From 2df6f67d199cfc0e0aa321830df2ee712057c61a Mon Sep 17 00:00:00 2001
From: Thomas Munro <thomas.munro@gmail.com>
Date: Mon, 21 Jul 2025 13:58:47 +1200
Subject: [PATCH 1/3] aio: Refactor read_stream.c end-of-stream state.

Previously, stream->distance was set to 0 to indicate that the end of
the stream had been reached.  In preparation for a later commit that
preserves the distance across reset operations, introduce a separate
end_of_stream flag instead, so it survives across restart operations.
No externally visible effects.

Discussion: https://postgr.es/m/CA%2BhUKG%2BWWr4-8TYemyU%3DucQsNe6bUBN_Sq3mCnBoBtxaJ9w3ug%40mail.gmail.com
---
 src/backend/storage/aio/read_stream.c | 21 ++++++++++++++-------
 1 file changed, 14 insertions(+), 7 deletions(-)

diff --git a/src/backend/storage/aio/read_stream.c b/src/backend/storage/aio/read_stream.c
index 0e7f5557f5c..5de6f83c253 100644
--- a/src/backend/storage/aio/read_stream.c
+++ b/src/backend/storage/aio/read_stream.c
@@ -137,6 +137,7 @@ struct ReadStream
 	int16		next_io_index;
 
 	bool		fast_path;
+	bool		end_of_stream;
 
 	/* Circular queue of buffers. */
 	int16		oldest_buffer_index;	/* Next pinned buffer to return */
@@ -429,6 +430,9 @@ read_stream_look_ahead(ReadStream *stream)
 			continue;
 		}
 
+		if (stream->end_of_stream)
+			break;
+
 		/*
 		 * See which block the callback wants next in the stream.  We need to
 		 * compute the index of the Nth block of the pending read including
@@ -443,7 +447,7 @@ read_stream_look_ahead(ReadStream *stream)
 		if (blocknum == InvalidBlockNumber)
 		{
 			/* End of stream. */
-			stream->distance = 0;
+			stream->end_of_stream = true;
 			break;
 		}
 
@@ -488,7 +492,7 @@ read_stream_look_ahead(ReadStream *stream)
 		(stream->pending_read_nblocks == stream->io_combine_limit ||
 		 (stream->pending_read_nblocks >= stream->distance &&
 		  stream->pinned_buffers == 0) ||
-		 stream->distance == 0) &&
+		 stream->end_of_stream) &&
 		stream->ios_in_progress < stream->max_ios)
 		read_stream_start_pending_read(stream);
 
@@ -498,7 +502,7 @@ read_stream_look_ahead(ReadStream *stream)
 	 * stream.  In the worst case we can always make progress one buffer at a
 	 * time.
 	 */
-	Assert(stream->pinned_buffers > 0 || stream->distance == 0);
+	Assert(stream->pinned_buffers > 0 || stream->end_of_stream);
 
 	if (stream->batch_mode)
 		pgaio_exit_batchmode();
@@ -789,6 +793,7 @@ read_stream_next_buffer(ReadStream *stream, void **per_buffer_data)
 		Assert(stream->forwarded_buffers == 0);
 		Assert(stream->pinned_buffers == 1);
 		Assert(stream->distance == 1);
+		Assert(stream->end_of_stream == false);
 		Assert(stream->pending_read_nblocks == 0);
 		Assert(stream->per_buffer_data_size == 0);
 		Assert(stream->initialized_buffers > stream->oldest_buffer_index);
@@ -841,7 +846,7 @@ read_stream_next_buffer(ReadStream *stream, void **per_buffer_data)
 		else
 		{
 			/* No more blocks, end of stream. */
-			stream->distance = 0;
+			stream->end_of_stream = true;
 			stream->oldest_buffer_index = stream->next_buffer_index;
 			stream->pinned_buffers = 0;
 			stream->buffers[oldest_buffer_index] = InvalidBuffer;
@@ -857,7 +862,7 @@ read_stream_next_buffer(ReadStream *stream, void **per_buffer_data)
 		Assert(stream->oldest_buffer_index == stream->next_buffer_index);
 
 		/* End of stream reached?  */
-		if (stream->distance == 0)
+		if (stream->end_of_stream)
 			return InvalidBuffer;
 
 		/*
@@ -871,7 +876,7 @@ read_stream_next_buffer(ReadStream *stream, void **per_buffer_data)
 		/* End of stream reached? */
 		if (stream->pinned_buffers == 0)
 		{
-			Assert(stream->distance == 0);
+			Assert(stream->end_of_stream);
 			return InvalidBuffer;
 		}
 	}
@@ -976,6 +981,7 @@ read_stream_next_buffer(ReadStream *stream, void **per_buffer_data)
 		stream->forwarded_buffers == 0 &&
 		stream->pinned_buffers == 1 &&
 		stream->distance == 1 &&
+		stream->end_of_stream == false &&
 		stream->pending_read_nblocks == 0 &&
 		stream->per_buffer_data_size == 0)
 	{
@@ -1013,7 +1019,7 @@ read_stream_reset(ReadStream *stream)
 	Buffer		buffer;
 
 	/* Stop looking ahead. */
-	stream->distance = 0;
+	stream->end_of_stream = true;
 
 	/* Forget buffered block number and fast path state. */
 	stream->buffered_blocknum = InvalidBlockNumber;
@@ -1046,6 +1052,7 @@ read_stream_reset(ReadStream *stream)
 
 	/* Start off assuming data is cached. */
 	stream->distance = 1;
+	stream->end_of_stream = false;
 }
 
 /*
-- 
2.39.5 (Apple Git-154)