[ Segmentation fault ] Using SQL\Query->toString__FOR_DEBUGGING_ONLY() causes a segmentation fault when a query is also running

[lexidor]

Describe the bug

A segmentation fault occurs when running when asking for the string repr of a SQL\Query while the connection is busy.

Standalone code, or other way to reproduce the problem

<?hh

use namespace HH\Lib\SQL;

<<__EntryPoint>>
async function main_async(): Awaitable<void> {
    require_once __DIR__.'/../vendor/autoload.hack';
    \Facebook\AutoloadMap\initialize();

    $config = /*my config*/;

    $async_conn = await AsyncMysqlClient::connect($config->servername, 3306, $config->dbname, $config->username, $config->password);

    concurrent {
        await func_async($async_conn, new SQL\Query('SELECT %s', 'something'));
        await func_async($async_conn, new SQL\Query('SELECT %s', 'something'));
    }
}

async function func_async(AsyncMysqlConnection $asyncMysql, SQL\Query $query): Awaitable<void> {
    $query->toString__FOR_DEBUGGING_ONLY($asyncMysql);
    await $asyncMysql->queryf('SELECT %s', 'something');
}

Steps to reproduce the behavior:

Have a SQL server running.
I used 10.4.11-MariaDB-1:10.4.11+maria~bionic-log.
Run the example code above.
Read stacktrace from /tmp/stacktrace.xxxxx.log

Expected behavior
Code completes silently.

Screenshots
NONE

Desktop (please complete the following information):

• OS: Ubuntu 18.04.4 LTS
• HHVM Version: 4.51.0

HipHop VM 4.51.0-dev (rel)
Compiler: 1585280615_594927537
Repo schema: ac933c8afa79ac54d3ec9e7de7983d3b2a7977f5

Additional context
Stacktrace

Host: lexidor-Cloud-VM
ProcessID: 27372
ThreadID: 140689418532416
ThreadPID: 27372
Name: /usr/bin/hhvm
CmdLine: hhvm library/autoload.php
Type: Segmentation fault
Runtime: hhvm
Version: 1585280615_594927537
DebuggerCount: 0

Arguments: library/autoload.php
ThreadType: CLI

# 0  000055811f319ee6
# 1  000055811f5e2a95
# 2  00007ff4cccaf890
# 3  00005581204c9fe5
# 4  0000558129214953
# 5  000055812921489b
# 6  0000558129215018
# 7  0000558125600491
# 8  000055811fc0b703
# 9  000055811f5fed22
# 10 000055812049cb96
# 11 00005581204af8b8
# 12 00005581204a9eab
# 13 0000558129200631
# 14 000055812920055d
# 15 0000558125600491
# 16 000055811fc0b703
# 17 000055811f978ae0
# 18 000055811f602dd6
# 19 0000558121296861
# 20 0000558121296688
# 21 00005581212963d0
# 22 000055811f684622
# 23 000055811f6855f2
# 24 000055811f68e3e5
# 25 000055811f6901ce
# 26 000055811f2a0974
# 27 00007ff4c61ccb97
# 28 000055811f29dc2a

PHP Stacktrace:

#0  HH\Lib\SQL\Query->toString__FOR_DEBUGGING_ONLY() called at [autoload.php:21]
#1  func_async() called at [autoload.php:16]
#2  main_async()
#3  Closure$__SystemLib\enter_async_entry_point()
#4  HH\Asio\join()
#5  __SystemLib\enter_async_entry_point()