Remove unused ParallelWorkerInfo.pid field

The pid was originally used in error context of messages propagated
from parallel workers, but commit 292794f removed that. If the need
arises in the future, you can also get the pid with
"shm_mq_get_sender(pcxt->worker[i].error_mqh)->pid".

Author: hlinnaka

src/backend/access/transam/parallel.c

@@ -1131,16 +1131,6 @@ HandleParallelMessage(ParallelContext *pcxt, int i, StringInfo msg)
 
 	switch (msgtype)
 	{
-		case PqMsg_BackendKeyData:
-			{
-				int32		pid = pq_getmsgint(msg, 4);
-
-				(void) pq_getmsgint(msg, 4);	/* discard cancel key */
-				(void) pq_getmsgend(msg);
-				pcxt->worker[i].pid = pid;
-				break;
-			}
-
 		case PqMsg_ErrorResponse:
 		case PqMsg_NoticeResponse:
 			{
@@ -1304,7 +1294,6 @@ ParallelWorkerMain(Datum main_arg)
 	char	   *relmapperspace;
 	char	   *uncommittedenumsspace;
 	char	   *clientconninfospace;
-	StringInfoData msgbuf;
 	char	   *session_dsm_handle_space;
 	Snapshot	tsnapshot;
 	Snapshot	asnapshot;
@@ -1369,18 +1358,6 @@ ParallelWorkerMain(Datum main_arg)
 	pq_set_parallel_leader(fps->parallel_leader_pid,
 						   fps->parallel_leader_proc_number);
 
-	/*
-	 * Send a BackendKeyData message to the process that initiated parallelism
-	 * so that it has access to our PID before it receives any other messages
-	 * from us.  Our cancel key is sent, too, since that's the way the
-	 * protocol message is defined, but it won't actually be used for anything
-	 * in this case.
-	 */
-	pq_beginmessage(&msgbuf, PqMsg_BackendKeyData);
-	pq_sendint32(&msgbuf, (int32) MyProcPid);
-	pq_sendint32(&msgbuf, (int32) MyCancelKey);
-	pq_endmessage(&msgbuf);
-
 	/*
 	 * Hooray! Primary initialization is complete.  Now, we need to set up our
 	 * backend-local state to match the original backend.

src/include/access/parallel.h

@@ -26,7 +26,6 @@ typedef struct ParallelWorkerInfo
 {
 	BackgroundWorkerHandle *bgwhandle;
 	shm_mq_handle *error_mqh;
-	int32		pid;
 } ParallelWorkerInfo;
 
 typedef struct ParallelContext