Remove PID from 'parallel worker' context message.

Discussion: <bfd204ab-ab1a-792a-b345-0274a09a4b5f@2ndquadrant.com>

src/backend/access/transam/parallel.c

@@ -788,7 +788,7 @@ HandleParallelMessage(ParallelContext *pcxt, int i, StringInfo msg)
 				 */
 				save_error_context_stack = error_context_stack;
 				errctx.callback = ParallelErrorContext;
-				errctx.arg = &pcxt->worker[i].pid;
+				errctx.arg = NULL;
 				errctx.previous = pcxt->error_context_stack;
 				error_context_stack = &errctx;
 
@@ -1095,7 +1095,7 @@ static void
 ParallelErrorContext(void *arg)
 {
 	if (force_parallel_mode != FORCE_PARALLEL_REGRESS)
-		errcontext("parallel worker, PID %d", *(int32 *) arg);
+		errcontext("parallel worker");
 }
 
 /*

src/test/regress/expected/select_parallel.out

@@ -64,8 +64,8 @@ explain (costs off)
 (5 rows)
 
 do $$begin
-  -- Provoke error in worker.  The original message CONTEXT contains a worker
-  -- PID that must be hidden in the test output.
+  -- Provoke error, possibly in worker.  If this error happens to occur in
+  -- the worker, there will be a CONTEXT line which must be hidden.
   perform stringu1::int2 from tenk1 where unique1 = 1;
   exception
 	when others then

src/test/regress/sql/select_parallel.sql

@@ -30,8 +30,8 @@ explain (costs off)
   select stringu1::int2 from tenk1 where unique1 = 1;
 
 do $$begin
-  -- Provoke error in worker.  The original message CONTEXT contains a worker
-  -- PID that must be hidden in the test output.
+  -- Provoke error, possibly in worker.  If this error happens to occur in
+  -- the worker, there will be a CONTEXT line which must be hidden.
   perform stringu1::int2 from tenk1 where unique1 = 1;
   exception
 	when others then