MDEV-20377: Enable MemorySanitizer user-poisoning

For ENGINE=Aria, we work around bugs in various tests that catch
writes of uninitialized bytes from the Aria page cache.
(Why do we even write anything on DROP TABLE?)

include/my_valgrind.h

@@ -50,13 +50,8 @@
 # define REDZONE_SIZE 8
 #elif __has_feature(memory_sanitizer)
 # include <sanitizer/msan_interface.h>
-# if 0 /* FIXME: these reveal lots of failures */
 # define MEM_UNDEFINED(a,len) __msan_allocated_memory(a,len)
 # define MEM_MAKE_DEFINED(a,len) __msan_unpoison(a,len)
-# else
-#  define MEM_UNDEFINED(a,len) ((void) 0)
-#  define MEM_MAKE_DEFINED(a,len) ((void) 0)
-# endif
 # define MEM_NOACCESS(a,len) ((void) 0)
 # define MEM_CHECK_ADDRESSABLE(a,len) ((void) 0)
 # define MEM_CHECK_DEFINED(a,len) __msan_check_mem_is_initialized(a,len)

storage/maria/ma_pagecache.c

@@ -681,6 +681,10 @@ static my_bool pagecache_fwrite(PAGECACHE *pagecache,
     DBUG_PRINT("error", ("write callback problem"));
     DBUG_RETURN(1);
   }
+#if __has_feature(memory_sanitizer) /* FIXME: encryption.aria_tiny etc. fail */
+  /* FIXME: ENGINE=Aria occasionally writes uninitialized data */
+  __msan_unpoison(args.page, pagecache->block_size);
+#endif
   res= (int)my_pwrite(filedesc->file, args.page, pagecache->block_size,
                  ((my_off_t) pageno << pagecache->shift), flags);
   (*filedesc->post_write_hook)(res, &args);