Fix read_stream.c for changing io_combine_limit.

In a couple of places, read_stream.c assumed that io_combine_limit would
be stable during the lifetime of a stream.  That is not true in at least
one unusual case: streams held by CURSORs where you could change the GUC
between FETCH commands, with unpredictable results.

Fix, by storing stream->io_combine_limit and referring only to that
after construction.  This mirrors the treatment of the other important
setting {effective,maintenance}_io_concurrency, which is stored in
stream->max_ios.

One of the cases was the queue overflow space, which was sized for
io_combine_limit and could be overrun if the GUC was increased.  Since
that coding was a little hard to follow, also introduce a variable for
better readability instead of open-coding the arithmetic.  Doing so
revealed an off-by-one thinko while clamping max_pinned_buffers to
INT16_MAX, though that wasn't a live bug due to the current limits on
GUC values.

Back-patch to 17.

Discussion: https://postgr.es/m/CA%2BhUKG%2B2T9p-%2BzM6Eeou-RAJjTML6eit1qn26f9twznX59qtCA%40mail.gmail.com

Author: macdice

src/backend/storage/aio/read_stream.c

@@ -109,6 +109,7 @@ typedef struct InProgressIO
 struct ReadStream
 {
 	int16		max_ios;
+	int16		io_combine_limit;
 	int16		ios_in_progress;
 	int16		queue_size;
 	int16		max_pinned_buffers;
@@ -219,7 +220,7 @@ read_stream_start_pending_read(ReadStream *stream, bool suppress_advice)
 
 	/* This should only be called with a pending read. */
 	Assert(stream->pending_read_nblocks > 0);
-	Assert(stream->pending_read_nblocks <= io_combine_limit);
+	Assert(stream->pending_read_nblocks <= stream->io_combine_limit);
 
 	/* We had better not exceed the pin limit by starting this read. */
 	Assert(stream->pinned_buffers + stream->pending_read_nblocks <=
@@ -307,7 +308,7 @@ read_stream_look_ahead(ReadStream *stream, bool suppress_advice)
 		int16		buffer_index;
 		void	   *per_buffer_data;
 
-		if (stream->pending_read_nblocks == io_combine_limit)
+		if (stream->pending_read_nblocks == stream->io_combine_limit)
 		{
 			read_stream_start_pending_read(stream, suppress_advice);
 			suppress_advice = false;
@@ -367,7 +368,7 @@ read_stream_look_ahead(ReadStream *stream, bool suppress_advice)
 	 * signaled end-of-stream, we start the read immediately.
 	 */
 	if (stream->pending_read_nblocks > 0 &&
-		(stream->pending_read_nblocks == io_combine_limit ||
+		(stream->pending_read_nblocks == stream->io_combine_limit ||
 		 (stream->pending_read_nblocks == stream->distance &&
 		  stream->pinned_buffers == 0) ||
 		 stream->distance == 0) &&
@@ -396,6 +397,7 @@ read_stream_begin_relation(int flags,
 	ReadStream *stream;
 	size_t		size;
 	int16		queue_size;
+	int16		queue_overflow;
 	int			max_ios;
 	int			strategy_pin_limit;
 	uint32		max_pinned_buffers;
@@ -429,6 +431,14 @@ read_stream_begin_relation(int flags,
 	/* Cap to INT16_MAX to avoid overflowing below */
 	max_ios = Min(max_ios, PG_INT16_MAX);
 
+	/*
+	 * If starting a multi-block I/O near the end of the queue, we might
+	 * temporarily need extra space for overflowing buffers before they are
+	 * moved to regular circular position.  This is the maximum extra space we
+	 * could need.
+	 */
+	queue_overflow = io_combine_limit - 1;
+
 	/*
 	 * Choose the maximum number of buffers we're prepared to pin.  We try to
 	 * pin fewer if we can, though.  We clamp it to at least io_combine_limit
@@ -439,7 +449,7 @@ read_stream_begin_relation(int flags,
 	 */
 	max_pinned_buffers = Max(max_ios * 4, io_combine_limit);
 	max_pinned_buffers = Min(max_pinned_buffers,
-							 PG_INT16_MAX - io_combine_limit - 1);
+							 PG_INT16_MAX - queue_overflow - 1);
 
 	/* Give the strategy a chance to limit the number of buffers we pin. */
 	strategy_pin_limit = GetAccessStrategyPinLimit(strategy);
@@ -465,18 +475,17 @@ read_stream_begin_relation(int flags,
 	 * one big chunk.  Though we have queue_size buffers, we want to be able
 	 * to assume that all the buffers for a single read are contiguous (i.e.
 	 * don't wrap around halfway through), so we allow temporary overflows of
-	 * up to the maximum possible read size by allocating an extra
-	 * io_combine_limit - 1 elements.
+	 * up to the maximum possible overflow size.
 	 */
 	size = offsetof(ReadStream, buffers);
-	size += sizeof(Buffer) * (queue_size + io_combine_limit - 1);
+	size += sizeof(Buffer) * (queue_size + queue_overflow);
 	size += sizeof(InProgressIO) * Max(1, max_ios);
 	size += per_buffer_data_size * queue_size;
 	size += MAXIMUM_ALIGNOF * 2;
 	stream = (ReadStream *) palloc(size);
 	memset(stream, 0, offsetof(ReadStream, buffers));
 	stream->ios = (InProgressIO *)
-		MAXALIGN(&stream->buffers[queue_size + io_combine_limit - 1]);
+		MAXALIGN(&stream->buffers[queue_size + queue_overflow]);
 	if (per_buffer_data_size > 0)
 		stream->per_buffer_data = (void *)
 			MAXALIGN(&stream->ios[Max(1, max_ios)]);
@@ -503,7 +512,14 @@ read_stream_begin_relation(int flags,
 	if (max_ios == 0)
 		max_ios = 1;
 
+	/*
+	 * Capture stable values for these two GUC-derived numbers for the
+	 * lifetime of this stream, so we don't have to worry about the GUCs
+	 * changing underneath us beyond this point.
+	 */
 	stream->max_ios = max_ios;
+	stream->io_combine_limit = io_combine_limit;
+
 	stream->per_buffer_data_size = per_buffer_data_size;
 	stream->max_pinned_buffers = max_pinned_buffers;
 	stream->queue_size = queue_size;
@@ -517,7 +533,7 @@ read_stream_begin_relation(int flags,
 	 * doing full io_combine_limit sized reads (behavior B).
 	 */
 	if (flags & READ_STREAM_FULL)
-		stream->distance = Min(max_pinned_buffers, io_combine_limit);
+		stream->distance = Min(max_pinned_buffers, stream->io_combine_limit);
 	else
 		stream->distance = 1;
 
@@ -683,14 +699,14 @@ read_stream_next_buffer(ReadStream *stream, void **per_buffer_data)
 		else
 		{
 			/* No advice; move towards io_combine_limit (behavior B). */
-			if (stream->distance > io_combine_limit)
+			if (stream->distance > stream->io_combine_limit)
 			{
 				stream->distance--;
 			}
 			else
 			{
 				distance = stream->distance * 2;
-				distance = Min(distance, io_combine_limit);
+				distance = Min(distance, stream->io_combine_limit);
 				distance = Min(distance, stream->max_pinned_buffers);
 				stream->distance = distance;
 			}