Consider refactoring proxy-mysql-default-version

[TeslaCN]

Refactor

The property proxy-mysql-default-version of server.yaml from #16796 was used to fix issue #16735.

But this solution confused many users:

• App connection Shardingsphere-proxy fails  #18482
• Use Jdbc to connect ShardingSphere-Proxy will failed when use MySQL 8  #19649
• JDBC connection Exception: Unknown system variable 'query_cache_size' #20075
• shardingsphere docker return Unknown system variable 'query_cache_size' #21276
• shardingsphere-proxy5.2.0 proxy mysql8.x [ERROR] java.sql.SQLException: Unknown system variable 'query_cache_size' #21455
• Unknown system variable 'query_cache_size' #22276
• The server version in Proxy is incorrect #23166

I'm wondering if we could support multi-version system variables query and remove this property, which could help simplifying usage of ShardingSphere-Proxy MySQL.

[TeslaCN]

We could generate values for Java Enum by executing the following codes in MySQL server source code.

#include <iostream>
#include <map>
#include <string>
#include "set_var.h"
#include "sys_vars.h"

using namespace std;

string generateFlag(const sys_var *v);

string generateDefaultValue(sys_var *v);

int main() {
  map<string, string> m;
  for (sys_var *v = all_sys_vars.first; v; v = v->next) {
    string name = v->name.str, generated;
    for (char c : name) {
      generated += (char)toupper(c);
    }
    generated += '(';
    string flag = generateFlag(v);
    generated += flag;
    generated += ", ";
    string def = generateDefaultValue(v);
    generated += def + ')';
    m[name] = ("\"TODO\"" == def ? "// " : "") + generated;
  }
  for (auto &iterator : m) {
    cout << iterator.second << "," << endl << endl;
  }
  return 0;
}

string generateFlag(const sys_var *v) {
  string flag;
  if (v->scope() & sys_var::GLOBAL) flag += " | Flag.GLOBAL";
  if (v->scope() & sys_var::SESSION) flag += " | Flag.SESSION";
  if (v->scope() & sys_var::ONLY_SESSION) flag += " | Flag.ONLY_SESSION";
  if (v->is_readonly()) flag += " | Flag.READONLY";
  if (v->not_visible()) flag += " | Flag.INVISIBLE";
  if (v->is_trilevel()) flag += " | Flag.TRI_LEVEL";
  if (v->is_hint_updateable()) flag += " | Flag.HINT_UPDATEABLE";
  if (v->is_persist_readonly()) flag += " | Flag.PERSIST_AS_READ_ONLY";
  if (v->is_sensitive()) flag += " | Flag.SENSITIVE";
  return flag.substr(3);
}

string generateDefaultValue(sys_var *v) {
  stringstream stream;
  stream << '"';
  switch (v->get_option()->var_type) {
    case GET_STR: {
      char *c = reinterpret_cast<char *>(v->get_option()->def_value);
      if (nullptr != c) stream << c;
      break;
    }
    case GET_INT:
    case GET_LONG:
    case GET_LL:
    case GET_BOOL: {
      stream << v->get_option()->def_value;
      break;
    }
    case GET_UINT:
    case GET_ULONG:
    case GET_ULL: {
      stream << (ulonglong)v->get_option()->def_value;
      break;
    }
    case GET_DOUBLE: {
      stream << (double)v->get_option()->def_value;
      break;
    }
    default: {
      stream << "TODO";
    }
  }
  stream << '"';
  return stream.str();
}