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//! CowCell - A concurrently readable cell with Arc
//!
//! A CowCell can be used in place of a `RwLock`. Readers are guaranteed that
//! the data will not change during the lifetime of the read. Readers do
//! not block writers, and writers do not block readers. Writers are serialised
//! same as the write in a RwLock.
//!
//! This is the `Arc` collected implementation. `Arc` is slightly slower than `EbrCell`
//! but has better behaviour with very long running read operations, and more
//! accurate memory reclaim behaviour.
#[cfg(feature = "asynch")]
pub mod asynch;
use std::ops::{Deref, DerefMut};
use std::sync::Arc;
use std::sync::{Mutex, MutexGuard};
/// A conncurrently readable cell.
///
/// This structure behaves in a similar manner to a `RwLock<T>`. However unlike
/// a `RwLock`, writes and parallel reads can be performed at the same time. This
/// means readers and writers do no block either other. Writers are serialised.
///
/// To achieve this a form of "copy-on-write" (or for Rust, clone on write) is
/// used. As a write transaction begins, we clone the existing data to a new
/// location that is capable of being mutated.
///
/// Readers are guaranteed that the content of the `CowCell` will live as long
/// as the read transaction is open, and will be consistent for the duration
/// of the transaction. There can be an "unlimited" number of readers in parallel
/// accessing different generations of data of the `CowCell`.
///
/// Writers are serialised and are guaranteed they have exclusive write access
/// to the structure.
///
/// # Examples
/// ```
/// use concread::cowcell::CowCell;
///
/// let data: i64 = 0;
/// let cowcell = CowCell::new(data);
///
/// // Begin a read transaction
/// let read_txn = cowcell.read();
/// assert_eq!(*read_txn, 0);
/// {
/// // Now create a write, and commit it.
/// let mut write_txn = cowcell.write();
/// *write_txn = 1;
/// // Commit the change
/// write_txn.commit();
/// }
/// // Show the previous generation still reads '0'
/// assert_eq!(*read_txn, 0);
/// let new_read_txn = cowcell.read();
/// // And a new read transaction has '1'
/// assert_eq!(*new_read_txn, 1);
/// ```
#[derive(Debug)]
pub struct CowCell<T> {
write: Mutex<()>,
active: Mutex<Arc<T>>,
}
/// A `CowCell` Write Transaction handle.
///
/// This allows mutation of the content of the `CowCell` without blocking or
/// affecting current readers.
///
/// Changes are only stored in this structure until you call commit. To abort/
/// rollback a change, don't call commit and allow the write transaction to
/// be dropped. This causes the `CowCell` to unlock allowing the next writer
/// to proceed.
pub struct CowCellWriteTxn<'a, T: 'a> {
// Hold open the guard, and initiate the copy to here.
work: Option<T>,
read: Arc<T>,
// This way we know who to contact for updating our data ....
caller: &'a CowCell<T>,
_guard: MutexGuard<'a, ()>,
}
/// A `CowCell` Read Transaction handle.
///
/// This allows safe reading of the value within the `CowCell`, that allows
/// no mutation of the value, and without blocking writers.
#[derive(Debug)]
pub struct CowCellReadTxn<T>(Arc<T>);
impl<T> Clone for CowCellReadTxn<T> {
fn clone(&self) -> Self {
CowCellReadTxn(self.0.clone())
}
}
impl<T> CowCell<T>
where
T: Clone,
{
/// Create a new `CowCell` for storing type `T`. `T` must implement `Clone`
/// to enable clone-on-write.
pub fn new(data: T) -> Self {
CowCell {
write: Mutex::new(()),
active: Mutex::new(Arc::new(data)),
}
}
/// Begin a read transaction, returning a read guard. The content of
/// the read guard is guaranteed to be consistent for the life time of the
/// read - even if writers commit during.
pub fn read(&self) -> CowCellReadTxn<T> {
let rwguard = self.active.lock().unwrap();
CowCellReadTxn(rwguard.clone())
// rwguard ends here
}
/// Begin a write transaction, returning a write guard. The content of the
/// write is only visible to this thread, and is not visible to any reader
/// until `commit()` is called.
pub fn write(&self) -> CowCellWriteTxn<T> {
/* Take the exclusive write lock first */
let mguard = self.write.lock().unwrap();
// We delay copying until the first get_mut.
let read = {
let rwguard = self.active.lock().unwrap();
rwguard.clone()
};
/* Now build the write struct */
CowCellWriteTxn {
work: None,
read,
caller: self,
_guard: mguard,
}
}
/// Attempt to create a write transaction. If it fails, and err
/// is returned. On success the `Ok(guard)` is returned. See also
/// `write(&self)`
pub fn try_write(&self) -> Option<CowCellWriteTxn<T>> {
/* Take the exclusive write lock first */
self.write.try_lock().ok().map(|mguard| {
// We delay copying until the first get_mut.
let read = {
let rwguard = self.active.lock().unwrap();
rwguard.clone()
};
/* Now build the write struct */
CowCellWriteTxn {
work: None,
read,
caller: self,
_guard: mguard,
}
})
}
fn commit(&self, newdata: Option<T>) {
if let Some(nd) = newdata {
let mut rwguard = self.active.lock().unwrap();
let new_inner = Arc::new(nd);
// now over-write the last value in the mutex.
*rwguard = new_inner;
}
// If not some, we do nothing.
// Done
}
}
impl<T> Deref for CowCellReadTxn<T> {
type Target = T;
#[inline]
fn deref(&self) -> &T {
&self.0
}
}
impl<'a, T> CowCellWriteTxn<'a, T>
where
T: Clone,
{
/// Access a mutable pointer of the data in the `CowCell`. This data is only
/// visible to the write transaction object in this thread, until you call
/// `commit()`.
#[inline(always)]
pub fn get_mut(&mut self) -> &mut T {
if self.work.is_none() {
let mut data: Option<T> = Some((*self.read).clone());
std::mem::swap(&mut data, &mut self.work);
// Should be the none we previously had.
debug_assert!(data.is_none())
}
self.work.as_mut().expect("can not fail")
}
/// Commit the changes made in this write transactions to the `CowCell`.
/// This will consume the transaction so no further changes can be made
/// after this is called. Not calling this in a block, is equivalent to
/// an abort/rollback of the transaction.
pub fn commit(self) {
/* Write our data back to the CowCell */
self.caller.commit(self.work);
}
}
impl<'a, T> Deref for CowCellWriteTxn<'a, T>
where
T: Clone,
{
type Target = T;
#[inline(always)]
fn deref(&self) -> &T {
match &self.work {
Some(v) => v,
None => &(*self.read),
}
}
}
impl<'a, T> DerefMut for CowCellWriteTxn<'a, T>
where
T: Clone,
{
#[inline(always)]
fn deref_mut(&mut self) -> &mut T {
self.get_mut()
}
}
#[cfg(test)]
mod tests {
use super::CowCell;
use std::sync::atomic::{AtomicUsize, Ordering};
use crossbeam_utils::thread::scope;
#[test]
fn test_deref_mut() {
let data: i64 = 0;
let cc = CowCell::new(data);
{
/* Take a write txn */
let mut cc_wrtxn = cc.write();
*cc_wrtxn = 1;
cc_wrtxn.commit();
}
let cc_rotxn = cc.read();
assert_eq!(*cc_rotxn, 1);
}
#[test]
fn test_try_write() {
let data: i64 = 0;
let cc = CowCell::new(data);
/* Take a write txn */
let cc_wrtxn_a = cc.try_write();
assert!(cc_wrtxn_a.is_some());
/* Because we already hold the writ, the second is guaranteed to fail */
let cc_wrtxn_a = cc.try_write();
assert!(cc_wrtxn_a.is_none());
}
#[test]
fn test_simple_create() {
let data: i64 = 0;
let cc = CowCell::new(data);
let cc_rotxn_a = cc.read();
assert_eq!(*cc_rotxn_a, 0);
{
/* Take a write txn */
let mut cc_wrtxn = cc.write();
/* Get the data ... */
{
let mut_ptr = cc_wrtxn.get_mut();
/* Assert it's 0 */
assert_eq!(*mut_ptr, 0);
*mut_ptr = 1;
assert_eq!(*mut_ptr, 1);
}
assert_eq!(*cc_rotxn_a, 0);
let cc_rotxn_b = cc.read();
assert_eq!(*cc_rotxn_b, 0);
/* The write txn and it's lock is dropped here */
cc_wrtxn.commit();
}
/* Start a new txn and see it's still good */
let cc_rotxn_c = cc.read();
assert_eq!(*cc_rotxn_c, 1);
assert_eq!(*cc_rotxn_a, 0);
}
const MAX_TARGET: i64 = 2000;
#[test]
#[cfg_attr(miri, ignore)]
fn test_multithread_create() {
let start = time::Instant::now();
// Create the new cowcell.
let data: i64 = 0;
let cc = CowCell::new(data);
assert!(scope(|scope| {
let cc_ref = &cc;
let _readers: Vec<_> = (0..7)
.map(|_| {
scope.spawn(move |_| {
let mut last_value: i64 = 0;
while last_value < MAX_TARGET {
let cc_rotxn = cc_ref.read();
{
assert!(*cc_rotxn >= last_value);
last_value = *cc_rotxn;
}
}
})
})
.collect();
let _writers: Vec<_> = (0..3)
.map(|_| {
scope.spawn(move |_| {
let mut last_value: i64 = 0;
while last_value < MAX_TARGET {
let mut cc_wrtxn = cc_ref.write();
{
let mut_ptr = cc_wrtxn.get_mut();
assert!(*mut_ptr >= last_value);
last_value = *mut_ptr;
*mut_ptr = *mut_ptr + 1;
}
cc_wrtxn.commit();
}
})
})
.collect();
})
.is_ok());
let end = time::Instant::now();
print!("Arc MT create :{:?} ", end - start);
}
static GC_COUNT: AtomicUsize = AtomicUsize::new(0);
#[derive(Debug, Clone)]
struct TestGcWrapper<T> {
data: T,
}
impl<T> Drop for TestGcWrapper<T> {
fn drop(&mut self) {
// Add to the atomic counter ...
GC_COUNT.fetch_add(1, Ordering::Release);
}
}
fn test_gc_operation_thread(cc: &CowCell<TestGcWrapper<i64>>) {
while GC_COUNT.load(Ordering::Acquire) < 50 {
// thread::sleep(std::time::Duration::from_millis(200));
{
let mut cc_wrtxn = cc.write();
{
let mut_ptr = cc_wrtxn.get_mut();
mut_ptr.data = mut_ptr.data + 1;
}
cc_wrtxn.commit();
}
}
}
#[test]
#[cfg_attr(miri, ignore)]
fn test_gc_operation() {
GC_COUNT.store(0, Ordering::Release);
let data = TestGcWrapper { data: 0 };
let cc = CowCell::new(data);
assert!(scope(|scope| {
let cc_ref = &cc;
let _writers: Vec<_> = (0..3)
.map(|_| {
scope.spawn(move |_| {
test_gc_operation_thread(cc_ref);
})
})
.collect();
})
.is_ok());
assert!(GC_COUNT.load(Ordering::Acquire) >= 50);
}
}