Fix interaction of partitioned tables with BulkInsertState.

When copying into a partitioned table, the target heap may change from
one tuple to next.  We must ask ReadBufferBI() to get a new buffer
every time such change occurs.  To do that, use new function
ReleaseBulkInsertStatePin().  This fixes the bug that tuples ended up
being inserted into the wrong partition, which occurred exactly
because the wrong buffer was used.

Amit Langote, per a suggestion from Robert Haas.  Some cosmetic
adjustments by me.

Reports by 高增琦 (Gao Zengqi), Venkata B Nagothi, and
Ragnar Ouchterlony.

Discussion: http://postgr.es/m/CAFmBtr32FDOqofo8yG-4mjzL1HnYHxXK5S9OGFJ%3D%3DcJpgEW4vA%40mail.gmail.com
Discussion: http://postgr.es/m/CAEyp7J9WiX0L3DoiNcRrY-9iyw%3DqP%2Bj%3DDLsAnNFF1xT2J1ggfQ%40mail.gmail.com
Discussion: http://postgr.es/m/16d73804-c9cd-14c5-463e-5caad563ff77%40agama.tv
Discussion: http://postgr.es/m/CA+TgmoaiZpDVUUN8LZ4jv1qFE_QyR+H9ec+79f5vNczYarg5Zg@mail.gmail.com

src/backend/access/heap/heapam.c

@@ -2324,6 +2324,17 @@ FreeBulkInsertState(BulkInsertState bistate)
 	pfree(bistate);
 }
 
+/*
+ * ReleaseBulkInsertStatePin - release a buffer currently held in bistate
+ */
+void
+ReleaseBulkInsertStatePin(BulkInsertState bistate)
+{
+	if (bistate->current_buf != InvalidBuffer)
+		ReleaseBuffer(bistate->current_buf);
+	bistate->current_buf = InvalidBuffer;
+}
+
 
 /*
  *	heap_insert		- insert tuple into a heap

src/backend/commands/copy.c

@@ -2307,6 +2307,7 @@ CopyFrom(CopyState cstate)
 	uint64		processed = 0;
 	bool		useHeapMultiInsert;
 	int			nBufferedTuples = 0;
+	int			prev_leaf_part_index = -1;
 
 #define MAX_BUFFERED_TUPLES 1000
 	HeapTuple  *bufferedTuples = NULL;	/* initialize to silence warning */
@@ -2561,6 +2562,17 @@ CopyFrom(CopyState cstate)
 			Assert(leaf_part_index >= 0 &&
 				   leaf_part_index < cstate->num_partitions);
 
+			/*
+			 * If this tuple is mapped to a partition that is not same as the
+			 * previous one, we'd better make the bulk insert mechanism gets a
+			 * new buffer.
+			 */
+			if (prev_leaf_part_index != leaf_part_index)
+			{
+				ReleaseBulkInsertStatePin(bistate);
+				prev_leaf_part_index = leaf_part_index;
+			}
+
 			/*
 			 * Save the old ResultRelInfo and switch to the one corresponding
 			 * to the selected partition.

src/include/access/heapam.h

@@ -147,6 +147,7 @@ extern void setLastTid(const ItemPointer tid);
 
 extern BulkInsertState GetBulkInsertState(void);
 extern void FreeBulkInsertState(BulkInsertState);
+extern void ReleaseBulkInsertStatePin(BulkInsertState bistate);
 
 extern Oid heap_insert(Relation relation, HeapTuple tup, CommandId cid,
 			int options, BulkInsertState bistate);