How can I enclose a mysql::Conn in a lazy_static macro? - mysql

Question

This code:
#[macro_use]
extern crate lazy_static;
extern crate mysql;
use mysql::*;
fn some_fn() {
lazy_static! {
static ref CONNECTION: Conn = Conn::new("mysql://root:password#127.0.0.1:3306/mydb?prefer_socket=false").unwrap();
}
}
generates a very long error message:
error[E0277]: the trait bound `*mut std::os::raw::c_void: std::marker::Sync` is not satisfied in `winapi::minwinbase::OVERLAPPED`
--> src\main.rs:8:5
|
8 | / lazy_static! {
9 | | static ref CONNECTION: Conn = Conn::new("mysql://root:password#127.0.0.1:3306/mydb?prefer_socket=false").unwrap();
10 | | }
| |_____^ `*mut std::os::raw::c_void` cannot be shared between threads safely
|
= help: within `winapi::minwinbase::OVERLAPPED`, the trait `std::marker::Sync` is not implemented for `*mut std::os::raw::c_void`
= note: required because it appears within the type `winapi::minwinbase::OVERLAPPED`
= note: required because of the requirements on the impl of `std::marker::Sync` for `std::ptr::Unique<winapi::minwinbase::OVERLAPPED>`
= note: required because it appears within the type `std::boxed::Box<winapi::minwinbase::OVERLAPPED>`
= note: required because it appears within the type `named_pipe::Overlapped`
= note: required because it appears within the type `named_pipe::PipeClient`
= note: required because it appears within the type `std::option::Option<named_pipe::PipeClient>`
= note: required because it appears within the type `std::io::BufWriter<named_pipe::PipeClient>`
= note: required because it appears within the type `std::option::Option<std::io::BufWriter<named_pipe::PipeClient>>`
= note: required because it appears within the type `bufstream::InternalBufWriter<named_pipe::PipeClient>`
= note: required because it appears within the type `std::io::BufReader<bufstream::InternalBufWriter<named_pipe::PipeClient>>`
= note: required because it appears within the type `bufstream::BufStream<named_pipe::PipeClient>`
= note: required because it appears within the type `mysql::io::Stream`
= note: required because it appears within the type `std::option::Option<mysql::io::Stream>`
= note: required because it appears within the type `mysql::Conn`
= note: required by `lazy_static::lazy::Lazy`
= note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)
Is it because mysql::Conn is not intended to use in multithreaded apps?
How I can use lazy_static if my program is not multithreaded and I use non-thread-safe types?

Answer 1

Your code works just fine... on macOS. I believe this to be a bug with the
implementation of the MySQL crate, and the author agreed, fixing the bug in less than a day. You should be able to just upgrade the crate and use your original code.
As a temporary workaround, you can wrap the Conn in a Mutex:
use mysql::*;
use std::sync::Mutex;
fn some_fn() {
lazy_static! {
static ref CONNECTION: Mutex<Conn> = Mutex::new(Conn::new("mysql://root:password#127.0.0.1:3306/mydb?prefer_socket=false").unwrap());
}
}
if my program is not multithreaded and I use non-thread-safe types?
I always recommend against using global variables, threads or not. Instead, create your Conn at the top of your program and pass references to it into all of your functions.