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use parking_lot::RwLock;
use std::{
any::{Any, TypeId},
hash::{BuildHasher, Hash},
sync::Arc,
};
type ErrorObject = Arc<dyn Any + Send + Sync + 'static>;
type WaiterValue<V> = Option<Result<V, ErrorObject>>;
type Waiter<V> = Arc<RwLock<WaiterValue<V>>>;
pub(crate) enum InitResult<V, E> {
Initialized(V),
ReadExisting(V),
InitErr(Arc<E>),
}
pub(crate) struct ValueInitializer<K, V, S> {
// TypeId is the type ID of the concrete error type of generic type E in
// try_init_or_read(). We use the type ID as a part of the key to ensure that
// we can always downcast the trait object ErrorObject (in Waiter<V>) into
// its concrete type.
waiters: moka_cht::SegmentedHashMap<(Arc<K>, TypeId), Waiter<V>, S>,
}
impl<K, V, S> ValueInitializer<K, V, S>
where
Arc<K>: Eq + Hash,
V: Clone,
S: BuildHasher,
{
pub(crate) fn with_hasher(hasher: S) -> Self {
Self {
waiters: moka_cht::SegmentedHashMap::with_num_segments_and_hasher(16, hasher),
}
}
/// # Panics
/// Panics if the `init` future has been panicked.
pub(crate) fn init_or_read(&self, key: Arc<K>, init: impl FnOnce() -> V) -> InitResult<V, ()> {
// This closure will be called after the init closure has returned a value.
// It will convert the returned value (from init) into an InitResult.
let post_init = |_key, value: V, lock: &mut WaiterValue<V>| {
*lock = Some(Ok(value.clone()));
InitResult::Initialized(value)
};
let type_id = TypeId::of::<()>();
self.do_try_init(&key, type_id, init, post_init)
}
/// # Panics
/// Panics if the `init` future has been panicked.
pub(crate) fn try_init_or_read<F, E>(&self, key: Arc<K>, init: F) -> InitResult<V, E>
where
F: FnOnce() -> Result<V, E>,
E: Send + Sync + 'static,
{
let type_id = TypeId::of::<E>();
// This closure will be called after the init closure has returned a value.
// It will convert the returned value (from init) into an InitResult.
let post_init = |key, value: Result<V, E>, lock: &mut WaiterValue<V>| match value {
Ok(value) => {
*lock = Some(Ok(value.clone()));
InitResult::Initialized(value)
}
Err(e) => {
let err: ErrorObject = Arc::new(e);
*lock = Some(Err(Arc::clone(&err)));
self.remove_waiter(key, type_id);
InitResult::InitErr(err.downcast().unwrap())
}
};
self.do_try_init(&key, type_id, init, post_init)
}
/// # Panics
/// Panics if the `init` future has been panicked.
fn do_try_init<'a, F, O, C, E>(
&self,
key: &'a Arc<K>,
type_id: TypeId,
init: F,
mut post_init: C,
) -> InitResult<V, E>
where
F: FnOnce() -> O,
C: FnMut(&'a Arc<K>, O, &mut WaiterValue<V>) -> InitResult<V, E>,
E: Send + Sync + 'static,
{
use std::panic::{catch_unwind, resume_unwind, AssertUnwindSafe};
use InitResult::*;
const MAX_RETRIES: usize = 200;
let mut retries = 0;
loop {
let waiter = Arc::new(RwLock::new(None));
let mut lock = waiter.write();
match self.try_insert_waiter(key, type_id, &waiter) {
None => {
// Our waiter was inserted. Let's resolve the init future.
// Catching panic is safe here as we do not try to resolve the future again.
match catch_unwind(AssertUnwindSafe(init)) {
// Resolved.
Ok(value) => return post_init(key, value, &mut lock),
// Panicked.
Err(payload) => {
*lock = None;
// Remove the waiter so that others can retry.
self.remove_waiter(key, type_id);
resume_unwind(payload);
} // The write lock will be unlocked here.
}
}
Some(res) => {
// Somebody else's waiter already exists. Drop our write lock and wait
// for a read lock to become available.
std::mem::drop(lock);
match &*res.read() {
Some(Ok(value)) => return ReadExisting(value.clone()),
Some(Err(e)) => return InitErr(Arc::clone(e).downcast().unwrap()),
// None means somebody else's init closure has been panicked.
None => {
retries += 1;
if retries < MAX_RETRIES {
// Retry from the beginning.
continue;
} else {
panic!(
r#"Too many retries. Tried to read the return value from the `init` \
closure but failed {} times. Maybe the `init` kept panicking?"#,
retries
);
}
}
}
}
}
}
}
#[inline]
pub(crate) fn remove_waiter(&self, key: &Arc<K>, type_id: TypeId) {
let key = Arc::clone(key);
self.waiters.remove(&(key, type_id));
}
#[inline]
fn try_insert_waiter(
&self,
key: &Arc<K>,
type_id: TypeId,
waiter: &Waiter<V>,
) -> Option<Waiter<V>> {
let key = Arc::clone(key);
let waiter = Arc::clone(waiter);
self.waiters
.insert_with_or_modify((key, type_id), || waiter, |_, w| Arc::clone(w))
}
}