MDEV-20525 rocksdb debug compilation fails on Windows due to unresolved my_assert variable

MYSQL_PLUGIN_IMPORT did not work correctly for the RocksDB helper library
rocksdb_aux_lib, because that library was not compiled with
-DMYSQL_DYNAMIC_PLUGIN.

Fix DBUG such that it does not depend on exported data, only on functions
(which do not need MYSQL_PLUGIN_IMPORT decoration)

Use a "getter" function _db_my_assert() instead of DLL-exported variable.

Also, reduce object code duplication by moving more of the DBUG_ASSERT
logic inside the _db_my_assert() function, and add unlikely() and
ATTRIBUTE_COLD hints to ensure that the 'assertion failed' code will
be separated from the main control flow logic. Thus, the compiler can
move the unlikely() code to the end of the compiled function, reachable
via a forward conditional branch, which the processor's branch predictor
could assume 'not taken'.

Author: dr-m

dbug/dbug.c

@@ -2258,6 +2258,16 @@ static int default_my_dbug_sanity(void)
   return 0;
 }
 
+extern my_bool my_assert;
+ATTRIBUTE_COLD
+my_bool _db_my_assert(const char *file, int line, const char *msg)
+{
+  _db_flush_();
+  my_bool a = my_assert;
+  if (!a)
+    fprintf(stderr, "%s:%d: assert: %s\n", file, line, msg);
+  return a;
+}
 #else
 
 /*

include/my_dbug.h

@@ -35,7 +35,6 @@ struct _db_stack_frame_ {
 };
 
 struct  _db_code_state_;
-extern MYSQL_PLUGIN_IMPORT my_bool my_assert;
 extern  my_bool _dbug_on_;
 extern  my_bool _db_keyword_(struct _db_code_state_ *, const char *, int);
 extern  int _db_explain_(struct _db_code_state_ *cs, char *buf, size_t len);
@@ -59,6 +58,8 @@ extern  void _db_dump_(uint _line_,const char *keyword,
 extern  void _db_end_(void);
 extern  void _db_lock_file_(void);
 extern  void _db_unlock_file_(void);
+ATTRIBUTE_COLD
+extern  my_bool _db_my_assert(const char *file, int line, const char *msg);
 extern  FILE *_db_fp_(void);
 extern void _db_flush_(void);
 extern void dbug_swap_code_state(void **code_state_store);
@@ -104,10 +105,9 @@ extern int (*dbug_sanity)(void);
 #define DBUG_END()  _db_end_ ()
 #define DBUG_LOCK_FILE _db_lock_file_()
 #define DBUG_UNLOCK_FILE _db_unlock_file_()
-#define DBUG_ASSERT(A) do { if (!(A)) { _db_flush_();                      \
-      if (my_assert) assert(A);                                            \
-      else fprintf(stderr, "%s:%d: assert: %s\n", __FILE__, __LINE__, #A); \
-}} while (0)
+#define DBUG_ASSERT(A) do { \
+  if (unlikely(!(A)) && _db_my_assert(__FILE__, __LINE__, #A)) assert(A); \
+} while (0)
 #define DBUG_SLOW_ASSERT(A) DBUG_ASSERT(A)
 #define DBUG_ASSERT_EXISTS
 #define DBUG_EXPLAIN(buf,len) _db_explain_(0, (buf),(len))

include/my_sys.h

@@ -267,7 +267,7 @@ extern ulong    my_sync_count;
 extern uint	mysys_usage_id;
 extern int32    my_file_opened;
 extern my_bool	my_init_done, my_thr_key_mysys_exists;
-extern MYSQL_PLUGIN_IMPORT my_bool my_assert;
+extern my_bool my_assert;
 extern my_bool  my_assert_on_error;
 extern myf      my_global_flags;        /* Set to MY_WME for more error messages */
 					/* Point to current my_message() */