SSCCE example of database query packet out of order error
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This is a short, self-contained, correct example SSCCE for debugging purposes from a much larger project.

The Problem

This program runs a simple database SELECT query intended to pull a large quantity of results (100k+). This issue I see is that the program will randomly fail with a PacketOutOfOrder error.

This error only occurs on the release target. The debug target ran 355 consecutive times with zero errors.

Expected Behavior

If the query can run once successfully, I expect it to run successfully every time, barring any external factors (hardware failures, unreliable network, etc).

Steps to Reproduce

  1. Grab your favorite MySQL derivative and spin up a database server. I've tested against MySQL 5.5 and MariaDB 10.1.21.

  2. Add a test user to your database. If you prefer more security, specify your host IP instead of the host wildcard '%'.

    CREATE USER 'testbot'@'%' IDENTIFIED BY 'testbot';
    GRANT ALL PRIVILEGES ON `fake_data`.* TO 'testbot'@'%';
  3. Add a database for testing and put some fake data into it...

    1. Edit the global variables in tools/ script. Change db_host and db_port to reflect the database settings for your environment.

    2. Install python 2.7 and pip.

    3. Install two dependencies for the fake data script:

      • pip install faker

      • pip install mysql-connector==2.1.4. The latest version of this package would not install for me.

    4. Run python2 ./tools/ If no errors are reported, you should see a fake_data database on your MySQL server with a products table full of random data.

  4. Run the binary.

    export DSN="mysql://testbot:testbot@host:port/fake_data"
    RUST_LOG=bug_test_01=debug,mysql_async=debug cargo run --release --bin bug-test-01
  5. Observe that the programs fails with an error similar to:

    DEBUG:bug_test_01: running
    thread 'main' panicked at 'error resolving future: Error(PacketOutOfOrder, State { next_error: None, backtrace: None })', /checkout/src/libcore/
    note: Run with `RUST_BACKTRACE=1` for a backtrace.

    If you do not see this error, rerun the binary until you do. Like I said, the error occurs randomly. To improve your chances of the error occurring, remove the RUST_LOG environment variable to disable debugging print statements, or only remove mysql_async=debug from that log list.

System Info

$ rustc --version --verbose
rustc 1.19.0-nightly (f062832b2 2017-06-07)
binary: rustc
commit-hash: f062832b208e94f2f0f26ed7fb5c48c172069fbe
commit-date: 2017-06-07
host: x86_64-unknown-linux-gnu
release: 1.19.0-nightly
LLVM version: 4.0
$ uname -a
Linux 4.10.11-1-ARCH #1 SMP PREEMPT Tue Apr 18 08:39:42 CEST 2017 x86_64 GNU/Linux

Things I've Tried or...

How I know this isn't a network/database server issue.

On the very same machine I run cargo run --release --bin bug-test-01, I fired up Wireshark and captured the network packets to and from the database server on port 3306.

I then analyzed the SQL protocol in the packet capture and found no sequence numbers out of order, and other than the program closing the TCP connection early with RST packets after the Packet out of order error, everything appears to transferring smoothly from the database server to my computer's network interface card (NIC). NOTE, if you choose to pursue this exercise yourself, the MySQL dissector is invaluable, but buggy; you may run into "Malformed MySQL Packets" in Wireshark. I recommend compiling Wireshark from scratch with the patch mentioned in this bug report to disable SQL protocol compression detection.

I also used the command below (tshark is a commandline utility provided with Wireshark) to print all the MySQL packet sequence numbers captured. My captures showed all sequence numbers in order.

./tshark -r PATH-TO-CAPTURE-FILE -T fields -e mysql.packet_number | egrep -v '^$' | sed 's/,/\n/g' | less

Why I can't add more debug print statements to find the problem.

I did this, and the problem stopped manifesting. I forked mysql_async and added a debug!() statement in src/conn/futures/ prints with every attempt to process/validate each MySQL packet--and in src/ prints with every attempt to parse each MySQL packet.

Apparently, giving the event loop (that decodes the packets) the extra bit of work to print to stderr delays the program just enough that it doesn't fail (so long as I have RUST_LOG=mysql_async=debug enabled.

This indicates to me that I have a race condition of sorts in my program. Not necessarily in my code, but I rather suspect it's in the libraries I'm using. Either mysql_async itself, or the tokio-rs/futures/mio framework. While this example spins up a second thread, all the database work is done in one thread (specifically the thread created by tokio_core::reactor::Core. My second thread just waits for a result.

Besides, isn't Rust supposed to prevent race conditions from occurring? At least so far as my code is concerned, it's 100% Rust with no unsafe blocks.

Heck, while debugging a separate issue with this project, I introduced myself to the perftools on Linux and perf report showed that my program spent over 50% of its time zeroing the 4kB local stack buffer used by mysql_async::io::Stream. So I'm not even sure where the garbage value corrupting the MySQL packet sequence number could be coming from, if not the tokio-rs libraries further down the software stack, (e.g. tokio::net::TcpStream).

My Appeal to You

Where do I go from here to isolate and trap this bug?

I don't use gdb because I'm a novice at best with that program. I can print variables, step, continue, and show backtraces, but I'm not productive with the debugger. If I use it, I'm stepping through every line in the program and that's a huge time sink, not to mention that slowing down the rate at which the binary processes network traffic prevents the problem from occurring.

I'm not comfortable side-stepping the issue. Teaching my code to retry a fixed number of times and give up is an option, but it complicates my code and reduces its utility. If I can identify the cause and fix it, I'd rather do that. I've pretty much exhausted all the tricks in my toolbox, and I am stalled, so here's hoping someone out there has options in their toolbox to share.

I've also tried reading the source code for mysql_async, the various tokior-rs libraries I use, and it's a bit beyond my capacity to read that code and infer the why's and contexts and invariant assumptions that established such code. The very little I've gleaned from the tokio-rs guides is that tokio-rs has a layered approach to I/O, so if the protocol handler isn't given enough buffered data to decode the next packet/message, then its return value is used to indicate that the buffer wasn't used and needs more data.

Copyright (c) 2017 Justin Charette (boxofrox)