ab_data_retrieval/

segment_downloading.rs

//! Fetching segments of the archived history of Subspace Network.

use crate::piece_getter::PieceGetter;
use ab_archiving::archiver::Segment;
use ab_archiving::reconstructor::{Reconstructor, ReconstructorError};
use ab_core_primitives::pieces::Piece;
use ab_core_primitives::segments::{ArchivedHistorySegment, RecordedHistorySegment, SegmentIndex};
use ab_erasure_coding::ErasureCoding;
use futures::StreamExt;
use std::time::Duration;
use tokio::task::spawn_blocking;
use tokio::time::sleep;
use tracing::debug;

/// The number of times we try to download a segment before giving up.
/// This is a suggested default, callers can supply their own value if needed.
pub const SEGMENT_DOWNLOAD_RETRIES: usize = 3;

/// The amount of time we wait between segment download retries.
/// This is a suggested default, callers can supply their own value if needed.
pub const SEGMENT_DOWNLOAD_RETRY_DELAY: Duration = Duration::from_secs(10);

/// Segment getter errors.
#[derive(Debug, thiserror::Error)]
pub enum SegmentDownloadingError {
    /// Not enough pieces
    #[error(
        "Not enough ({downloaded_pieces}/{}) pieces for segment {segment_index}",
        RecordedHistorySegment::NUM_RAW_RECORDS
    )]
    NotEnoughPieces {
        /// The segment we were trying to download
        segment_index: SegmentIndex,
        /// Number of pieces that were downloaded
        downloaded_pieces: usize,
    },

    /// Piece getter error
    #[error("Piece getter error: {source}")]
    PieceGetterError {
        #[from]
        source: anyhow::Error,
    },

    /// Segment reconstruction error
    #[error("Segment reconstruction error: {source}")]
    SegmentReconstruction {
        #[from]
        source: ReconstructorError,
    },

    /// Segment decoding error
    #[error("Segment data decoding error: {source}")]
    SegmentDecoding {
        #[from]
        source: parity_scale_codec::Error,
    },
}

/// Concurrently downloads the pieces for `segment_index`, and reconstructs the segment.
pub async fn download_segment<PG>(
    segment_index: SegmentIndex,
    piece_getter: &PG,
    erasure_coding: ErasureCoding,
    retries: usize,
    retry_delay: Option<Duration>,
) -> Result<Segment, SegmentDownloadingError>
where
    PG: PieceGetter,
{
    let reconstructor = Reconstructor::new(erasure_coding);

    let segment_pieces =
        download_segment_pieces(segment_index, piece_getter, retries, retry_delay).await?;

    let segment =
        spawn_blocking(move || reconstructor.reconstruct_segment(segment_pieces.as_slice()))
            .await
            .expect("Panic if blocking task panicked")?;

    Ok(segment)
}

/// Downloads pieces of a segment so that segment can be reconstructed afterward.
/// Repeatedly attempts to download pieces until the required number of pieces is reached.
///
/// Prefers source pieces if available, on error returns the number of available pieces.
pub async fn download_segment_pieces<PG>(
    segment_index: SegmentIndex,
    piece_getter: &PG,
    retries: usize,
    retry_delay: Option<Duration>,
) -> Result<Box<[Option<Piece>; ArchivedHistorySegment::NUM_PIECES]>, SegmentDownloadingError>
where
    PG: PieceGetter,
{
    let mut existing_pieces = Box::new([const { None }; ArchivedHistorySegment::NUM_PIECES]);

    for retry in 0..=retries {
        match download_missing_segment_pieces(segment_index, piece_getter, existing_pieces).await {
            Ok(segment_pieces) => return Ok(segment_pieces),
            Err((error, incomplete_segment_pieces)) => {
                existing_pieces = incomplete_segment_pieces;

                if retry < retries {
                    debug!(
                        %segment_index,
                        %retry,
                        ?retry_delay,
                        ?error,
                        "Failed to download segment pieces once, retrying"
                    );
                    if let Some(retry_delay) = retry_delay {
                        // Wait before retrying to give the node a chance to find other peers
                        sleep(retry_delay).await;
                    }
                }
            }
        }
    }

    debug!(
        %segment_index,
        %retries,
        "Failed to download segment pieces"
    );

    Err(SegmentDownloadingError::NotEnoughPieces {
        segment_index,
        downloaded_pieces: existing_pieces
            .iter()
            .filter(|piece| piece.is_some())
            .count(),
    })
}

/// Tries to download pieces of a segment once, so that segment can be reconstructed afterward.
/// Pieces that are already available are passed in `segment_pieces`, the rest are `None`.
///
/// Prefers source pieces if available, on error returns the incomplete piece download (including
/// pieces that were already available).
async fn download_missing_segment_pieces<PG>(
    segment_index: SegmentIndex,
    piece_getter: &PG,
    mut segment_pieces: Box<[Option<Piece>; ArchivedHistorySegment::NUM_PIECES]>,
) -> Result<
    Box<[Option<Piece>; ArchivedHistorySegment::NUM_PIECES]>,
    (
        SegmentDownloadingError,
        Box<[Option<Piece>; ArchivedHistorySegment::NUM_PIECES]>,
    ),
>
where
    PG: PieceGetter,
{
    let required_pieces_number = RecordedHistorySegment::NUM_RAW_RECORDS;
    let mut downloaded_pieces = segment_pieces
        .iter()
        .filter(|piece| piece.is_some())
        .count();

    // Debugging failure patterns in piece downloads
    let mut first_success = None;
    let mut last_success = None;
    let mut first_failure = None;
    let mut last_failure = None;

    let mut pieces_iter = segment_index.segment_piece_indexes().into_iter();

    // Download in batches until we get enough or exhaust available pieces
    while !pieces_iter.is_empty() && downloaded_pieces != required_pieces_number {
        let piece_indices = pieces_iter
            .by_ref()
            .filter(|piece_index| segment_pieces[usize::from(piece_index.position())].is_none())
            .take(required_pieces_number - downloaded_pieces)
            .collect();

        let mut received_segment_pieces = match piece_getter.get_pieces(piece_indices).await {
            Ok(pieces) => pieces,
            Err(error) => return Err((error.into(), segment_pieces)),
        };

        while let Some((piece_index, result)) = received_segment_pieces.next().await {
            match result {
                Ok(Some(piece)) => {
                    downloaded_pieces += 1;
                    segment_pieces
                        .get_mut(usize::from(piece_index.position()))
                        .expect("Piece position is by definition within segment; qed")
                        .replace(piece);

                    if first_success.is_none() {
                        first_success = Some(piece_index.position());
                    }
                    last_success = Some(piece_index.position());
                }
                // We often see an error where 127 pieces are downloaded successfully, but the
                // other 129 fail. It seems like 1 request in a 128 piece batch fails, then 128
                // single piece requests are made, and also fail.
                // Delaying requests after a failure gives the node a chance to find other peers.
                Ok(None) => {
                    debug!(%piece_index, "Piece was not found");
                    if first_failure.is_none() {
                        first_failure = Some(piece_index.position());
                    }
                    last_failure = Some(piece_index.position());
                }
                Err(error) => {
                    debug!(%error, %piece_index, "Failed to get piece");
                    if first_failure.is_none() {
                        first_failure = Some(piece_index.position());
                    }
                    last_failure = Some(piece_index.position());
                }
            }
        }
    }

    if downloaded_pieces < required_pieces_number {
        debug!(
            %segment_index,
            %downloaded_pieces,
            %required_pieces_number,
            // Piece positions that succeeded/failed
            ?first_success,
            ?last_success,
            ?first_failure,
            ?last_failure,
            "Failed to retrieve pieces for segment"
        );

        return Err((
            SegmentDownloadingError::NotEnoughPieces {
                segment_index,
                downloaded_pieces: segment_pieces
                    .iter()
                    .filter(|piece| piece.is_some())
                    .count(),
            },
            segment_pieces,
        ));
    }

    Ok(segment_pieces)
}