From d572c9f86aee9fdebd91ec7f17662441e9d1e4c8 Mon Sep 17 00:00:00 2001
From: Andres Freund <andres@anarazel.de>
Date: Thu, 19 Mar 2026 23:06:58 -0400
Subject: [PATCH v17 18/18] Dirty hack to make read_stream_reset()/end() not
 wait for IO

Author:
Reviewed-by:
Discussion: https://postgr.es/m/
Backpatch:
---
 src/include/storage/aio.h             |  1 +
 src/backend/storage/aio/aio.c         | 27 ++++++++++++++++++++++
 src/backend/storage/aio/read_stream.c | 32 ++++++++++++++++++++++-----
 3 files changed, 54 insertions(+), 6 deletions(-)

diff --git a/src/include/storage/aio.h b/src/include/storage/aio.h
index ec543b784..c184e97a9 100644
--- a/src/include/storage/aio.h
+++ b/src/include/storage/aio.h
@@ -328,6 +328,7 @@ extern int	pgaio_wref_get_id(PgAioWaitRef *iow);
 extern void pgaio_wref_wait(PgAioWaitRef *iow);
 extern bool pgaio_wref_check_done(PgAioWaitRef *iow);
 
+extern void pgaio_wref_discard_result(PgAioWaitRef *iow);
 
 
 /* --------------------------------------------------------------------------------
diff --git a/src/backend/storage/aio/aio.c b/src/backend/storage/aio/aio.c
index 4e742038d..49eb677ad 100644
--- a/src/backend/storage/aio/aio.c
+++ b/src/backend/storage/aio/aio.c
@@ -1055,6 +1055,33 @@ pgaio_wref_check_done(PgAioWaitRef *iow)
 	return false;
 }
 
+void
+pgaio_wref_discard_result(PgAioWaitRef *iow)
+{
+	uint64		ref_generation;
+	bool		am_owner;
+	PgAioHandle *ioh;
+	PgAioHandleState state;
+
+	ioh = pgaio_io_from_wref(iow, &ref_generation);
+
+	am_owner = ioh->owner_procno == MyProcNumber;
+
+	if (!am_owner)
+		elog(ERROR, "not you");
+
+	if (pgaio_io_was_recycled(ioh, ref_generation, &state))
+		return;
+
+	pgaio_debug_io(DEBUG2, ioh,
+				   "discarding result %p",
+				   ioh->report_return);
+
+	if (ioh->resowner)
+		pgaio_io_release_resowner(&ioh->resowner_node, false);
+}
+
+
 
 
 /* --------------------------------------------------------------------------------
diff --git a/src/backend/storage/aio/read_stream.c b/src/backend/storage/aio/read_stream.c
index e3c16bd17..d30cda868 100644
--- a/src/backend/storage/aio/read_stream.c
+++ b/src/backend/storage/aio/read_stream.c
@@ -524,7 +524,7 @@ 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->distance <= 0) &&
 		stream->ios_in_progress < stream->max_ios)
 		read_stream_start_pending_read(stream);
 
@@ -534,7 +534,7 @@ 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->distance <= 0);
 
 	if (stream->batch_mode)
 		pgaio_exit_batchmode();
@@ -910,7 +910,7 @@ 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->distance <= 0)
 			return InvalidBuffer;
 
 		/*
@@ -924,7 +924,7 @@ 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->distance <= 0);
 			return InvalidBuffer;
 		}
 	}
@@ -952,7 +952,27 @@ read_stream_next_buffer(ReadStream *stream, void **per_buffer_data)
 		Assert(stream->ios[io_index].op.buffers ==
 			   &stream->buffers[oldest_buffer_index]);
 
-		needed_wait = WaitReadBuffers(&stream->ios[io_index].op);
+		/*
+		 * If the stream has been reset, don't even wait for the IO, just
+		 * discard it.
+		 */
+		if (stream->distance < 0)
+		{
+			if (pgaio_wref_valid(&stream->ios[io_index].op.io_wref) &&
+				!stream->ios[io_index].op.foreign_io)
+			{
+				pgaio_wref_discard_result(&stream->ios[io_index].op.io_wref);
+				pgaio_wref_clear(&stream->ios[io_index].op.io_wref);
+			}
+			else
+				WaitReadBuffers(&stream->ios[io_index].op);
+
+			needed_wait = false;
+		}
+		else
+		{
+			needed_wait = WaitReadBuffers(&stream->ios[io_index].op);
+		}
 
 		Assert(stream->ios_in_progress > 0);
 		stream->ios_in_progress--;
@@ -1129,7 +1149,7 @@ read_stream_reset(ReadStream *stream)
 	Buffer		buffer;
 
 	/* Stop looking ahead. */
-	stream->distance = 0;
+	stream->distance = -1;
 
 	/* Forget buffered block number and fast path state. */
 	stream->buffered_blocknum = InvalidBlockNumber;
-- 
2.53.0