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//! # Census
//!
//! Census' `Inventory` makes it possible to track a set of living items of a specific type.
//! Items are automatically removed from the `Inventory<T>` as the living item's are dropped.
//!
//! ```rust
//! use census::{Inventory, TrackedObject};
//!
//! let inventory = Inventory::new();
//!
//! // Each object tracked needs to be registered explicitely in the Inventory.
//! // A `TrackedObject<T>` wrapper is then returned.
//! let one = inventory.track("one".to_string());
//! let two = inventory.track("two".to_string());
//!
//! // A snapshot of the list of living instances can be obtained...
//! // (no guarantee on their order)
//! let living_instances: Vec<TrackedObject<String>> = inventory.list();
//! assert_eq!(living_instances.len(), 2);
//! ```
use std::borrow::Borrow;
use std::fmt;
use std::ops::Deref;
use std::sync::{Arc, Condvar, Mutex, MutexGuard, Weak};
use std::fmt::{Error, Formatter};
struct Items<T> {
alive_count: usize,
items: Vec<Weak<InnerTrackedObject<T>>>,
}
impl<T> Default for Items<T> {
fn default() -> Self {
Items {
alive_count: 0,
items: Vec::new(),
}
}
}
impl<T> Items<T> {
fn record_birth(&mut self) {
self.alive_count += 1;
}
fn record_death(&mut self) {
self.alive_count -= 1;
}
fn len(&mut self) -> usize {
self.alive_count()
}
fn list_arc(&mut self) -> Vec<TrackedObject<T>> {
self.items
.iter()
.flat_map(|weak| weak.upgrade())
.map(|v| TrackedObject { inner: v })
.collect()
}
fn gc_if_needed(&mut self) {
if !self.should_gc() {
return;
}
let mut i = 0;
while i < self.items.len() {
let should_remove = self.items[i].strong_count() == 0;
if should_remove {
self.items.swap_remove(i);
} else {
i += 1;
}
}
}
fn alive_count(&self) -> usize {
self.alive_count
}
fn should_gc(&self) -> bool {
self.alive_count * 2 <= self.items.len()
}
}
struct InnerInventory<T> {
items: Mutex<Items<T>>,
condvar: Condvar,
}
/// The `Inventory` register and keeps track of all of the objects alive.
pub struct Inventory<T> {
inner: Arc<InnerInventory<T>>,
}
impl<T> Default for Inventory<T> {
fn default() -> Self {
Inventory {
inner: Arc::new(InnerInventory {
items: Mutex::new(Items::default()),
condvar: Condvar::new(),
}),
}
}
}
impl<T> Clone for Inventory<T> {
fn clone(&self) -> Self {
Inventory {
inner: self.inner.clone(),
}
}
}
impl<T> Inventory<T> {
/// Creates a new inventory.
pub fn new() -> Inventory<T> {
Inventory::default()
}
fn lock_items(&self) -> MutexGuard<Items<T>> {
let mut guard = self.inner.items.lock().unwrap();
guard.gc_if_needed();
guard
}
/// Returns the number of tracked object.
pub fn len(&self) -> usize {
self.lock_items().len()
}
/// Takes a snapshot of the list of tracked object.
///
/// Note that the list is a simple `Vec` of tracked object.
/// As a result, it is a consistent snapshot of the
/// list of living instance at the time of the call,
///
/// Obviously, instances may have been created after the call.
/// They will obviously not appear in the snapshot.
///
/// ```rust
/// use census::{Inventory, TrackedObject};
///
/// let inventory = Inventory::new();
///
/// let one = inventory.track("one".to_string());
/// let living_instances: Vec<TrackedObject<String>> = inventory.list();
/// let two = inventory.track("two".to_string());
///
/// // our snapshot is a bit old.
/// assert_eq!(living_instances.len(), 1);
///
/// // a fresher snapshot would contain our new element.
/// assert_eq!(inventory.list().len(), 2);
/// ```
///
/// Also, the instance in the snapshot itself
/// are considered "living".
///
/// As a result, as long as a snapshot is not dropped,
/// all of its instances will be part of the inventory.
///
/// ```rust
/// # use census::{Inventory, TrackedObject};
///
/// let inventory = Inventory::new();
///
/// let one = inventory.track("one".to_string());
/// let living_instances: Vec<TrackedObject<String>> = inventory.list();
///
/// // let's drop one here
/// drop(one);
///
/// // The instance is technically still in the inventory
/// // as our previous snapshot is extending its life...
/// assert_eq!(inventory.list().len(), 1);
///
/// // If we drop our previous snapshot however...
/// drop(living_instances);
///
/// // `one` is really untracked.
/// assert!(inventory.list().is_empty());
/// ```
///
pub fn list(&self) -> Vec<TrackedObject<T>> {
self.lock_items().list_arc()
}
/// This function blocks until there are no more items in the inventory.
///
/// It is a helper calling
/// ```ignore
/// self.wait_until_predicate(|count| count == 0)
/// ```
///
/// Note it is very easy to misuse this function and create a deadlock.
/// For instance, if any living TrackedObject is on the stack at the moment of the call,
/// it will not get dropped, and the inventory cannot become empty.
pub fn wait_until_empty(&self) {
self.wait_until_predicate(|count| count == 0)
}
/// This function blocks until the number of items in the repository matches a specific
/// predicate.
///
/// See also `wait_until_empty`.
///
/// Note it is very easy to misuse this function and create a deadlock.
/// For instance, if any living TrackedObject is on the stack at the moment of the call,
/// it will not get dropped, and the inventory cannot become empty.
pub fn wait_until_predicate<F: Fn(usize) -> bool>(&self, predicate_on_count: F) {
let mut count = self.lock_items();
while !predicate_on_count(count.alive_count()) {
count = self.inner.condvar.wait(count).unwrap();
}
}
/// Starts tracking a given `T` object.
pub fn track(&self, item: T) -> TrackedObject<T> {
let item_arc = Arc::new(InnerTrackedObject {
census: self.clone(),
item,
});
let item_weak = Arc::downgrade(&item_arc);
let mut items_lock = self.lock_items();
items_lock.items.push(item_weak);
items_lock.record_birth();
self.inner.condvar.notify_all();
TrackedObject { inner: item_arc }
}
}
/// Your tracked object.
///
/// A tracked object contains reference counting logic and an
/// `Arc` to your object. It is cloneable but calling clone will
/// not clone your internal object.
///
/// Your object cannot be mutated. You can borrow it using
/// the `Deref` interface.
#[derive(Clone)]
pub struct TrackedObject<T> {
inner: Arc<InnerTrackedObject<T>>,
}
struct InnerTrackedObject<T> {
census: Inventory<T>,
item: T,
}
impl<T: fmt::Debug> fmt::Debug for TrackedObject<T> {
fn fmt(&self, f: &mut Formatter<'_>) -> Result<(), Error> {
write!(f, "Tracked({:?})", self.inner.item)
}
}
impl<T> TrackedObject<T> {
/// Creates a new object from an existing one.
///
/// The new object will be registered
/// in your original object's inventory.
///
/// ```rust
/// use census::{Inventory, TrackedObject};
///
/// let inventory = Inventory::new();
///
/// let seven = inventory.track(7);
/// let fourteen = seven.map(|i| i * 2);
/// assert_eq!(*fourteen, 14);
///
/// let living_instances = inventory.list();
/// assert_eq!(living_instances.len(), 2);
/// ```
pub fn map<F>(&self, f: F) -> TrackedObject<T>
where
F: FnOnce(&T) -> T,
{
let t = f(self);
self.inner.census.track(t)
}
}
impl<T> Drop for InnerTrackedObject<T> {
fn drop(&mut self) {
let mut lock = self.census.lock_items();
lock.record_death();
self.census.inner.condvar.notify_all();
}
}
impl<T> Deref for TrackedObject<T> {
type Target = T;
fn deref(&self) -> &T {
&self.inner.item
}
}
impl<T> AsRef<T> for TrackedObject<T> {
fn as_ref(&self) -> &T {
&self.inner.item
}
}
impl<T> Borrow<T> for TrackedObject<T> {
fn borrow(&self) -> &T {
&self.inner.item
}
}
#[cfg(test)]
mod tests {
use super::Inventory;
use std::sync::mpsc::channel;
use std::sync::{Arc, Barrier};
use std::thread;
#[test]
fn test_census_map() {
let census = Inventory::new();
let a = census.track(1);
let _b = a.map(|v| v * 7);
assert_eq!(census.len(), 2);
assert_eq!(
census.list().into_iter().map(|m| *m).collect::<Vec<_>>(),
vec![1, 7]
);
}
#[test]
fn test_census() {
let census = Inventory::new();
let _a = census.track(1);
let _b = census.track(3);
assert_eq!(census.len(), 2,);
assert_eq!(
census.list().into_iter().map(|m| *m).collect::<Vec<_>>(),
vec![1, 3]
);
}
#[test]
fn test_census_2() {
let census = Inventory::new();
{
let _a = census.track(1);
let _b = census.track(3);
// dropping both here
}
assert_eq!(census.len(), 0);
assert!(census.list().is_empty());
}
#[test]
fn test_census_3() {
let census = Inventory::new();
let a = census.track(1);
let _a2 = a.clone();
drop(a);
assert_eq!(census.len(), 1);
assert_eq!(
census.list().into_iter().map(|m| *m).collect::<Vec<_>>(),
vec![1]
);
}
#[test]
fn test_census_list_extends_life() {
let census = Inventory::new();
let a = census.track(1);
let living = census.list();
assert_eq!(living.len(), 1);
drop(a);
let living_2 = census.list();
assert_eq!(living_2.len(), 1);
drop(living_2);
drop(living);
assert_eq!(census.len(), 0);
assert!(census.list().is_empty());
}
#[test]
fn test_census_race_condition() {
let census = Inventory::new();
let census_clone = census.clone();
thread::spawn(move || {
for _ in 0..1_000 {
let _a = census_clone.track(1);
}
});
for i in 0..10_000 {
println!("i {}", i);
census.list();
}
}
#[test]
fn test_census_concurrent_drop() {
let census = Inventory::new();
let mut senders = Vec::new();
let mut handles = Vec::new();
let barrier = Arc::new(Barrier::new(2));
for _ in 0..2 {
let (send, recv) = channel();
let barrier = barrier.clone();
handles.push(thread::spawn(move || {
for obj in recv {
barrier.wait();
drop(obj);
}
}));
senders.push(send);
}
for i in 0..50_000 {
let tracked = census.track(i);
for send in &senders {
send.send(tracked.clone()).unwrap();
}
}
drop(senders);
for handle in handles {
handle.join().unwrap();
}
assert_eq!(census.len(), 0);
}
fn test_census_changes_iter_util(el: usize) {
let census = Inventory::new();
for i in 0..el {
let tracked = census.track(i);
thread::spawn(move || {
let _tracked = tracked;
});
}
census.wait_until_empty();
assert_eq!(census.len(), 0);
assert!(census.list().is_empty());
}
#[test]
fn test_census_changes_iter_many() {
for i in 1..200 {
test_census_changes_iter_util(i);
}
}
}