Improve API for storing data in read streams.

Read stream callbacks receive a void pointer into the per-buffer data
queue so that can store data there for later retrieval by the buffer
consumer.  We can improve readability and safety a bit by changing
cast-and-assign or raw memcpy() to:

	read_stream_put_value(stream, per_buffer_data, my_int);

This form infers the size and asserts that the storage space matches,
generally mirroring the read_stream_get_buffer_and_value() call used for
retrieving the streamed data later.

Author: macdice

src/backend/access/heap/vacuumlazy.c

@@ -1630,7 +1630,7 @@ heap_vac_scan_next_block(ReadStream *stream,
 		 */
 		vacrel->current_block = next_block;
 		blk_info |= VAC_BLK_ALL_VISIBLE_ACCORDING_TO_VM;
-		*((uint8 *) per_buffer_data) = blk_info;
+		read_stream_put_value(stream, per_buffer_data, blk_info);
 		return vacrel->current_block;
 	}
 	else
@@ -1646,7 +1646,7 @@ heap_vac_scan_next_block(ReadStream *stream,
 			blk_info |= VAC_BLK_ALL_VISIBLE_ACCORDING_TO_VM;
 		if (vacrel->next_unskippable_eager_scanned)
 			blk_info |= VAC_BLK_WAS_EAGER_SCANNED;
-		*((uint8 *) per_buffer_data) = blk_info;
+		read_stream_put_value(stream, per_buffer_data, blk_info);
 		return vacrel->current_block;
 	}
 }
@@ -2691,7 +2691,7 @@ vacuum_reap_lp_read_stream_next(ReadStream *stream,
 	 * Save the TidStoreIterResult for later, so we can extract the offsets.
 	 * It is safe to copy the result, according to TidStoreIterateNext().
 	 */
-	memcpy(per_buffer_data, iter_result, sizeof(*iter_result));
+	read_stream_put_value(stream, per_buffer_data, *iter_result);
 
 	return iter_result->blkno;
 }

src/include/storage/read_stream.h

@@ -157,4 +157,13 @@ read_stream_get_buffer_and_value_with_size(ReadStream *stream,
 	(AssertMacro(sizeof(**(pointer)) <= read_stream_per_buffer_data_size(stream)), \
 	 read_stream_next_buffer((stream), ((void **) (pointer))))
 
+/*
+ * Set the per-buffer data by value.  This can be called from inside a
+ * callback that is returning block numbers.  It asserts that the value's size
+ * matches the available space.
+ */
+#define read_stream_put_value(stream, per_buffer_data, value) \
+	(AssertMacro(sizeof(value) == read_stream_per_buffer_data_size(stream)), \
+	 memcpy((per_buffer_data), &(value), sizeof(value)))
+
 #endif							/* READ_STREAM_H */