You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: ticketer.rs
    path of file: ./repos/rustls/rustls/src
    
    the code of the file until where you have to start completion: use alloc::boxed::Box;
use alloc::vec::Vec;
use core::mem;
use std::sync::{Mutex, MutexGuard};

use pki_types::UnixTime;

use crate::server::ProducesTickets;
use crate::{rand, Error};

#[derive(Debug)]
pub(crate) struct TicketSwitcherState {
    next: Option<Box<dyn ProducesTickets>>,
    current: Box<dyn ProducesTickets>,
    previous: Option<Box<dyn ProducesTickets>>,
    next_switch_time: u64,
}

/// A ticketer that has a 'current' sub-ticketer and a single
/// 'previous' ticketer.  It creates a new ticketer every so
/// often, demoting the current ticketer.
#[derive(Debug)]
pub struct TicketSwitcher {
    pub(crate) generator: fn() -> Result<Box<dyn ProducesTickets>, rand::GetRandomFailed>,
    lifetime: u32,
    state: Mutex<TicketSwitcherState>,
}

impl TicketSwitcher {
    /// Creates a new `TicketSwitcher`, which rotates through sub-ticketers
    /// based on the passage of time.
    ///
    /// `lifetime` is in seconds, and is how long the current ticketer
    /// is used to generate new tickets.  Tickets are accepted for no
    /// longer than twice this duration.  `generator` produces a new
    /// `ProducesTickets` implementation.
    pub fn new(
        lifetime: u32,
        generator: fn() -> Result<Box<dyn ProducesTickets>, rand::GetRandomFailed>,
    ) -> Result<Self, Error> {
        Ok(Self {
            generator,
            lifetime,
            state: Mutex::new(TicketSwitcherState {
                next: Some(generator()?),
                current: generator()?,
                previous: None,
                next_switch_time: UnixTime::now()
                    .as_secs()
                    .saturating_add(u64::from(lifetime)),
            }),
        })
    }

    /// If it's time, demote the `current` ticketer to `previous` (so it
    /// does no new encryptions but can do decryption) and use next for a
    /// new `current` ticketer.
    ///
    /// Calling this regularly will ensure timely key erasure.  Otherwise,
    /// key erasure will be delayed until the next encrypt/decrypt call.
    ///
    /// For efficiency, this is also responsible for locking the state mutex
    /// and returning the mutexguard.
    pub(crate) fn maybe_roll(&self, now: UnixTime) -> Option<MutexGuard<TicketSwitcherState>> {
        // The code below aims to make switching as efficient as possible
        // in the common case that the generator never fails. To achieve this
        // we run the following steps:
        //  1. If no switch is necessary, just return the mutexguard
        //  2. Shift over all of the ticketers (so current becomes previous,
        //     and next becomes current). After this, other threads can
        //     start using the new current ticketer.
        //  3. unlock mutex and generate new ticketer.
        //  4. Place new ticketer in next and return current
        //
        // There are a few things to note here. First, we don't check whether
        // a new switch might
    
    pub(crate) fn maybe_roll(&self, now: UnixTime) -> Option<MutexGuard<TicketSwitcherState>> {
        // The code below aims to make switching as efficient as possible
        // in the common case that the generator never fails. To achieve this
        // we run the following steps:
        //  1. If no switch is necessary, just return the mutexguard
        //  2. Shift over all of the ticketers (so current becomes previous,
        //     and next becomes current). After this, other threads can
        //     start using the new current ticketer.
        //  3. unlock mutex and generate new ticketer.
        //  4. Place new ticketer in next and return current
        //
        // There are a few things to note here. First, we don't check whether
        // a new switch might be needed in step 4, even though, due to locking
        // and entropy collection, significant amounts of time may have passed.
        // This is to guarantee that the thread doing the switch will eventually
        // make progress.
        //
        // Second, because next may be None, step 2 can fail. In that case
        // we enter a recovery mode where we generate 2 new ticketers, one for
        // next and one for the current ticketer. We then take the mutex a
        // second time and redo the time check to see if a switch is still
        // necessary.
        //
        // This somewhat convoluted approach ensures good availability of the
        // mutex, by ensuring that the state is usable and the mutex not held
        // during generation. It also ensures that, so long as the inner
        // ticketer never generates panics during encryption/decryption,
        // we are guaranteed to never panic when holding the mutex.

        let now = now.as_secs();
        let mut are_recovering = false; // Are we recovering from previous failure?
        {
            // Scope the mutex so we only take it for as long as needed
            let mut state = self.state.lock().ok()?;

            // Fast path in case we do not need to switch to the next ticketer yet
            if now <= state.next_switch_time {
                return Some(state);
            }

            // Make the switch, or mark for recovery if not possible
            if let Some(next) = state.next.take() {
                state.previous = Some(mem::replace(&mut state.current, next));
                state.next_switch_time = now.saturating_add(u64::from(self.lifetime));
            } else {
                are_recovering = true;
            }
        }

        // We always need a next, so generate it now
        let next = (self.generator)().ok()?;
        if !are_recovering {
            // Normal path, generate new next and place it in the state
            let mut state = self.state.lock().ok()?;
            state.next = Some(next);
            Some(state)
        } else {
            // Recovering, generate also a new current ticketer, and modify state
            // as needed. (we need to redo the time check, otherwise this might
            // result in very rapid switching of ticketers)
            let new_current = (self.generator)().ok()?;
            let mut state = self.state.lock().ok()?;
            state.next = Some(next);
            if now > state.next_switch_time {
                state.previous = Some(mem::replace(&mut state.current, new_current));
                state.next_switch_time = now.saturating_add(u64::from(self.lifetime));
            }
            Some(state)
        }
    }