MAXALIGN the target address where we store flattened value.

The API (EOH_flatten_into) that flattens the expanded value representation
expects the target address to be maxaligned.  All it's usage adhere to that
principle except when serializing datums for parallel query.  Fix that
usage.

Diagnosed-by: Tom Lane
Author: Tom Lane and Amit Kapila
Backpatch-through: 9.6
Discussion: https://postgr.es/m/11629.1536550032@sss.pgh.pa.us

src/backend/utils/adt/datum.c

@@ -338,8 +338,19 @@ datumSerialize(Datum value, bool isnull, bool typByVal, int typLen,
 		}
 		else if (eoh)
 		{
-			EOH_flatten_into(eoh, (void *) *start_address, header);
+			char	   *tmp;
+
+			/*
+			 * EOH_flatten_into expects the target address to be maxaligned,
+			 * so we can't store directly to *start_address.
+			 */
+			tmp = (char *) palloc(header);
+			EOH_flatten_into(eoh, (void *) tmp, header);
+			memcpy(*start_address, tmp, header);
 			*start_address += header;
+
+			/* be tidy. */
+			pfree(tmp);
 		}
 		else
 		{

src/test/regress/expected/select_parallel.out

@@ -628,6 +628,33 @@ EXPLAIN (analyze, timing off, summary off, costs off) SELECT * FROM tenk1;
    ->  Parallel Seq Scan on tenk1 (actual rows=2000 loops=5)
 (4 rows)
 
+-- test passing expanded-value representations to workers
+CREATE FUNCTION make_some_array(int,int) returns int[] as
+$$declare x int[];
+  begin
+    x[1] := $1;
+    x[2] := $2;
+    return x;
+  end$$ language plpgsql parallel safe;
+CREATE TABLE fooarr(f1 text, f2 int[], f3 text);
+INSERT INTO fooarr VALUES('1', ARRAY[1,2], 'one');
+PREPARE pstmt(text, int[]) AS SELECT * FROM fooarr WHERE f1 = $1 AND f2 = $2;
+EXPLAIN (COSTS OFF) EXECUTE pstmt('1', make_some_array(1,2));
+                            QUERY PLAN                            
+------------------------------------------------------------------
+ Gather
+   Workers Planned: 3
+   ->  Parallel Seq Scan on fooarr
+         Filter: ((f1 = '1'::text) AND (f2 = '{1,2}'::integer[]))
+(4 rows)
+
+EXECUTE pstmt('1', make_some_array(1,2));
+ f1 |  f2   | f3  
+----+-------+-----
+ 1  | {1,2} | one
+(1 row)
+
+DEALLOCATE pstmt;
 -- provoke error in worker
 select stringu1::int2 from tenk1 where unique1 = 1;
 ERROR:  invalid input syntax for integer: "BAAAAA"

src/test/regress/sql/select_parallel.sql

@@ -218,6 +218,22 @@ explain (costs off)
 -- to increase the parallel query test coverage
 EXPLAIN (analyze, timing off, summary off, costs off) SELECT * FROM tenk1;
 
+-- test passing expanded-value representations to workers
+CREATE FUNCTION make_some_array(int,int) returns int[] as
+$$declare x int[];
+  begin
+    x[1] := $1;
+    x[2] := $2;
+    return x;
+  end$$ language plpgsql parallel safe;
+CREATE TABLE fooarr(f1 text, f2 int[], f3 text);
+INSERT INTO fooarr VALUES('1', ARRAY[1,2], 'one');
+
+PREPARE pstmt(text, int[]) AS SELECT * FROM fooarr WHERE f1 = $1 AND f2 = $2;
+EXPLAIN (COSTS OFF) EXECUTE pstmt('1', make_some_array(1,2));
+EXECUTE pstmt('1', make_some_array(1,2));
+DEALLOCATE pstmt;
+
 -- provoke error in worker
 select stringu1::int2 from tenk1 where unique1 = 1;