This library implements the ghetto lock described [here](https://github.com/memcached/memcached/wiki/ProgrammingTricks#ghetto-central-locking). The lock isn't resistant to server failures and should be used only in situations where strong locking guarantees are not required.
A popular use case for this lock is to avoid the [stampeding herd problem](https://en.wikipedia.org/wiki/Thundering_herd_problem) caused by a cache miss.
# Example:
```rust
use ghetto_lock::{LockOptions, LockError};
use memcache::Client;
use std::borrow::Cow;
fn expensive_computation() -> u64 {
2 * 2
}
fn main() {
let mut client = Client::connect("memcache://localhost:11211").expect("error creating client");
let mut lock = LockOptions::new(Cow::Borrowed("db-lock"), Cow::Borrowed("owner-1"))
.with_expiry(1)
.build()
.expect("failed to build client");
let value = client.get("key").expect("failed to get key");
let v = if value.is_none() {
lock.try_acquire()
.and_then(|_guard| {
// compute and update cache for other instances to consume
let v = expensive_computation();
client.set("key", v, 5).expect("failed to set key");
Ok(v)
})
.or_else(|_| loop {
// poll cache key until it is updated.
let v = client.get("key").expect("failed to get key");
if v.is_none() {
continue;
}
break Ok::<_, LockError>(v.unwrap());
}).unwrap()
} else { value.unwrap() };
assert_eq!(4, v);
}
```