runsync_transfer/

recv.rs

//! Receiver pipeline.
//!
//! ```text
//!   N stream tasks ──frames──> rayon pool (open → decrypt → decompress → pwrite) ──> disk
//!    read-ahead of                    (cfg.workers threads)
//!    queue_depth each
//! ```
//!
//! Frames are self-describing, so a chunk is written the moment it decodes —
//! there is no reassembly buffer, and a 100 GB file costs the same memory as a
//! small one. A file is renamed from its `.part` sidecar into place only after
//! every chunk has landed and, when enabled, its hash has been checked.

use crate::codec::compress;
use crate::codec::crypto::{Handshake, Role, Sealer, HANDSHAKE_MSG_LEN, TAG_LEN};
use crate::config::Config;
use crate::error::{Error, Result};
use crate::io::{FileWriters, WriteHandle};
use crate::manifest;
use crate::metrics::{Metrics, Progress, ProgressFn};
use crate::pool::{BufPool, ObjPool};
use crate::resume::ResumeState;
use crate::send::merkle_root;
use crate::transport::{BoxRecv, Transport};
use crate::wire::{self, Control, EntryKind, FileEntry, FrameHeader, LocalFileIndex, ResumeEntry};
use futures_util::stream::{FuturesOrdered, StreamExt};
use parking_lot::Mutex;
use std::collections::HashMap;
use std::path::{Path, PathBuf};
use std::sync::atomic::{AtomicBool, AtomicU64, Ordering};
use std::sync::Arc;
use tokio::io::{AsyncReadExt, AsyncWriteExt};
use tokio::sync::mpsc;

/// Persist the resume bitmap every this many chunks. Each checkpoint costs one
/// fsync of the data file plus a small rename, so it trades a little throughput
/// for how much work a crash can destroy: at 1 MiB chunks this caps the loss at
/// 256 MiB per file.
const CHECKPOINT_INTERVAL: u64 = 256;

/// Also checkpoint on a timer. Without this, a file with fewer chunks than
/// `CHECKPOINT_INTERVAL` would never persist any state, and an interruption
/// would throw away everything already written — exactly the case resume
/// exists for.
const CHECKPOINT_MAX_AGE: std::time::Duration = std::time::Duration::from_secs(5);

/// How long the receiver waits for the sender to acknowledge completion by
/// ending its control stream. Bounded so a peer that simply vanishes after the
/// last chunk cannot hold the transfer open.
const FAREWELL_TIMEOUT: std::time::Duration = std::time::Duration::from_secs(20);

struct RecvFile {
    entry: FileEntry,
    dest: PathBuf,
    /// Chunks already on disk when this session started; their hashes have to
    /// be recovered by reading them back before the file can be verified.
    preexisting: crate::resume::ChunkBitmap,
    /// No partial file existed when this session began, so unwritten regions of
    /// it are known to read as zeros and an all-zero chunk needs no write.
    ///
    /// Decided once, up front. Asking the write handle would be wrong: a handle
    /// re-opened later in the session finds the file already there and would
    /// report the opposite, silently turning every hole back into gigabytes of
    /// zeros.
    fresh_part: bool,
    state: Mutex<ResumeState>,
    chunk_hashes: Mutex<Vec<[u8; 32]>>,
    expected_hash: Mutex<Option<[u8; 32]>>,
    /// The existing copy at the destination, held open for the whole transfer
    /// so blocks can be reused from it without racing anything that might
    /// replace the file underneath us.
    existing: Option<crate::io::ChunkReader>,
    /// Set once the sender has announced the file, so we know no more chunks
    /// are coming and an incomplete file is a real failure.
    announced: AtomicBool,
    finalized: AtomicBool,
    since_checkpoint: AtomicU64,
    last_checkpoint: Mutex<std::time::Instant>,
}

struct RecvShared {
    cfg: Config,
    root: PathBuf,
    files: HashMap<u32, Arc<RecvFile>>,
    writers: FileWriters,
    metrics: Metrics,
    /// Files still to finalize. Hitting zero ends the transfer.
    pending: AtomicU64,
}

/// Receive into `dest_root`.
pub async fn receive(
    transport: Arc<dyn Transport>,
    dest_root: impl AsRef<Path>,
    cfg: &Config,
    progress: Option<ProgressFn>,
) -> Result<Progress> {
    cfg.validate()?;
    let root = dest_root.as_ref().to_path_buf();
    tokio::fs::create_dir_all(&root).await?;
    // Canonicalise once so every later path check compares against a real,
    // symlink-resolved root rather than whatever the caller typed.
    let root = tokio::fs::canonicalize(&root).await.unwrap_or(root);
    let metrics = Metrics::new();

    let (mut ctl_w, mut ctl_r) = transport.accept_bi().await?;

    let hs = Handshake::new(Role::Responder, &cfg.secrecy, cfg.cipher);
    let mut peer = [0u8; HANDSHAKE_MSG_LEN];
    tokio::time::timeout(cfg.handshake_timeout, ctl_r.read_exact(&mut peer))
        .await
        .map_err(|_| Error::Handshake("timed out waiting for the peer's handshake".into()))?
        .map_err(map_eof)?;
    ctl_w.write_all(hs.message()).await?;
    ctl_w.flush().await?;
    let crypto = Arc::new(hs.finish(&peer)?);

    // --- manifest ---------------------------------------------------------
    let entries = match wire::read_control(
        &mut ctl_r,
        cfg.max_frame_bytes,
        cfg.max_manifest_entries,
    )
    .await?
    {
        Control::Manifest(e) => e,
        Control::Abort { reason } => return Err(Error::Closed(reason)),
        other => return Err(Error::protocol(format!("expected Manifest, got {other:?}"))),
    };
    manifest::validate(&entries, cfg)?;

    // Directories first so file writes never race a missing parent. Symlinks
    // are deliberately deferred to the very end — see `create_symlinks`.
    let mut symlinks = Vec::new();
    let mut files = HashMap::new();
    let mut resume_reply = Vec::new();
    let mut local_index: Vec<LocalFileIndex> = Vec::new();
    // Hashes remembered from previous runs, so an unchanged file is a `stat`
    // rather than a full read.
    let mut cache = (cfg.delta && cfg.trust_mtime).then(|| crate::index::ChunkIndex::load(&root));
    #[allow(unused_mut)]
    let mut seen: std::collections::HashSet<String> = std::collections::HashSet::new();
    let mut total_bytes = 0u64;
    let mut file_count = 0u64;

    for entry in &entries {
        match entry.kind {
            EntryKind::Directory => {
                let dest = manifest::safe_join(&root, &entry.path)?;
                tokio::fs::create_dir_all(&dest).await?;
            }
            EntryKind::Symlink => {
                let (rel, target) = manifest::split_symlink(&entry.path)?;
                let dest = manifest::safe_join(&root, rel)?;
                symlinks.push((dest, target.to_string()));
            }
            EntryKind::File => {
                let dest = manifest::safe_join(&root, &entry.path)?;
                if let Some(parent) = dest.parent() {
                    tokio::fs::create_dir_all(parent).await?;
                }
                total_bytes += entry.size;
                file_count += 1;

                let part = crate::io::part_path_for(&dest);
                let fresh_part = !part.exists();
                // Resume state without the partial file it describes is stale;
                // trusting it would claim chunks that no longer exist anywhere.
                if fresh_part {
                    ResumeState::load_or_new(&dest, entry.size, entry.chunk_size).clear();
                }

                // With resume off, drop any state left by an earlier run before
                // loading, so the transfer genuinely starts from nothing.
                if !cfg.resume {
                    ResumeState::load_or_new(&dest, entry.size, entry.chunk_size).clear();
                }
                let state = ResumeState::load_or_new(&dest, entry.size, entry.chunk_size);
                // Hash whatever is already at this path, so the sender can
                // recognise the parts that have not changed. Opened once and
                // kept, so the blocks reused later come from the same bytes
                // that were hashed here.
                let (existing, local_hashes) = if cfg.delta && dest.exists() {
                    index_existing(
                        &dest,
                        &entry.path,
                        entry.chunk_size,
                        cfg.chunk_hash_budget,
                        cache.as_mut(),
                    )
                } else {
                    (None, Vec::new())
                };
                if !local_hashes.is_empty() {
                    local_index.push(LocalFileIndex {
                        file_id: entry.file_id,
                        hashes: local_hashes,
                    });
                }

                let preexisting = state.bitmap().clone();
                if preexisting.count() > 0 {
                    resume_reply.push(ResumeEntry {
                        file_id: entry.file_id,
                        have: preexisting.as_bytes().to_vec(),
                    });
                    metrics.chunk_skipped(0);
                }

                let n = entry.chunk_count() as usize;
                files.insert(
                    entry.file_id,
                    Arc::new(RecvFile {
                        entry: entry.clone(),
                        dest,
                        preexisting,
                        existing,
                        fresh_part,
                        state: Mutex::new(state),
                        chunk_hashes: Mutex::new(vec![[0u8; 32]; n]),
                        expected_hash: Mutex::new(None),
                        announced: AtomicBool::new(false),
                        finalized: AtomicBool::new(false),
                        since_checkpoint: AtomicU64::new(0),
                        last_checkpoint: Mutex::new(std::time::Instant::now()),
                    }),
                );
            }
        }
    }

    metrics.set_totals(file_count, total_bytes);
    let shared = Arc::new(RecvShared {
        cfg: cfg.clone(),
        root: root.clone(),
        files,
        writers: FileWriters::new(),
        metrics: metrics.clone(),
        pending: AtomicU64::new(file_count),
    });

    if !local_index.is_empty() {
        let reusable: usize = local_index.iter().map(|e| e.hashes.len()).sum();
        tracing::info!(
            files = local_index.len(),
            chunks = reusable,
            "offering existing blocks for reuse"
        );
        // Split across messages so one enormous index cannot exceed the frame
        // limit the sender will enforce on it.
        for batch in split_index(local_index, cfg.max_frame_bytes) {
            wire::write_control(&mut ctl_w, &Control::LocalIndex(batch)).await?;
        }
    }
    wire::write_control(&mut ctl_w, &Control::ResumeState(resume_reply)).await?;

    // Zero-length files have no chunks, so nothing would ever finalize them.
    for f in shared.files.values() {
        if f.entry.size == 0 {
            let h = WriteHandle::open(&f.dest, 0, false)?;
            h.commit(f.entry.mode, f.entry.mtime, cfg.preserve_metadata)?;
            f.finalized.store(true, Ordering::Release);
            shared.pending.fetch_sub(1, Ordering::AcqRel);
            metrics.file_done();
        }
    }

    // --- start ------------------------------------------------------------
    let stream_count = match wire::read_control(
        &mut ctl_r,
        cfg.max_frame_bytes,
        cfg.max_manifest_entries,
    )
    .await?
    {
        Control::Start { streams } => streams as usize,
        Control::Abort { reason } => return Err(Error::Closed(reason)),
        other => return Err(Error::protocol(format!("expected Start, got {other:?}"))),
    };
    if stream_count == 0 || stream_count > 1024 {
        return Err(Error::protocol(format!(
            "sender asked for {stream_count} data streams, which is outside the accepted range"
        )));
    }

    let cpu = Arc::new(
        rayon::ThreadPoolBuilder::new()
            .num_threads(cfg.workers)
            .thread_name(|i| format!("rst-decode-{i}"))
            .build()
            .map_err(|e| Error::Worker(e.to_string()))?,
    );
    let pool = BufPool::new(
        stream_count * cfg.queue_depth * 2 + cfg.workers * 2,
        cfg.chunk_size + cfg.chunk_size / 8,
    );
    let openers: Arc<ObjPool<Sealer>> = ObjPool::new(cfg.workers + stream_count);
    let progress_task = progress.map(|f| {
        let m = metrics.clone();
        tokio::spawn(async move {
            let mut tick = tokio::time::interval(std::time::Duration::from_millis(500));
            tick.set_missed_tick_behavior(tokio::time::MissedTickBehavior::Skip);
            loop {
                tick.tick().await;
                f(m.snapshot());
            }
        })
    });

    // Control reader runs alongside the data streams: `FileComplete` for a file
    // may arrive before, during, or after its chunks, since the control stream
    // is ordered only against itself.
    let (done_tx, mut done_rx) = mpsc::unbounded_channel::<Result<()>>();
    let ctl_task = {
        let shared = shared.clone();
        let done_tx = done_tx.clone();
        tokio::spawn(async move {
            let r = run_control(&mut ctl_r, shared).await;
            let _ = done_tx.send(r);
            ctl_r
        })
    };

    let mut stream_tasks = Vec::with_capacity(stream_count);
    for _ in 0..stream_count {
        let src = transport.accept_uni().await?;
        stream_tasks.push(tokio::spawn(run_stream(
            src,
            shared.clone(),
            openers.clone(),
            crypto.clone(),
            cpu.clone(),
            pool.clone(),
        )));
    }
    drop(done_tx);

    let mut first_err = None;
    for t in stream_tasks {
        match t.await {
            Ok(Ok(())) => {}
            Ok(Err(e)) => {
                tracing::error!(error = %e, "data stream failed");
                first_err.get_or_insert(e);
            }
            Err(e) => {
                first_err.get_or_insert(Error::Worker(e.to_string()));
            }
        }
    }

    // Control may still be delivering trailing FileComplete messages.
    let mut ctl_r = ctl_task.await.map_err(|e| Error::Worker(e.to_string()))?;
    while let Some(r) = done_rx.recv().await {
        if let Err(e) = r {
            first_err.get_or_insert(e);
        }
    }

    if let Some(e) = first_err {
        // Persist what did land before giving up, so the next attempt resumes
        // instead of starting over. The `.part` files themselves are left in
        // place; only a caller asking for a clean start should remove them.
        checkpoint_all(&shared);
        return Err(e);
    }

    let left = shared.pending.load(Ordering::Acquire);
    if left != 0 {
        // The sender stopped early. Same reasoning as the error path above:
        // keep what arrived so the next attempt can resume from it.
        checkpoint_all(&shared);
        let names: Vec<_> = shared
            .files
            .values()
            .filter(|f| !f.finalized.load(Ordering::Acquire))
            .take(5)
            .map(|f| f.entry.path.clone())
            .collect();
        return Err(Error::Protocol(format!(
            "sender finished with {left} files incomplete (e.g. {names:?})"
        )));
    }

    // Remember what was just written, so the next sync of this tree costs a
    // `stat` per file rather than a full read.
    if let Some(mut cache) = cache.take() {
        for f in shared.files.values() {
            if !f.finalized.load(Ordering::Acquire) {
                continue;
            }
            seen.insert(f.entry.path.clone());
            if let Ok(meta) = std::fs::metadata(&f.dest) {
                let hashes = f.chunk_hashes.lock().clone();
                if !hashes.is_empty() && hashes.iter().any(|h| *h != [0u8; 32]) {
                    cache.insert(
                        &f.entry.path,
                        meta.len(),
                        crate::index::mtime_of(&meta),
                        f.entry.chunk_size,
                        hashes,
                    );
                }
            }
        }
        cache.retain(&seen);
        if let Err(e) = cache.save() {
            tracing::warn!(error = %e, "could not persist the chunk index");
        }
    }

    create_symlinks(&root, &symlinks).await;
    apply_directory_metadata(&root, &entries, cfg).await;

    // Everything is committed by this point. Tell the sender, then wait for it
    // to end its control stream, which is its acknowledgement that it read us.
    //
    // Returning straight after the write would be a race the caller loses: it
    // typically closes the connection as soon as this function returns, and on
    // a slow sender that discards the confirmation the sender is still waiting
    // for. The wait is bounded so a vanished peer cannot hang the transfer.
    wire::write_control(&mut ctl_w, &Control::AllComplete).await?;
    match tokio::time::timeout(FAREWELL_TIMEOUT, drain_to_eof(&mut ctl_r)).await {
        Ok(_) => {}
        Err(_) => tracing::debug!("sender did not close its control stream in time"),
    }
    let _ = ctl_w.shutdown().await;

    if let Some(p) = progress_task {
        p.abort();
    }
    let snapshot = metrics.snapshot();
    tracing::info!(summary = %snapshot, "receive complete");
    Ok(snapshot)
}

async fn run_control(ctl_r: &mut BoxRecv, shared: Arc<RecvShared>) -> Result<()> {
    loop {
        let msg = match wire::read_control(
            ctl_r,
            shared.cfg.max_frame_bytes,
            shared.cfg.max_manifest_entries,
        )
        .await
        {
            Ok(m) => m,
            // A clean EOF here means the sender finished the control stream.
            Err(Error::Closed(_)) => return Ok(()),
            Err(e) => return Err(e),
        };
        match msg {
            Control::FileComplete { file_id, hash } => {
                if let Some(f) = shared.files.get(&file_id) {
                    *f.expected_hash.lock() = hash;
                    f.announced.store(true, Ordering::Release);
                    // The last chunk may already have landed, in which case
                    // this message is what unblocks verification.
                    finalize_if_ready(&shared, f)?;
                }
            }
            Control::AllComplete => return Ok(()),
            Control::Abort { reason } => return Err(Error::Closed(reason)),
            other => {
                tracing::debug!(?other, "ignoring unexpected control message");
            }
        }
    }
}

/// Read frames off one stream, decoding `queue_depth` of them concurrently.
async fn run_stream(
    mut src: BoxRecv,
    shared: Arc<RecvShared>,
    openers: Arc<ObjPool<Sealer>>,
    crypto: Arc<crate::codec::crypto::SessionCrypto>,
    cpu: Arc<rayon::ThreadPool>,
    pool: Arc<BufPool>,
) -> Result<()> {
    let mut inflight: FuturesOrdered<tokio::sync::oneshot::Receiver<Result<()>>> =
        FuturesOrdered::new();
    let max_frame = shared.cfg.max_frame_bytes;

    loop {
        // Drain finished work before reading more, so a slow disk applies
        // backpressure to the network instead of growing the queue.
        while inflight.len() >= shared.cfg.queue_depth {
            if let Some(res) = inflight.next().await {
                res.map_err(|_| Error::Worker("decode worker vanished".into()))??;
            }
        }

        let mut head = [0u8; wire::FRAME_HEADER_LEN];
        match src.read_exact(&mut head).await {
            Ok(_) => {}
            Err(e) if e.kind() == std::io::ErrorKind::UnexpectedEof => break,
            Err(e) => return Err(Error::Io(e)),
        }
        let header = FrameHeader::decode(&head, max_frame)?;

        let body_len = header.wire_payload_len();
        if body_len > max_frame {
            return Err(Error::FrameTooLarge {
                got: body_len,
                limit: max_frame,
            });
        }
        let mut body = pool.take();
        body.resize(body_len, 0);
        src.read_exact(&mut body).await.map_err(map_eof)?;

        let (tx, rx) = tokio::sync::oneshot::channel();
        let shared2 = shared.clone();
        let openers2 = openers.clone();
        let crypto2 = crypto.clone();
        let pool2 = pool.clone();
        cpu.spawn(move || {
            // Borrowed per chunk, so the disk write below never blocks another
            // worker's decrypt.
            let mut opener = openers2.take_or(|| crypto2.opener());
            let r = decode_and_write(header, head, body, &shared2, &mut opener, &pool2);
            openers2.put(opener);
            let _ = tx.send(r);
        });
        inflight.push_back(rx);
    }

    while let Some(res) = inflight.next().await {
        res.map_err(|_| Error::Worker("decode worker vanished".into()))??;
    }
    Ok(())
}

/// Decrypt, decompress, and write one chunk. Runs on a rayon worker.
fn decode_and_write(
    header: FrameHeader,
    head: [u8; wire::FRAME_HEADER_LEN],
    mut body: Vec<u8>,
    shared: &RecvShared,
    opener: &mut Sealer,
    pool: &BufPool,
) -> Result<()> {
    let file = shared.files.get(&header.file_id).ok_or_else(|| {
        Error::protocol(format!(
            "frame references file_id {} which is not in the manifest",
            header.file_id
        ))
    })?;

    let total_chunks = file.entry.chunk_count();
    if header.chunk_index >= total_chunks {
        return Err(Error::protocol(format!(
            "chunk {} is past the end of file {} ({} chunks)",
            header.chunk_index, header.file_id, total_chunks
        )));
    }

    let chunk_size_u64 = file.entry.chunk_size as u64;
    let offset = header.chunk_index * chunk_size_u64;
    let expect_len = chunk_size_u64.min(file.entry.size - offset) as usize;

    // The sender recognised this chunk as one we already hold. Copy it from the
    // old file rather than receiving it.
    if header.reuses_local() {
        if header.payload_len != 0 || !body.is_empty() {
            return Err(Error::protocol("reuse frame carries a payload"));
        }
        if header.raw_len as usize != expect_len {
            return Err(Error::protocol(format!(
                "reuse chunk {} of file {} declares {} bytes, expected {expect_len}",
                header.chunk_index, header.file_id, header.raw_len
            )));
        }
        pool.put(body);
        let Some(existing) = file.existing.as_ref() else {
            return Err(Error::protocol(
                "peer asked us to reuse a block from a file we never offered",
            ));
        };

        let mut buf = pool.take();
        buf.resize(expect_len, 0);
        let n = existing.read_at(offset, &mut buf)?;
        if n != expect_len {
            return Err(Error::Io(std::io::Error::new(
                std::io::ErrorKind::UnexpectedEof,
                format!("existing copy is short at chunk {}", header.chunk_index),
            )));
        }

        let handle = shared.writers.get_or_open(header.file_id, || {
            WriteHandle::open_cloned(&file.dest, file.entry.size, shared.cfg.preallocate)
        })?;
        // A cloned partial already holds these bytes; writing them again would
        // break the sharing the clone just bought.
        if !handle.matches_at(offset, &buf)? {
            handle.write_at(offset, &buf)?;
        }

        if shared.cfg.verify_hashes {
            let h = *blake3::hash(&buf).as_bytes();
            let mut hashes = file.chunk_hashes.lock();
            if let Some(slot) = hashes.get_mut(header.chunk_index as usize) {
                *slot = h;
            }
        }
        pool.put(buf);
        return record_chunk(
            shared,
            file,
            &handle,
            header,
            ChunkOutcome {
                raw_len: expect_len as u64,
                wire_len: wire::FRAME_HEADER_LEN as u64,
                compressed: false,
                was_hole: false,
                was_reuse: true,
            },
        );
    }

    // A hole. Nothing was sent, so there is nothing to open or decompress.
    if header.is_zero() {
        if header.payload_len != 0 || !body.is_empty() {
            return Err(Error::protocol("zero-chunk frame carries a payload"));
        }
        if header.raw_len as usize != expect_len {
            return Err(Error::protocol(format!(
                "zero chunk {} of file {} declares {} bytes, expected {expect_len}",
                header.chunk_index, header.file_id, header.raw_len
            )));
        }
        pool.put(body);
        let handle = shared.writers.get_or_open(header.file_id, || {
            WriteHandle::open(&file.dest, file.entry.size, shared.cfg.preallocate)
        })?;
        // Skipping the write is only sound when this session created the file,
        // so the region is known to be zeros. A `.part` left by an earlier run
        // may hold a torn partial write at this offset, so zero it explicitly.
        if !file.fresh_part {
            handle.write_zeros_at(offset, expect_len)?;
        }
        if shared.cfg.verify_hashes {
            let zeros = vec![0u8; expect_len];
            let h = *blake3::hash(&zeros).as_bytes();
            let mut hashes = file.chunk_hashes.lock();
            if let Some(slot) = hashes.get_mut(header.chunk_index as usize) {
                *slot = h;
            }
        }
        return record_chunk(
            shared,
            file,
            &handle,
            header,
            ChunkOutcome {
                raw_len: expect_len as u64,
                wire_len: wire::FRAME_HEADER_LEN as u64,
                compressed: false,
                was_hole: true,
                was_reuse: false,
            },
        );
    }

    // Verify before touching the plaintext, so a forged frame never reaches the
    // decompressor.
    if header.sealed() {
        if body.len() < TAG_LEN {
            return Err(Error::protocol("sealed frame is shorter than its tag"));
        }
        let split = body.len() - TAG_LEN;
        let mut tag = [0u8; TAG_LEN];
        tag.copy_from_slice(&body[split..]);
        body.truncate(split);
        opener.open(
            header.file_id,
            header.chunk_index,
            header.epoch,
            &head,
            &mut body,
            &tag,
        )?;
    } else if !opener.is_passthrough() {
        // We negotiated encryption; an unsealed frame is a downgrade attempt.
        return Err(Error::protocol(
            "peer sent an unsealed frame on an encrypted session",
        ));
    }

    // The declared plaintext length must match what this chunk's position
    // implies, or a peer could shift file contents around.
    if header.raw_len as usize != expect_len {
        return Err(Error::protocol(format!(
            "chunk {} of file {} declares {} plaintext bytes, expected {expect_len}",
            header.chunk_index, header.file_id, header.raw_len
        )));
    }

    let mut plain = pool.take();
    compress::decompress_into(header.algorithm, header.raw_len as usize, &body, &mut plain)?;
    let wire_len = (wire::FRAME_HEADER_LEN + body.len()) as u64;
    pool.put(body);

    let handle = shared.writers.get_or_open(header.file_id, || {
        WriteHandle::open(&file.dest, file.entry.size, shared.cfg.preallocate)
    })?;

    handle.write_at(offset, &plain)?;

    if shared.cfg.verify_hashes {
        let h = *blake3::hash(&plain).as_bytes();
        let mut hashes = file.chunk_hashes.lock();
        if let Some(slot) = hashes.get_mut(header.chunk_index as usize) {
            *slot = h;
        }
    }
    let raw_len = plain.len() as u64;
    pool.put(plain);

    record_chunk(
        shared,
        file,
        &handle,
        header,
        ChunkOutcome {
            raw_len,
            wire_len,
            compressed: header.algorithm != compress::Algorithm::None,
            was_hole: false,
            was_reuse: false,
        },
    )
}

/// What one decoded chunk contributed, for the metrics and checkpoint pass.
struct ChunkOutcome {
    raw_len: u64,
    wire_len: u64,
    compressed: bool,
    /// Arrived as a zero-chunk flag rather than as data.
    was_hole: bool,
    /// Copied from the receiver's own older copy rather than received.
    was_reuse: bool,
}

/// Mark a chunk present, checkpoint on schedule, and finalize if that was the
/// last one. Shared by the ordinary and the zero-chunk paths.
fn record_chunk(
    shared: &RecvShared,
    file: &Arc<RecvFile>,
    handle: &WriteHandle,
    header: FrameHeader,
    outcome: ChunkOutcome,
) -> Result<()> {
    let ChunkOutcome {
        raw_len,
        wire_len,
        compressed,
        was_hole,
        was_reuse,
    } = outcome;
    let complete = {
        let mut st = file.state.lock();
        st.record(header.chunk_index);
        // Checkpoint on an interval rather than every chunk: an fsync per chunk
        // would cost more than the transfer itself. Whichever of the two
        // triggers fires first wins, so both a huge file and a small one bound
        // how much an interruption can cost.
        if shared.cfg.resume {
            let by_count =
                file.since_checkpoint.fetch_add(1, Ordering::AcqRel) + 1 >= CHECKPOINT_INTERVAL;
            let by_age = file.last_checkpoint.lock().elapsed() >= CHECKPOINT_MAX_AGE;
            if by_count || by_age {
                file.since_checkpoint.store(0, Ordering::Release);
                *file.last_checkpoint.lock() = std::time::Instant::now();
                st.checkpoint(handle)?;
            }
        }
        st.is_complete()
    };

    if was_reuse {
        shared.metrics.chunk_reused(raw_len, wire_len);
    } else if was_hole {
        shared.metrics.chunk_zero(raw_len, wire_len);
    } else {
        shared.metrics.chunk_done(raw_len, wire_len, compressed);
    }

    if complete {
        finalize_if_ready(shared, file)?;
    }
    Ok(())
}

/// Commit a file once every chunk is present and the sender has announced it.
///
/// Both the last chunk and the `FileComplete` message call in here, in whatever
/// order they happen to arrive; the `finalized` flag makes the commit happen
/// exactly once.
fn finalize_if_ready(shared: &RecvShared, file: &Arc<RecvFile>) -> Result<()> {
    if file.finalized.load(Ordering::Acquire) {
        return Ok(());
    }
    if !file.state.lock().is_complete() {
        return Ok(());
    }
    if shared.cfg.verify_hashes && !file.announced.load(Ordering::Acquire) {
        // The expected root has not arrived yet.
        return Ok(());
    }
    // Claim the commit; whoever loses here simply returns.
    if file.finalized.swap(true, Ordering::AcqRel) {
        return Ok(());
    }

    let Some(handle) = shared.writers.take(file.entry.file_id) else {
        // No chunk ever opened the file: everything was resumed from disk.
        let h = WriteHandle::open(&file.dest, file.entry.size, false)?;
        return commit(shared, file, Arc::new(h));
    };
    commit(shared, file, handle)
}

fn commit(shared: &RecvShared, file: &Arc<RecvFile>, handle: Arc<WriteHandle>) -> Result<()> {
    if shared.cfg.verify_hashes {
        if let Some(expected) = *file.expected_hash.lock() {
            // Chunks carried over from an interrupted run were never hashed in
            // this session; read them back so the check covers the whole file.
            fill_resumed_hashes(file, &handle)?;
            let actual = merkle_root(&file.chunk_hashes.lock());
            if actual != expected {
                return Err(Error::Integrity {
                    path: file.entry.path.clone(),
                    expected: hex(&expected),
                    actual: hex(&actual),
                });
            }
        }
    }

    {
        let mut st = file.state.lock();
        st.checkpoint(&handle)?;
    }

    handle.commit(
        file.entry.mode,
        file.entry.mtime,
        shared.cfg.preserve_metadata,
    )?;
    file.state.lock().clear();

    shared.pending.fetch_sub(1, Ordering::AcqRel);
    shared.metrics.file_done();
    tracing::debug!(path = %file.entry.path, "file committed");
    let _ = &shared.root;
    Ok(())
}

/// Hash the chunks that were already on disk when this session started.
fn fill_resumed_hashes(file: &Arc<RecvFile>, handle: &WriteHandle) -> Result<()> {
    if file.preexisting.count() == 0 {
        return Ok(());
    }
    let chunk_size = file.entry.chunk_size as u64;
    let mut buf = vec![0u8; chunk_size as usize];
    let mut hashes = file.chunk_hashes.lock();
    for i in 0..file.entry.chunk_count() {
        if !file.preexisting.get(i) {
            continue;
        }
        let offset = i * chunk_size;
        let len = chunk_size.min(file.entry.size - offset) as usize;
        let n = handle.read_at(offset, &mut buf[..len])?;
        if n != len {
            return Err(Error::Io(std::io::Error::new(
                std::io::ErrorKind::UnexpectedEof,
                format!("partial file is short at chunk {i}"),
            )));
        }
        if let Some(slot) = hashes.get_mut(i as usize) {
            *slot = *blake3::hash(&buf[..len]).as_bytes();
        }
    }
    Ok(())
}

/// Create symlinks last, and only when they point inside the destination.
///
/// Ordering matters: if a symlink `a -> /etc` existed while files were being
/// written, a manifest entry for `a/passwd` would resolve through it and write
/// outside the root. Creating links only after every regular file is committed
/// removes that entirely, and the target check stops the link itself from being
/// a usable escape afterwards.
async fn create_symlinks(root: &Path, links: &[(PathBuf, String)]) {
    for (dest, target) in links {
        if !symlink_target_is_contained(root, dest, target) {
            tracing::warn!(
                link = %dest.display(),
                target = %target,
                "skipping symlink whose target escapes the destination root"
            );
            continue;
        }
        if let Some(parent) = dest.parent() {
            let _ = tokio::fs::create_dir_all(parent).await;
        }
        let _ = tokio::fs::remove_file(dest).await;
        #[cfg(unix)]
        if let Err(e) = tokio::fs::symlink(target, dest).await {
            tracing::warn!(link = %dest.display(), error = %e, "could not create symlink");
        }
        #[cfg(not(unix))]
        {
            let _ = (dest, target);
            tracing::warn!("symlinks are not created on this platform");
        }
    }
}

/// Lexically resolve `target` relative to the link's parent and check it lands
/// under `root`. Purely textual on purpose: the target need not exist yet, and
/// a filesystem check would be a race.
fn symlink_target_is_contained(root: &Path, link: &Path, target: &str) -> bool {
    if target.is_empty() {
        return false;
    }
    let t = Path::new(target);
    if t.is_absolute() {
        return false;
    }
    let Some(parent) = link.parent() else {
        return false;
    };
    let mut resolved = parent.to_path_buf();
    for comp in t.components() {
        match comp {
            std::path::Component::Normal(c) => resolved.push(c),
            std::path::Component::CurDir => {}
            std::path::Component::ParentDir => {
                if !resolved.pop() {
                    return false;
                }
            }
            _ => return false,
        }
    }
    resolved.starts_with(root)
}

async fn apply_directory_metadata(root: &Path, entries: &[FileEntry], cfg: &Config) {
    if !cfg.preserve_metadata {
        return;
    }
    // Deepest first, so setting a read-only parent cannot block its children.
    let mut dirs: Vec<_> = entries
        .iter()
        .filter(|e| e.kind == EntryKind::Directory)
        .collect();
    dirs.sort_by_key(|e| std::cmp::Reverse(e.path.matches('/').count()));
    for e in dirs {
        let Ok(dest) = manifest::safe_join(root, &e.path) else {
            continue;
        };
        #[cfg(unix)]
        if e.mode != 0 {
            use std::os::unix::fs::PermissionsExt;
            let _ =
                tokio::fs::set_permissions(&dest, std::fs::Permissions::from_mode(e.mode & 0o7777))
                    .await;
        }
        let _ = &dest;
    }
}

/// Hash an existing file chunk by chunk, so its blocks can be offered for reuse.
///
/// Returns the open reader alongside the hashes: the blocks handed out later
/// must come from the same bytes that were hashed, and reopening by path would
/// not guarantee that.
fn index_existing(
    dest: &Path,
    rel: &str,
    chunk_size: u32,
    budget: usize,
    cache: Option<&mut crate::index::ChunkIndex>,
) -> (Option<crate::io::ChunkReader>, Vec<[u8; 32]>) {
    if chunk_size == 0 {
        return (None, Vec::new());
    }
    let Ok(reader) = crate::io::ChunkReader::open(dest) else {
        return (None, Vec::new());
    };
    let len = reader.len();

    // If the file looks untouched since it was last hashed, take the cached
    // hashes and read nothing.
    let meta = std::fs::metadata(dest).ok();
    let mtime = meta.as_ref().map(crate::index::mtime_of).unwrap_or(0);
    if let Some(cache) = cache {
        if let Some(h) = cache.get(rel, len, mtime, chunk_size) {
            return (Some(reader), h.to_vec());
        }
        let hashes = hash_whole_file(&reader, chunk_size, budget);
        if !hashes.is_empty() {
            cache.insert(rel, len, mtime, chunk_size, hashes.clone());
        }
        return (Some(reader), hashes);
    }
    let hashes = hash_whole_file(&reader, chunk_size, budget);
    (Some(reader), hashes)
}

fn hash_whole_file(
    reader: &crate::io::ChunkReader,
    chunk_size: u32,
    budget: usize,
) -> Vec<[u8; 32]> {
    let len = reader.len();
    let chunks = len.div_ceil(chunk_size as u64);
    // Each hash costs 32 bytes on the wire; refuse to build an index so large
    // that announcing it would cost more than it can possibly save.
    if chunks == 0 || chunks as usize * 32 > budget {
        return Vec::new();
    }

    let mut buf = vec![0u8; chunk_size as usize];
    let mut hashes = Vec::with_capacity(chunks as usize);
    for i in 0..chunks {
        let offset = i * chunk_size as u64;
        let want = (chunk_size as u64).min(len - offset) as usize;
        match reader.read_at(offset, &mut buf[..want]) {
            Ok(n) if n == want => hashes.push(*blake3::hash(&buf[..want]).as_bytes()),
            _ => return Vec::new(),
        }
    }
    hashes
}

/// Break a local index into messages that each fit under the frame limit.
fn split_index(mut entries: Vec<LocalFileIndex>, max_frame: usize) -> Vec<Vec<LocalFileIndex>> {
    let cap = (max_frame / 2).max(1 << 20);
    let mut out = Vec::new();
    let mut batch = Vec::new();
    let mut size = 0usize;
    for e in entries.drain(..) {
        let cost = 8 + e.hashes.len() * 32;
        if size + cost > cap && !batch.is_empty() {
            out.push(std::mem::take(&mut batch));
            size = 0;
        }
        size += cost;
        batch.push(e);
    }
    if !batch.is_empty() {
        out.push(batch);
    }
    out
}

/// Read until the peer ends the stream, discarding anything still queued.
async fn drain_to_eof(r: &mut BoxRecv) {
    let mut scratch = [0u8; 256];
    loop {
        match r.read(&mut scratch).await {
            Ok(0) | Err(_) => return,
            Ok(_) => {}
        }
    }
}

/// Flush every in-progress file's resume state. Best effort: this runs on the
/// way out of a failing transfer, where a second error has nothing left to
/// abort.
fn checkpoint_all(shared: &RecvShared) {
    for file in shared.files.values() {
        if file.finalized.load(Ordering::Acquire) {
            continue;
        }
        let Some(handle) = shared.writers.take(file.entry.file_id) else {
            continue;
        };
        if let Err(e) = file.state.lock().checkpoint(&handle) {
            tracing::warn!(path = %file.entry.path, error = %e, "could not persist resume state");
        }
    }
}

fn hex(b: &[u8]) -> String {
    b.iter().map(|x| format!("{x:02x}")).collect()
}

fn map_eof(e: std::io::Error) -> Error {
    if e.kind() == std::io::ErrorKind::UnexpectedEof {
        Error::Closed("stream ended mid-frame".into())
    } else {
        Error::Io(e)
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn symlink_containment() {
        let root = Path::new("/dest");
        let link = Path::new("/dest/sub/link");
        assert!(symlink_target_is_contained(root, link, "sibling"));
        assert!(symlink_target_is_contained(root, link, "./a/b"));
        assert!(symlink_target_is_contained(root, link, "../other"));

        // Escapes.
        assert!(!symlink_target_is_contained(root, link, "/etc"));
        assert!(!symlink_target_is_contained(root, link, "../../etc"));
        assert!(!symlink_target_is_contained(root, link, "../../../"));
        assert!(!symlink_target_is_contained(root, link, ""));
    }
}