summaryrefslogtreecommitdiffstats
path: root/vendor/scopeguard/src/lib.rs
blob: d33c2b69c821e59d28898f9989295e6a681354e8 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
#![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);
    }
}