_autovacuum_worker.

Since commit 3a9b18b309, roles with privileges of pg_signal_backend
cannot signal autovacuum workers.  Many users treated the ability
to signal autovacuum workers as a feature instead of a bug, so we
are reintroducing it via a new predefined role.  Having privileges
of this new role, named pg_signal_autovacuum_worker, only permits
signaling autovacuum workers.  It does not permit signaling other
types of superuser backends.

Bumps catversion.

Author: Kirill Reshke
Reviewed-by: Anthony Leung, Michael Paquier, Andrey Borodin
Discussion: https://postgr.es/m/CALdSSPhC4GGmbnugHfB9G0%3DfAxjCSug_-rmL9oUh0LTxsyBfsg%40mail.gmail.com

doc/src/sgml/func.sgml

@@ -28163,6 +28163,10 @@ acl      | {postgres=arwdDxtm/postgres,foo=r/postgres}
         calling role is a member of the role whose backend is being canceled or
         the calling role has privileges of <literal>pg_signal_backend</literal>,
         however only superusers can cancel superuser backends.
+        As an exception, roles with privileges of
+        <literal>pg_signal_autovacuum_worker</literal> are permitted to
+        cancel autovacuum worker processes, which are otherwise considered
+        superuser backends.
        </para></entry>
       </row>
 
@@ -28237,6 +28241,10 @@ acl      | {postgres=arwdDxtm/postgres,foo=r/postgres}
         is a member of the role whose backend is being terminated or the
         calling role has privileges of <literal>pg_signal_backend</literal>,
         however only superusers can terminate superuser backends.
+        As an exception, roles with privileges of
+        <literal>pg_signal_autovacuum_worker</literal> are permitted to
+        terminate autovacuum worker processes, which are otherwise considered
+        superuser backends.
        </para>
        <para>
         If <parameter>timeout</parameter> is not specified or zero, this

doc/src/sgml/user-manag.sgml

@@ -661,6 +661,11 @@ DROP ROLE doomed_role;
        <entry>pg_signal_backend</entry>
        <entry>Signal another backend to cancel a query or terminate its session.</entry>
       </row>
+      <row>
+       <entry>pg_signal_autovacuum_worker</entry>
+       <entry>Signal an autovacuum worker to cancel the current table's vacuum
+       or terminate its session.</entry>
+      </row>
       <row>
        <entry>pg_read_server_files</entry>
        <entry>Allow reading files from any location the database can access on the server with COPY and

src/backend/storage/ipc/signalfuncs.c

@@ -34,8 +34,9 @@
  * role as the backend being signaled. For "dangerous" signals, an explicit
  * check for superuser needs to be done prior to calling this function.
  *
- * Returns 0 on success, 1 on general failure, 2 on normal permission error
- * and 3 if the caller needs to be a superuser.
+ * Returns 0 on success, 1 on general failure, 2 on normal permission error,
+ * 3 if the caller needs to be a superuser, and 4 if the caller needs to have
+ * privileges of pg_signal_autovacuum_worker.
  *
  * In the event of a general failure (return code 1), a warning message will
  * be emitted. For permission errors, doing that is the responsibility of
@@ -45,6 +46,7 @@
 #define SIGNAL_BACKEND_ERROR 1
 #define SIGNAL_BACKEND_NOPERMISSION 2
 #define SIGNAL_BACKEND_NOSUPERUSER 3
+#define SIGNAL_BACKEND_NOAUTOVAC 4
 static int
 pg_signal_backend(int pid, int sig)
 {
@@ -77,14 +79,26 @@ pg_signal_backend(int pid, int sig)
 	/*
 	 * Only allow superusers to signal superuser-owned backends.  Any process
 	 * not advertising a role might have the importance of a superuser-owned
-	 * backend, so treat it that way.
+	 * backend, so treat it that way.  As an exception, we allow roles with
+	 * privileges of pg_signal_autovacuum_worker to signal autovacuum workers
+	 * (which do not advertise a role).
+	 *
+	 * Otherwise, users can signal backends for roles they have privileges of.
 	 */
-	if ((!OidIsValid(proc->roleId) || superuser_arg(proc->roleId)) &&
-		!superuser())
-		return SIGNAL_BACKEND_NOSUPERUSER;
+	if (!OidIsValid(proc->roleId) || superuser_arg(proc->roleId))
+	{
+		ProcNumber	procNumber = GetNumberFromPGProc(proc);
+		PgBackendStatus *procStatus = pgstat_get_beentry_by_proc_number(procNumber);
 
-	/* Users can signal backends they have role membership in. */
-	if (!has_privs_of_role(GetUserId(), proc->roleId) &&
+		if (procStatus && procStatus->st_backendType == B_AUTOVAC_WORKER)
+		{
+			if (!has_privs_of_role(GetUserId(), ROLE_PG_SIGNAL_AUTOVACUUM_WORKER))
+				return SIGNAL_BACKEND_NOAUTOVAC;
+		}
+		else if (!superuser())
+			return SIGNAL_BACKEND_NOSUPERUSER;
+	}
+	else if (!has_privs_of_role(GetUserId(), proc->roleId) &&
 			 !has_privs_of_role(GetUserId(), ROLE_PG_SIGNAL_BACKEND))
 		return SIGNAL_BACKEND_NOPERMISSION;
 
@@ -130,6 +144,13 @@ pg_cancel_backend(PG_FUNCTION_ARGS)
 				 errdetail("Only roles with the %s attribute may cancel queries of roles with the %s attribute.",
 						   "SUPERUSER", "SUPERUSER")));
 
+	if (r == SIGNAL_BACKEND_NOAUTOVAC)
+		ereport(ERROR,
+				(errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
+				 errmsg("permission denied to cancel query"),
+				 errdetail("Only roles with privileges of the \"%s\" role may cancel autovacuum workers.",
+						   "pg_signal_autovacuum_worker")));
+
 	if (r == SIGNAL_BACKEND_NOPERMISSION)
 		ereport(ERROR,
 				(errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
@@ -236,6 +257,13 @@ pg_terminate_backend(PG_FUNCTION_ARGS)
 				 errdetail("Only roles with the %s attribute may terminate processes of roles with the %s attribute.",
 						   "SUPERUSER", "SUPERUSER")));
 
+	if (r == SIGNAL_BACKEND_NOAUTOVAC)
+		ereport(ERROR,
+				(errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
+				 errmsg("permission denied to terminate process"),
+				 errdetail("Only roles with privileges of the \"%s\" role may terminate autovacuum workers.",
+						   "pg_signal_autovacuum_worker")));
+
 	if (r == SIGNAL_BACKEND_NOPERMISSION)
 		ereport(ERROR,
 				(errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),

src/include/catalog/catversion.h

@@ -57,6 +57,6 @@
  */
 
 /*							yyyymmddN */
-#define CATALOG_VERSION_NO	202407082
+#define CATALOG_VERSION_NO	202407091
 
 #endif

src/include/catalog/pg_authid.dat

@@ -99,5 +99,10 @@
   rolcreaterole => 'f', rolcreatedb => 'f', rolcanlogin => 'f',
   rolreplication => 'f', rolbypassrls => 'f', rolconnlimit => '-1',
   rolpassword => '_null_', rolvaliduntil => '_null_' },
+{ oid => '8916', oid_symbol => 'ROLE_PG_SIGNAL_AUTOVACUUM_WORKER',
+  rolname => 'pg_signal_autovacuum_worker', rolsuper => 'f', rolinherit => 't',
+  rolcreaterole => 'f', rolcreatedb => 'f', rolcanlogin => 'f',
+  rolreplication => 'f', rolbypassrls => 'f', rolconnlimit => '-1',
+  rolpassword => '_null_', rolvaliduntil => '_null_' },
 
 ]