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//! Support for locking the package and index caches.
//!
//! This implements locking on the package and index caches (source files,
//! `.crate` files, and index caches) to coordinate when multiple cargos are
//! running at the same time.
//!
//! ## Usage
//!
//! There is a global [`CacheLocker`] held inside cargo's venerable
//! [`GlobalContext`]. The `CacheLocker` manages creating and tracking the locks
//! being held. There are methods on [`GlobalContext`] for managing the locks:
//!
//! - [`GlobalContext::acquire_package_cache_lock`] --- Acquires a lock. May block if
//! another process holds a lock.
//! - [`GlobalContext::try_acquire_package_cache_lock`] --- Acquires a lock, returning
//! immediately if it would block.
//! - [`GlobalContext::assert_package_cache_locked`] --- This is used to ensure the
//! proper lock is being held.
//!
//! Lower-level code that accesses the package cache typically just use
//! `assert_package_cache_locked` to ensure that the correct lock is being
//! held. Higher-level code is responsible for acquiring the appropriate lock,
//! and holding it during the duration that it is performing its operation.
//!
//! ## Types of locking
//!
//! There are three styles of locks:
//!
//! * [`CacheLockMode::DownloadExclusive`] -- This is an exclusive lock
//! acquired while downloading packages and doing resolution.
//! * [`CacheLockMode::Shared`] -- This is a shared lock acquired while a
//! build is running. In other words, whenever cargo just needs to read from
//! the cache, it should hold this lock. This is here to ensure that no
//! cargos are trying to read the source caches when cache garbage
//! collection runs.
//! * [`CacheLockMode::MutateExclusive`] -- This is an exclusive lock acquired
//! whenever needing to modify existing source files (for example, with
//! cache garbage collection). This is acquired to make sure that no other
//! cargo is reading from the cache.
//!
//! Importantly, a `DownloadExclusive` lock does *not* interfere with a
//! `Shared` lock. The download process generally does not modify source files
//! (it only adds new ones), so other cargos should be able to safely proceed
//! in reading source files[^1].
//!
//! See the [`CacheLockMode`] enum docs for more details on when the different
//! modes should be used.
//!
//! ## Locking implementation details
//!
//! This is implemented by two separate lock files, the "download" one and the
//! "mutate" one. The `MutateExclusive` lock acquired both the "mutate" and
//! "download" locks. The `Shared` lock acquires the "mutate" lock in share
//! mode.
//!
//! An important rule is that `MutateExclusive` acquires the locks in the
//! order "mutate" first and then the "download". That helps prevent
//! deadlocks. It is not allowed for a cargo to first acquire a
//! `DownloadExclusive` lock and then a `Shared` lock because that would open
//! it up for deadlock.
//!
//! Another rule is that there should be only one [`CacheLocker`] per process
//! to uphold the ordering rules. You could in theory have multiple if you
//! could ensure that other threads would make progress and drop a lock, but
//! cargo is not architected that way.
//!
//! It is safe to recursively acquire a lock as many times as you want.
//!
//! ## Interaction with older cargos
//!
//! Before version 1.74, cargo only acquired the `DownloadExclusive` lock when
//! downloading and doing resolution. Newer cargos that acquire
//! `MutateExclusive` should still correctly block when an old cargo is
//! downloading (because it also acquires `DownloadExclusive`), but they do
//! not properly coordinate when an old cargo is in the build phase (because
//! it holds no locks). This isn't expected to be much of a problem because
//! the intended use of mutating the cache is only to delete old contents
//! which aren't currently being used. It is possible for there to be a
//! conflict, particularly if the user manually deletes the entire cache, but
//! it is not expected for this scenario to happen too often, and the only
//! consequence is that one side or the other encounters an error and needs to
//! retry.
//!
//! [^1]: A minor caveat is that downloads will delete an existing `src`
//! directory if it was extracted via an old cargo. See
//! [`crate::sources::registry::RegistrySource::unpack_package`]. This
//! should probably be fixed, but is unlikely to be a problem if the user is
//! only using versions of cargo with the same deletion logic.
use super::FileLock;
use crate::CargoResult;
use crate::GlobalContext;
use anyhow::Context as _;
use std::cell::RefCell;
use std::io;
/// The style of lock to acquire.
#[derive(Copy, Clone, Debug, PartialEq)]
pub enum CacheLockMode {
/// A `DownloadExclusive` lock ensures that only one cargo is doing
/// resolution and downloading new packages.
///
/// You should use this when downloading new packages or doing resolution.
///
/// If another cargo has a `MutateExclusive` lock, then an attempt to get
/// a `DownloadExclusive` lock will block.
///
/// If another cargo has a `Shared` lock, then both can operate
/// concurrently.
DownloadExclusive,
/// A `Shared` lock allows multiple cargos to read from the source files.
///
/// You should use this when cargo is reading source files from the
/// package cache. This is typically done during the build phase, since
/// cargo only needs to read files during that time. This allows multiple
/// cargo processes to build concurrently without interfering with one
/// another, while guarding against other cargos using `MutateExclusive`.
///
/// If another cargo has a `MutateExclusive` lock, then an attempt to get
/// a `Shared` will block.
///
/// If another cargo has a `DownloadExclusive` lock, then they both can
/// operate concurrently under the assumption that downloading does not
/// modify existing source files.
Shared,
/// A `MutateExclusive` lock ensures no other cargo is reading or writing
/// from the package caches.
///
/// You should use this when modifying existing files in the package
/// cache. For example, things like garbage collection want to avoid
/// deleting files while other cargos are trying to read (`Shared`) or
/// resolve or download (`DownloadExclusive`).
///
/// If another cargo has a `DownloadExclusive` or `Shared` lock, then this
/// will block until they all release their locks.
MutateExclusive,
}
/// Whether or not a lock attempt should block.
#[derive(Copy, Clone)]
enum BlockingMode {
Blocking,
NonBlocking,
}
use BlockingMode::*;
/// Whether or not a lock attempt blocked or succeeded.
#[derive(PartialEq, Copy, Clone)]
#[must_use]
enum LockingResult {
LockAcquired,
WouldBlock,
}
use LockingResult::*;
/// A file lock, with a counter to assist with recursive locking.
#[derive(Debug)]
struct RecursiveLock {
/// The file lock.
///
/// An important note is that locks can be `None` even when they are held.
/// This can happen on things like old NFS mounts where locking isn't
/// supported. We otherwise pretend we have a lock via the lock count. See
/// [`FileLock`] for more detail on that.
lock: Option<FileLock>,
/// Number locks held, to support recursive locking.
count: u32,
/// If this is `true`, it is an exclusive lock, otherwise it is shared.
is_exclusive: bool,
/// The filename of the lock.
filename: &'static str,
}
impl RecursiveLock {
fn new(filename: &'static str) -> RecursiveLock {
RecursiveLock {
lock: None,
count: 0,
is_exclusive: false,
filename,
}
}
/// Low-level lock count increment routine.
fn increment(&mut self) {
self.count = self.count.checked_add(1).unwrap();
}
/// Unlocks a previously acquired lock.
fn decrement(&mut self) {
let new_cnt = self.count.checked_sub(1).unwrap();
self.count = new_cnt;
if new_cnt == 0 {
// This will drop, releasing the lock.
self.lock = None;
}
}
/// Acquires a shared lock.
fn lock_shared(
&mut self,
gctx: &GlobalContext,
description: &'static str,
blocking: BlockingMode,
) -> LockingResult {
match blocking {
Blocking => {
self.lock_shared_blocking(gctx, description);
LockAcquired
}
NonBlocking => self.lock_shared_nonblocking(gctx),
}
}
/// Acquires a shared lock, blocking if held by another locker.
fn lock_shared_blocking(&mut self, gctx: &GlobalContext, description: &'static str) {
if self.count == 0 {
self.is_exclusive = false;
self.lock = match gctx
.home()
.open_ro_shared_create(self.filename, gctx, description)
{
Ok(lock) => Some(lock),
Err(e) => {
// There is no error here because locking is mostly a
// best-effort attempt. If cargo home is read-only, we don't
// want to fail just because we couldn't create the lock file.
tracing::warn!("failed to acquire cache lock {}: {e:?}", self.filename);
None
}
};
}
self.increment();
}
/// Acquires a shared lock, returns [`WouldBlock`] if held by another locker.
fn lock_shared_nonblocking(&mut self, gctx: &GlobalContext) -> LockingResult {
if self.count == 0 {
self.is_exclusive = false;
self.lock = match gctx.home().try_open_ro_shared_create(self.filename) {
Ok(Some(lock)) => Some(lock),
Ok(None) => {
return WouldBlock;
}
Err(e) => {
// Pretend that the lock was acquired (see lock_shared_blocking).
tracing::warn!("failed to acquire cache lock {}: {e:?}", self.filename);
None
}
};
}
self.increment();
LockAcquired
}
/// Acquires an exclusive lock.
fn lock_exclusive(
&mut self,
gctx: &GlobalContext,
description: &'static str,
blocking: BlockingMode,
) -> CargoResult<LockingResult> {
if self.count > 0 && !self.is_exclusive {
// Lock upgrades are dicey. It might be possible to support
// this but would take a bit of work, and so far it isn't
// needed.
panic!("lock upgrade from shared to exclusive not supported");
}
match blocking {
Blocking => {
self.lock_exclusive_blocking(gctx, description)?;
Ok(LockAcquired)
}
NonBlocking => self.lock_exclusive_nonblocking(gctx),
}
}
/// Acquires an exclusive lock, blocking if held by another locker.
fn lock_exclusive_blocking(
&mut self,
gctx: &GlobalContext,
description: &'static str,
) -> CargoResult<()> {
if self.count == 0 {
self.is_exclusive = true;
match gctx
.home()
.open_rw_exclusive_create(self.filename, gctx, description)
{
Ok(lock) => self.lock = Some(lock),
Err(e) => {
if maybe_readonly(&e) {
// This is a best-effort attempt to at least try to
// acquire some sort of lock. This can help in the
// situation where this cargo only has read-only access,
// but maybe some other cargo has read-write. This will at
// least attempt to coordinate with it.
//
// We don't want to fail on a read-only mount because
// cargo grabs an exclusive lock in situations where it
// may only be reading from the package cache. In that
// case, cargo isn't writing anything, and we don't want
// to fail on that.
self.lock_shared_blocking(gctx, description);
// This has to pretend it is exclusive for recursive locks to work.
self.is_exclusive = true;
return Ok(());
} else {
return Err(e).context("failed to acquire package cache lock");
}
}
}
}
self.increment();
Ok(())
}
/// Acquires an exclusive lock, returns [`WouldBlock`] if held by another locker.
fn lock_exclusive_nonblocking(&mut self, gctx: &GlobalContext) -> CargoResult<LockingResult> {
if self.count == 0 {
self.is_exclusive = true;
match gctx.home().try_open_rw_exclusive_create(self.filename) {
Ok(Some(lock)) => self.lock = Some(lock),
Ok(None) => return Ok(WouldBlock),
Err(e) => {
if maybe_readonly(&e) {
let result = self.lock_shared_nonblocking(gctx);
// This has to pretend it is exclusive for recursive locks to work.
self.is_exclusive = true;
return Ok(result);
} else {
return Err(e).context("failed to acquire package cache lock");
}
}
}
}
self.increment();
Ok(LockAcquired)
}
}
/// The state of the [`CacheLocker`].
#[derive(Debug)]
struct CacheState {
/// The cache lock guards the package cache used for download and
/// resolution (append operations that should not interfere with reading
/// from existing src files).
cache_lock: RecursiveLock,
/// The mutate lock is used to either guard the entire package cache for
/// destructive modifications (in exclusive mode), or for reading the
/// package cache src files (in shared mode).
///
/// Note that [`CacheLockMode::MutateExclusive`] holds both
/// [`CacheState::mutate_lock`] and [`CacheState::cache_lock`].
mutate_lock: RecursiveLock,
}
impl CacheState {
fn lock(
&mut self,
gctx: &GlobalContext,
mode: CacheLockMode,
blocking: BlockingMode,
) -> CargoResult<LockingResult> {
use CacheLockMode::*;
if mode == Shared && self.cache_lock.count > 0 && self.mutate_lock.count == 0 {
// Shared lock, when a DownloadExclusive is held.
//
// This isn't supported because it could cause a deadlock. If
// one cargo is attempting to acquire a MutateExclusive lock,
// and acquires the mutate lock, but is blocked on the
// download lock, and the cargo that holds the download lock
// attempts to get a shared lock, they would end up blocking
// each other.
panic!("shared lock while holding download lock is not allowed");
}
match mode {
Shared => {
if self.mutate_lock.lock_shared(gctx, SHARED_DESCR, blocking) == WouldBlock {
return Ok(WouldBlock);
}
}
DownloadExclusive => {
if self
.cache_lock
.lock_exclusive(gctx, DOWNLOAD_EXCLUSIVE_DESCR, blocking)?
== WouldBlock
{
return Ok(WouldBlock);
}
}
MutateExclusive => {
if self
.mutate_lock
.lock_exclusive(gctx, MUTATE_EXCLUSIVE_DESCR, blocking)?
== WouldBlock
{
return Ok(WouldBlock);
}
// Part of the contract of MutateExclusive is that it doesn't
// allow any processes to have a lock on the package cache, so
// this acquires both locks.
match self
.cache_lock
.lock_exclusive(gctx, DOWNLOAD_EXCLUSIVE_DESCR, blocking)
{
Ok(LockAcquired) => {}
Ok(WouldBlock) => return Ok(WouldBlock),
Err(e) => {
self.mutate_lock.decrement();
return Err(e);
}
}
}
}
Ok(LockAcquired)
}
}
/// A held lock guard.
///
/// When this is dropped, the lock will be released.
#[must_use]
pub struct CacheLock<'lock> {
mode: CacheLockMode,
locker: &'lock CacheLocker,
}
impl Drop for CacheLock<'_> {
fn drop(&mut self) {
use CacheLockMode::*;
let mut state = self.locker.state.borrow_mut();
match self.mode {
Shared => {
state.mutate_lock.decrement();
}
DownloadExclusive => {
state.cache_lock.decrement();
}
MutateExclusive => {
state.cache_lock.decrement();
state.mutate_lock.decrement();
}
}
}
}
/// The filename for the [`CacheLockMode::DownloadExclusive`] lock.
const CACHE_LOCK_NAME: &str = ".package-cache";
/// The filename for the [`CacheLockMode::MutateExclusive`] and
/// [`CacheLockMode::Shared`] lock.
const MUTATE_NAME: &str = ".package-cache-mutate";
// Descriptions that are displayed in the "Blocking" message shown to the user.
const SHARED_DESCR: &str = "shared package cache";
const DOWNLOAD_EXCLUSIVE_DESCR: &str = "package cache";
const MUTATE_EXCLUSIVE_DESCR: &str = "package cache mutation";
/// A locker that can be used to acquire locks.
///
/// See the [`crate::util::cache_lock`] module documentation for an overview
/// of how cache locking works.
#[derive(Debug)]
pub struct CacheLocker {
/// The state of the locker.
///
/// [`CacheLocker`] uses interior mutability because it is stuffed inside
/// [`GlobalContext`], which does not allow mutation.
state: RefCell<CacheState>,
}
impl CacheLocker {
/// Creates a new `CacheLocker`.
pub fn new() -> CacheLocker {
CacheLocker {
state: RefCell::new(CacheState {
cache_lock: RecursiveLock::new(CACHE_LOCK_NAME),
mutate_lock: RecursiveLock::new(MUTATE_NAME),
}),
}
}
/// Acquires a lock with the given mode, possibly blocking if another
/// cargo is holding the lock.
pub fn lock(&self, gctx: &GlobalContext, mode: CacheLockMode) -> CargoResult<CacheLock<'_>> {
let mut state = self.state.borrow_mut();
let _ = state.lock(gctx, mode, Blocking)?;
Ok(CacheLock { mode, locker: self })
}
/// Acquires a lock with the given mode, returning `None` if another cargo
/// is holding the lock.
pub fn try_lock(
&self,
gctx: &GlobalContext,
mode: CacheLockMode,
) -> CargoResult<Option<CacheLock<'_>>> {
let mut state = self.state.borrow_mut();
if state.lock(gctx, mode, NonBlocking)? == LockAcquired {
Ok(Some(CacheLock { mode, locker: self }))
} else {
Ok(None)
}
}
/// Returns whether or not a lock is held for the given mode in this locker.
///
/// This does not tell you whether or not it is locked in some other
/// locker (such as in another process).
///
/// Note that `Shared` will return true if a `MutateExclusive` lock is
/// held, since `MutateExclusive` is just an upgraded `Shared`. Likewise,
/// `DownloadExclusive` will return true if a `MutateExclusive` lock is
/// held since they overlap.
pub fn is_locked(&self, mode: CacheLockMode) -> bool {
let state = self.state.borrow();
match (
mode,
state.cache_lock.count,
state.mutate_lock.count,
state.mutate_lock.is_exclusive,
) {
(CacheLockMode::Shared, _, 1.., _) => true,
(CacheLockMode::MutateExclusive, _, 1.., true) => true,
(CacheLockMode::DownloadExclusive, 1.., _, _) => true,
_ => false,
}
}
}
/// Returns whether or not the error appears to be from a read-only filesystem.
fn maybe_readonly(err: &anyhow::Error) -> bool {
err.chain().any(|err| {
if let Some(io) = err.downcast_ref::<io::Error>() {
if io.kind() == io::ErrorKind::PermissionDenied {
return true;
}
#[cfg(unix)]
return io.raw_os_error() == Some(libc::EROFS);
}
false
})
}