read_stream: Move logic about IO combining & issuing to helpers

The long if statements were hard to read and hard to document. Splitting them
into inline helpers makes it much easier to explain each part separately.

This is done in preparation for making the logic more complicated...

Reviewed-by: Melanie Plageman <melanieplageman@gmail.com>
Reviewed-by: Nazir Bilal Yavuz <byavuz81@gmail.com>
Discussion: https://postgr.es/m/f3xxfrkafjxpyqxywcxricxgyizjirfceychyxsgn7bwjp5eda@kwbduhy7tfmu

src/backend/storage/aio/read_stream.c

@@ -440,6 +440,78 @@ read_stream_start_pending_read(ReadStream *stream)
 	return true;
 }
 
+/*
+ * Should we continue to perform look ahead?  Looking ahead may allow us to
+ * make the pending IO larger via IO combining or to issue more read-ahead.
+ */
+static inline bool
+read_stream_should_look_ahead(ReadStream *stream)
+{
+	/* If the callback has signaled end-of-stream, we're done */
+	if (stream->distance == 0)
+		return false;
+
+	/* never start more IOs than our cap */
+	if (stream->ios_in_progress >= stream->max_ios)
+		return false;
+
+	/*
+	 * Don't start more read-ahead if that'd put us over the distance limit
+	 * for doing read-ahead. As stream->distance is capped by
+	 * max_pinned_buffers, this prevents us from looking ahead so far that it
+	 * would put us over the pin limit.
+	 */
+	if (stream->pinned_buffers + stream->pending_read_nblocks >= stream->distance)
+		return false;
+
+	return true;
+}
+
+/*
+ * We don't start the pending read just because we've hit the distance limit,
+ * preferring to give it another chance to grow to full io_combine_limit size
+ * once more buffers have been consumed.  But this is not desirable in all
+ * situations - see below.
+ */
+static inline bool
+read_stream_should_issue_now(ReadStream *stream)
+{
+	int16		pending_read_nblocks = stream->pending_read_nblocks;
+
+	/* there is no pending IO that could be issued */
+	if (pending_read_nblocks == 0)
+		return false;
+
+	/* never start more IOs than our cap */
+	if (stream->ios_in_progress >= stream->max_ios)
+		return false;
+
+	/*
+	 * If the callback has signaled end-of-stream, start the pending read
+	 * immediately. There is no further potential for IO combining.
+	 */
+	if (stream->distance == 0)
+		return true;
+
+	/*
+	 * If we've already reached io_combine_limit, there's no chance of growing
+	 * the read further.
+	 */
+	if (pending_read_nblocks >= stream->io_combine_limit)
+		return true;
+
+	/*
+	 * If we currently have no reads in flight or prepared, issue the IO once
+	 * we are not looking ahead further. This ensures there's always at least
+	 * one IO prepared.
+	 */
+	if (stream->pinned_buffers == 0 &&
+		!read_stream_should_look_ahead(stream))
+		return true;
+
+	return false;
+}
+
 static void
 read_stream_look_ahead(ReadStream *stream)
 {
@@ -452,14 +524,13 @@ read_stream_look_ahead(ReadStream *stream)
 	if (stream->batch_mode)
 		pgaio_enter_batchmode();
 
-	while (stream->ios_in_progress < stream->max_ios &&
-		   stream->pinned_buffers + stream->pending_read_nblocks < stream->distance)
+	while (read_stream_should_look_ahead(stream))
 	{
 		BlockNumber blocknum;
 		int16		buffer_index;
 		void	   *per_buffer_data;
 
-		if (stream->pending_read_nblocks == stream->io_combine_limit)
+		if (read_stream_should_issue_now(stream))
 		{
 			read_stream_start_pending_read(stream);
 			continue;
@@ -511,21 +582,13 @@ read_stream_look_ahead(ReadStream *stream)
 	}
 
 	/*
-	 * We don't start the pending read just because we've hit the distance
-	 * limit, preferring to give it another chance to grow to full
-	 * io_combine_limit size once more buffers have been consumed.  However,
-	 * if we've already reached io_combine_limit, or we've reached the
-	 * distance limit and there isn't anything pinned yet, or the callback has
-	 * signaled end-of-stream, we start the read immediately.  Note that the
-	 * pending read can exceed the distance goal, if the latter was reduced
-	 * after hitting the per-backend buffer limit.
+	 * Check if the pending read should be issued now, or if we should give it
+	 * another chance to grow to the full size.
+	 *
+	 * Note that the pending read can exceed the distance goal, if the latter
+	 * was reduced after hitting the per-backend buffer limit.
 	 */
-	if (stream->pending_read_nblocks > 0 &&
-		(stream->pending_read_nblocks == stream->io_combine_limit ||
-		 (stream->pending_read_nblocks >= stream->distance &&
-		  stream->pinned_buffers == 0) ||
-		 stream->distance == 0) &&
-		stream->ios_in_progress < stream->max_ios)
+	if (read_stream_should_issue_now(stream))
 		read_stream_start_pending_read(stream);
 
 	/*