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//! This library implements the 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 macgyver_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); //!} //!``` #[macro_use] extern crate log; use memcache::Client; use std::{ borrow::Cow, collections::HashMap, fmt, time::{Duration, Instant, SystemTime, UNIX_EPOCH}, }; pub use memcache::{Connectable, MemcacheError}; const MEMCACHE_DEFAULT_URL: &'static str = "memcache://localhost:11211"; /// The lock configuration along with the connection to memcache server. pub struct MacGyverLock<'a> { /// Name of the lock. Also used as the key in memcached. name: Cow<'a, str>, /// Memcache client instance. memcache: Client, /// Expiry in seconds. expiry: u32, /// The value set in the key to identify the current owner of the key. owner: Cow<'a, str>, } /// Lock guard returned after successfully acquiring a lock. /// The lock is automatically released when the `Guard` is dropped. /// /// Note: The `PartialEq` impl only exists to make error handling simple. /// `Guard`s should never be compared. It always returns `false`. pub struct Guard<'l, 'b> { released: bool, lock: &'l mut MacGyverLock<'b>, } impl<'l, 'b> PartialEq for Guard<'l, 'b> { // There can never be two mutable references to MacGyverLock. fn eq(&self, _: &Self) -> bool { false } } impl<'l, 'b> fmt::Debug for Guard<'l, 'b> { fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { write!(f, "Guard {{ released: {} }}", self.released) } } impl<'a, 'b> Guard<'a, 'b> { /// Releases the lock by deleting the key in memcache. /// /// Returns `Ok(())` when the lock is successfully released. pub fn try_release(mut self) -> Result<(), LockError> { if !self.released { let result = self.lock.release().map_err(Into::into); self.released = result.is_ok(); result } else { Err(LockError::AlreadyReleased) } } } impl<'a, 'b> Drop for Guard<'a, 'b> { fn drop(&mut self) { if !self.released { let _ = self.lock.release(); } } } /// Result type used by the `try_acquire` function. pub type LockResult<'a, 'b> = Result<Guard<'a, 'b>, LockError>; impl<'a> MacGyverLock<'a> { fn is_expired(&self, lock_time: Instant) -> bool { self.expiry != 0 // denotes no expiry && if self.expiry <= 60 * 60 * 24 * 30 { // memcache interprets these values as seconds lock_time.elapsed().as_secs() >= u64::from(self.expiry) } else { // check unix time. UNIX_EPOCH + Duration::from_secs(u64::from(self.expiry)) <= SystemTime::now() } } // 'l is the scope for which the lock should live. /// Try to acquire the lock, returning the guard if the lock was successfully acquired. /// Otherwise returns an error denoting why the lock couldn't be acquired. pub fn try_acquire<'l>(&'l mut self) -> LockResult<'l, 'a> { debug!(target: "macgyver-lock", "trying to acquire lock: {} for user: {}", &self.name, &self.owner); let instant = Instant::now(); match self.memcache.add(&self.name, &*self.owner, self.expiry) { Ok(()) => { // if lock expired before the call could return, we don't have the lock, so error. if self.is_expired(instant) { debug!( target: "macgyver-lock", "failed to acquire lock: {} for user: {}, memcache.add latency: {}", &self.name, &self.owner, instant.elapsed().as_secs() ); Err(LockError::TimedOut) } else { debug!(target: "macgyver-lock", "acquired lock: {} for user: {}", &self.name, &self.owner); Ok(Guard { released: false, lock: self, }) } } Err(e) => match e { // key already exists in server, so retry MemcacheError::ServerError(0x02) => { debug!(target: "macgyver-lock", "failed to acquire lock: {}, already taken", &self.name); Err(LockError::FailedToAcquire) } e => Err(LockError::MemcacheError(e)), }, } } /// Using CAS with an expiry value in the past to atomically delete the lock as /// `delete` doesn't accept `cas_id`. /// /// Returns `true` if the key has been successfully deleted. `false` if key expired. fn delete(&mut self, cas: u64) -> Result<(), LockError> { match self.memcache.cas(&self.name, "", 1576412321, cas)? { true => Ok(()), false => Err(LockError::AlreadyReleased), } } /// Releases the lock by deleting the key in memcache. fn release(&mut self) -> Result<(), LockError> { let result: HashMap<String, (Vec<u8>, u32, Option<u64>)> = self.memcache.gets(&[&self.name])?; if let Some((current_owner, _, cas)) = result.get(&*self.name) { if *current_owner == &*self.owner.as_bytes() { debug!(target: "macgyver-lock", "trying to release: {}", &self.name); self.delete(cas.expect("cas should be present")) } else { debug!(target: "macgyver-lock", "trying to release: {}, not owned", &self.name); Err(LockError::NotOwned) } } else { debug!(target: "macgyver-lock", "trying to release: {}, already released", &self.name); Err(LockError::AlreadyReleased) } } } /// Error type #[derive(Debug)] pub enum LockError { /// When trying to release a lock, it was found to be already released. /// This could be because the key has expired. AlreadyReleased, /// Tried to release the lock but it wasn't owned by the current instance. NotOwned, /// An other user succeeded in acquiring the lock. FailedToAcquire, /// The `add` call was successful, but a possible latency in network/process /// scheduling caused the lock to be released. TimedOut, /// Other errors returned by the underlying memcache client. MemcacheError(MemcacheError), } impl PartialEq for LockError { fn eq(&self, other: &Self) -> bool { match (self, other) { (LockError::AlreadyReleased, LockError::AlreadyReleased) => true, (LockError::NotOwned, LockError::NotOwned) => true, (LockError::FailedToAcquire, LockError::FailedToAcquire) => true, (LockError::TimedOut, LockError::TimedOut) => true, (LockError::MemcacheError(l), LockError::MemcacheError(r)) => match (l, r) { (MemcacheError::Io(l), MemcacheError::Io(r)) => l.kind() == r.kind(), (MemcacheError::ClientError(l), MemcacheError::ClientError(r)) => l == r, (MemcacheError::ServerError(l), MemcacheError::ServerError(r)) => l == r, (MemcacheError::FromUtf8(_), MemcacheError::FromUtf8(_)) => true, (MemcacheError::ParseIntError(_), MemcacheError::ParseIntError(_)) => true, (MemcacheError::ParseFloatError(_), MemcacheError::ParseFloatError(_)) => true, (MemcacheError::ParseBoolError(_), MemcacheError::ParseBoolError(_)) => true, _ => false, }, _ => false, } } } impl From<MemcacheError> for LockError { fn from(error: MemcacheError) -> Self { Self::MemcacheError(error) } } /// Builder for `MacGyverLock` pub struct LockOptions<'a, C> { /// Name of the lock. Also used as the memcache key name: Cow<'a, str>, /// `Connectable` which can resolve to a list of memcache servers. connectable: C, /// Expiry to use for the memcache lock key in seconds. expiry: Option<u32>, /// Unique identifier representing the current owner of the lock. owner: Cow<'a, str>, /// Read timeout for the underlying connection. read_timeout: Option<Duration>, /// Write timeout for the underlying connection. write_timeout: Option<Duration>, } impl<'a> LockOptions<'a, &str> { /// Initialize the lock builder. /// `name` is used as the memcache key while trying to acquire the lock. /// `owner` is used to uniquely identify the current owner of the lock. /// Each individual user trying to acquire the lock should have a unique owner value. pub fn new(name: Cow<'a, str>, owner: Cow<'a, str>) -> Self { Self { name, connectable: MEMCACHE_DEFAULT_URL, expiry: None, owner: owner, read_timeout: None, write_timeout: None, } } } impl<'a, C: Connectable> LockOptions<'a, C> { /// Set the expiry in seconds for the the lock key. The lock will be /// released after this number of seconds elapse. /// Defaults to `10` seconds. /// /// Note: A value of `0` denotes no expiry and a value greater than 24*60*60*30 /// is interpreted as a unix timestamp. /// /// Note 2: `std::time::SystemTime::now` is used to check if the lock has expired /// when expiry is interepreted as UNIX timestamp. The check might fail if the system /// time has been tampered with. /// /// If a user holding the lock panics, it could lead to a deadlock when there is no /// expiry. pub fn with_expiry(mut self, expiry: u32) -> Self { self.expiry = Some(expiry); self } /// The memcache servers to use. /// /// Defaults to `memcache://localhost:11211` /// /// Note: The `Connectable` shouldn't use ASCII protocol because the underlying memcache /// library doesn't give a way to detect if acquiring the lock succeeded. pub fn with_connectable<K: Connectable>(self, connectable: K) -> LockOptions<'a, K> { LockOptions { connectable: connectable, name: self.name, expiry: self.expiry, owner: self.owner, read_timeout: self.read_timeout, write_timeout: self.write_timeout, } } /// Set the read timeout of the underlying connection to the server. The value should be very /// small compared to the key expiry time. Otherwise, we might never acquire the lock in case /// of network delays. pub fn with_read_timeout(mut self, timeout: Duration) -> Self { self.read_timeout = Some(timeout); self } /// Set the write timeout of the underlying connection to the server. The value should be very /// small compared to the expiry seconds. Otherwise, we might never acquire of the lock in case /// of network delays. pub fn with_write_timeout(mut self, timeout: Duration) -> Self { self.write_timeout = Some(timeout); self } /// Build the lock. pub fn build(self) -> Result<MacGyverLock<'a>, LockError> { let mut memcache = Client::connect(self.connectable)?; memcache.set_read_timeout(self.read_timeout)?; memcache.set_write_timeout(self.write_timeout)?; Ok(MacGyverLock { name: self.name, memcache, owner: self.owner, expiry: self.expiry.unwrap_or(10), }) } }