aio: Add io_method=io_uring

Performing AIO using io_uring can be considerably faster than
io_method=worker, particularly when lots of small IOs are issued, as
a) the context-switch overhead for worker based AIO becomes more significant
b) the number of IO workers can become limiting

io_uring, however, is linux specific and requires an additional compile-time
dependency (liburing).

This implementation is fairly simple and there are substantial optimization
opportunities.

The description of the existing AIO_IO_COMPLETION wait event is updated to
make the difference between it and the new AIO_IO_URING_EXECUTION clearer.

Reviewed-by: Noah Misch <noah@leadboat.com>
Reviewed-by: Jakub Wartak <jakub.wartak@enterprisedb.com>
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.h

@@ -22,12 +22,20 @@
 #include "storage/procnumber.h"
 
 
+/* io_uring is incompatible with EXEC_BACKEND */
+#if defined(USE_LIBURING) && !defined(EXEC_BACKEND)
+#define IOMETHOD_IO_URING_ENABLED
+#endif
+
 
 /* Enum for io_method GUC. */
 typedef enum IoMethod
 {
 	IOMETHOD_SYNC = 0,
 	IOMETHOD_WORKER,
+#ifdef IOMETHOD_IO_URING_ENABLED
+	IOMETHOD_IO_URING,
+#endif
 } IoMethod;
 
 /* We'll default to worker based execution. */

src/include/storage/aio_internal.h

@@ -248,6 +248,15 @@ typedef struct PgAioCtl
  */
 typedef struct IoMethodOps
 {
+	/* properties */
+
+	/*
+	 * If an FD is about to be closed, do we need to wait for all in-flight
+	 * IOs referencing that FD?
+	 */
+	bool		wait_on_fd_before_close;
+
+
 	/* global initialization */
 
 	/*
@@ -314,6 +323,7 @@ extern PgAioResult pgaio_io_call_complete_local(PgAioHandle *ioh);
 /* aio_io.c */
 extern void pgaio_io_perform_synchronously(PgAioHandle *ioh);
 extern const char *pgaio_io_get_op_name(PgAioHandle *ioh);
+extern bool pgaio_io_uses_fd(PgAioHandle *ioh, int fd);
 
 /* aio_target.c */
 extern bool pgaio_io_can_reopen(PgAioHandle *ioh);
@@ -386,6 +396,9 @@ extern PgAioHandle *pgaio_inj_io_get(void);
 /* Declarations for the tables of function pointers exposed by each IO method. */
 extern PGDLLIMPORT const IoMethodOps pgaio_sync_ops;
 extern PGDLLIMPORT const IoMethodOps pgaio_worker_ops;
+#ifdef IOMETHOD_IO_URING_ENABLED
+extern PGDLLIMPORT const IoMethodOps pgaio_uring_ops;
+#endif
 
 extern PGDLLIMPORT const IoMethodOps *pgaio_method_ops;
 extern PGDLLIMPORT PgAioCtl *pgaio_ctl;

src/include/storage/lwlock.h

@@ -218,6 +218,7 @@ typedef enum BuiltinTrancheIds
 	LWTRANCHE_SUBTRANS_SLRU,
 	LWTRANCHE_XACT_SLRU,
 	LWTRANCHE_PARALLEL_VACUUM_DSA,
+	LWTRANCHE_AIO_URING_COMPLETION,
 	LWTRANCHE_FIRST_USER_DEFINED,
 }			BuiltinTrancheIds;