MDEV-36482: Make liburing work WITH_MSAN=ON

A read from liburing has its output buffer as defined
by the nature of the system call.

Instrumenting the library proved difficult as the
completion events don't include the opcode or pointers
that where there on submission. It was just easier
to do this in the application code.

Author: grooverdan

tpool/aio_liburing.cc

@@ -15,6 +15,7 @@ Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02111 - 1301 USA*/
 
 #include "tpool_structs.h"
 #include "tpool.h"
+#include "my_valgrind.h"
 #include "mysql/service_my_print_error.h"
 #include "mysqld_error.h"
 
@@ -172,6 +173,10 @@ class aio_uring final : public tpool::aio
       {
         iocb->m_err= 0;
         iocb->m_ret_len= res;
+#if __has_feature(memory_sanitizer)
+        if (iocb->m_opcode == aio_opcode::AIO_PREAD)
+          MEM_MAKE_DEFINED(iocb->m_buffer, res);
+#endif
       }
 
       io_uring_cqe_seen(&aio->uring_, cqe);