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John Doty 2022-12-19 08:27:18 -08:00
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#![cfg_attr(not(any(test, feature = "use_std")), no_std)]
#![doc(html_root_url = "https://docs.rs/scopeguard/1/")]
//! A scope guard will run a given closure when it goes out of scope,
//! even if the code between panics.
//! (as long as panic doesn't abort)
//!
//! # Examples
//!
//! ## Hello World
//!
//! This example creates a scope guard with an example function:
//!
//! ```
//! extern crate scopeguard;
//!
//! fn f() {
//! let _guard = scopeguard::guard((), |_| {
//! println!("Hello Scope Exit!");
//! });
//!
//! // rest of the code here.
//!
//! // Here, at the end of `_guard`'s scope, the guard's closure is called.
//! // It is also called if we exit this scope through unwinding instead.
//! }
//! # fn main() {
//! # f();
//! # }
//! ```
//!
//! ## `defer!`
//!
//! Use the `defer` macro to run an operation at scope exit,
//! either regular scope exit or during unwinding from a panic.
//!
//! ```
//! #[macro_use(defer)] extern crate scopeguard;
//!
//! use std::cell::Cell;
//!
//! fn main() {
//! // use a cell to observe drops during and after the scope guard is active
//! let drop_counter = Cell::new(0);
//! {
//! // Create a scope guard using `defer!` for the current scope
//! defer! {
//! drop_counter.set(1 + drop_counter.get());
//! }
//!
//! // Do regular operations here in the meantime.
//!
//! // Just before scope exit: it hasn't run yet.
//! assert_eq!(drop_counter.get(), 0);
//!
//! // The following scope end is where the defer closure is called
//! }
//! assert_eq!(drop_counter.get(), 1);
//! }
//! ```
//!
//! ## Scope Guard with Value
//!
//! If the scope guard closure needs to access an outer value that is also
//! mutated outside of the scope guard, then you may want to use the scope guard
//! with a value. The guard works like a smart pointer, so the inner value can
//! be accessed by reference or by mutable reference.
//!
//! ### 1. The guard owns a file
//!
//! In this example, the scope guard owns a file and ensures pending writes are
//! synced at scope exit.
//!
//! ```
//! extern crate scopeguard;
//!
//! use std::fs::*;
//! use std::io::{self, Write};
//! # // Mock file so that we don't actually write a file
//! # struct MockFile;
//! # impl MockFile {
//! # fn create(_s: &str) -> io::Result<Self> { Ok(MockFile) }
//! # fn write_all(&self, _b: &[u8]) -> io::Result<()> { Ok(()) }
//! # fn sync_all(&self) -> io::Result<()> { Ok(()) }
//! # }
//! # use self::MockFile as File;
//!
//! fn try_main() -> io::Result<()> {
//! let f = File::create("newfile.txt")?;
//! let mut file = scopeguard::guard(f, |f| {
//! // ensure we flush file at return or panic
//! let _ = f.sync_all();
//! });
//! // Access the file through the scope guard itself
//! file.write_all(b"test me\n").map(|_| ())
//! }
//!
//! fn main() {
//! try_main().unwrap();
//! }
//!
//! ```
//!
//! ### 2. The guard restores an invariant on scope exit
//!
//! ```
//! extern crate scopeguard;
//!
//! use std::mem::ManuallyDrop;
//! use std::ptr;
//!
//! // This function, just for this example, takes the first element
//! // and inserts it into the assumed sorted tail of the vector.
//! //
//! // For optimization purposes we temporarily violate an invariant of the
//! // Vec, that it owns all of its elements.
//! //
//! // The safe approach is to use swap, which means two writes to memory,
//! // the optimization is to use a “hole” which uses only one write of memory
//! // for each position it moves.
//! //
//! // We *must* use a scope guard to run this code safely. We
//! // are running arbitrary user code (comparison operators) that may panic.
//! // The scope guard ensures we restore the invariant after successful
//! // exit or during unwinding from panic.
//! fn insertion_sort_first<T>(v: &mut Vec<T>)
//! where T: PartialOrd
//! {
//! struct Hole<'a, T: 'a> {
//! v: &'a mut Vec<T>,
//! index: usize,
//! value: ManuallyDrop<T>,
//! }
//!
//! unsafe {
//! // Create a moved-from location in the vector, a “hole”.
//! let value = ptr::read(&v[0]);
//! let mut hole = Hole { v: v, index: 0, value: ManuallyDrop::new(value) };
//!
//! // Use a scope guard with a value.
//! // At scope exit, plug the hole so that the vector is fully
//! // initialized again.
//! // The scope guard owns the hole, but we can access it through the guard.
//! let mut hole_guard = scopeguard::guard(hole, |hole| {
//! // plug the hole in the vector with the value that was // taken out
//! let index = hole.index;
//! ptr::copy_nonoverlapping(&*hole.value, &mut hole.v[index], 1);
//! });
//!
//! // run algorithm that moves the hole in the vector here
//! // move the hole until it's in a sorted position
//! for i in 1..hole_guard.v.len() {
//! if *hole_guard.value >= hole_guard.v[i] {
//! // move the element back and the hole forward
//! let index = hole_guard.index;
//! ptr::copy_nonoverlapping(&hole_guard.v[index + 1], &mut hole_guard.v[index], 1);
//! hole_guard.index += 1;
//! } else {
//! break;
//! }
//! }
//!
//! // When the scope exits here, the Vec becomes whole again!
//! }
//! }
//!
//! fn main() {
//! let string = String::from;
//! let mut data = vec![string("c"), string("a"), string("b"), string("d")];
//! insertion_sort_first(&mut data);
//! assert_eq!(data, vec!["a", "b", "c", "d"]);
//! }
//!
//! ```
//!
//!
//! # Crate Features
//!
//! - `use_std`
//! + Enabled by default. Enables the `OnUnwind` and `OnSuccess` strategies.
//! + Disable to use `no_std`.
//!
//! # Rust Version
//!
//! This version of the crate requires Rust 1.20 or later.
//!
//! The scopeguard 1.x release series will use a carefully considered version
//! upgrade policy, where in a later 1.x version, we will raise the minimum
//! required Rust version.
#[cfg(not(any(test, feature = "use_std")))]
extern crate core as std;
use std::fmt;
use std::marker::PhantomData;
use std::mem::{self, ManuallyDrop};
use std::ops::{Deref, DerefMut};
use std::ptr;
/// Controls in which cases the associated code should be run
pub trait Strategy {
/// Return `true` if the guard’s associated code should run
/// (in the context where this method is called).
fn should_run() -> bool;
}
/// Always run on scope exit.
///
/// “Always” run: on regular exit from a scope or on unwinding from a panic.
/// Can not run on abort, process exit, and other catastrophic events where
/// destructors don’t run.
#[derive(Debug)]
pub enum Always {}
/// Run on scope exit through unwinding.
///
/// Requires crate feature `use_std`.
#[cfg(feature = "use_std")]
#[derive(Debug)]
pub enum OnUnwind {}
/// Run on regular scope exit, when not unwinding.
///
/// Requires crate feature `use_std`.
#[cfg(feature = "use_std")]
#[derive(Debug)]
pub enum OnSuccess {}
impl Strategy for Always {
#[inline(always)]
fn should_run() -> bool { true }
}
#[cfg(feature = "use_std")]
impl Strategy for OnUnwind {
#[inline]
fn should_run() -> bool { std::thread::panicking() }
}
#[cfg(feature = "use_std")]
impl Strategy for OnSuccess {
#[inline]
fn should_run() -> bool { !std::thread::panicking() }
}
/// Macro to create a `ScopeGuard` (always run).
///
/// The macro takes statements, which are the body of a closure
/// that will run when the scope is exited.
#[macro_export]
macro_rules! defer {
($($t:tt)*) => {
let _guard = $crate::guard((), |()| { $($t)* });
};
}
/// Macro to create a `ScopeGuard` (run on successful scope exit).
///
/// The macro takes statements, which are the body of a closure
/// that will run when the scope is exited.
///
/// Requires crate feature `use_std`.
#[cfg(feature = "use_std")]
#[macro_export]
macro_rules! defer_on_success {
($($t:tt)*) => {
let _guard = $crate::guard_on_success((), |()| { $($t)* });
};
}
/// Macro to create a `ScopeGuard` (run on unwinding from panic).
///
/// The macro takes statements, which are the body of a closure
/// that will run when the scope is exited.
///
/// Requires crate feature `use_std`.
#[cfg(feature = "use_std")]
#[macro_export]
macro_rules! defer_on_unwind {
($($t:tt)*) => {
let _guard = $crate::guard_on_unwind((), |()| { $($t)* });
};
}
/// `ScopeGuard` is a scope guard that may own a protected value.
///
/// If you place a guard in a local variable, the closure can
/// run regardless how you leave the scope — through regular return or panic
/// (except if panic or other code aborts; so as long as destructors run).
/// It is run only once.
///
/// The `S` parameter for [`Strategy`](trait.Strategy.html) determines if
/// the closure actually runs.
///
/// The guard's closure will be called with the held value in the destructor.
///
/// The `ScopeGuard` implements `Deref` so that you can access the inner value.
pub struct ScopeGuard<T, F, S = Always>
where F: FnOnce(T),
S: Strategy,
{
value: ManuallyDrop<T>,
dropfn: ManuallyDrop<F>,
// fn(S) -> S is used, so that the S is not taken into account for auto traits.
strategy: PhantomData<fn(S) -> S>,
}
impl<T, F, S> ScopeGuard<T, F, S>
where F: FnOnce(T),
S: Strategy,
{
/// Create a `ScopeGuard` that owns `v` (accessible through deref) and calls
/// `dropfn` when its destructor runs.
///
/// The `Strategy` decides whether the scope guard's closure should run.
#[inline]
pub fn with_strategy(v: T, dropfn: F) -> ScopeGuard<T, F, S> {
ScopeGuard {
value: ManuallyDrop::new(v),
dropfn: ManuallyDrop::new(dropfn),
strategy: PhantomData,
}
}
/// “Defuse” the guard and extract the value without calling the closure.
///
/// ```
/// extern crate scopeguard;
///
/// use scopeguard::{guard, ScopeGuard};
///
/// fn conditional() -> bool { true }
///
/// fn main() {
/// let mut guard = guard(Vec::new(), |mut v| v.clear());
/// guard.push(1);
///
/// if conditional() {
/// // a condition maybe makes us decide to
/// // “defuse” the guard and get back its inner parts
/// let value = ScopeGuard::into_inner(guard);
/// } else {
/// // guard still exists in this branch
/// }
/// }
/// ```
#[inline]
pub fn into_inner(guard: Self) -> T {
// Cannot move out of Drop-implementing types, so
// ptr::read the value and forget the guard.
unsafe {
let value = ptr::read(&*guard.value);
// read the closure so that it is dropped, and assign it to a local
// variable to ensure that it is only dropped after the guard has
// been forgotten. (In case the Drop impl of the closure, or that
// of any consumed captured variable, panics).
let _dropfn = ptr::read(&*guard.dropfn);
mem::forget(guard);
value
}
}
}
/// Create a new `ScopeGuard` owning `v` and with deferred closure `dropfn`.
#[inline]
pub fn guard<T, F>(v: T, dropfn: F) -> ScopeGuard<T, F, Always>
where F: FnOnce(T)
{
ScopeGuard::with_strategy(v, dropfn)
}
/// Create a new `ScopeGuard` owning `v` and with deferred closure `dropfn`.
///
/// Requires crate feature `use_std`.
#[cfg(feature = "use_std")]
#[inline]
pub fn guard_on_success<T, F>(v: T, dropfn: F) -> ScopeGuard<T, F, OnSuccess>
where F: FnOnce(T)
{
ScopeGuard::with_strategy(v, dropfn)
}
/// Create a new `ScopeGuard` owning `v` and with deferred closure `dropfn`.
///
/// Requires crate feature `use_std`.
///
/// ## Examples
///
/// For performance reasons, or to emulate “only run guard on unwind” in
/// no-std environments, we can also use the default guard and simply manually
/// defuse it at the end of scope like the following example. (The performance
/// reason would be if the [`OnUnwind`]'s call to [std::thread::panicking()] is
/// an issue.)
///
/// ```
/// extern crate scopeguard;
///
/// use scopeguard::ScopeGuard;
/// # fn main() {
/// {
/// let guard = scopeguard::guard((), |_| {});
///
/// // rest of the code here
///
/// // we reached the end of scope without unwinding - defuse it
/// ScopeGuard::into_inner(guard);
/// }
/// # }
/// ```
#[cfg(feature = "use_std")]
#[inline]
pub fn guard_on_unwind<T, F>(v: T, dropfn: F) -> ScopeGuard<T, F, OnUnwind>
where F: FnOnce(T)
{
ScopeGuard::with_strategy(v, dropfn)
}
// ScopeGuard can be Sync even if F isn't because the closure is
// not accessible from references.
// The guard does not store any instance of S, so it is also irrelevant.
unsafe impl<T, F, S> Sync for ScopeGuard<T, F, S>
where T: Sync,
F: FnOnce(T),
S: Strategy
{}
impl<T, F, S> Deref for ScopeGuard<T, F, S>
where F: FnOnce(T),
S: Strategy
{
type Target = T;
fn deref(&self) -> &T {
&*self.value
}
}
impl<T, F, S> DerefMut for ScopeGuard<T, F, S>
where F: FnOnce(T),
S: Strategy
{
fn deref_mut(&mut self) -> &mut T {
&mut *self.value
}
}
impl<T, F, S> Drop for ScopeGuard<T, F, S>
where F: FnOnce(T),
S: Strategy
{
fn drop(&mut self) {
// This is OK because the fields are `ManuallyDrop`s
// which will not be dropped by the compiler.
let (value, dropfn) = unsafe {
(ptr::read(&*self.value), ptr::read(&*self.dropfn))
};
if S::should_run() {
dropfn(value);
}
}
}
impl<T, F, S> fmt::Debug for ScopeGuard<T, F, S>
where T: fmt::Debug,
F: FnOnce(T),
S: Strategy
{
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
f.debug_struct(stringify!(ScopeGuard))
.field("value", &*self.value)
.finish()
}
}
#[cfg(test)]
mod tests {
use super::*;
use std::cell::Cell;
use std::panic::catch_unwind;
use std::panic::AssertUnwindSafe;
#[test]
fn test_defer() {
let drops = Cell::new(0);
defer!(drops.set(1000));
assert_eq!(drops.get(), 0);
}
#[cfg(feature = "use_std")]
#[test]
fn test_defer_success_1() {
let drops = Cell::new(0);
{
defer_on_success!(drops.set(1));
assert_eq!(drops.get(), 0);
}
assert_eq!(drops.get(), 1);
}
#[cfg(feature = "use_std")]
#[test]
fn test_defer_success_2() {
let drops = Cell::new(0);
let _ = catch_unwind(AssertUnwindSafe(|| {
defer_on_success!(drops.set(1));
panic!("failure")
}));
assert_eq!(drops.get(), 0);
}
#[cfg(feature = "use_std")]
#[test]
fn test_defer_unwind_1() {
let drops = Cell::new(0);
let _ = catch_unwind(AssertUnwindSafe(|| {
defer_on_unwind!(drops.set(1));
assert_eq!(drops.get(), 0);
panic!("failure")
}));
assert_eq!(drops.get(), 1);
}
#[cfg(feature = "use_std")]
#[test]
fn test_defer_unwind_2() {
let drops = Cell::new(0);
{
defer_on_unwind!(drops.set(1));
}
assert_eq!(drops.get(), 0);
}
#[test]
fn test_only_dropped_by_closure_when_run() {
let value_drops = Cell::new(0);
let value = guard((), |()| value_drops.set(1 + value_drops.get()));
let closure_drops = Cell::new(0);
let guard = guard(value, |_| closure_drops.set(1 + closure_drops.get()));
assert_eq!(value_drops.get(), 0);
assert_eq!(closure_drops.get(), 0);
drop(guard);
assert_eq!(value_drops.get(), 1);
assert_eq!(closure_drops.get(), 1);
}
#[cfg(feature = "use_std")]
#[test]
fn test_dropped_once_when_not_run() {
let value_drops = Cell::new(0);
let value = guard((), |()| value_drops.set(1 + value_drops.get()));
let captured_drops = Cell::new(0);
let captured = guard((), |()| captured_drops.set(1 + captured_drops.get()));
let closure_drops = Cell::new(0);
let guard = guard_on_unwind(value, |value| {
drop(value);
drop(captured);
closure_drops.set(1 + closure_drops.get())
});
assert_eq!(value_drops.get(), 0);
assert_eq!(captured_drops.get(), 0);
assert_eq!(closure_drops.get(), 0);
drop(guard);
assert_eq!(value_drops.get(), 1);
assert_eq!(captured_drops.get(), 1);
assert_eq!(closure_drops.get(), 0);
}
#[test]
fn test_into_inner() {
let dropped = Cell::new(false);
let value = guard(42, |_| dropped.set(true));
let guard = guard(value, |_| dropped.set(true));
let inner = ScopeGuard::into_inner(guard);
assert_eq!(dropped.get(), false);
assert_eq!(*inner, 42);
}
}