What's related here:
is pretty scary.
Apologies that this is a members-only story. I think I'll past the pertinent part here:
What the Hell Is a Lifetime Bug?
Rust has this thing called lifetimes.
They’re supposed to help you prove to the compiler that your references are valid. It’s like having a friend constantly look over your shoulder and slap your hand if you’re about to dereference a dangling pointer.
Most of the time, lifetimes protect you.
Sometimes, they do exactly what you ask.
And that’s where the danger begins.
We had code like this (simplified):
use std::collections::HashMap;
struct UserRef<'a> {
name: &'a str,
email: &'a str,
}
fn get_user<'a>(store: &'a HashMap<u32, (String, String)>, id: u32) -> Option<UserRef<'a>> {
store.get(&id).map(|(name, email)| UserRef {
name: name.as_str(),
email: email.as_str(),
})
}
This compiled fine. Passed unit tests.
Returned the right values in staging.
Then broke under pressure.
How I Wasted 3 Months on a Bug That Compiled
Here’s the problem.
We were returning a reference to a String that belonged to a map. But that map lived somewhere else, and under async conditions, the map could be dropped while the reference still lived.
The Rust compiler didn’t catch it, because we told it that 'a was safe.
It believed us.
But in production, under certain concurrent workloads, that reference became invalid. And Rust doesn’t null it or throw an exception.
It just gives you… empty strings.
...
The Fix That Felt Like Giving Up (But Wasn’t)
I resisted cloning for weeks. I thought:
“I’m writing in Rust! I don’t need to clone!”
Eventually, I tried this:
struct User {
name: String,
email: String,
}
fn get_user(store: &HashMap<u32, (String, String)>, id: u32) -> Option<User> {
store.get(&id).map(|(name, email)| User {
name: name.clone(),
email: email.clone(),
})
}
Boom. Bug gone. Clean output. No more ghost responses.
Memory usage increased by ~0.3% on stress tests.
That was it.