MDEV-16264: Minor cleanup

aio_linux::m_max_io_count: Unused data member; remove.

aiocb::m_ret_len: Declare as the more compatible type size_t.
Unfortunately, ssize_t is not available on Microsoft Visual Studio.

tpool/aio_linux.cc

@@ -39,7 +39,6 @@ namespace tpool
 
 class aio_linux : public aio
 {
-  int m_max_io_count;
   thread_pool* m_pool;
   io_context_t m_io_ctx;
   bool m_in_shutdown;
@@ -62,7 +61,7 @@ class aio_linux : public aio
         long long res = event.res;
         if (res < 0)
         {
-          iocb->m_err = -res;
+          iocb->m_err = static_cast<int>(-res);
           iocb->m_ret_len = 0;
         }
         else
@@ -93,8 +92,8 @@ class aio_linux : public aio
   }
 
 public:
-  aio_linux(io_context_t ctx, thread_pool* pool, size_t max_count)
-    : m_max_io_count(max_count), m_pool(pool), m_io_ctx(ctx),
+  aio_linux(io_context_t ctx, thread_pool* pool)
+    : m_pool(pool), m_io_ctx(ctx),
     m_in_shutdown(), m_getevent_thread(getevent_thread_routine, this)
   {
   }
@@ -146,7 +145,7 @@ aio* create_linux_aio(thread_pool* pool, int max_io)
     fprintf(stderr, "io_setup(%d) returned %d\n", max_io, ret);
     return nullptr;
   }
-  return new aio_linux(ctx, pool, max_io);
+  return new aio_linux(ctx, pool);
 }
 #else
 aio* create_linux_aio(thread_pool* pool, int max_aio)

tpool/aio_simulated.cc

@@ -133,7 +133,11 @@ class simulated_aio : public aio
   static void simulated_aio_callback(void *param)
   {
     aiocb *cb= (aiocb *) param;
-    int ret_len;
+#ifdef _WIN32
+    size_t ret_len;
+#else
+    ssize_t ret_len;
+#endif
     int err= 0;
     switch (cb->m_opcode)
     {
@@ -146,14 +150,13 @@ class simulated_aio : public aio
     default:
       abort();
     }
-    if (ret_len < 0)
-    {
 #ifdef _WIN32
+    if (static_cast<int>(ret_len) < 0)
       err= GetLastError();
 #else
+    if (ret_len < 0)
       err= errno;
 #endif
-    }
     cb->m_ret_len = ret_len;
     cb->m_err = err;
     cb->m_callback(cb);

tpool/tpool.h

@@ -110,7 +110,6 @@ enum class aio_opcode
   AIO_PWRITE
 };
 const int MAX_AIO_USERDATA_LEN= 40;
-struct aiocb;
 
 /** IO control block, includes parameters for the IO, and the callback*/
 
@@ -129,7 +128,7 @@ struct aiocb
   callback_func m_callback;
   task_group* m_group;
   /* Returned length and error code*/
-  int m_ret_len;
+  size_t m_ret_len;
   int m_err;
   void *m_internal;
   task m_internal_task;