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
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
#![doc(
    html_root_url = "https://docs.rs/signal-hook-registry/1.2.2/signal-hook-registry/",
    test(attr(deny(warnings)))
)]
#![warn(missing_docs)]
#![allow(unknown_lints, renamed_and_remove_lints, bare_trait_objects)]

//! Backend of the [signal-hook] crate.
//!
//! The [signal-hook] crate tries to provide an API to the unix signals, which are a global
//! resource. Therefore, it is desirable an application contains just one version of the crate
//! which manages this global resource. But that makes it impossible to make breaking changes in
//! the API.
//!
//! Therefore, this crate provides very minimal and low level API to the signals that is unlikely
//! to have to change, while there may be multiple versions of the [signal-hook] that all use this
//! low-level API to provide different versions of the high level APIs.
//!
//! It is also possible some other crates might want to build a completely different API. This
//! split allows these crates to still reuse the same low-level routines in this crate instead of
//! going to the (much more dangerous) unix calls.
//!
//! # What this crate provides
//!
//! The only thing this crate does is multiplexing the signals. An application or library can add
//! or remove callbacks and have multiple callbacks for the same signal.
//!
//! It handles dispatching the callbacks and managing them in a way that uses only the
//! [async-signal-safe] functions inside the signal handler. Note that the callbacks are still run
//! inside the signal handler, so it is up to the caller to ensure they are also
//! [async-signal-safe].
//!
//! # What this is for
//!
//! This is a building block for other libraries creating reasonable abstractions on top of
//! signals. The [signal-hook] is the generally preferred way if you need to handle signals in your
//! application and provides several safe patterns of doing so.
//!
//! # Rust version compatibility
//!
//! Currently builds on 1.26.0 an newer and this is very unlikely to change. However, tests
//! require dependencies that don't build there, so tests need newer Rust version (they are run on
//! stable).
//!
//! # Portability
//!
//! This crate includes a limited support for Windows, based on `signal`/`raise` in the CRT.
//! There are differences in both API and behavior:
//!
//! - Due to lack of `siginfo_t`, we don't provide `register_sigaction` or `register_unchecked`.
//! - Due to lack of signal blocking, there's a race condition.
//!   After the call to `signal`, there's a moment where we miss a signal.
//!   That means when you register a handler, there may be a signal which invokes
//!   neither the default handler or the handler you register.
//! - Handlers registered by `signal` in Windows are cleared on first signal.
//!   To match behavior in other platforms, we re-register the handler each time the handler is
//!   called, but there's a moment where we miss a handler.
//!   That means when you receive two signals in a row, there may be a signal which invokes
//!   the default handler, nevertheless you certainly have registered the handler.
//!
//! [signal-hook]: https://docs.rs/signal-hook
//! [async-signal-safe]: http://www.man7.org/linux/man-pages/man7/signal-safety.7.html

extern crate libc;

mod half_lock;

use std::collections::hash_map::Entry;
use std::collections::{BTreeMap, HashMap};
use std::io::Error;
use std::mem;
#[cfg(not(windows))]
use std::ptr;
// Once::new is now a const-fn. But it is not stable in all the rustc versions we want to support
// yet.
#[allow(deprecated)]
use std::sync::ONCE_INIT;
use std::sync::{Arc, Once};

#[cfg(not(windows))]
use libc::{c_int, c_void, sigaction, siginfo_t};
#[cfg(windows)]
use libc::{c_int, sighandler_t};

#[cfg(not(windows))]
use libc::{SIGFPE, SIGILL, SIGKILL, SIGSEGV, SIGSTOP};
#[cfg(windows)]
use libc::{SIGFPE, SIGILL, SIGSEGV};

use half_lock::HalfLock;

// These constants are not defined in the current version of libc, but it actually
// exists in Windows CRT.
#[cfg(windows)]
const SIG_DFL: sighandler_t = 0;
#[cfg(windows)]
const SIG_IGN: sighandler_t = 1;
#[cfg(windows)]
const SIG_GET: sighandler_t = 2;
#[cfg(windows)]
const SIG_ERR: sighandler_t = !0;

// To simplify implementation. Not to be exposed.
#[cfg(windows)]
#[allow(non_camel_case_types)]
struct siginfo_t;

// # Internal workings
//
// This uses a form of RCU. There's an atomic pointer to the current action descriptors (in the
// form of IndependentArcSwap, to be able to track what, if any, signal handlers still use the
// version). A signal handler takes a copy of the pointer and calls all the relevant actions.
//
// Modifications to that are protected by a mutex, to avoid juggling multiple signal handlers at
// once (eg. not calling sigaction concurrently). This should not be a problem, because modifying
// the signal actions should be initialization only anyway. To avoid all allocations and also
// deallocations inside the signal handler, after replacing the pointer, the modification routine
// needs to busy-wait for the reference count on the old pointer to drop to 1 and take ownership ‒
// that way the one deallocating is the modification routine, outside of the signal handler.

#[derive(Copy, Clone, Debug, Eq, PartialEq, Ord, PartialOrd, Hash)]
struct ActionId(u128);

/// An ID of registered action.
///
/// This is returned by all the registration routines and can be used to remove the action later on
/// with a call to [`unregister`](fn.unregister.html).
#[derive(Copy, Clone, Debug, Eq, PartialEq, Ord, PartialOrd, Hash)]
pub struct SigId {
    signal: c_int,
    action: ActionId,
}

// This should be dyn Fn(...), but we want to support Rust 1.26.0 and that one doesn't allow dyn
// yet.
#[allow(unknown_lints, bare_trait_objects)]
type Action = Fn(&siginfo_t) + Send + Sync;

#[derive(Clone)]
struct Slot {
    prev: Prev,
    // We use BTreeMap here, because we want to run the actions in the order they were inserted.
    // This works, because the ActionIds are assigned in an increasing order.
    actions: BTreeMap<ActionId, Arc<Action>>,
}

impl Slot {
    #[cfg(windows)]
    fn new(signal: libc::c_int) -> Result<Self, Error> {
        let old = unsafe { libc::signal(signal, handler as sighandler_t) };
        if old == SIG_ERR {
            return Err(Error::last_os_error());
        }
        Ok(Slot {
            prev: Prev {
                signal,
                info: old,
            },
            actions: BTreeMap::new(),
        })
    }

    #[cfg(not(windows))]
    fn new(signal: libc::c_int) -> Result<Self, Error> {
        // C data structure, expected to be zeroed out.
        let mut new: libc::sigaction = unsafe { mem::zeroed() };
        new.sa_sigaction = handler as usize;
        // Android is broken and uses different int types than the rest (and different depending on
        // the pointer width). This converts the flags to the proper type no matter what it is on
        // the given platform.
        let flags = libc::SA_RESTART | libc::SA_NOCLDSTOP;
        #[allow(unused_assignments)]
        let mut siginfo = flags;
        siginfo = libc::SA_SIGINFO as _;
        let flags = flags | siginfo;
        new.sa_flags = flags as _;
        // C data structure, expected to be zeroed out.
        let mut old: libc::sigaction = unsafe { mem::zeroed() };
        // FFI ‒ pointers are valid, it doesn't take ownership.
        if unsafe { libc::sigaction(signal, &new, &mut old) } != 0 {
            return Err(Error::last_os_error());
        }
        Ok(Slot {
            prev: Prev {
                signal,
                info: old,
            },
            actions: BTreeMap::new(),
        })
    }
}

#[derive(Clone)]
struct SignalData {
    signals: HashMap<c_int, Slot>,
    next_id: u128,
}

#[derive(Clone)]
struct Prev {
    signal: c_int,
    #[cfg(windows)]
    info: sighandler_t,
    #[cfg(not(windows))]
    info: sigaction,
}

impl Prev {
    #[cfg(windows)]
    fn detect(signal: c_int) -> Result<Self, Error> {
        let old = unsafe { libc::signal(signal, SIG_GET) };
        if old == SIG_ERR {
            return Err(Error::last_os_error());
        }
        Ok(Prev {
            signal,
            info: old,
        })
    }

    #[cfg(not(windows))]
    fn detect(signal: c_int) -> Result<Self, Error> {
        // C data structure, expected to be zeroed out.
        let mut old: libc::sigaction = unsafe { mem::zeroed() };
        // FFI ‒ pointers are valid, it doesn't take ownership.
        if unsafe { libc::sigaction(signal, ptr::null(), &mut old) } != 0 {
            return Err(Error::last_os_error());
        }

        Ok(Prev {
            signal,
            info: old,
        })
    }

    #[cfg(windows)]
    fn execute(&self, sig: c_int) {
        let fptr = self.info;
        if fptr != 0 && fptr != SIG_DFL && fptr != SIG_IGN {
            // FFI ‒ calling the original signal handler.
            unsafe {
                let action = mem::transmute::<usize, extern "C" fn(c_int)>(fptr);
                action(sig);
            }
        }
    }

    #[cfg(not(windows))]
    unsafe fn execute(&self, sig: c_int, info: *mut siginfo_t, data: *mut c_void) {
        let fptr = self.info.sa_sigaction;
        if fptr != 0 && fptr != libc::SIG_DFL && fptr != libc::SIG_IGN {
            // Android is broken and uses different int types than the rest (and different
            // depending on the pointer width). This converts the flags to the proper type no
            // matter what it is on the given platform.
            //
            // The trick is to create the same-typed variable as the sa_flags first and then
            // set it to the proper value (does Rust have a way to copy a type in a different
            // way?)
            #[allow(unused_assignments)]
            let mut siginfo = self.info.sa_flags;
            siginfo = libc::SA_SIGINFO as _;
            if self.info.sa_flags & siginfo == 0 {
                let action = mem::transmute::<usize, extern "C" fn(c_int)>(fptr);
                action(sig);
            } else {
                type SigAction = extern "C" fn(c_int, *mut siginfo_t, *mut c_void);
                let action = mem::transmute::<usize, SigAction>(fptr);
                action(sig, info, data);
            }
        }
    }
}

/// Lazy-initiated data structure with our global variables.
///
/// Used inside a structure to cut down on boilerplate code to lazy-initialize stuff. We don't dare
/// use anything fancy like lazy-static or once-cell, since we are not sure they are
/// async-signal-safe in their access. Our code uses the [Once], but only on the write end outside
/// of signal handler. The handler assumes it has already been initialized.
struct GlobalData {
    /// The data structure describing what needs to be run for each signal.
    data: HalfLock<SignalData>,

    /// A fallback to fight/minimize a race condition during signal initialization.
    ///
    /// See the comment inside [`register_unchecked_impl`].
    race_fallback: HalfLock<Option<Prev>>,
}

static mut GLOBAL_DATA: Option<GlobalData> = None;
#[allow(deprecated)]
static GLOBAL_INIT: Once = ONCE_INIT;

impl GlobalData {
    fn get() -> &'static Self {
        unsafe { GLOBAL_DATA.as_ref().unwrap() }
    }
    fn ensure() -> &'static Self {
        GLOBAL_INIT.call_once(|| unsafe {
            GLOBAL_DATA = Some(GlobalData {
                data: HalfLock::new(SignalData {
                    signals: HashMap::new(),
                    next_id: 1,
                }),
                race_fallback: HalfLock::new(None),
            });
        });
        Self::get()
    }
}

#[cfg(windows)]
extern "C" fn handler(sig: c_int) {
    if sig != SIGFPE {
        // Windows CRT `signal` resets handler every time, unless for SIGFPE.
        // Reregister the handler to retain maximal compatibility.
        // Problems:
        // - It's racy. But this is inevitably racy in Windows.
        // - Interacts poorly with handlers outside signal-hook-registry.
        let old = unsafe { libc::signal(sig, handler as sighandler_t) };
        if old == SIG_ERR {
            // MSDN doesn't describe which errors might occur,
            // but we can tell from the Linux manpage that
            // EINVAL (invalid signal number) is mostly the only case.
            // Therefore, this branch must not occur.
            // In any case we can do nothing useful in the signal handler,
            // so we're going to abort silently.
            unsafe {
                libc::abort();
            }
        }
    }

    let globals = GlobalData::get();
    let fallback = globals.race_fallback.read();
    let sigdata = globals.data.read();

    if let Some(ref slot) = sigdata.signals.get(&sig) {
        slot.prev.execute(sig);

        for action in slot.actions.values() {
            action(&siginfo_t);
        }
    } else if let Some(prev) = fallback.as_ref() {
        // In case we get called but don't have the slot for this signal set up yet, we are under
        // the race condition. We may have the old signal handler stored in the fallback
        // temporarily.
        if sig == prev.signal {
            prev.execute(sig);
        }
        // else -> probably should not happen, but races with other threads are possible so
        // better safe
    }
}

#[cfg(not(windows))]
extern "C" fn handler(sig: c_int, info: *mut siginfo_t, data: *mut c_void) {
    let globals = GlobalData::get();
    let fallback = globals.race_fallback.read();
    let sigdata = globals.data.read();

    if let Some(ref slot) = sigdata.signals.get(&sig) {
        unsafe { slot.prev.execute(sig, info, data) };

        let info = unsafe { info.as_ref() };
        let info = info.unwrap_or_else(|| {
            // The info being null seems to be illegal according to POSIX, but has been observed on
            // some probably broken platform. We can't do anything about that, that is just broken,
            // but we are not allowed to panic in a signal handler, so we are left only with simply
            // aborting. We try to write a message what happens, but using the libc stuff
            // (`eprintln` is not guaranteed to be async-signal-safe).
            unsafe {
                const MSG: &[u8] =
                    b"Platform broken, got NULL as siginfo to signal handler. Aborting";
                libc::write(2, MSG.as_ptr() as *const _, MSG.len());
                libc::abort();
            }
        });

        for action in slot.actions.values() {
            action(info);
        }
    } else if let Some(ref prev) = fallback.as_ref() {
        // In case we get called but don't have the slot for this signal set up yet, we are under
        // the race condition. We may have the old signal handler stored in the fallback
        // temporarily.
        if prev.signal == sig {
            unsafe { prev.execute(sig, info, data) };
        }
        // else -> probably should not happen, but races with other threads are possible so
        // better safe
    }
}

/// List of forbidden signals.
///
/// Some signals are impossible to replace according to POSIX and some are so special that this
/// library refuses to handle them (eg. SIGSEGV). The routines panic in case registering one of
/// these signals is attempted.
///
/// See [`register`](fn.register.html).
pub const FORBIDDEN: &[c_int] = FORBIDDEN_IMPL;

#[cfg(windows)]
const FORBIDDEN_IMPL: &[c_int] = &[SIGILL, SIGFPE, SIGSEGV];
#[cfg(not(windows))]
const FORBIDDEN_IMPL: &[c_int] = &[SIGKILL, SIGSTOP, SIGILL, SIGFPE, SIGSEGV];

/// Registers an arbitrary action for the given signal.
///
/// This makes sure there's a signal handler for the given signal. It then adds the action to the
/// ones called each time the signal is delivered. If multiple actions are set for the same signal,
/// all are called, in the order of registration.
///
/// If there was a previous signal handler for the given signal, it is chained ‒ it will be called
/// as part of this library's signal handler, before any actions set through this function.
///
/// On success, the function returns an ID that can be used to remove the action again with
/// [`unregister`](fn.unregister.html).
///
/// # Panics
///
/// If the signal is one of (see [`FORBIDDEN`]):
///
/// * `SIGKILL`
/// * `SIGSTOP`
/// * `SIGILL`
/// * `SIGFPE`
/// * `SIGSEGV`
///
/// The first two are not possible to override (and the underlying C functions simply ignore all
/// requests to do so, which smells of possible bugs, or return errors). The rest can be set, but
/// generally needs very special handling to do so correctly (direct manipulation of the
/// application's address space, `longjmp` and similar). Unless you know very well what you're
/// doing, you'll shoot yourself into the foot and this library won't help you with that.
///
/// # Errors
///
/// Since the library manipulates signals using the low-level C functions, all these can return
/// errors. Generally, the errors mean something like the specified signal does not exist on the
/// given platform ‒ after a program is debugged and tested on a given OS, it should never return
/// an error.
///
/// However, if an error *is* returned, there are no guarantees if the given action was registered
/// or not.
///
/// # Safety
///
/// This function is unsafe, because the `action` is run inside a signal handler. The set of
/// functions allowed to be called from within is very limited (they are called async-signal-safe
/// functions by POSIX). These specifically do *not* contain mutexes and memory
/// allocation/deallocation. They *do* contain routines to terminate the program, to further
/// manipulate signals (by the low-level functions, not by this library) and to read and write file
/// descriptors. Calling program's own functions consisting only of these is OK, as is manipulating
/// program's variables ‒ however, as the action can be called on any thread that does not have the
/// given signal masked (by default no signal is masked on any thread), and mutexes are a no-go,
/// this is harder than it looks like at first.
///
/// As panicking from within a signal handler would be a panic across FFI boundary (which is
/// undefined behavior), the passed handler must not panic.
///
/// If you find these limitations hard to satisfy, choose from the helper functions in the
/// [signal-hook](https://docs.rs/signal-hook) crate ‒ these provide safe interface to use some
/// common signal handling patters.
///
/// # Race condition
///
/// Upon registering the first hook for a given signal into this library, there's a short race
/// condition under the following circumstances:
///
/// * The program already has a signal handler installed for this particular signal (through some
///   other library, possibly).
/// * Concurrently, some other thread installs a different signal handler while it is being
///   installed by this library.
/// * At the same time, the signal is delivered.
///
/// Under such conditions signal-hook might wrongly "chain" to the older signal handler for a short
/// while (until the registration is fully complete).
///
/// Note that the exact conditions of the race condition might change in future versions of the
/// library. The recommended way to avoid it is to register signals before starting any additional
/// threads, or at least not to register signals concurrently.
///
/// Alternatively, make sure all signals are handled through this library.
///
/// # Performance
///
/// Even when it is possible to repeatedly install and remove actions during the lifetime of a
/// program, the installation and removal is considered a slow operation and should not be done
/// very often. Also, there's limited (though huge) amount of distinct IDs (they are `u128`).
///
/// # Examples
///
/// ```rust
/// extern crate signal_hook;
///
/// use std::io::Error;
/// use std::process;
///
/// fn main() -> Result<(), Error> {
///     let signal = unsafe { signal_hook::register(signal_hook::SIGTERM, || process::abort()) }?;
///     // Stuff here...
///     signal_hook::unregister(signal); // Not really necessary.
///     Ok(())
/// }
/// ```
pub unsafe fn register<F>(signal: c_int, action: F) -> Result<SigId, Error>
where
    F: Fn() + Sync + Send + 'static,
{
    register_sigaction_impl(signal, move |_: &_| action())
}

/// Register a signal action.
///
/// This acts in the same way as [`register`], including the drawbacks, panics and performance
/// characteristics. The only difference is the provided action accepts a [`siginfo_t`] argument,
/// providing information about the received signal.
///
/// # Safety
///
/// See the details of [`register`].
#[cfg(not(windows))]
pub unsafe fn register_sigaction<F>(signal: c_int, action: F) -> Result<SigId, Error>
where
    F: Fn(&siginfo_t) + Sync + Send + 'static,
{
    register_sigaction_impl(signal, action)
}

unsafe fn register_sigaction_impl<F>(signal: c_int, action: F) -> Result<SigId, Error>
where
    F: Fn(&siginfo_t) + Sync + Send + 'static,
{
    assert!(
        !FORBIDDEN.contains(&signal),
        "Attempted to register forbidden signal {}",
        signal,
    );
    register_unchecked_impl(signal, action)
}

/// Register a signal action without checking for forbidden signals.
///
/// This acts in the same way as [`register_unchecked`], including the drawbacks, panics and
/// performance characteristics. The only difference is the provided action doesn't accept a
/// [`siginfo_t`] argument.
///
/// # Safety
///
/// See the details of [`register`].
pub unsafe fn register_signal_unchecked<F>(signal: c_int, action: F) -> Result<SigId, Error>
where
    F: Fn() + Sync + Send + 'static,
{
    register_unchecked_impl(signal, move |_: &_| action())
}

/// Register a signal action without checking for forbidden signals.
///
/// This acts the same way as [`register_sigaction`], but without checking for the [`FORBIDDEN`]
/// signals. All the signals passed are registered and it is up to the caller to make some sense of
/// them.
///
/// Note that you really need to know what you're doing if you change eg. the `SIGSEGV` signal
/// handler. Generally, you don't want to do that. But unlike the other functions here, this
/// function still allows you to do it.
///
/// # Safety
///
/// See the details of [`register`].
#[cfg(not(windows))]
pub unsafe fn register_unchecked<F>(signal: c_int, action: F) -> Result<SigId, Error>
where
    F: Fn(&siginfo_t) + Sync + Send + 'static,
{
    register_unchecked_impl(signal, action)
}

unsafe fn register_unchecked_impl<F>(signal: c_int, action: F) -> Result<SigId, Error>
where
    F: Fn(&siginfo_t) + Sync + Send + 'static,
{
    let globals = GlobalData::ensure();
    let action = Arc::from(action);

    let mut lock = globals.data.write();

    let mut sigdata = SignalData::clone(&lock);
    let id = ActionId(sigdata.next_id);
    sigdata.next_id += 1;

    match sigdata.signals.entry(signal) {
        Entry::Occupied(mut occupied) => {
            assert!(occupied.get_mut().actions.insert(id, action).is_none());
        }
        Entry::Vacant(place) => {
            // While the sigaction/signal exchanges the old one atomically, we are not able to
            // atomically store it somewhere a signal handler could read it. That poses a race
            // condition where we could lose some signals delivered in between changing it and
            // storing it.
            //
            // Therefore we first store the old one in the fallback storage. The fallback only
            // covers the cases where the slot is not yet active and becomes "inert" after that,
            // even if not removed (it may get overwritten by some other signal, but for that the
            // mutex in globals.data must be unlocked here - and by that time we already stored the
            // slot.
            //
            // And yes, this still leaves a short race condition when some other thread could
            // replace the signal handler and we would be calling the outdated one for a short
            // time, until we install the slot.
            globals.race_fallback.write().store(Some(Prev::detect(signal)?));

            let mut slot = Slot::new(signal)?;
            slot.actions.insert(id, action);
            place.insert(slot);
        }
    }

    lock.store(sigdata);

    Ok(SigId { signal, action: id })
}

/// Removes a previously installed action.
///
/// This function does nothing if the action was already removed. It returns true if it was removed
/// and false if the action wasn't found.
///
/// It can unregister all the actions installed by [`register`](fn.register.html) as well as the
/// ones from downstream crates (like [`signal-hook`](https://docs.rs/signal-hook)).
///
/// # Warning
///
/// This does *not* currently return the default/previous signal handler if the last action for a
/// signal was just unregistered. That means that if you replaced for example `SIGTERM` and then
/// removed the action, the program will effectively ignore `SIGTERM` signals from now on, not
/// terminate on them as is the default action. This is OK if you remove it as part of a shutdown,
/// but it is not recommended to remove termination actions during the normal runtime of
/// application (unless the desired effect is to create something that can be terminated only by
/// SIGKILL).
pub fn unregister(id: SigId) -> bool {
    let globals = GlobalData::ensure();
    let mut replace = false;
    let mut lock = globals.data.write();
    let mut sigdata = SignalData::clone(&lock);
    if let Some(slot) = sigdata.signals.get_mut(&id.signal) {
        replace = slot.actions.remove(&id.action).is_some();
    }
    if replace {
        lock.store(sigdata);
    }
    replace
}

/// Removes all previously installed actions for a given signal.
///
/// This is similar to the [`unregister`](fn.unregister.html) function, with the sole difference it
/// removes all actions for the given signal.
///
/// Returns if any hooks were actually removed (returns false if there was no hook registered for
/// the signal).
///
/// # Warning
///
/// Similar to [`unregister`](fn.unregister.html), this does not manipulate the signal handler in
/// the OS, it only removes the hooks on the Rust side.
///
/// Furthermore, this will remove *all* signal hooks of the given signal. These may have been
/// registered by some library or unrelated part of the program. Therefore, this should be only
/// used by the top-level application.
pub fn unregister_signal(signal: c_int) -> bool {
    let globals = GlobalData::ensure();
    let mut replace = false;
    let mut lock = globals.data.write();
    let mut sigdata = SignalData::clone(&lock);
    if let Some(slot) = sigdata.signals.get_mut(&signal) {
        if !slot.actions.is_empty() {
            slot.actions.clear();
            replace = true;
        }
    }
    if replace {
        lock.store(sigdata);
    }
    replace
}

#[cfg(test)]
mod tests {
    use std::sync::atomic::{AtomicUsize, Ordering};
    use std::sync::Arc;
    use std::thread;
    use std::time::Duration;

    #[cfg(not(windows))]
    use libc::{pid_t, SIGUSR1, SIGUSR2};

    #[cfg(windows)]
    use libc::SIGTERM as SIGUSR1;
    #[cfg(windows)]
    use libc::SIGTERM as SIGUSR2;

    use super::*;

    #[test]
    #[should_panic]
    fn panic_forbidden() {
        let _ = unsafe { register(SIGILL, || ()) };
    }

    /// Registering the forbidden signals is allowed in the _unchecked version.
    #[test]
    fn forbidden_raw() {
        unsafe { register_signal_unchecked(SIGFPE, || std::process::abort()).unwrap() };
    }

    #[test]
    fn signal_without_pid() {
        let status = Arc::new(AtomicUsize::new(0));
        let action = {
            let status = Arc::clone(&status);
            move || {
                status.store(1, Ordering::Relaxed);
            }
        };
        unsafe {
            register(SIGUSR2, action).unwrap();
            libc::raise(SIGUSR2);
        }
        for _ in 0..10 {
            thread::sleep(Duration::from_millis(100));
            let current = status.load(Ordering::Relaxed);
            match current {
                // Not yet
                0 => continue,
                // Good, we are done with the correct result
                _ if current == 1 => return,
                _ => panic!("Wrong result value {}", current),
            }
        }
        panic!("Timed out waiting for the signal");
    }

    #[test]
    #[cfg(not(windows))]
    fn signal_with_pid() {
        let status = Arc::new(AtomicUsize::new(0));
        let action = {
            let status = Arc::clone(&status);
            move |siginfo: &siginfo_t| {
                // Hack: currently, libc exposes only the first 3 fields of siginfo_t. The pid
                // comes somewhat later on. Therefore, we do a Really Ugly Hack and define our
                // own structure (and hope it is correct on all platforms). But hey, this is
                // only the tests, so we are going to get away with this.
                #[repr(C)]
                struct SigInfo {
                    _fields: [c_int; 3],
                    #[cfg(all(target_pointer_width = "64", target_os = "linux"))]
                    _pad: c_int,
                    pid: pid_t,
                }
                let s: &SigInfo = unsafe {
                    (siginfo as *const _ as usize as *const SigInfo)
                        .as_ref()
                        .unwrap()
                };
                status.store(s.pid as usize, Ordering::Relaxed);
            }
        };
        let pid;
        unsafe {
            pid = libc::getpid();
            register_sigaction(SIGUSR2, action).unwrap();
            libc::raise(SIGUSR2);
        }
        for _ in 0..10 {
            thread::sleep(Duration::from_millis(100));
            let current = status.load(Ordering::Relaxed);
            match current {
                // Not yet (PID == 0 doesn't happen)
                0 => continue,
                // Good, we are done with the correct result
                _ if current == pid as usize => return,
                _ => panic!("Wrong status value {}", current),
            }
        }
        panic!("Timed out waiting for the signal");
    }

    /// Check that registration works as expected and that unregister tells if it did or not.
    #[test]
    fn register_unregister() {
        let signal = unsafe { register(SIGUSR1, || ()).unwrap() };
        // It was there now, so we can unregister
        assert!(unregister(signal));
        // The next time unregistering does nothing and tells us so.
        assert!(!unregister(signal));
    }
}