# Copyright (c) Facebook, Inc. and its affiliates.
#
# This source code is licensed under the BSD license found in the
# LICENSE file in the root directory of this source tree.

from contextlib import contextmanager
import functools
import threading
from typing import Any, Dict, Generator, Optional, Tuple
import weakref

import torch
from torch import Tensor
import torch.nn as nn
import torch.utils.checkpoint as torch_checkpoint

from fairscale.utils.containers import pack_kwargs, split_non_tensors, unpack_kwargs, unpack_non_tensors

from .checkpoint_utils import dec_counter, inc_counter, init_counter, patch_batchnorm


# https://docs.python.org/3/library/threading.html#thread-local-data
# Manage the checkpoint context with thread-local data.
class ThreadLocal(threading.local):
    def __init__(self) -> None:
        self.is_checkpointing = False
        self.is_recomputing = False


thread_local = ThreadLocal()


@contextmanager
def enable_checkpointing() -> Generator[None, None, None]:
    """Makes :func:`is_checkpointing` return :data:`True` within a context."""
    orig = thread_local.is_checkpointing
    thread_local.is_checkpointing = True
    try:
        yield
    finally:
        thread_local.is_checkpointing = orig


@contextmanager
def enable_recomputing() -> Generator[None, None, None]:
    """Makes :func:`is_recomputing` return :data:`True` within a context."""
    orig = thread_local.is_recomputing
    thread_local.is_recomputing = True
    try:
        yield
    finally:
        thread_local.is_recomputing = orig


def is_checkpointing() -> bool:
    """Whether the current forward propagation is under checkpointing.

    Returns:
        bool: :data:`True` if it's under checkpointing.

    """
    return thread_local.is_checkpointing


def is_recomputing() -> bool:
    """Whether the current forward propagation is under checkpoint
    recomputation. Use this to prevent duplicated side-effects at forward
    propagation::

        class Counter(nn.Module):
            def __init__(self):
                super().__init__()
                self.counter = 0

            def forward(self, input):
                if not is_recomputing():
                    self.counter += 1
                return input

    Returns:
        bool: :data:`True` if it's under checkpoint recomputation.
    """
    return thread_local.is_recomputing


def checkpoint_wrapper(
    module: nn.Module, offload_to_cpu: bool = False, maintain_forward_counter: bool = False
) -> nn.Module:
    """
    A friendlier wrapper for performing activation checkpointing.

    Compared to the PyTorch version, this version:

        - wraps an nn.Module, so that all subsequent calls will use checkpointing
        - handles keyword arguments in the forward
        - handles non-Tensor outputs from the forward
        - supports offloading activations to CPU

    Usage::

        checkpointed_module = checkpoint_wrapper(my_module, offload_to_cpu=True)
        a, b = checkpointed_module(x, y=3, z=torch.Tensor([1]))

    To understand the benefits of checkpointing and the `offload_to_cpu` flag,
    let's divide activations into 2 types: inner activations and outer
    activations w.r.t. the checkpointed modules. The inner ones are saved
    by activation checkpointing, the outer ones are saved by offload_to_cpu.

    In terms of GPU memory savings:

        - When inner ones are large in size and outer ones are small,
          checkpointing helps a lot, offload_to_cpu may help a little.
        - When inner ones are small and outer ones are large,
          checkpointing helps little, offload_to_cpu helps a lot.
        - When both inner and outer are large, both help and the
          benefit is additive.

    ..Note::

        The first and last layers are not likely to benefit from the `offload_to_cpu` flag
        because (1) there are typically other references to the first layer's input, so
        the GPU memory won't be freed; (2) the input to the last layer is immediately
        used by the backward pass and won't result in memory savings.

    Args:
        module (nn.Module):
            The module to be wrapped
        offload_to_cpu (bool):
            Whether to offload activations to CPU.
        maintain_forward_counter (bool):
            If True, maintain a forward counter per inner module. The counter will first
            increases in forward calls of outer forward pass and then decreases in the
            forward calls of outer backward pass. It is used by FullyShardedDataParallel.

    Returns:
        (nn.Module):
            Wrapped module
    """
    # Patch the batchnorm layers in case there are any in this module.
    patch_batchnorm(module)

    if maintain_forward_counter:
        init_counter(module)

    # The use of weakref here is to prevent creating a ref cycle: m -> m.forward -> m.
    # When such cycle exists, gc won't collect the module when the module is freed.
    # That causes GPU memory to be leaked. See the unit test for how we catch that.
    #
    # We prefer this over a class wrapper since the class wrapper would have to
    # proxy a lot of fields and methods.
    module.forward = functools.partial(  # type: ignore
        _checkpointed_forward, type(module).forward, weakref.ref(module), offload_to_cpu
    )
    return module


def _checkpointed_forward(
    original_forward: Any, weak_self: Any, offload_to_cpu: bool, *args: Any, **kwargs: Any
) -> Any:
    module = weak_self()

    # If gradients are disabled, just use original `.forward()` method directly.
    # Doing so also ensures the internal fwd counter is not incremented in the forward pass,
    # which would be an issue during eval since there wouldn't be a corresponding backward pass
    # to decrement the fwd counter.
    # See https://github.com/facebookresearch/fairscale/pull/709.
    if not torch.is_grad_enabled():
        return original_forward(module, *args, **kwargs)

    # Autograd Functions in PyTorch work best with positional args, since
    # the backward must return gradients (or None) for every input argument.
    # We can flatten keyword arguments to make this easier.
    args = (module,) + args
    kwarg_keys, flat_args = pack_kwargs(*args, **kwargs)
    parent_ctx_dict: Dict[str, Any] = {
        "offload": offload_to_cpu,
    }
    # Dummy tensor with grad is used to ensure the backward pass is called. This is needed
    # when original_forward's input are non-tensor (i.e. a tuple). Using this dummy tensor
    # avoids requiring users to set their input tensors's requires_grad flag. In the case
    # of tuple type inputs, setting the flag won't even trigger the backward pass.
    output = CheckpointFunction.apply(
        torch.tensor([], requires_grad=True), original_forward, parent_ctx_dict, kwarg_keys, *flat_args
    )
    if not isinstance(output, torch.Tensor):
        packed_non_tensor_outputs = parent_ctx_dict["packed_non_tensor_outputs"]
        if packed_non_tensor_outputs:
            output = unpack_non_tensors(output, packed_non_tensor_outputs)
    return output


def get_rng_state() -> Dict[str, Any]:
    state = {"torch_rng_state": torch.get_rng_state()}
    if torch.cuda.is_available():
        state["cuda_rng_state"] = torch.cuda.get_rng_state()
    return state


def set_rng_state(state: Dict[str, Any]) -> None:
    torch.set_rng_state(state["torch_rng_state"])
    if torch.cuda.is_available():
        torch.cuda.set_rng_state(state["cuda_rng_state"])


def is_autocast_enabled() -> bool:
    """Similar to torch.is_autocast_enabled, but compatible with torch 1.5.1"""
    if hasattr(torch, "is_autocast_enabled"):
        return torch.is_autocast_enabled()
    return False


@contextmanager
def autocast(enabled: bool) -> Generator:
    """Similar to torch.cuda.amp.autocast, but compatible with torch 1.5.1"""
    if enabled:
        with torch.cuda.amp.autocast(enabled):
            yield
    else:
        yield


class CheckpointFunction(torch.autograd.Function):
    """Similar to the torch version, but support non-Tensor outputs.

    The caller is expected to provide a dict (*parent_ctx_dict*) that will hold
    the non-Tensor outputs. These should be combined with the Tensor *outputs*
    by calling :func:`unpack_non_tensors`.
    """

    @staticmethod
    def forward(  # type: ignore
        ctx: Any,
        dummy_tensor_requires_grad: torch.Tensor,
        run_function: Any,
        parent_ctx_dict: Dict[str, Any],
        kwarg_keys: Tuple[str, ...],
        *args: Any,
        **kwargs: Any
    ) -> Any:
        torch_checkpoint.check_backward_validity(args)

        ctx.run_function = run_function
        ctx.kwarg_keys = kwarg_keys
        ctx.fwd_rng_state = get_rng_state()
        ctx.had_autocast_in_fwd = is_autocast_enabled()

        tensor_inputs, packed_non_tensor_inputs = split_non_tensors(args)
        if parent_ctx_dict["offload"]:
            ctx.fwd_device = tuple(x.device for x in tensor_inputs)
            ctx.grad_requirements = tuple(x.requires_grad for x in tensor_inputs)
            tensor_inputs = tuple(x.cpu() for x in tensor_inputs)
        else:
            ctx.fwd_device, ctx.grad_requirements = None, None

        ctx.save_for_backward(*tensor_inputs)
        ctx.packed_non_tensor_inputs = packed_non_tensor_inputs

        with torch.no_grad(), enable_checkpointing():
            unpacked_args, unpacked_kwargs = unpack_kwargs(kwarg_keys, args)
            outputs = run_function(*unpacked_args, **unpacked_kwargs)
            the_module = unpacked_args[0]
            inc_counter(the_module)

        if not isinstance(outputs, torch.Tensor):
            # Autograd Functions don't like non-Tensor outputs. We can split the
            # non-Tensor and Tensor outputs, returning the former by reference
            # through *parent_ctx_dict* and returning the latter directly.
            outputs, packed_non_tensor_outputs = split_non_tensors(outputs)
            parent_ctx_dict["packed_non_tensor_outputs"] = packed_non_tensor_outputs
        return outputs

    @staticmethod
    def backward(ctx: Any, *args: Any) -> Tuple[Optional[Tensor], ...]:
        if not torch.autograd._is_checkpoint_valid():
            raise RuntimeError("Checkpointing is not compatible with .grad(), please use .backward() if possible")

        tensor_inputs: Tuple = ctx.saved_tensors
        tensor_inputs = torch_checkpoint.detach_variable(tensor_inputs)
        if ctx.fwd_device is not None:
            tensor_inputs = tuple(t.to(ctx.fwd_device[i]) for i, t in enumerate(tensor_inputs))
            for i, need_grad in enumerate(ctx.grad_requirements):
                tensor_inputs[i].requires_grad = need_grad
        inputs = unpack_non_tensors(tensor_inputs, ctx.packed_non_tensor_inputs)

        # Store the current states.
        bwd_rng_state = get_rng_state()

        # Set the states to what it used to be before the forward pass.
        set_rng_state(ctx.fwd_rng_state)

        with torch.enable_grad(), enable_recomputing(), autocast(ctx.had_autocast_in_fwd):
            unpacked_args, unpacked_kwargs = unpack_kwargs(ctx.kwarg_keys, inputs)
            outputs = ctx.run_function(*unpacked_args, **unpacked_kwargs)
            tensor_outputs, _ = split_non_tensors(outputs)
            the_module = unpacked_args[0]
            dec_counter(the_module)

        # Set the states back to what it was at the start of this function.
        set_rng_state(bwd_rng_state)

        # Run backward() with only Tensors that require grad
        outputs_with_grad = []
        args_with_grad = []
        for i in range(len(tensor_outputs)):
            if tensor_outputs[i].requires_grad:
                outputs_with_grad.append(tensor_outputs[i])
                args_with_grad.append(args[i])
        if len(outputs_with_grad) == 0:
            raise RuntimeError("None of the outputs have requires_grad=True, " "this checkpoint() is not necessary")

        torch.autograd.backward(outputs_with_grad, args_with_grad)

        grads = tuple(inp.grad if isinstance(inp, torch.Tensor) else None for inp in inputs)
        return (None, None, None, None) + grads
