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    // Copyright 2018 Developers of the Rand project.
    //
    // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
    // https://www.apache.org/licenses/LICENSE-2.0> or the MIT license
    // <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your
    // option. This file may not be copied, modified, or distributed
    // except according to those terms.

    //! Implementations that just need to read from a file
    use crate::{
        util::LazyUsize,
        util_libc::{open_readonly, sys_fill_exact},
        Error,
    };
    use core::{
        cell::UnsafeCell,
        sync::atomic::{AtomicUsize, Ordering::Relaxed},
    };

    #[cfg(any(
        target_os = "dragonfly",
        target_os = "emscripten",
        target_os = "haiku",
        target_os = "macos",
        target_os = "solaris",
        target_os = "illumos"
    ))]
    const FILE_PATH: &str = "/dev/random\0";
    #[cfg(any(target_os = "android", target_os = "linux", target_os = "redox"))]
    const FILE_PATH: &str = "/dev/urandom\0";

    pub fn getrandom_inner(dest: &mut [u8]) -> Result<(), Error> {
        let fd = get_rng_fd()?;
        let read = |buf: &mut [u8]| unsafe { libc::read(fd, buf.as_mut_ptr() as *mut _, buf.len()) };

        if cfg!(target_os = "emscripten") {
            // `Crypto.getRandomValues` documents `dest` should be at most 65536 bytes.
            for chunk in dest.chunks_mut(65536) {
                sys_fill_exact(chunk, read)?;
            }
        } else {
            sys_fill_exact(dest, read)?;
        }
        Ok(())
    }

    // Returns the file descriptor for the device file used to retrieve random
    // bytes. The file will be opened exactly once. All subsequent calls will
    // return the same file descriptor. This file descriptor is never closed.
    fn get_rng_fd() -> Result<libc::c_int, Error> {
        static FD: AtomicUsize = AtomicUsize::new(LazyUsize::UNINIT);
        fn get_fd() -> Option<libc::c_int> {
            match FD.load(Relaxed) {
                LazyUsize::UNINIT => None,
                val => Some(val as libc::c_int),
            }
        }

        // Use double-checked locking to avoid acquiring the lock if possible.
        if let Some(fd) = get_fd() {
            return Ok(fd);
        }

        // SAFETY: We use the mutex only in this method, and we always unlock it
        // before returning, making sure we don't violate the pthread_mutex_t API.
        static MUTEX: Mutex = Mutex::new();
        unsafe { MUTEX.lock() };
        let _guard = DropGuard(|| unsafe { MUTEX.unlock() });

        if let Some(fd) = get_fd() {
            return Ok(fd);
        }

        // On Linux, /dev/urandom might return insecure values.
        #[cfg(any(target_os = "android", target_os = "linux"))]
        wait_until_rng_ready()?;

        let fd = unsafe { open_readonly(FILE_PATH)? };
        // The fd always fits in a usize without conflicting with UNINIT.
        debug_assert!(fd >= 0 && (fd as usize) < LazyUsize::UNINIT);
        FD.store(fd as usize, Relaxed);

        Ok(fd)
    }

    // Succeeds once /dev/urandom is safe to read from
    #[cfg(any(target_os = "android", target_os = "linux"))]
    fn wait_until_rng_ready() -> Result<(), Error> {
        // Poll /dev/random to make sure it is ok to read from /dev/urandom.
        let fd = unsafe { open_readonly("/dev/random\0")? };
        let mut pfd = libc::pollfd {
            fd,
            events: libc::POLLIN,
            revents: 0,
        };
        let _guard = DropGuard(|| unsafe {
            libc::close(fd);
        });

        loop {
            // A negative timeout means an infinite timeout.
            let res = unsafe { libc::poll(&mut pfd, 1, -1) };
            if res >= 0 {
                debug_assert_eq!(res, 1); // We only used one fd, and cannot timeout.
                return Ok(());
            }
            let err = crate::util_libc::last_os_error();
            match err.raw_os_error() {
                Some(libc::EINTR) | Some(libc::EAGAIN) => continue,
                _ => return Err(err),
            }
        }
    }

    struct Mutex(UnsafeCell<libc::pthread_mutex_t>);

    impl Mutex {
        const fn new() -> Self {
            Self(UnsafeCell::new(libc::PTHREAD_MUTEX_INITIALIZER))
        }
        unsafe fn lock(&self) {
            let r = libc::pthread_mutex_lock(self.0.get());
            debug_assert_eq!(r, 0);
        }
        unsafe fn unlock(&self) {
            let r = libc::pthread_mutex_unlock(self.0.get());
            debug_assert_eq!(r, 0);
        }
    }

    unsafe impl Sync for Mutex {}

    struct DropGuard<F: FnMut()>(F);

    impl<F: FnMut()> Drop for DropGuard<F> {
        fn drop(&mut self) {
            self.0()
        }
    }