Make AIO handle callbacks extensible

The pg_tde extension's custom storage manager needs to add its own AIO
callback to decrypt pages before the buffer manager's callback is called
but since the set of AIO callbacks is fixed we need to expose this API
so that extensions can register their own types of callbacks. This new
function for registering a callback is supposed to be called when
intializing shared memory.

We try to keep the number of callbacks low, setting it to 15, so that we
have the 4 built-in callbacks and 11 slots that extensions can use.

Author: jeltz

src/backend/storage/aio/aio_callback.c

@@ -27,16 +27,16 @@ static const PgAioHandleCallbacks aio_invalid_cb = {0};
 
 typedef struct PgAioHandleCallbacksEntry
 {
-	const PgAioHandleCallbacks *const cb;
-	const char *const name;
+	const PgAioHandleCallbacks *cb;
+	const char *name;
 } PgAioHandleCallbacksEntry;
 
 /*
  * Callback definition for the callbacks that can be registered on an IO
  * handle.  See PgAioHandleCallbackID's definition for an explanation for why
  * callbacks are not identified by a pointer.
  */
-static const PgAioHandleCallbacksEntry aio_handle_cbs[] = {
+static PgAioHandleCallbacksEntry aio_handle_cbs[PGAIO_HCB_MAX] = {
 #define CALLBACK_ENTRY(id, callback)  [id] = {.cb = &callback, .name = #callback}
 	CALLBACK_ENTRY(PGAIO_HCB_INVALID, aio_invalid_cb),
 
@@ -48,7 +48,21 @@ static const PgAioHandleCallbacksEntry aio_handle_cbs[] = {
 #undef CALLBACK_ENTRY
 };
 
+static PgAioHandleCallbackID aio_handle_max_cb_id = PGAIO_HCB_LOCAL_BUFFER_READV;
 
+PgAioHandleCallbackID
+pgaio_io_register_callback_entry(const PgAioHandleCallbacks *callback, const char *name)
+{
+	PgAioHandleCallbackID cb_id = ++aio_handle_max_cb_id;
+
+	if (cb_id > PGAIO_HCB_MAX)
+		elog(FATAL, "There can be at most %d AIO callback entries", PGAIO_HCB_MAX);
+
+	aio_handle_cbs[cb_id].cb = callback;
+	aio_handle_cbs[cb_id].name = name;
+
+	return cb_id;
+}
 
 /* --------------------------------------------------------------------------------
  * Public callback related functions operating on IO Handles

src/include/storage/aio.h

@@ -200,7 +200,7 @@ typedef enum PgAioHandleCallbackID
 	PGAIO_HCB_LOCAL_BUFFER_READV,
 } PgAioHandleCallbackID;
 
-#define PGAIO_HCB_MAX	PGAIO_HCB_LOCAL_BUFFER_READV
+#define PGAIO_HCB_MAX	15
 StaticAssertDecl(PGAIO_HCB_MAX < (1 << PGAIO_RESULT_ID_BITS),
 				 "PGAIO_HCB_MAX is too big for PGAIO_RESULT_ID_BITS");
 
@@ -308,6 +308,7 @@ extern PgAioTargetData *pgaio_io_get_target_data(PgAioHandle *ioh);
 extern char *pgaio_io_get_target_description(PgAioHandle *ioh);
 
 /* functions in aio_callback.c */
+extern PgAioHandleCallbackID pgaio_io_register_callback_entry(const PgAioHandleCallbacks *callback, const char *name);
 extern void pgaio_io_register_callbacks(PgAioHandle *ioh, PgAioHandleCallbackID cb_id,
 										uint8 cb_data);
 extern void pgaio_io_set_handle_data_64(PgAioHandle *ioh, uint64 *data, uint8 len);