aio: Basic subsystem initialization

This commit just does the minimal wiring up of the AIO subsystem, added in the
next commit, to the rest of the system. The next commit contains more details
about motivation and architecture.

This commit is kept separate to make it easier to review, separating the
changes across the tree, from the implementation of the new subsystem.

We discussed squashing this commit with the main commit before merging AIO,
but there has been a mild preference for keeping it separate.

Reviewed-by: Heikki Linnakangas <hlinnaka@iki.fi>
Reviewed-by: Noah Misch <noah@leadboat.com>
Discussion: https://postgr.es/m/uvrtrknj4kdytuboidbhwclo4gxhswwcpgadptsjvjqcluzmah%40brqs62irg4dt

5 files changed, 134 insertions, 0 deletions

diff --git a/src/backend/storage/aio/Makefile b/src/backend/storage/aio/Makefile
index 2f29a9ec4d1..eaeaeeee8e3 100644
--- a/src/backend/storage/aio/Makefile
+++ b/src/backend/storage/aio/Makefile
@@ -9,6 +9,8 @@ top_builddir = ../../../..
 include $(top_builddir)/src/Makefile.global
 
 OBJS = \
+	aio.o \
+	aio_init.o \
 	read_stream.o
 
 include $(top_srcdir)/src/backend/common.mk
diff --git a/src/backend/storage/aio/aio.c b/src/backend/storage/aio/aio.c
new file mode 100644
index 00000000000..828a94efdc3
--- /dev/null
+++ b/src/backend/storage/aio/aio.c
@@ -0,0 +1,90 @@
+/*-------------------------------------------------------------------------
+ *
+ * aio.c
+ *    AIO - Core Logic
+ *
+ * Portions Copyright (c) 1996-2025, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * IDENTIFICATION
+ *    src/backend/storage/aio/aio.c
+ *
+ *-------------------------------------------------------------------------
+ */
+
+#include "postgres.h"
+
+#include "lib/ilist.h"
+#include "storage/aio.h"
+#include "storage/aio_subsys.h"
+#include "utils/guc.h"
+#include "utils/guc_hooks.h"
+
+
+/* Options for io_method. */
+const struct config_enum_entry io_method_options[] = {
+	{"sync", IOMETHOD_SYNC, false},
+	{NULL, 0, false}
+};
+
+/* GUCs */
+int			io_method = DEFAULT_IO_METHOD;
+int			io_max_concurrency = -1;
+
+
+
+/*
+ * Release IO handle during resource owner cleanup.
+ */
+void
+pgaio_io_release_resowner(dlist_node *ioh_node, bool on_error)
+{
+}
+
+/*
+ * Perform AIO related cleanup after an error.
+ *
+ * This should be called early in the error recovery paths, as later steps may
+ * need to issue AIO (e.g. to record a transaction abort WAL record).
+ */
+void
+pgaio_error_cleanup(void)
+{
+}
+
+/*
+ * Perform AIO related checks at (sub-)transactional boundaries.
+ *
+ * This should be called late during (sub-)transactional commit/abort, after
+ * all steps that might need to perform AIO, so that we can verify that the
+ * AIO subsystem is in a valid state at the end of a transaction.
+ */
+void
+AtEOXact_Aio(bool is_commit)
+{
+}
+
+void
+assign_io_method(int newval, void *extra)
+{
+}
+
+bool
+check_io_max_concurrency(int *newval, void **extra, GucSource source)
+{
+	if (*newval == -1)
+	{
+		/*
+		 * Auto-tuning will be applied later during startup, as auto-tuning
+		 * depends on the value of various GUCs.
+		 */
+		return true;
+	}
+	else if (*newval == 0)
+	{
+		GUC_check_errdetail("Only -1 or values bigger than 0 are valid.");
+		return false;
+	}
+
+	return true;
+}
diff --git a/src/backend/storage/aio/aio_init.c b/src/backend/storage/aio/aio_init.c
new file mode 100644
index 00000000000..aeacc144149
--- /dev/null
+++ b/src/backend/storage/aio/aio_init.c
@@ -0,0 +1,37 @@
+/*-------------------------------------------------------------------------
+ *
+ * aio_init.c
+ *    AIO - Subsystem Initialization
+ *
+ * Portions Copyright (c) 1996-2025, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * IDENTIFICATION
+ *    src/backend/storage/aio/aio_init.c
+ *
+ *-------------------------------------------------------------------------
+ */
+
+#include "postgres.h"
+
+#include "storage/aio_subsys.h"
+
+
+
+Size
+AioShmemSize(void)
+{
+	Size		sz = 0;
+
+	return sz;
+}
+
+void
+AioShmemInit(void)
+{
+}
+
+void
+pgaio_init_backend(void)
+{
+}
diff --git a/src/backend/storage/aio/meson.build b/src/backend/storage/aio/meson.build
index 8abe0eb4863..c822fd4ddf7 100644
--- a/src/backend/storage/aio/meson.build
+++ b/src/backend/storage/aio/meson.build
@@ -1,5 +1,7 @@
 # Copyright (c) 2024-2025, PostgreSQL Global Development Group
 
 backend_sources += files(
+  'aio.c',
+  'aio_init.c',
   'read_stream.c',
 )
diff --git a/src/backend/storage/ipc/ipci.c b/src/backend/storage/ipc/ipci.c
index 174eed70367..2fa045e6b0f 100644
--- a/src/backend/storage/ipc/ipci.c
+++ b/src/backend/storage/ipc/ipci.c
@@ -37,6 +37,7 @@
 #include "replication/slotsync.h"
 #include "replication/walreceiver.h"
 #include "replication/walsender.h"
+#include "storage/aio_subsys.h"
 #include "storage/bufmgr.h"
 #include "storage/dsm.h"
 #include "storage/dsm_registry.h"
@@ -148,6 +149,7 @@ CalculateShmemSize(int *num_semaphores)
 	size = add_size(size, WaitEventCustomShmemSize());
 	size = add_size(size, InjectionPointShmemSize());
 	size = add_size(size, SlotSyncShmemSize());
+	size = add_size(size, AioShmemSize());
 
 	/* include additional requested shmem from preload libraries */
 	size = add_size(size, total_addin_request);
@@ -340,6 +342,7 @@ CreateOrAttachShmemStructs(void)
 	StatsShmemInit();
 	WaitEventCustomShmemInit();
 	InjectionPointShmemInit();
+	AioShmemInit();
 }
 
 /*