// Copyright 2021 TiKV Project Authors. Licensed under Apache-2.0.

use std::alloc::{GlobalAlloc, Layout};

use atomic::{Atomic, Ordering};

type AllocFn = unsafe fn(Layout) -> *mut u8;
type DeallocFn = unsafe fn(*mut u8, Layout);

/// Used to initialize the plugin's allocator.
///
/// A `HostAllocatorPtr` contains the relevant pointers to initialize the
/// allocator of to plugin. It will be passed from TiKV to the plugin.
#[repr(C)]
pub struct HostAllocatorPtr {
    pub alloc_fn: AllocFn,
    pub dealloc_fn: DeallocFn,
}

/// An allocator that forwards invocations to the host (TiKV) of the plugin.
pub struct HostAllocator {
    alloc_fn: Atomic<Option<AllocFn>>,
    dealloc_fn: Atomic<Option<DeallocFn>>,
}

impl HostAllocator {
    /// Creates a new [`HostAllocator`].
    ///
    /// The internal function pointers are initially `None`, so any attempt to
    /// allocate memory before a call to [`set_allocator()`] will result in
    /// a panic.
    pub const fn new() -> Self {
        HostAllocator {
            alloc_fn: Atomic::new(None),
            dealloc_fn: Atomic::new(None),
        }
    }

    /// Updates the function pointers of the [`HostAllocator`] to the given
    /// [`HostAllocatorPtr`]. This function needs to be called before _any_
    /// allocation with this allocator is performed, because otherwise the
    /// [`HostAllocator`] is in an invalid state.
    pub fn set_allocator(&self, allocator: HostAllocatorPtr) {
        self.alloc_fn
            .store(Some(allocator.alloc_fn), Ordering::SeqCst);
        self.dealloc_fn
            .store(Some(allocator.dealloc_fn), Ordering::SeqCst);
    }
}

unsafe impl GlobalAlloc for HostAllocator {
    unsafe fn alloc(&self, layout: Layout) -> *mut u8 {
        self.alloc_fn.load(Ordering::Relaxed).unwrap()(layout)
    }
    unsafe fn dealloc(&self, ptr: *mut u8, layout: Layout) {
        self.dealloc_fn.load(Ordering::Relaxed).unwrap()(ptr, layout)
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn atomic_is_lock_free() {
        assert!(Atomic::<Option<AllocFn>>::is_lock_free());
    }
}