//===----------------------------------------------------------------------===//

//

// This source file is part of the SwiftNIO open source project

//

// Copyright (c) 2017-2021 Apple Inc. and the SwiftNIO project authors

// Licensed under Apache License v2.0

//

// See LICENSE.txt for license information

// See CONTRIBUTORS.txt for the list of SwiftNIO project authors

//

// SPDX-License-Identifier: Apache-2.0

//

//===----------------------------------------------------------------------===//

import NIOCore

#if canImport(Dispatch)

import Dispatch

#endif

/// A DNS resolver built on top of the libc `getaddrinfo` function.

///

/// This is the lowest-common-denominator resolver available to NIO. It's not really a very good

/// solution because the `getaddrinfo` call blocks during the DNS resolution, meaning that this resolver

/// will block a thread for as long as it takes to perform the getaddrinfo call. To prevent it from blocking `EventLoop`

/// threads, it will offload the blocking `getaddrinfo` calls to a `DispatchQueue`.

/// One advantage from leveraging `getaddrinfo` is the automatic conformance to RFC 6724, which removes some of the work

/// needed to implement it.

///

/// This resolver is a single-use object: it can only be used to perform a single host resolution.

#if os(Linux) || os(FreeBSD) || os(Android)

import CNIOLinux

#endif

#if os(Windows)

import let WinSDK.AF_INET

import let WinSDK.AF_INET6

import func WinSDK.FreeAddrInfoW

import func WinSDK.GetAddrInfoW

import func WinSDK.gai_strerrorA

import struct WinSDK.ADDRESS_FAMILY

import struct WinSDK.ADDRINFOW

import struct WinSDK.SOCKADDR_IN

import struct WinSDK.SOCKADDR_IN6

#endif

// A thread-specific variable where we store the offload queue if we're on an `SelectableEventLoop`.

let offloadQueueTSV = ThreadSpecificVariable<DispatchQueue>()

internal final class GetaddrinfoResolver: Resolver, Sendable {

    private let loop: EventLoop

    private let v4Future: EventLoopPromise<[SocketAddress]>

    private let v6Future: EventLoopPromise<[SocketAddress]>

    private let aiSocktype: NIOBSDSocket.SocketType

    private let aiProtocol: NIOBSDSocket.OptionLevel

    /// Create a new resolver.

    ///

    /// - Parameters:

    ///   - loop: The `EventLoop` whose thread this resolver will block.

    ///   - aiSocktype: The sock type to use as hint when calling getaddrinfo.

    ///   - aiProtocol: the protocol to use as hint when calling getaddrinfo.

    init(

        loop: EventLoop,

        aiSocktype: NIOBSDSocket.SocketType,

        aiProtocol: NIOBSDSocket.OptionLevel

    ) {

        self.loop = loop

        self.v4Future = loop.makePromise()

        self.v6Future = loop.makePromise()

        self.aiSocktype = aiSocktype

        self.aiProtocol = aiProtocol

    }

    /// Initiate a DNS A query for a given host.

    ///

    /// Due to the nature of `getaddrinfo`, we only actually call the function once, in the AAAA query.

    /// That means this just returns the future for the A results, which in practice will always have been

    /// satisfied by the time this function is called.

    ///

    /// - Parameters:

    ///   - host: The hostname to do an A lookup on.

    ///   - port: The port we'll be connecting to.

    /// - Returns: An `EventLoopFuture` that fires with the result of the lookup.

    func initiateAQuery(host: String, port: Int) -> EventLoopFuture<[SocketAddress]> {

        v4Future.futureResult

    }

    /// Initiate a DNS AAAA query for a given host.

    ///

    /// Due to the nature of `getaddrinfo`, we only actually call the function once, in this function.

    ///

    /// - Parameters:

    ///   - host: The hostname to do an AAAA lookup on.

    ///   - port: The port we'll be connecting to.

    /// - Returns: An `EventLoopFuture` that fires with the result of the lookup.

    func initiateAAAAQuery(host: String, port: Int) -> EventLoopFuture<[SocketAddress]> {

        self.offloadQueue().async {

            self.resolveBlocking(host: host, port: port)

        }

        return v6Future.futureResult

    }

    private func offloadQueue() -> DispatchQueue {

        if let offloadQueue = offloadQueueTSV.currentValue {

            return offloadQueue

        } else {

            if MultiThreadedEventLoopGroup.currentEventLoop != nil {

                // Okay, we're on an SelectableEL thread. Let's stuff our queue into the thread local.

                let offloadQueue = DispatchQueue(label: "io.swiftnio.GetaddrinfoResolver.offloadQueue")

                offloadQueueTSV.currentValue = offloadQueue

                return offloadQueue

            } else {

                return DispatchQueue.global()

            }

        }

    }

    /// Cancel all outstanding DNS queries.

    ///

    /// This method is called whenever queries that have not completed no longer have their

    /// results needed. The resolver should, if possible, abort any outstanding queries and

    /// clean up their state.

    ///

    /// In the getaddrinfo case this is a no-op, as the resolver blocks.

    func cancelQueries() {}

    /// Perform the DNS queries and record the result.

    ///

    /// - Parameters:

    ///   - host: The hostname to do the DNS queries on.

    ///   - port: The port we'll be connecting to.

    private func resolveBlocking(host: String, port: Int) {

        #if os(Windows)

        host.withCString(encodedAs: UTF16.self) { wszHost in

            String(port).withCString(encodedAs: UTF16.self) { wszPort in

                var pResult: UnsafeMutablePointer<ADDRINFOW>?

                var aiHints: ADDRINFOW = ADDRINFOW()

                aiHints.ai_socktype = self.aiSocktype.rawValue

                aiHints.ai_protocol = self.aiProtocol.rawValue

                let iResult = GetAddrInfoW(wszHost, wszPort, &aiHints, &pResult)

                guard iResult == 0 else {

                    self.fail(SocketAddressError.unknown(host: host, port: port))

                    return

                }

                if let pResult = pResult {

                    self.parseAndPublishResults(pResult, host: host)

                    FreeAddrInfoW(pResult)

                } else {

                    self.fail(SocketAddressError.unsupported)

                }

            }

        }

        #else

        var info: UnsafeMutablePointer<addrinfo>?

        var hint = addrinfo()

        hint.ai_socktype = self.aiSocktype.rawValue

        hint.ai_protocol = self.aiProtocol.rawValue

        guard getaddrinfo(host, String(port), &hint, &info) == 0 else {

            self.fail(SocketAddressError.unknown(host: host, port: port))

            return

        }

        if let info = info {

            self.parseAndPublishResults(info, host: host)

            freeaddrinfo(info)

        } else {

            // this is odd, getaddrinfo returned NULL

            self.fail(SocketAddressError.unsupported)

        }

        #endif

    }

    /// Parses the DNS results from the `addrinfo` linked list.

    ///

    /// - Parameters:

    ///   - info: The pointer to the first of the `addrinfo` structures in the list.

    ///   - host: The hostname we resolved.

    #if os(Windows)

    internal typealias CAddrInfo = ADDRINFOW

    #else

    internal typealias CAddrInfo = addrinfo

    #endif

    private func parseAndPublishResults(_ info: UnsafeMutablePointer<CAddrInfo>, host: String) {

        var v4Results: [SocketAddress] = []

        var v6Results: [SocketAddress] = []

        var info: UnsafeMutablePointer<CAddrInfo> = info

        while true {

            let addressBytes = UnsafeRawPointer(info.pointee.ai_addr)

            switch NIOBSDSocket.AddressFamily(rawValue: info.pointee.ai_family) {

            case .inet:

                // Force-unwrap must be safe, or libc did the wrong thing.

                v4Results.append(.init(addressBytes!.load(as: sockaddr_in.self), host: host))

            case .inet6:

                // Force-unwrap must be safe, or libc did the wrong thing.

                v6Results.append(.init(addressBytes!.load(as: sockaddr_in6.self), host: host))

            default:

                self.fail(SocketAddressError.unsupported)

                return

            }

            guard let nextInfo = info.pointee.ai_next else {

                break

            }

            info = nextInfo

        }

        // Ensure that both futures are succeeded in the same tick

        // to avoid racing and potentially leaking a promise

        self.loop.execute { [v4Results, v6Results] in

            self.v6Future.succeed(v6Results)

            self.v4Future.succeed(v4Results)

        }

    }

    /// Record an error and fail the lookup process.

    ///

    /// - Parameters:

    ///   - error: The error encountered during lookup.

    private func fail(_ error: Error) {

        // Ensure that both futures are succeeded in the same tick

        // to avoid racing and potentially leaking a promise

        self.loop.execute {

            self.v6Future.fail(error)

            self.v4Future.fail(error)

        }

    }

}