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// Copyright 2023 The Matrix.org Foundation C.I.C.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
//! Collection of small helpers that implement store-based locks.
//!
//! This is a per-process lock that may be used only for very specific use
//! cases, where multiple processes might concurrently write to the same
//! database at the same time; this would invalidate store caches, so
//! that should be done mindfully. Such a lock can be acquired multiple times by
//! the same process, and it remains active as long as there's at least one user
//! in a given process.
//!
//! The lock is implemented using time-based leases to values inserted in a
//! store. The store maintains the lock identifier (key), who's the
//! current holder (value), and an expiration timestamp on the side; see also
//! `CryptoStore::try_take_leased_lock` for more details.
//!
//! The lock is initially acquired for a certain period of time (namely, the
//! duration of a lease, aka `LEASE_DURATION_MS`), and then a "heartbeat" task
//! renews the lease to extend its duration, every so often (namely, every
//! `EXTEND_LEASE_EVERY_MS`). Since the tokio scheduler might be busy, the
//! extension request should happen way more frequently than the duration of a
//! lease, in case a deadline is missed. The current values have been chosen to
//! reflect that, with a ratio of 1:10 as of 2023-06-23.
//!
//! Releasing the lock happens naturally, by not renewing a lease. It happens
//! automatically after the duration of the last lease, at most.
use std::{
error::Error,
sync::{
atomic::{self, AtomicU32},
Arc,
},
time::Duration,
};
use tokio::{sync::Mutex, time::sleep};
use tracing::{debug, error, info, instrument, trace};
use crate::{
executor::{spawn, JoinHandle},
SendOutsideWasm,
};
/// Backing store for a cross-process lock.
#[cfg_attr(target_arch = "wasm32", async_trait::async_trait(?Send))]
#[cfg_attr(not(target_arch = "wasm32"), async_trait::async_trait)]
pub trait BackingStore {
type Error: Error + Send + Sync;
/// Try to take a lock using the given store.
async fn try_lock(
&self,
lease_duration_ms: u32,
key: &str,
holder: &str,
) -> Result<bool, Self::Error>;
}
/// Small state machine to handle wait times.
#[derive(Clone, Debug)]
enum WaitingTime {
/// Some time to wait, in milliseconds.
Some(u32),
/// Stop waiting when seeing this value.
Stop,
}
/// A guard on the store lock.
///
/// The lock will be automatically released a short period of time after all the
/// guards have dropped.
#[derive(Debug)]
pub struct CrossProcessStoreLockGuard {
num_holders: Arc<AtomicU32>,
}
impl Drop for CrossProcessStoreLockGuard {
fn drop(&mut self) {
self.num_holders.fetch_sub(1, atomic::Ordering::SeqCst);
}
}
/// A store-based lock for a `Store`.
///
/// See the doc-comment of this module for more information.
#[derive(Clone, Debug)]
pub struct CrossProcessStoreLock<S: BackingStore + Clone + SendOutsideWasm + 'static> {
/// The store we're using to lock.
store: S,
/// Number of holders of the lock in this process.
///
/// If greater than 0, this means we've already acquired this lock, in this
/// process, and the store lock mustn't be touched.
///
/// When the number of holders is decreased to 0, then the lock must be
/// released in the store.
num_holders: Arc<AtomicU32>,
/// A mutex to control an attempt to take the lock, to avoid making it
/// reentrant.
locking_attempt: Arc<Mutex<()>>,
/// Current renew task spawned by `try_lock_once`.
renew_task: Arc<Mutex<Option<JoinHandle<()>>>>,
/// The key used in the key/value mapping for the lock entry.
lock_key: String,
/// A specific value to identify the lock's holder.
lock_holder: String,
/// Backoff time, in milliseconds.
backoff: Arc<Mutex<WaitingTime>>,
}
/// Amount of time a lease of the lock should last, in milliseconds.
pub const LEASE_DURATION_MS: u32 = 500;
/// Period of time between two attempts to extend the lease. We'll
/// re-request a lease for an entire duration of `LEASE_DURATION_MS`
/// milliseconds, every `EXTEND_LEASE_EVERY_MS`, so this has to
/// be an amount safely low compared to `LEASE_DURATION_MS`, to make sure
/// that we can miss a deadline without compromising the lock.
pub const EXTEND_LEASE_EVERY_MS: u64 = 50;
/// Initial backoff, in milliseconds. This is the time we wait the first
/// time, if taking the lock initially failed.
const INITIAL_BACKOFF_MS: u32 = 10;
/// Maximal backoff, in milliseconds. This is the maximum amount of time
/// we'll wait for the lock, *between two attempts*.
pub const MAX_BACKOFF_MS: u32 = 1000;
impl<S: BackingStore + Clone + SendOutsideWasm + 'static> CrossProcessStoreLock<S> {
/// Create a new store-based lock implemented as a value in the store.
///
/// # Parameters
///
/// - `lock_key`: key in the key-value store to store the lock's state.
/// - `lock_holder`: identify the lock's holder with this given value.
pub fn new(store: S, lock_key: String, lock_holder: String) -> Self {
Self {
store,
lock_key,
lock_holder,
backoff: Arc::new(Mutex::new(WaitingTime::Some(INITIAL_BACKOFF_MS))),
num_holders: Arc::new(0.into()),
locking_attempt: Arc::new(Mutex::new(())),
renew_task: Default::default(),
}
}
/// Try to lock once, returns whether the lock was obtained or not.
#[instrument(skip(self), fields(?self.lock_key, ?self.lock_holder))]
pub async fn try_lock_once(
&self,
) -> Result<Option<CrossProcessStoreLockGuard>, LockStoreError> {
// Hold onto the locking attempt mutex for the entire lifetime of this
// function, to avoid multiple reentrant calls.
let mut _attempt = self.locking_attempt.lock().await;
// If another thread obtained the lock, make sure to only superficially increase
// the number of holders, and carry on.
if self.num_holders.load(atomic::Ordering::SeqCst) > 0 {
// Note: between the above load and the fetch_add below, another thread may
// decrement `num_holders`. That's fine because that means the lock
// was taken by at least one thread, and after this call it will be
// taken by at least one thread.
trace!("We already had the lock, incrementing holder count");
self.num_holders.fetch_add(1, atomic::Ordering::SeqCst);
let guard = CrossProcessStoreLockGuard { num_holders: self.num_holders.clone() };
return Ok(Some(guard));
}
let acquired = self
.store
.try_lock(LEASE_DURATION_MS, &self.lock_key, &self.lock_holder)
.await
.map_err(|err| LockStoreError::BackingStoreError(Box::new(err)))?;
if !acquired {
trace!("Couldn't acquire the lock immediately.");
return Ok(None);
}
trace!("Acquired the lock, spawning the lease extension task.");
// This is the first time we've acquired the lock. We're going to spawn the task
// that will renew the lease.
// Clone data to be owned by the task.
let this = (*self).clone();
let mut renew_task = self.renew_task.lock().await;
// Cancel the previous task, if any. That's safe to do, because:
// - either the task was done,
// - or it was still running, but taking a lock in the db has to be an atomic
// operation running in a transaction.
if let Some(_prev) = renew_task.take() {
#[cfg(not(target_arch = "wasm32"))]
if !_prev.is_finished() {
info!("aborting the previous renew task");
_prev.abort();
}
}
// Restart a new one.
*renew_task = Some(spawn(async move {
loop {
{
// First, check if there are still users of this lock.
//
// This is not racy, because:
// - the `locking_attempt` mutex makes sure we don't have unexpected
// interactions with the non-atomic sequence above in `try_lock_once`
// (check > 0, then add 1).
// - other entities holding onto the `num_holders` atomic will only
// decrease it over time.
let _guard = this.locking_attempt.lock().await;
// If there are no more users, we can quit.
if this.num_holders.load(atomic::Ordering::SeqCst) == 0 {
info!("exiting the lease extension loop");
// Cancel the lease with another 0ms lease.
// If we don't get the lock, that's (weird but) fine.
let fut = this.store.try_lock(0, &this.lock_key, &this.lock_holder);
let _ = fut.await;
// Exit the loop.
break;
}
}
sleep(Duration::from_millis(EXTEND_LEASE_EVERY_MS)).await;
let fut = this.store.try_lock(LEASE_DURATION_MS, &this.lock_key, &this.lock_holder);
if let Err(err) = fut.await {
error!("error when extending lock lease: {err:#}");
// Exit the loop.
break;
}
}
}));
self.num_holders.fetch_add(1, atomic::Ordering::SeqCst);
let guard = CrossProcessStoreLockGuard { num_holders: self.num_holders.clone() };
Ok(Some(guard))
}
/// Attempt to take the lock, with exponential backoff if the lock has
/// already been taken before.
///
/// The `max_backoff` parameter is the maximum time (in milliseconds) that
/// should be waited for, between two attempts. When that time is
/// reached a second time, the lock will stop attempting to get the lock
/// and will return a timeout error upon locking. If not provided,
/// will wait for [`MAX_BACKOFF_MS`].
#[instrument(skip(self), fields(?self.lock_key, ?self.lock_holder))]
pub async fn spin_lock(
&self,
max_backoff: Option<u32>,
) -> Result<CrossProcessStoreLockGuard, LockStoreError> {
let max_backoff = max_backoff.unwrap_or(MAX_BACKOFF_MS);
// Note: reads/writes to the backoff are racy across threads in theory, but the
// lock in `try_lock_once` should sequentialize it all.
loop {
if let Some(guard) = self.try_lock_once().await? {
// Reset backoff before returning, for the next attempt to lock.
*self.backoff.lock().await = WaitingTime::Some(INITIAL_BACKOFF_MS);
return Ok(guard);
}
// Exponential backoff! Multiply by 2 the time we've waited before, cap it to
// max_backoff.
let mut backoff = self.backoff.lock().await;
let wait = match &mut *backoff {
WaitingTime::Some(ref mut val) => {
let wait = *val;
*val = val.saturating_mul(2);
if *val >= max_backoff {
*backoff = WaitingTime::Stop;
}
wait
}
WaitingTime::Stop => {
// We've reached the maximum backoff, abandon.
return Err(LockStoreError::LockTimeout);
}
};
debug!("Waiting {wait} before re-attempting to take the lock");
sleep(Duration::from_millis(wait.into())).await;
}
}
/// Returns the value in the database that represents the holder's
/// identifier.
pub fn lock_holder(&self) -> &str {
&self.lock_holder
}
}
/// Error related to the locking API of the store.
#[derive(Debug, thiserror::Error)]
pub enum LockStoreError {
/// Spent too long waiting for a database lock.
#[error("a lock timed out")]
LockTimeout,
#[error(transparent)]
BackingStoreError(#[from] Box<dyn Error + Send + Sync>),
}
#[cfg(test)]
#[cfg(not(target_arch = "wasm32"))] // These tests require tokio::time, which is not implemented on wasm.
mod tests {
use std::{
collections::HashMap,
sync::{atomic, Arc, Mutex},
time::Instant,
};
use assert_matches::assert_matches;
use matrix_sdk_test::async_test;
use tokio::{
spawn,
time::{sleep, Duration},
};
use super::{
BackingStore, CrossProcessStoreLock, CrossProcessStoreLockGuard, LockStoreError,
EXTEND_LEASE_EVERY_MS,
};
#[derive(Clone, Default)]
struct TestStore {
leases: Arc<Mutex<HashMap<String, (String, Instant)>>>,
}
impl TestStore {
fn try_take_leased_lock(&self, lease_duration_ms: u32, key: &str, holder: &str) -> bool {
let now = Instant::now();
let expiration = now + Duration::from_millis(lease_duration_ms.into());
let mut leases = self.leases.lock().unwrap();
if let Some(prev) = leases.get_mut(key) {
if prev.0 == holder {
// We had the lease before, extend it.
prev.1 = expiration;
true
} else {
// We didn't have it.
if prev.1 < now {
// Steal it!
prev.0 = holder.to_owned();
prev.1 = expiration;
true
} else {
// We tried our best.
false
}
}
} else {
leases.insert(
key.to_owned(),
(
holder.to_owned(),
Instant::now() + Duration::from_millis(lease_duration_ms.into()),
),
);
true
}
}
}
#[derive(Debug, thiserror::Error)]
enum DummyError {}
#[cfg_attr(target_arch = "wasm32", async_trait::async_trait(?Send))]
#[cfg_attr(not(target_arch = "wasm32"), async_trait::async_trait)]
impl BackingStore for TestStore {
type Error = DummyError;
/// Try to take a lock using the given store.
async fn try_lock(
&self,
lease_duration_ms: u32,
key: &str,
holder: &str,
) -> Result<bool, Self::Error> {
Ok(self.try_take_leased_lock(lease_duration_ms, key, holder))
}
}
async fn release_lock(guard: Option<CrossProcessStoreLockGuard>) {
drop(guard);
sleep(Duration::from_millis(EXTEND_LEASE_EVERY_MS)).await;
}
type TestResult = Result<(), LockStoreError>;
#[async_test]
async fn test_simple_lock_unlock() -> TestResult {
let store = TestStore::default();
let lock = CrossProcessStoreLock::new(store, "key".to_owned(), "first".to_owned());
// The lock plain works when used with a single holder.
let acquired = lock.try_lock_once().await?;
assert!(acquired.is_some());
assert_eq!(lock.num_holders.load(atomic::Ordering::SeqCst), 1);
// Releasing works.
release_lock(acquired).await;
assert_eq!(lock.num_holders.load(atomic::Ordering::SeqCst), 0);
// Spin locking on the same lock always works, assuming no concurrent access.
let acquired = lock.spin_lock(None).await.unwrap();
// Releasing still works.
release_lock(Some(acquired)).await;
assert_eq!(lock.num_holders.load(atomic::Ordering::SeqCst), 0);
Ok(())
}
#[async_test]
async fn test_self_recovery() -> TestResult {
let store = TestStore::default();
let lock = CrossProcessStoreLock::new(store.clone(), "key".to_owned(), "first".to_owned());
// When a lock is acquired...
let acquired = lock.try_lock_once().await?;
assert!(acquired.is_some());
assert_eq!(lock.num_holders.load(atomic::Ordering::SeqCst), 1);
// But then forgotten... (note: no need to release the guard)
drop(lock);
// And when rematerializing the lock with the same key/value...
let lock = CrossProcessStoreLock::new(store.clone(), "key".to_owned(), "first".to_owned());
// We still got it.
let acquired = lock.try_lock_once().await?;
assert!(acquired.is_some());
assert_eq!(lock.num_holders.load(atomic::Ordering::SeqCst), 1);
Ok(())
}
#[async_test]
async fn test_multiple_holders_same_process() -> TestResult {
let store = TestStore::default();
let lock = CrossProcessStoreLock::new(store, "key".to_owned(), "first".to_owned());
// Taking the lock twice...
let acquired = lock.try_lock_once().await?;
assert!(acquired.is_some());
let acquired2 = lock.try_lock_once().await?;
assert!(acquired2.is_some());
assert_eq!(lock.num_holders.load(atomic::Ordering::SeqCst), 2);
// ...means we can release it twice.
release_lock(acquired).await;
assert_eq!(lock.num_holders.load(atomic::Ordering::SeqCst), 1);
release_lock(acquired2).await;
assert_eq!(lock.num_holders.load(atomic::Ordering::SeqCst), 0);
Ok(())
}
#[async_test]
async fn test_multiple_processes() -> TestResult {
let store = TestStore::default();
let lock1 = CrossProcessStoreLock::new(store.clone(), "key".to_owned(), "first".to_owned());
let lock2 = CrossProcessStoreLock::new(store, "key".to_owned(), "second".to_owned());
// When the first process takes the lock...
let acquired1 = lock1.try_lock_once().await?;
assert!(acquired1.is_some());
// The second can't take it immediately.
let acquired2 = lock2.try_lock_once().await?;
assert!(acquired2.is_none());
let lock2_clone = lock2.clone();
let handle = spawn(async move { lock2_clone.spin_lock(Some(1000)).await });
sleep(Duration::from_millis(100)).await;
drop(acquired1);
// lock2 in the background manages to get the lock at some point.
let _acquired2 = handle
.await
.expect("join handle is properly awaited")
.expect("lock was obtained after spin-locking");
// Now if lock1 tries to get the lock with a small timeout, it will fail.
assert_matches!(lock1.spin_lock(Some(200)).await, Err(LockStoreError::LockTimeout));
Ok(())
}
}