//! Download Management for [`super`].

//!

//! This module consists of [`DownloadManager`], a helper for

//! downloading network documents.

//!

//! More information about the proper use can be found in the documentation

//! of the respective data type.

use std::{collections::VecDeque, fmt::Debug, net::SocketAddr};

use rand::{seq::IndexedRandom, Rng};

use retry_error::RetryError;

use tokio::net::TcpStream;

use tokio_util::compat::TokioAsyncReadCompatExt;

use tor_basic_utils::retry::RetryDelay;

use tor_dirclient::request::Requestable;

use tor_error::internal;

use tor_rtcompat::{PreferredRuntime, SleepProvider};

use tracing::{debug, warn};

use crate::err::AuthorityCommunicationError;

/// Download manager for authority requests.

///

/// This structure serves as the main interface for downloading documents from

/// a directory authority.  It implements the logic for retrying failed

/// downloads properly.

///

/// Technically, this structure does not need to be a structure and could be a

/// simple method instead.  However, because many settings stay the same across

/// all download attempts throughout the run-time of this program, making this

/// a separate structure is convenient for making the method signature smaller.

/// No state is kept *within* this structure, instead

/// [`DownloadManager::download()`] accepts an optional reference to a preferred

/// authority while returning the actual used authority, which the caller may

/// store in order to use that authority again in the future.

///

/// It may be worth to note that two round-robin loops, with one of them being

/// nested inside the other, are being used here.  The first serves as an

/// implementation of the specification in order to retry a download from a

/// different authority.  The second, inner, round-robin loop serves as an

/// implementation for happy-eyeballs, which, most commonly,  tries to connect

/// to both, the IPv4 and IPv6 (if present), utilizing the first one that

/// succeeds.  Keep in mind that error handling between these two is different.

/// The outer round-robin loop uses [`RetryError`], keeping track of all errors

/// in case that the download fails from all authorities, whereas the inner

/// round-robin loop uses [`TcpStream::connect()`], which only returns the error

/// of the last failed connection attempt, in the case that all attempts have

/// failed.

///

/// # Algorithm

///

/// 1. Shuffle the list of authorities in a randomized fashion.

/// 2. If there is a preferred authority, swap it with the first item in the list.

/// 3. Iterate through the list, calling [`tor_dirclient::send_request`].

///    3.1. If successful, set preferred authority to the current one and return.

///    3.2. If it failed, timeout with [`RetryDelay`] and go to 3.

///

/// # Specifications

///

/// * <https://spec.torproject.org/dir-spec/directory-cache-operation.html#general-download-behavior>

/// * <https://spec.torproject.org/dir-spec/directory-cache-operation.html#retry-as-cache>

#[derive(Debug)]

pub(super) struct DownloadManager<'a, 'b> {

    /// The list of download authorities.

    ///

    /// TODO DIRMIRROR: Consider accepting an AuthorityContacts and extract the

    /// download addresses ourselves?

    authorities: &'a Vec<Vec<SocketAddr>>,

    /// A handle to the runtime that is being used.

    rt: &'b PreferredRuntime,

}

impl<'a, 'b> DownloadManager<'a, 'b> {

    /// Creates a new [`DownloadManager`] with a set of download authorities.

    /// Performs a download to a single authority.

    ///

    /// To implement the retry algorithm from the spec, `endpoints` must be the

    /// available addresses (for all address families) for a single authority.

        // This check is important because tokio will panic otherwise.

        // Fortunately, Tokio's TcpStream::connect already offers round-robin.

        );

        // Perform the actual request.

        {

        }

    /// Downloads a [`Requestable`] from the download authorities.

    ///

    /// The relevant algorithm is non-trivial, but well-documented in the

    /// [`DownloadManager`], which is why we will leave it out here by

    /// just referencing to it.

    ///

    /// Returns the actual used authority as well as the response, or a

    /// collection of errors.

    #[allow(clippy::cognitive_complexity)]

        // Because this is a round-robin approach, we want to collect errors.

        // Use this struct to calculate delays between iterations.

        // Shuffle the list of authorities in a randomized order.

        // If we have a preferred authority, move it to the front.

            // In this case, we first throw it out and insert it to the start.

                }

                }

            }

        }

}

#[cfg(test)]

mod test {

    // @@ begin test lint list maintained by maint/add_warning @@

    #![allow(clippy::bool_assert_comparison)]

    #![allow(clippy::clone_on_copy)]

    #![allow(clippy::dbg_macro)]

    #![allow(clippy::mixed_attributes_style)]

    #![allow(clippy::print_stderr)]

    #![allow(clippy::print_stdout)]

    #![allow(clippy::single_char_pattern)]

    #![allow(clippy::unwrap_used)]

    #![allow(clippy::unchecked_time_subtraction)]

    #![allow(clippy::useless_vec)]

    #![allow(clippy::needless_pass_by_value)]

    //! <!-- @@ end test lint list maintained by maint/add_warning @@ -->

    use std::{

        io::ErrorKind,

        sync::{

            atomic::{AtomicUsize, Ordering},

            Arc, Mutex,

        },

    };

    use tokio::{

        io::{AsyncReadExt, AsyncWriteExt},

        net::TcpListener,

    };

    use tor_basic_utils::test_rng::testing_rng;

    use tor_dirclient::{request::ConsensusRequest, RequestError};

    use tor_netdoc::doc::netstatus::ConsensusFlavor;

    use super::*;

    /// Testing a request that is immediately successful.

    #[tokio::test]

    async fn request_legit() {

        let server = TcpListener::bind("[::]:0").await.unwrap();

        let server_addr = server.local_addr().unwrap();

        tokio::task::spawn(async move {

            let mut conn = server.accept().await.unwrap().0;

            let mut buf = vec![0; 1024];

            let _ = conn.read(&mut buf).await.unwrap();

            conn.write_all(b"HTTP/1.0 200 OK\r\nContent-Length: 3\r\n\r\nfoo")

                .await

                .unwrap();

        });

        let authorities = vec![vec![server_addr]];

        let rt = PreferredRuntime::current().unwrap();

        let mgr = DownloadManager::new(&authorities, &rt);

        let (preferred, resp) = mgr

            .download(

                &ConsensusRequest::new(ConsensusFlavor::Plain),

                None,

                &mut testing_rng(),

            )

            .await

            .unwrap();

        assert_eq!(resp, b"foo");

        assert_eq!(preferred, &authorities[0]);

    }

    /// Testing for a request that initially fails by returning a 404 but later succeeds.

    #[tokio::test(start_paused = true)]

    async fn request_fail_but_succeed() {

        let mut server_addrs = Vec::new();

        let requ_counter = Arc::new(AtomicUsize::new(0));

        let last = Arc::new(Mutex::new(Vec::new()));

        for _ in 0..8 {

            let server = TcpListener::bind("[::]:0").await.unwrap();

            let server_addr = server.local_addr().unwrap();

            let requ_counter = requ_counter.clone();

            let last = last.clone();

            server_addrs.push(vec![server_addr]);

            tokio::task::spawn(async move {

                loop {

                    let (mut conn, _) = server.accept().await.unwrap();

                    // Store which server_addr was active last, because only the

                    // last one will succeed.

                    *last.lock().unwrap() = vec![server_addr];

                    // This read is important!

                    // Otherwise this server will terminate the connection with

                    // RST instead of FIN, causing everything to fail.

                    let mut buf = vec![0; 1024];

                    let _ = conn.read(&mut buf).await.unwrap();

                    let cur_req = requ_counter.fetch_add(1, Ordering::AcqRel);

                    if cur_req < 7 {

                        // Send a failure.

                        conn.write_all(b"HTTP/1.0 404 Not Found\r\n\r\n")

                            .await

                            .unwrap();

                    } else {

                        // Send a success.

                        conn.write_all(b"HTTP/1.0 200 OK\r\nContent-Length: 3\r\n\r\nfoo")

                            .await

                            .unwrap();

                    }

                }

            });

        }

        let rt = PreferredRuntime::current().unwrap();

        let mgr = DownloadManager::new(&server_addrs, &rt);

        let (preferred, resp) = mgr

            .download(

                &ConsensusRequest::new(ConsensusFlavor::Plain),

                None,

                &mut testing_rng(),

            )

            .await

            .unwrap();

        assert_eq!(resp, b"foo");

        assert_eq!(*preferred, *last.lock().unwrap());

    }

    /// Request that fails all the time.

    ///

    /// Failures are done by a server accept and then immediately closing the

    /// connection.

    #[tokio::test(start_paused = true)]

    async fn request_fail_ultimately() {

        let mut server_addrs = Vec::new();

        for _ in 0..8 {

            let server = TcpListener::bind("[::]:0").await.unwrap();

            let server_addr = server.local_addr().unwrap();

            server_addrs.push(vec![server_addr]);

            tokio::task::spawn(async move {

                loop {

                    let _ = server.accept().await.unwrap();

                }

            });

        }

        let rt = PreferredRuntime::current().unwrap();

        let mgr = DownloadManager::new(&server_addrs, &rt);

        let errs = mgr

            .download(

                &ConsensusRequest::new(ConsensusFlavor::Plain),

                None,

                &mut testing_rng(),

            )

            .await

            .unwrap_err();

        // This is just a longer loop to assert all errors are either resets or truncated headers.

        //

        // Because the detection of TCP RST in itself tends to be stochastic at best,

        // we also check for TruncatedHeaders, which is what tor-dirclient will return

        // when it performs a successful(!) read(2) returning zero bytes, indicating

        // a successful closure of the connection.

        for err in errs {

            match err {

                AuthorityCommunicationError::Dirclient(e) => match *e {

                    tor_dirclient::Error::RequestFailed(e) => match e.error {

                        RequestError::IoError(e) => match e.kind() {

                            ErrorKind::ConnectionReset => {}

                            e => unreachable!("{e}"),

                        },

                        RequestError::TruncatedHeaders => {}

                        e => unreachable!("{e}"),

                    },

                    e => unreachable!("{e}"),

                },

                e => unreachable!("{e}"),

            }

        }

    }

}