Partially Revert "MDEV-24270: Collect multiple completed events at a time"

This partially reverts commit 6479006e.

Remove the constant tpool::aio::N_PENDING, which has no
intrinsic meaning for the tpool.

storage/innobase/buf/buf0buf.cc

@@ -1467,7 +1467,7 @@ bool buf_pool_t::create()
   ut_d(lru_scan_itr.m_mutex= &mutex);
 
   io_buf.create((srv_n_read_io_threads + srv_n_write_io_threads) *
-                tpool::aio::N_PENDING);
+                OS_AIO_N_PENDING_IOS_PER_THREAD);
 
   /* FIXME: remove some of these variables */
   srv_buf_pool_curr_size= curr_pool_size;

storage/innobase/include/os0file.h

@@ -264,6 +264,9 @@ struct os_file_size_t {
 	os_offset_t	m_alloc_size;
 };
 
+/** Win NT does not allow more than 64 */
+static const ulint OS_AIO_N_PENDING_IOS_PER_THREAD = 256;
+
 extern ulint	os_n_file_reads;
 extern ulint	os_n_file_writes;
 extern ulint	os_n_fsyncs;

storage/innobase/os/os0file.cc

@@ -4027,8 +4027,8 @@ static bool is_linux_native_aio_supported()
 
 bool os_aio_init(ulint n_reader_threads, ulint n_writer_threads, ulint)
 {
-  int max_write_events= int(n_writer_threads * tpool::aio::N_PENDING);
-  int max_read_events= int(n_reader_threads * tpool::aio::N_PENDING);
+  int max_write_events= int(n_writer_threads * OS_AIO_N_PENDING_IOS_PER_THREAD);
+  int max_read_events= int(n_reader_threads * OS_AIO_N_PENDING_IOS_PER_THREAD);
   int max_events = max_read_events + max_write_events;
 	int ret;
 

tpool/aio_linux.cc

@@ -51,6 +51,8 @@ namespace tpool
 {
 #ifdef LINUX_NATIVE_AIO
 
+#define MAX_EVENTS 256
+
 class aio_linux final : public aio
 {
   thread_pool *m_pool;
@@ -60,10 +62,10 @@ class aio_linux final : public aio
 
   static void getevent_thread_routine(aio_linux *aio)
   {
-    io_event events[N_PENDING];
+    io_event events[MAX_EVENTS];
     for (;;)
     {
-      switch (int ret= my_getevents(aio->m_io_ctx, 1, N_PENDING, events)) {
+      switch (int ret= my_getevents(aio->m_io_ctx, 1, MAX_EVENTS, events)) {
       case -EINTR:
         continue;
       case -EINVAL:

tpool/tpool.h

@@ -155,8 +155,6 @@ struct aiocb
 class aio
 {
 public:
-  /** Maximum number of pending requests per thread */
-  static constexpr unsigned N_PENDING= 256;
   /**
     Submit asyncronous IO.
     On completion, cb->m_callback is executed.