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    // Copyright 2018 Developers of the Rand project.
    // Copyright 2013 The Rust Project Developers.
    //
    // 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.

    //! A wrapper around another PRNG that reseeds it after it
    //! generates a certain number of random bytes.

    use core::mem::size_of;

    use rand_core::{RngCore, CryptoRng, SeedableRng, Error, ErrorKind};
    use rand_core::block::{BlockRngCore, BlockRng};

    /// A wrapper around any PRNG that implements [`BlockRngCore`], that adds the
    /// ability to reseed it.
    ///
    /// `ReseedingRng` reseeds the underlying PRNG in the following cases:
    ///
    /// - On a manual call to [`reseed()`].
    /// - After `clone()`, the clone will be reseeded on first use.
    /// - After a process is forked, the RNG in the child process is reseeded within
    ///   the next few generated values, depending on the block size of the
    ///   underlying PRNG. For [`ChaChaCore`] and [`Hc128Core`] this is a maximum of
    ///   15 `u32` values before reseeding.
    /// - After the PRNG has generated a configurable number of random bytes.
    ///
    /// # When should reseeding after a fixed number of generated bytes be used?
    ///
    /// Reseeding after a fixed number of generated bytes is never strictly
    /// *necessary*. Cryptographic PRNGs don't have a limited number of bytes they
    /// can output, or at least not a limit reachable in any practical way. There is
    /// no such thing as 'running out of entropy'.
    ///
    /// Occasionally reseeding can be seen as some form of 'security in depth'. Even
    /// if in the future a cryptographic weakness is found in the CSPRNG being used,
    /// or a flaw in the implementation, occasionally reseeding should make
    /// exploiting it much more difficult or even impossible.
    ///
    /// Use [`ReseedingRng::new`] with a `threshold` of `0` to disable reseeding
    /// after a fixed number of generated bytes.
    ///
    /// # Error handling
    ///
    /// Although unlikely, reseeding the wrapped PRNG can fail. `ReseedingRng` will
    /// never panic but try to handle the error intelligently through some
    /// combination of retrying and delaying reseeding until later.
    /// If handling the source error fails `ReseedingRng` will continue generating
    /// data from the wrapped PRNG without reseeding.
    ///
    /// Manually calling [`reseed()`] will not have this retry or delay logic, but
    /// reports the error.
    ///
    /// # Example
    ///
    /// ```
    /// # extern crate rand;
    /// # extern crate rand_chacha;
    /// # fn main() {
    /// use rand::prelude::*;
    /// use rand_chacha::ChaChaCore; // Internal part of ChaChaRng that
    ///                              // implements BlockRngCore
    /// use rand::rngs::OsRng;
    /// use rand::rngs::adapter::ReseedingRng;
    ///
    /// let prng = ChaChaCore::from_entropy();
    // FIXME: it is better to use EntropyRng as reseeder, but that doesn't implement
    // clone yet.
    /// let reseeder = OsRng::new().unwrap();
    /// let mut reseeding_rng = ReseedingRng::new(prng, 0, reseeder);
    ///
    /// println!("{}", reseeding_rng.gen::<u64>());
    ///
    /// let mut cloned_rng = reseeding_rng.clone();
    /// assert!(reseeding_rng.gen::<u64>() != cloned_rng.gen::<u64>());
    /// # }
    /// ```
    ///
    /// [`ChaChaCore`]: rand_chacha::ChaChaCore
    /// [`Hc128Core`]: rand_hc::Hc128Core
    /// [`BlockRngCore`]: rand_core::block::BlockRngCore
    /// [`ReseedingRng::new`]: ReseedingRng::new
    /// [`reseed()`]: ReseedingRng::reseed
    #[derive(Debug)]
    pub struct ReseedingRng<R, Rsdr>(BlockRng<ReseedingCore<R, Rsdr>>)
    where R: BlockRngCore + SeedableRng,
          Rsdr: RngCore;

    impl<R, Rsdr> ReseedingRng<R, Rsdr>
    where R: BlockRngCore + SeedableRng,
          Rsdr: RngCore
    {
        /// Create a new `ReseedingRng` from an existing PRNG, combined with a RNG
        /// to use as reseeder.
        ///
        /// `threshold` sets the number of generated bytes after which to reseed the
        /// PRNG. Set it to zero to never reseed based on the number of generated
        /// values.
        pub fn new(rng: R, threshold: u64, reseeder: Rsdr) -> Self {
            ReseedingRng(BlockRng::new(ReseedingCore::new(rng, threshold, reseeder)))
        }

        /// Reseed the internal PRNG.
        pub fn reseed(&mut self) -> Result<(), Error> {
            self.0.core.reseed()
        }
    }

    // TODO: this should be implemented for any type where the inner type
    // implements RngCore, but we can't specify that because ReseedingCore is private
    impl<R, Rsdr: RngCore> RngCore for ReseedingRng<R, Rsdr>
    where R: BlockRngCore<Item = u32> + SeedableRng,
        <R as BlockRngCore>::Results: AsRef<[u32]> + AsMut<[u32]>
    {
        #[inline(always)]
        fn next_u32(&mut self) -> u32 {
            self.0.next_u32()
        }

        #[inline(always)]
        fn next_u64(&mut self) -> u64 {
            self.0.next_u64()
        }

        fn fill_bytes(&mut self, dest: &mut [u8]) {
            self.0.fill_bytes(dest)
        }

        fn try_fill_bytes(&mut self, dest: &mut [u8]) -> Result<(), Error> {
            self.0.try_fill_bytes(dest)
        }
    }

    impl<R, Rsdr> Clone for ReseedingRng<R, Rsdr>
    where R: BlockRngCore + SeedableRng + Clone,
          Rsdr: RngCore + Clone
    {
        fn clone(&self) -> ReseedingRng<R, Rsdr> {
            // Recreating `BlockRng` seems easier than cloning it and resetting
            // the index.
            ReseedingRng(BlockRng::new(self.0.core.clone()))
        }
    }

    impl<R, Rsdr> CryptoRng for ReseedingRng<R, Rsdr>
    where R: BlockRngCore + SeedableRng + CryptoRng,
          Rsdr: RngCore + CryptoRng {}

    #[derive(Debug)]
    struct ReseedingCore<R, Rsdr> {
        inner: R,
        reseeder: Rsdr,
        threshold: i64,
        bytes_until_reseed: i64,
        fork_counter: usize,
    }

    impl<R, Rsdr> BlockRngCore for ReseedingCore<R, Rsdr>
    where R: BlockRngCore + SeedableRng,
          Rsdr: RngCore
    {
        type Item = <R as BlockRngCore>::Item;
        type Results = <R as BlockRngCore>::Results;

        fn generate(&mut self, results: &mut Self::Results) {
            let global_fork_counter = fork::get_fork_counter();
            if self.bytes_until_reseed <= 0 ||
               self.is_forked(global_fork_counter) {
                // We get better performance by not calling only `reseed` here
                // and continuing with the rest of the function, but by directly
                // returning from a non-inlined function.
                return self.reseed_and_generate(results, global_fork_counter);
            }
            let num_bytes = results.as_ref().len() * size_of::<Self::Item>();
            self.bytes_until_reseed -= num_bytes as i64;
            self.inner.generate(results);
        }
    }

    impl<R, Rsdr> ReseedingCore<R, Rsdr>
    where R: BlockRngCore + SeedableRng,
          Rsdr: RngCore
    {
        /// Create a new `ReseedingCore`.
        fn new(rng: R, threshold: u64, reseeder: Rsdr) -> Self {
            use ::core::i64::MAX;
            fork::register_fork_handler();

            // Because generating more values than `i64::MAX` takes centuries on
            // current hardware, we just clamp to that value.
            // Also we set a threshold of 0, which indicates no limit, to that
            // value.
            let threshold =
                if threshold == 0 { MAX }
                else if threshold <= MAX as u64 { threshold as i64 }
                else { MAX };

            ReseedingCore {
                inner: rng,
                reseeder,
                threshold: threshold as i64,
                bytes_until_reseed: threshold as i64,
                fork_counter: 0,
            }
        }

        /// Reseed the internal PRNG.
        fn reseed(&mut self) -> Result<(), Error> {
            R::from_rng(&mut self.reseeder).map(|result| {
                self.bytes_until_reseed = self.threshold;
                self.inner = result
            })
        }

        fn is_forked(&self, global_fork_counter: usize) -> bool {
            // In theory, on 32-bit platforms, it is possible for
            // `global_fork_counter` to wrap around after ~4e9 forks.
            //
            // This check will detect a fork in the normal case where
            // `fork_counter < global_fork_counter`, and also when the difference
            // between both is greater than `isize::MAX` (wrapped around).
            //
            // It will still fail to detect a fork if there have been more than
            // `isize::MAX` forks, without any reseed in between. Seems unlikely
            // enough.
            (self.fork_counter.wrapping_sub(global_fork_counter) as isize) < 0
        }

        #[inline(never)]
        fn reseed_and_generate(&mut self,
                               results: &mut <Self as BlockRngCore>::Results,
                               global_fork_counter: usize)
        {
            if self.is_forked(global_fork_counter) {
                info!("Fork detected, reseeding RNG");
            } else {
                trace!("Reseeding RNG (periodic reseed)");
            }

            let num_bytes =
                results.as_ref().len() * size_of::<<R as BlockRngCore>::Item>();

            let threshold = if let Err(e) = self.reseed() {
                let delay = match e.kind {
                    ErrorKind::Transient => num_bytes as i64,
                    kind @ _ if kind.should_retry() => self.threshold >> 8,
                    _ => self.threshold,
                };
                warn!("Reseeding RNG delayed reseeding by {} bytes due to \
                       error from source: {}", delay, e);
                delay
            } else {
                self.fork_counter = global_fork_counter;
                self.threshold
            };

            self.bytes_until_reseed = threshold - num_bytes as i64;
            self.inner.generate(results);
        }
    }

    impl<R, Rsdr> Clone for ReseedingCore<R, Rsdr>
    where R: BlockRngCore + SeedableRng + Clone,
          Rsdr: RngCore + Clone
    {
        fn clone(&self) -> ReseedingCore<R, Rsdr> {
            ReseedingCore {
                inner: self.inner.clone(),
                reseeder: self.reseeder.clone(),
                threshold: self.threshold,
                bytes_until_reseed: 0, // reseed clone on first use
                fork_counter: self.fork_counter,
            }
        }
    }

    impl<R, Rsdr> CryptoRng for ReseedingCore<R, Rsdr>
    where R: BlockRngCore + SeedableRng + CryptoRng,
          Rsdr: RngCore + CryptoRng {}

    #[cfg(all(feature="std", unix, not(target_os="emscripten")))]
    mod fork {
        extern crate libc;

        use std::sync::atomic::{AtomicUsize, ATOMIC_USIZE_INIT, Ordering};
        use std::sync::atomic::{AtomicBool, ATOMIC_BOOL_INIT};

        // Fork protection
        //
        // We implement fork protection on Unix using `pthread_atfork`.
        // When the process is forked, we increment `RESEEDING_RNG_FORK_COUNTER`.
        // Every `ReseedingRng` stores the last known value of the static in
        // `fork_counter`. If the cached `fork_counter` is less than
        // `RESEEDING_RNG_FORK_COUNTER`, it is time to reseed this RNG.
        //
        // If reseeding fails, we don't deal with this by setting a delay, but just
        // don't update `fork_counter`, so a reseed is attempted as soon as
        // possible.

        static RESEEDING_RNG_FORK_COUNTER: AtomicUsize = ATOMIC_USIZE_INIT;

        pub fn get_fork_counter() -> usize {
            RESEEDING_RNG_FORK_COUNTER.load(Ordering::Relaxed)
        }

        static FORK_HANDLER_REGISTERED: AtomicBool = ATOMIC_BOOL_INIT;

        extern fn fork_handler() {
            // Note: fetch_add is defined to wrap on overflow
            // (which is what we want).
            RESEEDING_RNG_FORK_COUNTER.fetch_add(1, Ordering::Relaxed);
        }

        pub fn register_fork_handler() {
            if FORK_HANDLER_REGISTERED.load(Ordering::Relaxed) == false {
                unsafe { libc::pthread_atfork(None, None, Some(fork_handler)) };
                FORK_HANDLER_REGISTERED.store(true, Ordering::Relaxed);
            }
        }
    }

    #[cfg(not(all(feature="std", unix, not(target_os="emscripten"))))]
    mod fork {
        pub fn get_fork_counter() -> usize { 0 }
        pub fn register_fork_handler() {}
    }

    #[cfg(test)]
    mod test {
        use {Rng, SeedableRng};
        use rand_chacha::ChaChaCore;
        use rngs::mock::StepRng;
        use super::ReseedingRng;

        #[test]
        fn test_reseeding() {
            let mut zero = StepRng::new(0, 0);
            let rng = ChaChaCore::from_rng(&mut zero).unwrap();
            let mut reseeding = ReseedingRng::new(rng, 32*4, zero);

            // Currently we only support for arrays up to length 32.
            // TODO: cannot generate seq via Rng::gen because it uses different alg
            let mut buf = [0u32; 32]; // Needs to be a multiple of the RNGs result
                                      // size to test exactly.
            reseeding.fill(&mut buf);
            let seq = buf;
            for _ in 0..10 {
                reseeding.fill(&mut buf);
                assert_eq!(buf, seq);
            }
        }

        #[test]
        fn test_clone_reseeding() {
            let mut zero = StepRng::new(0, 0);
            let rng = ChaChaCore::from_rng(&mut zero).unwrap();
            let mut rng1 = ReseedingRng::new(rng, 32*4, zero);

            let first: u32 = rng1.gen();
            for _ in 0..10 { let _ = rng1.gen::<u32>(); }

            let mut rng2 = rng1.clone();
            assert_eq!(first, rng2.gen::<u32>());
        }
    }

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