Merge pull request #37 from wwwehr/feature/hlfs-evm-blocks

Feature: Archive Node File Sharing

Cargo.lock

@@ -6659,6 +6659,7 @@ dependencies = [
  "clap",
  "eyre",
  "futures",
+ "futures-util",
  "jsonrpsee",
  "jsonrpsee-core",
  "lz4_flex",
@@ -6687,6 +6688,7 @@ dependencies = [
  "reth-execution-types",
  "reth-exex",
  "reth-fs-util",
+ "reth-hlfs",
  "reth-hyperliquid-types",
  "reth-network",
  "reth-network-api",
@@ -7966,6 +7968,20 @@ dependencies = [
  "thiserror 2.0.11",
 ]
 
+[[package]]
+name = "reth-hlfs"
+version = "1.2.0"
+dependencies = [
+ "bytes",
+ "parking_lot",
+ "rand 0.8.5",
+ "reth-tracing",
+ "tempfile",
+ "thiserror 2.0.11",
+ "tokio",
+ "tracing",
+]
+
 [[package]]
 name = "reth-hyperliquid-types"
 version = "1.2.0"

Cargo.toml

@@ -54,6 +54,7 @@ members = [
     "crates/net/ecies/",
     "crates/net/eth-wire-types",
     "crates/net/eth-wire/",
+    "crates/net/hlfs/",
     "crates/net/nat/",
     "crates/net/network-api/",
     "crates/net/network-types/",
@@ -356,6 +357,7 @@ reth-exex = { path = "crates/exex/exex" }
 reth-exex-test-utils = { path = "crates/exex/test-utils" }
 reth-exex-types = { path = "crates/exex/types" }
 reth-fs-util = { path = "crates/fs-util" }
+reth-hlfs = { path = "crates/net/hlfs" }
 reth-invalid-block-hooks = { path = "crates/engine/invalid-block-hooks" }
 reth-ipc = { path = "crates/rpc/ipc" }
 reth-libmdbx = { path = "crates/storage/libmdbx-rs" }

bin/reth/Cargo.toml

@@ -35,6 +35,7 @@ reth-cli-runner.workspace = true
 reth-cli-commands.workspace = true
 reth-cli-util.workspace = true
 reth-consensus-common.workspace = true
+reth-hlfs.workspace = true
 reth-rpc-builder.workspace = true
 reth-rpc.workspace = true
 reth-rpc-types-compat.workspace = true
@@ -81,8 +82,9 @@ tracing.workspace = true
 serde_json.workspace = true
 
 # async
-tokio = { workspace = true, features = ["sync", "macros", "time", "rt-multi-thread"] }
+tokio = { workspace = true, features = ["sync", "macros", "time", "rt-multi-thread", "net", "fs"] }
 futures.workspace = true
+futures-util.workspace = true
 
 # time
 time = { workspace = true }

bin/reth/src/block_ingest.rs

@@ -29,6 +29,7 @@ use tokio::sync::Mutex;
 use tracing::{debug, info};
 
 use crate::serialized::{BlockAndReceipts, EvmBlock};
+use crate::share_blocks::ShareBlocks;
 use crate::spot_meta::erc20_contract_to_spot_token;
 
 /// Poll interval when tailing an *open* hourly file.
@@ -41,6 +42,7 @@ pub(crate) struct BlockIngest {
     pub local_ingest_dir: Option<PathBuf>,
     pub local_blocks_cache: Arc<Mutex<BTreeMap<u64, BlockAndReceipts>>>, // height → block
     pub precompiles_cache: PrecompilesCache,
+    pub hlfs: Option<ShareBlocks>,
 }
 
 #[derive(Deserialize)]
@@ -155,9 +157,9 @@ impl BlockIngest {
         if let Some(block) = self.try_collect_local_block(height).await {
             info!("Returning locally synced block for @ Height [{height}]");
             return Some(block);
-        } else {
-            self.try_collect_s3_block(height)
         }
+
+        self.try_collect_s3_block(height)
     }
 
     pub(crate) fn try_collect_s3_block(&self, height: u64) -> Option<BlockAndReceipts> {

bin/reth/src/main.rs

@@ -6,6 +6,7 @@ static ALLOC: reth_cli_util::allocator::Allocator = reth_cli_util::allocator::ne
 mod block_ingest;
 mod call_forwarder;
 mod serialized;
+mod share_blocks;
 mod spot_meta;
 mod tx_forwarder;
 
@@ -18,6 +19,7 @@ use reth::cli::Cli;
 use reth_ethereum_cli::chainspec::EthereumChainSpecParser;
 use reth_hyperliquid_types::PrecompilesCache;
 use reth_node_ethereum::EthereumNode;
+use share_blocks::ShareBlocksArgs;
 use tokio::sync::Mutex;
 use tracing::info;
 use tx_forwarder::EthForwarderApiServer;
@@ -40,6 +42,10 @@ struct HyperliquidExtArgs {
     /// 3. filters out logs and transactions from subscription.
     #[arg(long, default_value = "false")]
     pub hl_node_compliant: bool,
+
+    /// Enable hlfs to backfill archive blocks
+    #[command(flatten)]
+    pub hlfs: ShareBlocksArgs,
 }
 
 fn main() {
@@ -85,8 +91,24 @@ fn main() {
                 .launch()
                 .await?;
 
-            let ingest =
-                BlockIngest { ingest_dir, local_ingest_dir, local_blocks_cache, precompiles_cache };
+            // start HLFS (serve + peer-backed backfill) using the node's network
+            let hlfs = if ext_args.hlfs.share_blocks {
+                let net = handle.node.network.clone();
+                Some(
+                    crate::share_blocks::ShareBlocks::start_with_network(&ext_args.hlfs, net)
+                        .await?,
+                )
+            } else {
+                None
+            };
+
+            let ingest = BlockIngest {
+                ingest_dir,
+                local_ingest_dir,
+                local_blocks_cache,
+                precompiles_cache,
+                hlfs,
+            };
             ingest.run(handle.node).await.unwrap();
             handle.node_exit_future.await
         },

bin/reth/src/share_blocks.rs

@@ -0,0 +1,167 @@
+use clap::Args;
+use reth_hlfs::{Backfiller, Client, PeerRecord, Server, OP_REQ_MAX_BLOCK, OP_RES_MAX_BLOCK};
+use reth_network_api::{events::NetworkEvent, FullNetwork};
+use std::{
+    collections::HashSet,
+    net::{IpAddr, SocketAddr},
+    path::PathBuf,
+    sync::Arc,
+};
+use tokio::{
+    task::JoinHandle,
+    time::{sleep, timeout, Duration},
+};
+use tracing::{debug, info, warn};
+
+// use futures_util::StreamExt;
+use futures_util::stream::StreamExt;
+
+#[derive(Args, Clone, Debug)]
+pub(crate) struct ShareBlocksArgs {
+    #[arg(long, default_value_t = false)]
+    pub share_blocks: bool,
+    #[arg(long, default_value = "0.0.0.0")]
+    pub share_blocks_host: String,
+    #[arg(long, default_value_t = 9595)]
+    pub share_blocks_port: u16,
+    #[arg(long, default_value = "evm-blocks")]
+    pub archive_dir: PathBuf,
+}
+
+pub(crate) struct ShareBlocks {
+    _server: JoinHandle<()>,
+    _autodetect: JoinHandle<()>,
+}
+
+impl ShareBlocks {
+    pub(crate) async fn start_with_network<Net>(
+        args: &ShareBlocksArgs,
+        network: Net,
+    ) -> eyre::Result<Self>
+    where
+        Net: FullNetwork + Clone + 'static,
+    {
+        let host: IpAddr = args
+            .share_blocks_host
+            .parse()
+            .map_err(|e| eyre::eyre!("invalid --share-blocks-host: {e}"))?;
+        let bind: SocketAddr = (host, args.share_blocks_port).into();
+
+        let srv = Server::new(bind, &args.archive_dir).with_limits(512, 50);
+        let _server = tokio::spawn(async move {
+            if let Err(e) = srv.run().await {
+                warn!(error=%e, "hlfs: server exited");
+            }
+        });
+
+        let _autodetect =
+            spawn_autodetect(network, host, args.share_blocks_port, args.archive_dir.clone());
+
+        info!(%bind, dir=%args.archive_dir.display(), "hlfs: enabled (reth peers)");
+        Ok(Self { _server, _autodetect })
+    }
+}
+
+fn spawn_autodetect<Net>(
+    network: Net,
+    self_ip: IpAddr,
+    hlfs_port: u16,
+    archive_dir: PathBuf,
+) -> JoinHandle<()>
+where
+    Net: FullNetwork + Clone + 'static,
+{
+    let client = Client::new(&archive_dir, Vec::new()).with_timeout(Duration::from_secs(5));
+    let backfiller = Arc::new(tokio::sync::Mutex::new(Backfiller::new(client, &archive_dir)));
+    let good: Arc<tokio::sync::Mutex<HashSet<PeerRecord>>> =
+        Arc::new(tokio::sync::Mutex::new(HashSet::new()));
+
+    tokio::spawn({
+        warn!("hlfs: backfiller started");
+        let backfiller = backfiller.clone();
+        async move {
+            loop {
+                let mut bf = backfiller.lock().await;
+                if bf.client.max_block < bf.max_block_seen {
+                    let block = bf.client.max_block + 1;
+                    let new_height = bf.fetch_if_missing(block).await.expect("new height");
+                    bf.client.max_block = new_height.unwrap();
+                }
+                sleep(Duration::from_millis(50)).await;
+            }
+        }
+    });
+
+    tokio::spawn({
+        let backfiller = backfiller.clone();
+        async move {
+            let mut events = network.event_listener();
+            loop {
+                let mut bf = backfiller.lock().await;
+                match events.next().await {
+                    Some(NetworkEvent::ActivePeerSession { info, .. }) => {
+                        let ip = info.remote_addr.ip();
+                        if ip.is_unspecified() {
+                            debug!(%ip, "hlfs: skip unspecified");
+                            continue;
+                        }
+                        if ip == self_ip {
+                            debug!(%ip, "hlfs: skip self");
+                            continue;
+                        }
+                        let addr = SocketAddr::new(info.remote_addr.ip(), hlfs_port);
+                        let max_block = probe_hlfs(addr).await;
+                        if max_block != 0 {
+                            let mut g = good.lock().await;
+                            if g.insert(PeerRecord { addr, max_block }) {
+                                let v: Vec<_> = g.iter().copied().collect();
+                                bf.set_peers(v.clone());
+                                info!(%addr, %max_block, total=v.len(), "hlfs: peer added");
+                            }
+                        } else {
+                            debug!(%addr, "hlfs: peer has no HLFS");
+                        }
+                    }
+                    Some(_) => {}
+                    None => {
+                        warn!("hlfs: network event stream ended");
+                        break;
+                    }
+                }
+            }
+        }
+    })
+}
+
+pub(crate) async fn probe_hlfs(addr: SocketAddr) -> u64 {
+    use tokio::{
+        io::{AsyncReadExt, AsyncWriteExt},
+        net::TcpStream,
+    };
+
+    let fut = async {
+        let mut s = TcpStream::connect(addr).await.ok()?;
+
+        // send [OP][8 zero bytes]
+        let mut msg = [0u8; 9];
+        msg[0] = OP_REQ_MAX_BLOCK;
+        s.write_all(&msg).await.ok()?;
+
+        // read 1-byte opcode
+        let mut op = [0u8; 1];
+        s.read_exact(&mut op).await.ok()?;
+        if op[0] != OP_RES_MAX_BLOCK {
+            return None;
+        }
+
+        // read 8-byte little-endian block number
+        let mut blk = [0u8; 8];
+        s.read_exact(&mut blk).await.ok()?;
+        Some(u64::from_le_bytes(blk))
+    };
+
+    match timeout(Duration::from_secs(2), fut).await {
+        Ok(Some(n)) => n,
+        _ => 0,
+    }
+}

crates/net/hlfs/Cargo.toml

@@ -0,0 +1,25 @@
+# crates/net/hlfs/Cargo.toml
+[package]
+name = "reth-hlfs"
+version.workspace = true
+edition.workspace = true
+rust-version.workspace = true
+license.workspace = true
+homepage.workspace = true
+repository.workspace = true
+description = "Implementation of archive block downloader"
+authors = ["@wwwehr"]
+
+
+[dependencies]
+tokio = { workspace = true, features = ["rt-multi-thread","macros","net","time","fs","sync","io-util"] }
+bytes.workspace = true
+parking_lot.workspace = true
+thiserror.workspace = true
+tracing.workspace = true
+reth-tracing.workspace = true
+
+[dev-dependencies]
+rand.workspace = true
+tempfile.workspace = true
+

crates/net/hlfs/src/lib.rs

@@ -0,0 +1,374 @@
+//! HLFS TCP micro-protocol for historical backfill (single-block, RR per block).
+
+use bytes::{Buf, BufMut, Bytes, BytesMut};
+use parking_lot::Mutex;
+use reth_tracing::tracing::{debug, info, trace, warn};
+use std::{
+    fs,
+    hash::{Hash, Hasher},
+    io,
+    net::SocketAddr,
+    path::{Path, PathBuf},
+    sync::Arc,
+    time::Duration,
+};
+use thiserror::Error;
+use tokio::{
+    io::{AsyncReadExt, AsyncWriteExt},
+    net::{TcpListener, TcpStream},
+    time::timeout,
+};
+
+type Result<T, E = HlfsError> = std::result::Result<T, E>;
+
+pub const OP_REQ_BLOCK: u8 = 0x01;
+pub const OP_RES_BLOCK: u8 = 0x02;
+pub const OP_REQ_MAX_BLOCK: u8 = 0x03;
+pub const OP_RES_MAX_BLOCK: u8 = 0x04;
+pub const OP_ERR_TOO_BUSY: u8 = 0x05;
+pub const OP_ERR_NOT_FOUND: u8 = 0x06;
+
+#[derive(Error, Debug)]
+pub enum HlfsError {
+    #[error("io: {0}")]
+    Io(#[from] io::Error),
+    #[error("proto")]
+    Proto,
+    #[error("no peers")]
+    NoPeers,
+    #[error("timeout")]
+    Timeout,
+    #[error("busy: retry_ms={0}")]
+    Busy(u32),
+    #[error("not found")]
+    NotFound,
+}
+
+#[inline]
+fn put_u64(b: &mut BytesMut, v: u64) {
+    b.put_u64_le(v)
+}
+#[inline]
+fn put_u32(b: &mut BytesMut, v: u32) {
+    b.put_u32_le(v)
+}
+
+async fn ensure_parent_dirs(path: &str) -> std::io::Result<()> {
+    if let Some(parent) = Path::new(path).parent() {
+        fs::create_dir_all(parent)
+    } else {
+        Ok(())
+    }
+}
+
+/// Client: tries each peer once; rotates starting index per call
+#[derive(Debug, Copy, Clone)]
+pub struct PeerRecord {
+    pub addr: SocketAddr,
+    pub max_block: u64,
+}
+
+impl PartialEq for PeerRecord {
+    fn eq(&self, o: &Self) -> bool {
+        self.addr == o.addr
+    }
+}
+impl Eq for PeerRecord {}
+impl Hash for PeerRecord {
+    fn hash<H: Hasher>(&self, s: &mut H) {
+        self.addr.hash(s);
+    }
+}
+
+#[derive(Clone)]
+pub struct Client {
+    root: PathBuf,
+    pub peers: Arc<Mutex<Vec<PeerRecord>>>,
+    timeout: Duration,
+    pub max_block: u64,
+}
+impl Client {
+    pub fn new(root: impl Into<PathBuf>, peers: Vec<PeerRecord>) -> Self {
+        let root: PathBuf = root.into();
+        let n = find_max_number_file(&root).unwrap();
+        debug!(max_block = n, "hlfs: our archive");
+        Self {
+            root,
+            peers: Arc::new(Mutex::new(peers)),
+            timeout: Duration::from_secs(3),
+            max_block: n,
+        }
+    }
+    pub fn update_peers(&self, peers: Vec<PeerRecord>) {
+        *self.peers.lock() = peers;
+    }
+    pub fn with_timeout(mut self, d: Duration) -> Self {
+        self.timeout = d;
+        self
+    }
+    pub async fn wants_block(&self, number: u64, rr_index: usize) -> Result<Vec<u8>, HlfsError> {
+        let peers = self.peers.lock().clone();
+        debug!(peer_count = peers.len(), "hlfs: peers");
+        if peers.is_empty() {
+            return Err(HlfsError::NoPeers);
+        }
+
+        let mut all = (0..peers.len()).map(|i| (rr_index + i) % peers.len());
+        let mut last_busy: Option<u32> = None;
+        while let Some(i) = all.next() {
+            let addr = peers[i];
+            trace!(%addr.addr, "hlfs: dialing");
+            match timeout(self.timeout, TcpStream::connect(addr.addr)).await {
+                Err(_) => continue,
+                Ok(Err(_)) => continue,
+                Ok(Ok(mut sock)) => {
+                    let mut req = BytesMut::with_capacity(1 + 8);
+                    req.put_u8(OP_REQ_BLOCK);
+                    put_u64(&mut req, number);
+                    if let Err(e) = sock.write_all(&req).await {
+                        debug!(%addr.addr, "hlfs: write err: {e}");
+                        continue;
+                    }
+                    let mut op = [0u8; 1];
+                    if let Err(e) = timeout(self.timeout, sock.read_exact(&mut op)).await {
+                        debug!(%addr.addr, "hlfs: read op timeout {e:?}");
+                        continue;
+                    }
+                    let op = op[0];
+                    match op {
+                        OP_RES_BLOCK => {
+                            // DATA
+                            let mut len = [0u8; 4];
+                            sock.read_exact(&mut len).await?;
+                            let len = u32::from_le_bytes(len) as usize;
+                            let mut buf = vec![0u8; len];
+                            sock.read_exact(&mut buf).await?;
+                            return Ok(buf);
+                        }
+                        OP_ERR_TOO_BUSY => {
+                            let mut ms = [0u8; 4];
+                            sock.read_exact(&mut ms).await?;
+                            last_busy = Some(u32::from_le_bytes(ms));
+                            continue;
+                        }
+                        OP_ERR_NOT_FOUND => {
+                            return Err(HlfsError::NotFound);
+                        }
+                        _ => {
+                            continue;
+                        }
+                    }
+                }
+            }
+        }
+        if let Some(ms) = last_busy {
+            return Err(HlfsError::Busy(ms));
+        }
+        Err(HlfsError::NotFound)
+    }
+}
+
+fn find_max_number_file(root: &Path) -> Result<u64> {
+    fn parse_num(name: &str) -> Option<u64> {
+        name.strip_suffix(".rmp.lz4")?.parse::<u64>().ok()
+    }
+
+    fn walk(dir: &Path, best: &mut Option<u64>) -> io::Result<()> {
+        for entry in fs::read_dir(dir)? {
+            let entry = entry?;
+            let path = entry.path();
+            let ft = entry.file_type()?;
+            if ft.is_dir() {
+                walk(&path, best)?;
+            } else if ft.is_file() {
+                if let Some(name) = path.file_name().and_then(|s| s.to_str()) {
+                    if let Some(n) = parse_num(name) {
+                        if best.map_or(true, |b| n > b) {
+                            *best = Some(n);
+                        }
+                    }
+                }
+            }
+        }
+        Ok(())
+    }
+
+    let mut best = Some(0);
+    let top: PathBuf = fs::read_dir(root)?
+        .filter_map(|e| e.ok())
+        .filter(|e| e.file_type().map(|t| t.is_dir()).unwrap_or(false))
+        .filter_map(|e| {
+            let name = e.file_name();
+            let s = name.to_str()?;
+            let n: u64 = s.parse().ok()?;
+            Some((n, e.path()))
+        })
+        .max_by_key(|(n, _)| *n)
+        .map(|(_, p)| p)
+        .ok_or_else(|| io::Error::new(io::ErrorKind::NotFound, "no numeric top-level dirs"))?;
+
+    walk(&top, &mut best)?;
+    Ok(best.expect("cannot find block files"))
+}
+
+/// Server: serves `{root}/{number}.rlp`.
+pub struct Server {
+    bind: SocketAddr,
+    root: PathBuf,
+    max_conns: usize,
+    inflight: Arc<Mutex<usize>>,
+    busy_retry_ms: u32,
+    max_block: u64,
+}
+
+impl Server {
+    pub fn new(bind: SocketAddr, root: impl Into<PathBuf>) -> Self {
+        let root: PathBuf = root.into();
+        let n = find_max_number_file(&root).unwrap();
+        Self {
+            bind,
+            root,
+            max_conns: 512,
+            inflight: Arc::new(Mutex::new(0)),
+            busy_retry_ms: 100,
+            max_block: n,
+        }
+    }
+    pub fn with_limits(mut self, max_conns: usize, busy_retry_ms: u32) -> Self {
+        self.max_conns = max_conns;
+        self.busy_retry_ms = busy_retry_ms;
+        self
+    }
+    pub async fn run(self) -> Result<(), HlfsError> {
+        fs::create_dir_all(&self.root).ok();
+        info!(%self.bind, root=%self.root.display(), max_conns=%self.max_conns, "hlfs: server listening");
+        let lst = TcpListener::bind(self.bind).await?;
+        loop {
+            let (mut sock, addr) = lst.accept().await?;
+            if *self.inflight.lock() >= self.max_conns {
+                let mut b = BytesMut::with_capacity(5);
+                b.put_u8(OP_ERR_TOO_BUSY);
+                put_u32(&mut b, self.busy_retry_ms);
+                let _ = sock.write_all(&b).await;
+                continue;
+            }
+            *self.inflight.lock() += 1;
+            let root = self.root.clone();
+            let inflight = self.inflight.clone();
+            let busy = self.busy_retry_ms;
+            tokio::spawn(async move {
+                let _ = handle_conn(&mut sock, &root, busy, addr, self.max_block).await;
+                *inflight.lock() -= 1;
+            });
+        }
+    }
+}
+async fn handle_conn(
+    sock: &mut TcpStream,
+    root: &Path,
+    busy_ms: u32,
+    addr: SocketAddr,
+    max_block: u64,
+) -> Result<(), HlfsError> {
+    let mut op = [0u8; 1];
+    sock.read_exact(&mut op).await?;
+    if op[0] != OP_REQ_BLOCK && op[0] != OP_REQ_MAX_BLOCK {
+        warn!(%addr, "hlfs: bad op");
+        return Err(HlfsError::Proto);
+    }
+
+    if op[0] == OP_REQ_MAX_BLOCK {
+        let mut b = BytesMut::with_capacity(1 + 8);
+        b.put_u8(OP_RES_MAX_BLOCK);
+        put_u64(&mut b, max_block);
+        let _ = sock.write_all(&b).await;
+        return Ok(());
+    }
+
+    let mut num = [0u8; 8];
+    sock.read_exact(&mut num).await?;
+    let number = u64::from_le_bytes(num);
+
+    let n = number.saturating_sub(1); // 0 -> 0, others -> number-1
+    let f = (n / 1_000_000) * 1_000_000;
+    let s = (n / 1_000) * 1_000;
+    let path = format!("{}/{f}/{s}/{number}.rmp.lz4", root.to_string_lossy());
+
+    trace!(%addr, number, %path, "hlfs: req");
+    if let Err(e) = ensure_parent_dirs(&path).await {
+        warn!(%addr, %path, "hlfs: mkdirs failed: {e}");
+    }
+
+    match fs::read(&path) {
+        Ok(data) => {
+            let mut b = BytesMut::with_capacity(1 + 4 + data.len());
+            b.put_u8(OP_RES_BLOCK);
+            put_u32(&mut b, data.len() as u32);
+            b.extend_from_slice(&data);
+            let _ = sock.write_all(&b).await;
+        }
+        Err(e) if e.kind() == io::ErrorKind::NotFound => {
+            let mut b = BytesMut::with_capacity(1);
+            b.put_u8(OP_ERR_NOT_FOUND);
+            let _ = sock.write_all(&b).await;
+        }
+        Err(e) => {
+            warn!(%addr, %path, "hlfs: read error: {e}");
+            let _ = sock.shutdown().await;
+        }
+    }
+    Ok(())
+}
+
+/// Backfiller: ask client per missing block; rotate peers every block.
+#[derive(Clone)]
+pub struct Backfiller {
+    pub client: Client,
+    root: PathBuf,
+    pub max_block_seen: u64,
+}
+impl Backfiller {
+    pub fn new(client: Client, root: impl Into<PathBuf>) -> Self {
+        Self { client, root: root.into(), max_block_seen: 0 }
+    }
+    pub fn set_peers(&mut self, peers: Vec<PeerRecord>) {
+        self.client.update_peers(peers);
+        let _peers = self.client.peers.lock().clone();
+        for p in _peers {
+            if p.max_block > self.max_block_seen {
+                self.max_block_seen = p.max_block
+            }
+        }
+    }
+    pub async fn fetch_if_missing(&mut self, number: u64) -> Result<Option<u64>, HlfsError> {
+        let rr_index = number as usize;
+        let n = number.saturating_sub(1); // 0 -> 0, others -> number-1
+        let f = (n / 1_000_000) * 1_000_000;
+        let s = (n / 1_000) * 1_000;
+
+        let path = format!("{}/{f}/{s}/{number}.rmp.lz4", self.root.to_string_lossy());
+        if Path::new(&path).exists() {
+            debug!(block = number, "hlfs: already have");
+            return Ok(None);
+        }
+        match self.client.wants_block(number, rr_index).await {
+            Err(HlfsError::NotFound) => Ok(None),
+            Err(HlfsError::Busy(ms)) => {
+                tokio::time::sleep(Duration::from_millis(ms as u64)).await;
+                Ok(None)
+            }
+            Err(e) => Err(e),
+            Ok(data) => {
+                if let Err(e) = ensure_parent_dirs(&path).await {
+                    warn!(%path, "hlfs: mkdirs failed: {e}");
+                }
+                if let Err(e) = fs::write(&path, &data) {
+                    warn!(%path, "hlfs: write failed: {e}");
+                    return Ok(None);
+                }
+                debug!(block = number, "hlfs: got block");
+                Ok(Some(number))
+            }
+        }
+    }
+}