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/backend/storage/aio/Makefile

@@ -15,6 +15,7 @@ OBJS = \
 	aio_io.o \
 	aio_target.o \
 	method_sync.o \
+	method_worker.o \
 	read_stream.o
 
 include $(top_srcdir)/src/backend/common.mk

src/backend/storage/aio/meson.build

@@ -7,5 +7,6 @@ backend_sources += files(
   'aio_io.c',
   'aio_target.c',
   'method_sync.c',
+  'method_worker.c',
   'read_stream.c',
 )

src/backend/storage/aio/method_worker.c

@@ -0,0 +1,88 @@
+/*-------------------------------------------------------------------------
+ *
+ * method_worker.c
+ *    AIO - perform AIO using worker processes
+ *
+ * Portions Copyright (c) 1996-2025, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * IDENTIFICATION
+ *	  src/backend/storage/aio/method_worker.c
+ *
+ *-------------------------------------------------------------------------
+ */
+
+#include "postgres.h"
+
+#include "libpq/pqsignal.h"
+#include "miscadmin.h"
+#include "postmaster/auxprocess.h"
+#include "postmaster/interrupt.h"
+#include "storage/aio_subsys.h"
+#include "storage/io_worker.h"
+#include "storage/ipc.h"
+#include "storage/latch.h"
+#include "storage/proc.h"
+#include "tcop/tcopprot.h"
+#include "utils/wait_event.h"
+
+
+/* GUCs */
+int			io_workers = 3;
+
+
+void
+IoWorkerMain(const void *startup_data, size_t startup_data_len)
+{
+	sigjmp_buf	local_sigjmp_buf;
+
+	MyBackendType = B_IO_WORKER;
+	AuxiliaryProcessMainCommon();
+
+	pqsignal(SIGHUP, SignalHandlerForConfigReload);
+	pqsignal(SIGINT, die);		/* to allow manually triggering worker restart */
+
+	/*
+	 * Ignore SIGTERM, will get explicit shutdown via SIGUSR2 later in the
+	 * shutdown sequence, similar to checkpointer.
+	 */
+	pqsignal(SIGTERM, SIG_IGN);
+	/* SIGQUIT handler was already set up by InitPostmasterChild */
+	pqsignal(SIGALRM, SIG_IGN);
+	pqsignal(SIGPIPE, SIG_IGN);
+	pqsignal(SIGUSR1, procsignal_sigusr1_handler);
+	pqsignal(SIGUSR2, SignalHandlerForShutdownRequest);
+
+	/* see PostgresMain() */
+	if (sigsetjmp(local_sigjmp_buf, 1) != 0)
+	{
+		error_context_stack = NULL;
+		HOLD_INTERRUPTS();
+
+		EmitErrorReport();
+
+		proc_exit(1);
+	}
+
+	/* We can now handle ereport(ERROR) */
+	PG_exception_stack = &local_sigjmp_buf;
+
+	sigprocmask(SIG_SETMASK, &UnBlockSig, NULL);
+
+	while (!ShutdownRequestPending)
+	{
+		WaitLatch(MyLatch, WL_LATCH_SET | WL_EXIT_ON_PM_DEATH, -1,
+				  WAIT_EVENT_IO_WORKER_MAIN);
+		ResetLatch(MyLatch);
+		CHECK_FOR_INTERRUPTS();
+	}
+
+	proc_exit(0);
+}
+
+bool
+pgaio_workers_enabled(void)
+{
+	/* placeholder for future commit */
+	return false;
+}