Fix documentation about DROP DATABASE FORCE process termination rights.

Specifically, it terminates a background worker even if the caller
couldn't terminate the background worker with pg_terminate_backend().
Commit 3a9b18b neglected to update
this.  Back-patch to v13, which introduced DROP DATABASE FORCE.

Reviewed by Amit Kapila.  Reported by Kirill Reshke.

Discussion: https://postgr.es/m/20240429212756.60.nmisch@google.com

Author: nmisch

doc/src/sgml/ref/drop_database.sgml

@@ -79,12 +79,14 @@ DROP DATABASE [ IF EXISTS ] <replaceable class="parameter">name</replaceable> [
       It doesn't terminate if prepared transactions, active logical replication
       slots or subscriptions are present in the target database.
      </para>
+     <!-- not mentioning exception for autovacuum workers, since those are an
+     implementation detail and the exception is not specific to FORCE -->
      <para>
-      This will fail if the current user has no permissions to terminate other
-      connections. Required permissions are the same as with
-      <literal>pg_terminate_backend</literal>, described in
-      <xref linkend="functions-admin-signal"/>.  This will also fail if we
-      are not able to terminate connections.
+      This terminates background worker connections and connections that the
+      current user has permission to terminate
+      with <function>pg_terminate_backend</function>, described in
+      <xref linkend="functions-admin-signal"/>.  If connections would remain,
+      this command will fail.
      </para>
     </listitem>
    </varlistentry>

src/backend/storage/ipc/procarray.c

@@ -3808,8 +3808,8 @@ CountOtherDBBackends(Oid databaseId, int *nbackends, int *nprepared)
  * The current backend is always ignored; it is caller's responsibility to
  * check whether the current backend uses the given DB, if it's important.
  *
- * It doesn't allow to terminate the connections even if there is a one
- * backend with the prepared transaction in the target database.
+ * If the target database has a prepared transaction or permissions checks
+ * fail for a connection, this fails without terminating anything.
  */
 void
 TerminateOtherDBBackends(Oid databaseId)
@@ -3854,14 +3854,19 @@ TerminateOtherDBBackends(Oid databaseId)
 		ListCell   *lc;
 
 		/*
-		 * Check whether we have the necessary rights to terminate other
-		 * sessions.  We don't terminate any session until we ensure that we
-		 * have rights on all the sessions to be terminated.  These checks are
-		 * the same as we do in pg_terminate_backend.
+		 * Permissions checks relax the pg_terminate_backend checks in two
+		 * ways, both by omitting the !OidIsValid(proc->roleId) check:
 		 *
-		 * In this case we don't raise some warnings - like "PID %d is not a
-		 * PostgreSQL server process", because for us already finished session
-		 * is not a problem.
+		 * - Accept terminating autovacuum workers, since DROP DATABASE
+		 * without FORCE terminates them.
+		 *
+		 * - Accept terminating bgworkers.  For bgworker authors, it's
+		 * convenient to be able to recommend FORCE if a worker is blocking
+		 * DROP DATABASE unexpectedly.
+		 *
+		 * Unlike pg_terminate_backend, we don't raise some warnings - like
+		 * "PID %d is not a PostgreSQL server process", because for us already
+		 * finished session is not a problem.
 		 */
 		foreach(lc, pids)
 		{
@@ -3870,15 +3875,13 @@ TerminateOtherDBBackends(Oid databaseId)
 
 			if (proc != NULL)
 			{
-				/* Only allow superusers to signal superuser-owned backends. */
 				if (superuser_arg(proc->roleId) && !superuser())
 					ereport(ERROR,
 							(errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
 							 errmsg("permission denied to terminate process"),
 							 errdetail("Only roles with the %s attribute may terminate processes of roles with the %s attribute.",
 									   "SUPERUSER", "SUPERUSER")));
 
-				/* Users can signal backends they have role membership in. */
 				if (!has_privs_of_role(GetUserId(), proc->roleId) &&
 					!has_privs_of_role(GetUserId(), ROLE_PG_SIGNAL_BACKEND))
 					ereport(ERROR,