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: signals.rs
    path of file: ./repos/wasmtime/crates/runtime/src/sys/unix
    
    the code of the file until where you have to start completion: //! Trap handling on Unix based on POSIX signals.

use crate::traphandlers::tls;
use crate::VMContext;
use std::cell::RefCell;
use std::io;
use std::mem::{self, MaybeUninit};
use std::ptr::{self, null_mut};

#[link(name = "wasmtime-helpers")]
extern "C" {
    #[wasmtime_versioned_export_macros::versioned_link]
    #[allow(improper_ctypes)]
    pub fn wasmtime_setjmp(
        jmp_buf: *mut *const u8,
        callback: extern "C" fn(*mut u8, *mut VMContext),
        payload: *mut u8,
        callee: *mut VMContext,
    ) -> i32;

    #[wasmtime_versioned_export_macros::versioned_link]
    pub fn wasmtime_longjmp(jmp_buf: *const u8) -> !;
}

/// Function which may handle custom signals while processing traps.
pub type SignalHandler<'a> =
    dyn Fn(libc::c_int, *const libc::siginfo_t, *const libc::c_void) -> bool + Send + Sync + 'a;

static mut PREV_SIGSEGV: MaybeUninit<libc::sigaction> = MaybeUninit::uninit();
static mut PREV_SIGBUS: MaybeUninit<libc::sigaction> = MaybeUninit::uninit();
static mut PREV_SIGILL: MaybeUninit<libc::sigaction> = MaybeUninit::uninit();
static mut PREV_SIGFPE: MaybeUninit<libc::sigaction> = MaybeUninit::uninit();

pub unsafe fn platform_init(macos_use_mach_ports: bool) {
    // Either mach ports shouldn't be in use or we shouldn't be on macOS,
    // otherwise the `machports.rs` module should be used instead.
    assert!(!macos_use_mach_ports || !cfg!(target_os = "macos"));

    let register = |slot: *mut libc::sigaction, signal: i32| {
        let mut handler: libc::sigaction = mem::zeroed();
        // The flags here are relatively careful, and they are...
        //
        // SA_SIGINFO gives us access to information like the program
        // counter from where the fault happened.
        //
        // SA_ONSTACK allows us to handle signals on an alternate stack,
        // so that the handler can run in response to running out
    
pub fn lazy_per_thread_init() {
    // This thread local is purely used to register a `Stack` to get deallocated
    // when the thread exists. Otherwise this function is only ever called at
    // most once per-thread.
    thread_local! {
        static STACK: RefCell<Option<Stack>> = const { RefCell::new(None) };
    }

    /// The size of the sigaltstack (not including the guard, which will be
    /// added). Make this large enough to run our signal handlers.
    ///
    /// The main current requirement of the signal handler in terms of stack
    /// space is that `malloc`/`realloc` are called to create a `Backtrace` of
    /// wasm frames.
    ///
    /// Historically this was 16k. Turns out jemalloc requires more than 16k of
    /// stack space in debug mode, so this was bumped to 64k.
    const MIN_STACK_SIZE: usize = 64 * 4096;

    struct Stack {
        mmap_ptr: *mut libc::c_void,
        mmap_size: usize,
    }

    return STACK.with(|s| {
        *s.borrow_mut() = unsafe { allocate_sigaltstack() };
    });

    unsafe fn allocate_sigaltstack() -> Option<Stack> {
        // Check to see if the existing sigaltstack, if it exists, is big
        // enough. If so we don't need to allocate our own.
        let mut old_stack = mem::zeroed();
        let r = libc::sigaltstack(ptr::null(), &mut old_stack);
        assert_eq!(
            r,
            0,
            "learning about sigaltstack failed: {}",
            io::Error::last_os_error()
        );
        if old_stack.ss_flags & libc::SS_DISABLE == 0 && old_stack.ss_size >= MIN_STACK_SIZE {
            return None;
        }

        // ... but failing that we need to allocate our own, so do all that
        // here.
        let page_size = crate::page_size();
        let guard_size = page_size;
        let alloc_size = guard_size + MIN_STACK_SIZE;

        let ptr = rustix::mm::mmap_anonymous(
            null_mut(),
            alloc_size,
            rustix::mm::ProtFlags::empty(),
            rustix::mm::MapFlags::PRIVATE,
        )
        .expect("failed to allocate memory for sigaltstack");

        // Prepare the stack with readable/writable memory and then register it
        // with `sigaltstack`.
        let stack_ptr = (ptr as usize + guard_size) as *mut std::ffi::c_void;
        rustix::mm::mprotect(
            stack_ptr,
            MIN_STACK_SIZE,
            rustix::mm::MprotectFlags::READ | rustix::mm::MprotectFlags::WRITE,
        )
        .expect("mprotect to configure memory for sigaltstack failed");
        let new_stack = libc::stack_t {
            ss_sp: stack_ptr,
            ss_flags: 0,
            ss_size: MIN_STACK_SIZE,
        };
        let r = libc::sigaltstack(&new_stack, ptr::null_mut());
        assert_eq!(
            r,
            0,
            "registering new sigaltstack failed: {}",
            io::Error::last_os_error()
        );

        Some(Stack {
            mmap_ptr: ptr,
            mmap_size: alloc_size,
        })
    }

    impl Drop for Stack {
        fn drop(&mut self) {
            unsafe {
                // Deallocate the stack memory.
                let r = rustix::mm::munmap(self.mmap_ptr, self.mmap_size);
                debug_assert!(r.is_ok(), "munmap failed during thread shutdown");
            }
        }
    }
}