// Copyright © 2019 Intel Corporation
//
// SPDX-License-Identifier: Apache-2.0
//

//! The internal VMM API for Cloud Hypervisor.
//!
//! This API is a synchronous, [mpsc](https://doc.rust-lang.org/std/sync/mpsc/)
//! based IPC for sending commands to the VMM thread, from other
//! Cloud Hypervisor threads. The IPC follows a command-response protocol, i.e.
//! each command will receive a response back.
//!
//! The main Cloud Hypervisor thread creates an API event file descriptor
//! to notify the VMM thread about pending API commands, together with an
//! API mpsc channel. The former is the IPC control plane, the latter is the
//! IPC data plane.
//! In order to use the IPC, a Cloud Hypervisor thread needs to have a clone
//! of both the API event file descriptor and the channel Sender. Then it must
//! go through the following steps:
//!
//! 1. The thread creates an mpsc channel for receiving the command response.
//! 2. The thread sends an ApiRequest to the Sender endpoint. The ApiRequest
//!    contains the response channel Sender, for the VMM API server to be able
//!    to send the response back.
//! 3. The thread writes to the API event file descriptor to notify the VMM
//!    API server about a pending command.
//! 4. The thread reads the response back from the VMM API server, from the
//!    response channel Receiver.
//! 5. The thread handles the response and forwards potential errors.

use crate::config::VmConfig;
use crate::vm::Error;
use std::sync::mpsc::Sender;
use std::sync::Arc;

/// API errors are sent back from the VMM API server through the ApiResponse.
#[derive(Debug)]
pub enum ApiError {
    /// The VM could not be created.
    VmCreate(Error),

    /// The VM could not start.
    VmStart(Error),
}

pub enum ApiResponsePayload {
    /// No data is sent on the channel.
    Empty,
}

/// This is the response sent by the VMM API server through the mpsc channel.
pub type ApiResponse = std::result::Result<ApiResponsePayload, ApiError>;

#[allow(clippy::large_enum_variant)]
pub enum ApiRequest {
    /// Create the virtual machine. This request payload is a VM configuration
    /// (VmConfig).
    /// If the VMM API server could not create the VM, it will send a VmCreate
    /// error back.
    VmCreate(Arc<VmConfig>, Sender<ApiResponse>),

    /// Start the previously created virtual machine.
    /// If the VM was not previously created, the VMM API server will send a
    /// VmStart error back.
    VmStart(Sender<ApiResponse>),
}