libsql: avoid redundant threaded locale resets

src/libsql/AsyncSQL.cpp

@@ -46,10 +46,14 @@ bool CAsyncSQL::QueryLocaleSet()
 		return true;
 	}
 
+	const char* current_charset = mysql_character_set_name(&m_hDB);
+	if (current_charset && m_stLocale == current_charset)
+		return true;
+
 	if (mysql_set_character_set(&m_hDB, m_stLocale.c_str()))
 	{
-		sys_err("cannot set locale %s by 'mysql_set_character_set', errno %u %s",
-			m_stLocale.c_str(), mysql_errno(&m_hDB), mysql_error(&m_hDB));
+		sys_err("cannot set locale %s by 'mysql_set_character_set' (current=%s), errno %u %s",
+			m_stLocale.c_str(), current_charset ? current_charset : "<unknown>", mysql_errno(&m_hDB), mysql_error(&m_hDB));
 		return false;
 	}
 
@@ -614,5 +618,7 @@ size_t CAsyncSQL::EscapeString(char* dst, size_t dstSize, const char* src, size_
 void CAsyncSQL2::SetLocale(const std::string& stLocale)
 {
 	m_stLocale = stLocale;
-	QueryLocaleSet();
+
+	if (!HasWorkerThread())
+		QueryLocaleSet();
 }