Remove pg_wait_for_backend_termination().

It was unable to wait on a backend that had already left the procarray.
Users tolerant of that limitation can poll pg_stat_activity.  Other
users can employ the "timeout" argument of pg_terminate_backend().

Reviewed by Bharath Rupireddy.

Discussion: https://postgr.es/m/20210605013236.GA208701@rfd.leadboat.com

Author: nmisch

doc/src/sgml/func.sgml

@@ -25002,23 +25002,6 @@ SELECT collation for ('foo' COLLATE "de_DE");
         <literal>false</literal> is returned.
        </para></entry>
       </row>
-
-      <row>
-       <entry role="func_table_entry"><para role="func_signature">
-        <indexterm>
-         <primary>pg_wait_for_backend_termination</primary>
-        </indexterm>
-        <function>pg_wait_for_backend_termination</function> ( <parameter>pid</parameter> <type>integer</type>, <parameter>timeout</parameter> <type>bigint</type> <literal>DEFAULT</literal> <literal>5000</literal> )
-        <returnvalue>boolean</returnvalue>
-       </para>
-       <para>
-        Waits for the backend process with the specified Process ID to
-        terminate.  If the process terminates before
-        the <parameter>timeout</parameter> (in milliseconds)
-        expires, <literal>true</literal> is returned.  On timeout, a warning
-        is emitted and <literal>false</literal> is returned.
-       </para></entry>
-      </row>
      </tbody>
     </tgroup>
    </table>

doc/src/sgml/release-14.sgml

@@ -640,13 +640,7 @@ Author: Magnus Hagander <magnus@hagander.net>
 -->
 
       <para>
-       Add function <link
-       linkend="functions-admin-signal"><function>pg_wait_for_backend_termination()</function></link>
-       that waits for session exit (Bharath Rupireddy)
-      </para>
-
-      <para>
-       Also add a similar optional wait parameter to <link
+       Add an optional timeout parameter to <link
        linkend="functions-admin-signal"><function>pg_terminate_backend()</function></link>
       </para>
      </listitem>

src/backend/catalog/system_functions.sql

@@ -397,11 +397,6 @@ CREATE OR REPLACE FUNCTION
   RETURNS boolean STRICT VOLATILE LANGUAGE INTERNAL AS 'pg_terminate_backend'
   PARALLEL SAFE;
 
-CREATE OR REPLACE FUNCTION
-  pg_wait_for_backend_termination(pid integer, timeout int8 DEFAULT 5000)
-  RETURNS boolean STRICT VOLATILE LANGUAGE INTERNAL AS 'pg_wait_for_backend_termination'
-  PARALLEL SAFE;
-
 -- legacy definition for compatibility with 9.3
 CREATE OR REPLACE FUNCTION
   json_populate_record(base anyelement, from_json json, use_json_as_text boolean DEFAULT false)

src/backend/storage/ipc/signalfuncs.c

@@ -230,42 +230,6 @@ pg_terminate_backend(PG_FUNCTION_ARGS)
 		PG_RETURN_BOOL(r == SIGNAL_BACKEND_SUCCESS);
 }
 
-/*
- * Wait for a backend process with the given PID to exit or until the given
- * timeout milliseconds occurs. Returns true if the backend has exited. On
- * timeout a warning is emitted and false is returned.
- *
- * We allow any user to call this function, consistent with any user being
- * able to view the pid of the process in pg_stat_activity etc.
- */
-Datum
-pg_wait_for_backend_termination(PG_FUNCTION_ARGS)
-{
-	int			pid;
-	int64		timeout;
-	PGPROC	   *proc = NULL;
-
-	pid = PG_GETARG_INT32(0);
-	timeout = PG_GETARG_INT64(1);
-
-	if (timeout <= 0)
-		ereport(ERROR,
-				(errcode(ERRCODE_NUMERIC_VALUE_OUT_OF_RANGE),
-				 errmsg("\"timeout\" must not be negative or zero")));
-
-	proc = BackendPidGetProc(pid);
-
-	if (proc == NULL)
-	{
-		ereport(WARNING,
-				(errmsg("PID %d is not a PostgreSQL server process", pid)));
-
-		PG_RETURN_BOOL(false);
-	}
-
-	PG_RETURN_BOOL(pg_wait_until_termination(pid, timeout));
-}
-
 /*
  * Signal to reload the database configuration
  *

src/include/catalog/catversion.h

@@ -53,6 +53,6 @@
  */
 
 /*							yyyymmddN */
-#define CATALOG_VERSION_NO	202106101
+#define CATALOG_VERSION_NO	202106151
 
 #endif

src/include/catalog/pg_proc.dat

@@ -6190,10 +6190,6 @@
   proname => 'pg_terminate_backend', provolatile => 'v', prorettype => 'bool',
   proargtypes => 'int4 int8', proargnames => '{pid,timeout}',
   prosrc => 'pg_terminate_backend' },
-{ oid => '2137', descr => 'wait for a backend process exit or timeout occurs',
-  proname => 'pg_wait_for_backend_termination', provolatile => 'v',
-  prorettype => 'bool', proargtypes => 'int4 int8',
-  proargnames => '{pid,timeout}', prosrc => 'pg_wait_for_backend_termination' },
 { oid => '2172', descr => 'prepare for taking an online backup',
   proname => 'pg_start_backup', provolatile => 'v', proparallel => 'r',
   prorettype => 'pg_lsn', proargtypes => 'text bool bool',