// macros.rs

//

// Copyright 2019 Raphaël Gomès <rgomes@octobus.net>

//

// This software may be used and distributed according to the terms of the

// GNU General Public License version 2 or any later version.

//! Macros for use in the `hg-cpython` bridge library.

use crate::exceptions::AlreadyBorrowed;

use cpython::{PyResult, Python};

use std::cell::{Cell, Ref, RefCell, RefMut};

/// Manages the shared state between Python and Rust

#[derive(Default)]

pub struct PySharedState {

    leak_count: Cell<usize>,

    mutably_borrowed: Cell<bool>,

}

impl PySharedState {

    pub fn borrow_mut<'a, T>(

        &'a self,

        py: Python<'a>,

        pyrefmut: RefMut<'a, T>,

    ) -> PyResult<PyRefMut<'a, T>> {

        if self.mutably_borrowed.get() {

            return Err(AlreadyBorrowed::new(

                py,

                "Cannot borrow mutably while there exists another \

                 mutable reference in a Python object",

            ));

        }

        match self.leak_count.get() {

            0 => {

                self.mutably_borrowed.replace(true);

                Ok(PyRefMut::new(py, pyrefmut, self))

            }

            // TODO

            // For now, this works differently than Python references

            // in the case of iterators.

            // Python does not complain when the data an iterator

            // points to is modified if the iterator is never used

            // afterwards.

            // Here, we are stricter than this by refusing to give a

            // mutable reference if it is already borrowed.

            // While the additional safety might be argued for, it

            // breaks valid programming patterns in Python and we need

            // to fix this issue down the line.

            _ => Err(AlreadyBorrowed::new(

                py,

                "Cannot borrow mutably while there are \

                 immutable references in Python objects",

            )),

        }

    }

    /// Return a reference to the wrapped data with an artificial static

    /// lifetime.

    /// We need to be protected by the GIL for thread-safety.

    ///

    /// # Safety

    ///

    /// This is highly unsafe since the lifetime of the given data can be

    /// extended. Do not call this function directly.

    pub unsafe fn leak_immutable<T>(

        &self,

        py: Python,

        data: &PySharedRefCell<T>,

    ) -> PyResult<&'static T> {

        if self.mutably_borrowed.get() {

            return Err(AlreadyBorrowed::new(

                py,

                "Cannot borrow immutably while there is a \

                 mutable reference in Python objects",

            ));

        }

        let ptr = data.as_ptr();

        self.leak_count.replace(self.leak_count.get() + 1);

        Ok(&*ptr)

    }

    /// # Safety

    ///

    /// It's unsafe to update the reference count without knowing the

    /// reference is deleted. Do not call this function directly.

    pub unsafe fn decrease_leak_count(&self, _py: Python, mutable: bool) {

        self.leak_count

            .replace(self.leak_count.get().saturating_sub(1));

        if mutable {

            self.mutably_borrowed.replace(false);

        }

    }

}

/// `RefCell` wrapper to be safely used in conjunction with `PySharedState`.

///

/// Only immutable operation is allowed through this interface.

#[derive(Debug)]

pub struct PySharedRefCell<T> {

    inner: RefCell<T>,

}

impl<T> PySharedRefCell<T> {

    pub const fn new(value: T) -> PySharedRefCell<T> {

        Self {

            inner: RefCell::new(value),

        }

    }

    pub fn borrow(&self) -> Ref<T> {

        // py_shared_state isn't involved since

        // - inner.borrow() would fail if self is mutably borrowed,

        // - and inner.borrow_mut() would fail while self is borrowed.

        self.inner.borrow()

    }

    pub fn as_ptr(&self) -> *mut T {

        self.inner.as_ptr()

    }

    pub unsafe fn borrow_mut(&self) -> RefMut<T> {

        // must be borrowed by self.py_shared_state(py).borrow_mut().

        self.inner.borrow_mut()

    }

}

/// Holds a mutable reference to data shared between Python and Rust.

pub struct PyRefMut<'a, T> {

    inner: RefMut<'a, T>,

    py_shared_state: &'a PySharedState,

}

impl<'a, T> PyRefMut<'a, T> {

    // Must be constructed by PySharedState after checking its leak_count.

    // Otherwise, drop() would incorrectly update the state.

    fn new(

        _py: Python<'a>,

        inner: RefMut<'a, T>,

        py_shared_state: &'a PySharedState,

    ) -> Self {

        Self {

            inner,

            py_shared_state,

        }

    }

}

impl<'a, T> std::ops::Deref for PyRefMut<'a, T> {

    type Target = RefMut<'a, T>;

    fn deref(&self) -> &Self::Target {

        &self.inner

    }

}

impl<'a, T> std::ops::DerefMut for PyRefMut<'a, T> {

    fn deref_mut(&mut self) -> &mut Self::Target {

        &mut self.inner

    }

}

impl<'a, T> Drop for PyRefMut<'a, T> {

    fn drop(&mut self) {

        let gil = Python::acquire_gil();

        let py = gil.python();

        unsafe {

            self.py_shared_state.decrease_leak_count(py, true);

        }

    }

}

/// Allows a `py_class!` generated struct to share references to one of its

/// data members with Python.

///

/// # Warning

///

/// The targeted `py_class!` needs to have the

/// `data py_shared_state: PySharedState;` data attribute to compile.

/// A better, more complicated macro is needed to automatically insert it,

/// but this one is not yet really battle tested (what happens when

/// multiple references are needed?). See the example below.

///

/// TODO allow Python container types: for now, integration with the garbage

///     collector does not extend to Rust structs holding references to Python

///     objects. Should the need surface, `__traverse__` and `__clear__` will

///     need to be written as per the `rust-cpython` docs on GC integration.

///

/// # Parameters

///

/// * `$name` is the same identifier used in for `py_class!` macro call.

/// * `$inner_struct` is the identifier of the underlying Rust struct

/// * `$data_member` is the identifier of the data member of `$inner_struct`

/// that will be shared.

/// * `$leaked` is the identifier to give to the struct that will manage

/// references to `$name`, to be used for example in other macros like

/// `py_shared_iterator`.

///

/// # Example

///

/// ```

/// struct MyStruct {

///     inner: Vec<u32>;

/// }

///

/// py_class!(pub class MyType |py| {

///     data inner: PySharedRefCell<MyStruct>;

///     data py_shared_state: PySharedState;

/// });

///

/// py_shared_ref!(MyType, MyStruct, inner, MyTypeLeakedRef);

/// ```

macro_rules! py_shared_ref {

    (

        $name: ident,

        $inner_struct: ident,

        $data_member: ident,

        $leaked: ident,

    ) => {

        impl $name {

            fn borrow_mut<'a>(

                &'a self,

                py: Python<'a>,

            ) -> PyResult<crate::ref_sharing::PyRefMut<'a, $inner_struct>>

            {

                // assert $data_member type

                use crate::ref_sharing::PySharedRefCell;

                let data: &PySharedRefCell<_> = self.$data_member(py);

                self.py_shared_state(py)

                    .borrow_mut(py, unsafe { data.borrow_mut() })

            }

            /// Returns a leaked reference and its management object.

            ///

            /// # Safety

            ///

            /// It's up to you to make sure that the management object lives

            /// longer than the leaked reference. Otherwise, you'll get a

            /// dangling reference.

            unsafe fn leak_immutable<'a>(

                &'a self,

                py: Python<'a>,

            ) -> PyResult<($leaked, &'static $inner_struct)> {

                // assert $data_member type

                use crate::ref_sharing::PySharedRefCell;

                let data: &PySharedRefCell<_> = self.$data_member(py);

                let static_ref =

                    self.py_shared_state(py).leak_immutable(py, data)?;

                let leak_handle = $leaked::new(py, self);

                Ok((leak_handle, static_ref))

            }

        }

        /// Manage immutable references to `$name` leaked into Python

        /// iterators.

        ///

        /// In truth, this does not represent leaked references themselves;

        /// it is instead useful alongside them to manage them.

        pub struct $leaked {

            inner: $name,

        }

        impl $leaked {

            // Marked as unsafe so client code wouldn't construct $leaked

            // struct by mistake. Its drop() is unsafe.

            unsafe fn new(py: Python, inner: &$name) -> Self {

                Self {

                    inner: inner.clone_ref(py),

                }

            }

        }

        impl Drop for $leaked {

            fn drop(&mut self) {

                let gil = Python::acquire_gil();

                let py = gil.python();

                let state = self.inner.py_shared_state(py);

                unsafe {

                    state.decrease_leak_count(py, false);

                }

            }

        }

    };

}

/// Defines a `py_class!` that acts as a Python iterator over a Rust iterator.

///

/// TODO: this is a bit awkward to use, and a better (more complicated)

///     procedural macro would simplify the interface a lot.

///

/// # Parameters

///

/// * `$name` is the identifier to give to the resulting Rust struct.

/// * `$leaked` corresponds to `$leaked` in the matching `py_shared_ref!` call.

/// * `$iterator_type` is the type of the Rust iterator.

/// * `$success_func` is a function for processing the Rust `(key, value)`

/// tuple on iteration success, turning it into something Python understands.

/// * `$success_func` is the return type of `$success_func`

///

/// # Example

///

/// ```

/// struct MyStruct {

///     inner: HashMap<Vec<u8>, Vec<u8>>;

/// }

///

/// py_class!(pub class MyType |py| {

///     data inner: PySharedRefCell<MyStruct>;

///     data py_shared_state: PySharedState;

///

///     def __iter__(&self) -> PyResult<MyTypeItemsIterator> {

///         let (leak_handle, leaked_ref) = unsafe { self.leak_immutable(py)? };

///             py,

///         )

///     }

/// });

///

/// impl MyType {

///     fn translate_key_value(

///         py: Python,

///         res: (&Vec<u8>, &Vec<u8>),

///     ) -> PyResult<Option<(PyBytes, PyBytes)>> {

///         let (f, entry) = res;

///         Ok(Some((

///             PyBytes::new(py, f),

///             PyBytes::new(py, entry),

///         )))

///     }

/// }

///

/// py_shared_ref!(MyType, MyStruct, inner, MyTypeLeakedRef);

///

/// py_shared_iterator!(

///     MyTypeItemsIterator,

///     MyTypeLeakedRef,

///     HashMap<'static, Vec<u8>, Vec<u8>>,

///     MyType::translate_key_value,

///     Option<(PyBytes, PyBytes)>

/// );

/// ```

macro_rules! py_shared_iterator {

    (

        $name: ident,

        $leaked: ident,

        $iterator_type: ty,

        $success_func: expr,

        $success_type: ty

    ) => {

        py_class!(pub class $name |py| {

            data inner: RefCell<Option<$leaked>>;

            data it: RefCell<$iterator_type>;

            def __next__(&self) -> PyResult<$success_type> {

                let mut inner_opt = self.inner(py).borrow_mut();

                if inner_opt.is_some() {

                    match self.it(py).borrow_mut().next() {

                        None => {

                            // replace Some(inner) by None, drop $leaked

                            inner_opt.take();

                            Ok(None)

                        }

                        Some(res) => {

                            $success_func(py, res)

                        }

                    }

                } else {

                    Ok(None)

                }

            }

            def __iter__(&self) -> PyResult<Self> {

                Ok(self.clone_ref(py))

            }

        });

        impl $name {

            pub fn from_inner(

                py: Python,

                leaked: $leaked,

                it: $iterator_type

            ) -> PyResult<Self> {

                Self::create_instance(

                    py,

                    RefCell::new(Some(leaked)),

                    RefCell::new(it)

                )

            }

        }

    };

}