aio: Infrastructure for io_method=worker

This commit contains the basic, system-wide, infrastructure for
io_method=worker. It does not yet actually execute IO, this commit just
provides the infrastructure for running IO workers, kept separate for easier
review.

The number of IO workers can be adjusted with a PGC_SIGHUP GUC. Eventually
we'd like to make the number of workers dynamically scale up/down based on the
current "IO load".

To allow the number of IO workers to be increased without a restart, we need
to reserve PGPROC entries for the workers unconditionally. This has been
judged to be worth the cost. If it turns out to be problematic, we can
introduce a PGC_POSTMASTER GUC to control the maximum number.

As io workers might be needed during shutdown, e.g. for AIO during the
shutdown checkpoint, a new PMState phase is added. IO workers are shut down
after the shutdown checkpoint has been performed and walsender/archiver have
shut down, but before the checkpointer itself shuts down. See also
87a6690cc6.

Updates PGSTAT_FILE_FORMAT_ID due to the addition of a new BackendType.

Reviewed-by: Noah Misch <noah@leadboat.com>
Co-authored-by: Thomas Munro <thomas.munro@gmail.com>
Co-authored-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/uvrtrknj4kdytuboidbhwclo4gxhswwcpgadptsjvjqcluzmah%40brqs62irg4dt
Discussion: https://postgr.es/m/20210223100344.llw5an2aklengrmn@alap3.anarazel.de
Discussion: https://postgr.es/m/stj36ea6yyhoxtqkhpieia2z4krnam7qyetc57rfezgk4zgapf@gcnactj4z56m

src/include/storage/aio_subsys.h

@@ -30,4 +30,8 @@ extern void pgaio_init_backend(void);
 extern void pgaio_error_cleanup(void);
 extern void AtEOXact_Aio(bool is_commit);
 
+
+/* aio_worker.c */
+extern bool pgaio_workers_enabled(void);
+
 #endif							/* AIO_SUBSYS_H */

src/include/storage/io_worker.h

@@ -0,0 +1,22 @@
+/*-------------------------------------------------------------------------
+ *
+ * io_worker.h
+ *    IO worker for implementing AIO "ourselves"
+ *
+ *
+ * Portions Copyright (c) 1996-2025, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * src/include/storage/io.h
+ *
+ *-------------------------------------------------------------------------
+ */
+#ifndef IO_WORKER_H
+#define IO_WORKER_H
+
+
+pg_noreturn extern void IoWorkerMain(const void *startup_data, size_t startup_data_len);
+
+extern PGDLLIMPORT int io_workers;
+
+#endif							/* IO_WORKER_H */

src/include/storage/proc.h

@@ -449,7 +449,9 @@ extern PGDLLIMPORT PGPROC *PreparedXactProcs;
  * 2 slots, but WAL writer is launched only after startup has exited, so we
  * only need 6 slots.
  */
-#define NUM_AUXILIARY_PROCS		6
+#define MAX_IO_WORKERS          32
+#define NUM_AUXILIARY_PROCS		(6 + MAX_IO_WORKERS)
+
 
 /* configurable options */
 extern PGDLLIMPORT int DeadlockTimeout;