seaweedfs/seaweed-volume/src/storage/erasure_coding/ec_encoder.rs

//! EC encoding: convert a .dat file into 10 data + 4 parity shards.
//!
//! Uses Reed-Solomon erasure coding. The .dat file is split into blocks
//! (1GB large, 1MB small) and encoded across 14 shard files.

use std::fs::File;
use std::io;
#[cfg(not(unix))]
use std::io::{Read, Seek, SeekFrom};

use reed_solomon_erasure::galois_8::ReedSolomon;

use crate::pb::volume_server_pb::{
    ChecksumAlgorithm, EcBitrotProtection, EcShardChecksums,
};
use crate::storage::erasure_coding::ec_bitrot::{
    self, ShardChecksumBuilder, DEFAULT_BITROT_BLOCK_SIZE,
};
use crate::storage::erasure_coding::ec_shard::*;
use crate::storage::idx;
use crate::storage::types::*;
use crate::storage::volume::volume_file_name;

/// Encode a .dat file into EC shard files.
///
/// Creates .ec00-.ec13 files in the same directory.
/// Also creates a sorted .ecx index from the .idx file.
///
/// Always encodes with the uniform block layout, sized for this .dat, and
/// returns the block size so the caller can persist it to .vif. Mirrors Go's
/// WriteEcFiles.
pub fn write_ec_files(
    dir: &str,
    idx_dir: &str,
    collection: &str,
    volume_id: VolumeId,
    data_shards: usize,
    parity_shards: usize,
) -> io::Result<i64> {
    let base = volume_file_name(dir, collection, volume_id);
    let dat_path = format!("{}.dat", base);
    let idx_base = volume_file_name(idx_dir, collection, volume_id);
    let idx_path = format!("{}.idx", idx_base);

    // Create sorted .ecx from .idx
    write_sorted_ecx_from_idx(&idx_path, &format!("{}.ecx", base))?;

    // Encode .dat into shards
    let dat_file = File::open(&dat_path)?;
    let dat_size = dat_file.metadata()?.len() as i64;

    let rs = ReedSolomon::new(data_shards, parity_shards)
        .map_err(|e| io::Error::new(io::ErrorKind::Other, format!("reed-solomon init: {:?}", e)))?;

    // Create shard files
    let total_shards = data_shards + parity_shards;
    let mut shards: Vec<EcVolumeShard> = (0..total_shards as u8)
        .map(|i| EcVolumeShard::new(dir, collection, volume_id, i))
        .collect();

    for shard in &mut shards {
        shard.create()?;
    }

    // Per-shard bitrot checksum builders: accumulate a CRC32C for every
    // DEFAULT_BITROT_BLOCK_SIZE block of each shard's byte stream as it is
    // written, so the resulting `.ecsum` sidecar can later detect silent
    // corruption in any shard (including cold parity).
    let mut builders: Vec<ShardChecksumBuilder> = (0..total_shards)
        .map(|_| ShardChecksumBuilder::new(DEFAULT_BITROT_BLOCK_SIZE as i64))
        .collect();

    let block_size = uniform_block_size(dat_size, data_shards);
    encode_dat_file(
        &dat_file,
        dat_size,
        &rs,
        &mut shards,
        &mut builders,
        data_shards,
        parity_shards,
        ENCODE_BUFFER_SIZE,
        block_size as usize,
        block_size as usize,
    )?;

    // Close all shards
    for shard in &mut shards {
        shard.close();
    }

    // Write the generation-0 bitrot sidecar (`<base>.ecsum`). Finalizing each
    // builder yields covered_size (== total bytes written to that shard) and
    // the packed little-endian CRC32C array.
    let mut shard_checksums: Vec<EcShardChecksums> = Vec::with_capacity(total_shards);
    for (i, builder) in builders.into_iter().enumerate() {
        let (covered_size, packed) = builder.finalize();
        shard_checksums.push(EcShardChecksums {
            shard_id: i as u32,
            covered_size,
            block_crc32c: packed,
        });
    }
    let prot = EcBitrotProtection {
        algorithm: ChecksumAlgorithm::ChecksumCrc32c as i32,
        block_size: DEFAULT_BITROT_BLOCK_SIZE as u32,
        generation: 0,
        ec_shard_config: Some(ec_bitrot::ec_shard_config(
            data_shards as u32,
            parity_shards as u32,
            block_size,
        )),
        shards: shard_checksums,
        encode_uuid: ec_bitrot::new_encode_uuid(),
    };
    let sidecar_path = ec_bitrot::bitrot_sidecar_path(&base, 0);
    if let Err(e) = ec_bitrot::save_bitrot_sidecar(&sidecar_path, &prot) {
        // A failed sidecar must not fail the encode — the shards are already
        // written and valid. The volume simply runs with bitrot protection
        // off for this generation until the sidecar is regenerated.
        tracing::warn!(
            volume_id = volume_id.0,
            path = %sidecar_path,
            error = %e,
            "ec encode: failed to write bitrot sidecar; protection off for this generation",
        );
    }

    Ok(block_size)
}

/// uniform_block_size returns the per-shard block size of the uniform layout
/// for a .dat of the given size: ceil(dat_file_size/data_shards) rounded up to
/// a whole small block. For every input this equals the legacy layout's padded
/// shard size, so only the byte placement differs between the two layouts,
/// never the shard length. Mirrors Go's UniformBlockSize.
pub fn uniform_block_size(dat_file_size: i64, data_shards: usize) -> i64 {
    let small = ERASURE_CODING_SMALL_BLOCK_SIZE as i64;
    let per_shard = (dat_file_size + data_shards as i64 - 1) / data_shards as i64;
    let blocks = ((per_shard + small - 1) / small).max(1);
    blocks * small
}

/// Rebuild missing EC shard files from existing shards using Reed-Solomon reconstruct.
///
/// This does not require the `.dat` file, only the existing `.ecXX` shard files.
///
/// `additional_dirs` lists sibling disk locations to search for existing shards when
/// they are not found in `dir`. This is required on multi-disk volume servers where
/// shards for the same volume may be spread across disks.
pub fn rebuild_ec_files(
    dir: &str,
    collection: &str,
    volume_id: VolumeId,
    missing_shard_ids: &[u32],
    data_shards: usize,
    parity_shards: usize,
    additional_dirs: &[&str],
) -> io::Result<()> {
    if missing_shard_ids.is_empty() {
        return Ok(());
    }

    let rs = ReedSolomon::new(data_shards, parity_shards)
        .map_err(|e| io::Error::new(io::ErrorKind::Other, format!("reed-solomon init: {:?}", e)))?;

    let total_shards = data_shards + parity_shards;
    let mut shards: Vec<EcVolumeShard> = (0..total_shards as u8)
        .map(|i| EcVolumeShard::new(dir, collection, volume_id, i))
        .collect();

    // Determine the exact shard size from the first available existing shard.
    // When a shard is not found in `dir`, search `additional_dirs` (sibling disks
    // on the same volume server) before failing — mirrors Go's findShardFile logic.
    let mut shard_size = 0;
    for (i, shard) in shards.iter_mut().enumerate() {
        if !missing_shard_ids.contains(&(i as u32)) {
            if let Ok(_) = shard.open() {
                let size = shard.file_size();
                if size > shard_size {
                    shard_size = size;
                }
            } else {
                // Try sibling disk locations before giving up.
                let mut found = false;
                for &other_dir in additional_dirs {
                    let mut alt = EcVolumeShard::new(other_dir, collection, volume_id, i as u8);
                    if let Ok(_) = alt.open() {
                        let size = alt.file_size();
                        if size > shard_size {
                            shard_size = size;
                        }
                        *shard = alt;
                        found = true;
                        break;
                    }
                }
                if !found {
                    return Err(io::Error::new(
                        io::ErrorKind::NotFound,
                        format!("missing non-rebuild shard {}", i),
                    ));
                }
            }
        }
    }

    if shard_size == 0 {
        return Err(io::Error::new(
            io::ErrorKind::InvalidData,
            "all existing shards are empty or cannot find an existing shard to determine size",
        ));
    }

    // Create the missing shards for writing
    for i in missing_shard_ids {
        if let Some(shard) = shards.get_mut(*i as usize) {
            shard.create()?;
        }
    }

    let block_size = ERASURE_CODING_SMALL_BLOCK_SIZE;
    let mut remaining = shard_size;
    let mut offset: u64 = 0;

    // Process all data in blocks
    while remaining > 0 {
        let to_process = remaining.min(block_size as i64) as usize;

        // Allocate buffers for all shards. Option<Vec<u8>> is required by rs.reconstruct()
        let mut buffers: Vec<Option<Vec<u8>>> = vec![None; total_shards];

        // Read available shards. A short read means a truncated/corrupt input
        // shard; treat it as an error rather than reconstructing over a
        // zero-padded tail and publishing the result as restored redundancy.
        for (i, shard) in shards.iter().enumerate() {
            if !missing_shard_ids.contains(&(i as u32)) {
                let mut buf = vec![0u8; to_process];
                let n = shard.read_at(&mut buf, offset)?;
                if n != to_process {
                    return Err(io::Error::new(
                        io::ErrorKind::UnexpectedEof,
                        format!(
                            "ec rebuild short read shard {} at {}: got {} want {}",
                            i, offset, n, to_process
                        ),
                    ));
                }
                buffers[i] = Some(buf);
            }
        }

        // Reconstruct missing shards
        rs.reconstruct(&mut buffers).map_err(|e| {
            io::Error::new(
                io::ErrorKind::Other,
                format!("reed-solomon reconstruct: {:?}", e),
            )
        })?;

        // Write recovered data into the missing shards
        for i in missing_shard_ids {
            let idx = *i as usize;
            if let Some(buf) = buffers[idx].take() {
                shards[idx].write_all(&buf)?;
            }
        }

        offset += to_process as u64;
        remaining -= to_process as i64;
    }

    // Close all shards
    for shard in &mut shards {
        shard.close();
    }

    Ok(())
}

/// Reed-Solomon parity check over the locally-held shards: recompute parity from
/// the data shards and flag any shard whose bytes disagree. Wired into FULL
/// (scrub mode 2) as a TEMPORARY local parity/cold-region check — the per-needle
/// FULL walk only reads live data-shard intervals, so on its own it can't catch
/// bitrot in a parity shard or an unwalked region. Move to mode 4 (CHECKSUM) and
/// drop it from mode 2 once the `.ecsum` subsystem lands.
pub fn verify_ec_shards(
    dir: &str,
    collection: &str,
    volume_id: VolumeId,
    data_shards: usize,
    parity_shards: usize,
) -> io::Result<(Vec<u32>, Vec<String>)> {
    let rs = ReedSolomon::new(data_shards, parity_shards)
        .map_err(|e| io::Error::new(io::ErrorKind::Other, format!("reed-solomon init: {:?}", e)))?;

    let total_shards = data_shards + parity_shards;
    let mut shards: Vec<EcVolumeShard> = (0..total_shards as u8)
        .map(|i| EcVolumeShard::new(dir, collection, volume_id, i))
        .collect();

    let mut shard_size = 0;
    let mut broken_shards = std::collections::HashSet::new();
    let mut details = Vec::new();

    for (i, shard) in shards.iter_mut().enumerate() {
        if let Ok(_) = shard.open() {
            let size = shard.file_size();
            if size > shard_size {
                shard_size = size;
            }
        } else {
            broken_shards.insert(i as u32);
            details.push(format!("failed to open or missing shard {}", i));
        }
    }

    if shard_size == 0 || broken_shards.len() >= parity_shards {
        // Can't do much if we don't know the size or have too many missing
        return Ok((broken_shards.into_iter().collect(), details));
    }

    let block_size = ERASURE_CODING_SMALL_BLOCK_SIZE;
    let mut remaining = shard_size;
    let mut offset: u64 = 0;

    while remaining > 0 {
        let to_process = remaining.min(block_size as i64) as usize;
        let mut buffers = vec![vec![0u8; to_process]; total_shards];

        let mut read_failed = false;
        for i in 0..total_shards {
            if !broken_shards.contains(&(i as u32)) {
                if let Err(e) = shards[i].read_at(&mut buffers[i], offset) {
                    broken_shards.insert(i as u32);
                    details.push(format!("read error shard {}: {}", i, e));
                    read_failed = true;
                }
            } else {
                read_failed = true;
            }
        }

        // Only do verification if all shards were readable
        if !read_failed {
            // Need to convert Vec<Vec<u8>> to &[&[u8]] for rs.verify
            let slice_ptrs: Vec<&[u8]> = buffers.iter().map(|v| v.as_slice()).collect();
            if let Ok(is_valid) = rs.verify(&slice_ptrs) {
                if !is_valid {
                    // Reed-Solomon verification failed. We cannot easily pinpoint which shard
                    // is corrupted without recalculating parities or syndromes, so we just
                    // log that this batch has corruption. Wait, we can test each parity shard!
                    // Let's re-encode from the first `data_shards` and compare to the actual `parity_shards`.

                    let mut verify_buffers = buffers.clone();
                    // Clear the parity parts
                    for i in data_shards..total_shards {
                        verify_buffers[i].fill(0);
                    }
                    if rs.encode(&mut verify_buffers).is_ok() {
                        for i in 0..total_shards {
                            if buffers[i] != verify_buffers[i] {
                                broken_shards.insert(i as u32);
                                details.push(format!(
                                    "parity mismatch on shard {} at offset {}",
                                    i, offset
                                ));
                            }
                        }
                    }
                }
            }
        }

        offset += to_process as u64;
        remaining -= to_process as i64;
    }

    // Close all shards
    for shard in &mut shards {
        shard.close();
    }

    let mut broken_vec: Vec<u32> = broken_shards.into_iter().collect();
    broken_vec.sort_unstable();

    Ok((broken_vec, details))
}

/// Write sorted .ecx index from .idx file.
pub(crate) fn write_sorted_ecx_from_idx(idx_path: &str, ecx_path: &str) -> io::Result<()> {
    if !std::path::Path::new(idx_path).exists() {
        return Err(io::Error::new(
            io::ErrorKind::NotFound,
            "idx file not found",
        ));
    }

    // Read all idx entries
    let mut idx_file = File::open(idx_path)?;
    let mut entries: Vec<(NeedleId, Offset, Size)> = Vec::new();

    idx::walk_index_file(&mut idx_file, 0, |key, offset, size| {
        entries.push((key, offset, size));
        Ok(())
    })?;

    // Sort by NeedleId, then by actual offset so later entries come last
    entries.sort_by_key(|&(key, offset, _)| (key, offset.to_actual_offset()));

    // Remove duplicates (keep last/latest entry for each key).
    // dedup_by_key keeps the first in each run, so we reverse first,
    // dedup, then reverse back.
    entries.reverse();
    entries.dedup_by_key(|entry| entry.0);
    entries.reverse();

    // Write sorted entries to .ecx
    let mut ecx_file = File::create(ecx_path)?;
    for &(key, offset, size) in &entries {
        idx::write_index_entry(&mut ecx_file, key, offset, size)?;
    }

    Ok(())
}

/// Rebuild the .ecx index file by walking needles in the EC data shards.
///
/// This is the equivalent of Go's `RebuildEcxFile`. It reads the logical .dat
/// content from the EC data shards, walks through needle headers to extract
/// (needle_id, offset, size) entries, deduplicates them, and writes a sorted
/// .ecx index file.
///
/// `additional_dirs` lists sibling disk locations to search for data shards when
/// they are not found in `dir` — mirrors the same fallback used by rebuild_ec_files,
/// required on multi-disk volume servers where shards may be spread across disks.
pub fn rebuild_ecx_file(
    dir: &str,
    collection: &str,
    volume_id: VolumeId,
    data_shards: usize,
    block_size: i64,
    dat_file_size: i64,
    additional_dirs: &[&str],
) -> io::Result<()> {
    use crate::storage::needle::needle::get_actual_size;
    use crate::storage::super_block::SUPER_BLOCK_SIZE;

    let base = volume_file_name(dir, collection, volume_id);
    let ecx_path = format!("{}.ecx", base);

    // Open data shards to read logical .dat content. When a shard isn't found
    // in `dir`, search `additional_dirs` (sibling disks on the same volume
    // server) before giving up.
    let mut shards: Vec<EcVolumeShard> = (0..data_shards as u8)
        .map(|i| EcVolumeShard::new(dir, collection, volume_id, i))
        .collect();

    for (i, shard) in shards.iter_mut().enumerate() {
        if let Err(_) = shard.open() {
            let mut found = false;
            for &other_dir in additional_dirs {
                let mut alt = EcVolumeShard::new(other_dir, collection, volume_id, i as u8);
                if alt.open().is_ok() {
                    *shard = alt;
                    found = true;
                    break;
                }
            }
            if !found {
                // If a data shard is missing, we can't rebuild ecx
                for s in &mut shards {
                    s.close();
                }
                return Err(io::Error::new(
                    io::ErrorKind::NotFound,
                    format!("cannot open data shard for ecx rebuild"),
                ));
            }
        }
    }

    // Determine total logical data size from shard sizes
    let shard_size = shards.iter().map(|s| s.file_size()).max().unwrap_or(0);
    let total_data_size = shard_size as i64 * data_shards as i64;
    // The volume's shard block layout: the .vif-recorded uniform block size,
    // or the legacy two-tier sizes when 0. The row count comes from the shard
    // length; -1 disambiguates a legacy shard that is an exact large-block
    // multiple (mirrors the ecdFileSize-1 fallback in the read path).
    let (large_block, small_block) = if block_size > 0 {
        (block_size, block_size)
    } else {
        (
            ERASURE_CODING_LARGE_BLOCK_SIZE as i64,
            ERASURE_CODING_SMALL_BLOCK_SIZE as i64,
        )
    };
    // The row count the de-stripe walks with. The encode-time .dat size is the
    // authority — the same value the read path divides by data_shards — and the
    // padded extent is only a fallback: under the legacy layout a shard that is
    // an exact large-block multiple reads as one row too many, which
    // re-interprets its last large row as small blocks and scrambles the
    // recovered offsets. Subtracting one keeps that fallback on the safe side of
    // the boundary, exactly as the read path's own fallback does.
    let locate_shard_size = if dat_file_size > 0 {
        dat_file_size / data_shards as i64
    } else {
        (shard_size as i64 - 1).max(0)
    };

    // Read version from superblock (first byte of logical data)
    let mut sb_buf = [0u8; SUPER_BLOCK_SIZE];
    read_from_data_shards(
        &shards,
        &mut sb_buf,
        0,
        data_shards,
        locate_shard_size,
        large_block,
        small_block,
    )?;
    let version = Version(sb_buf[0]);

    // Walk needles starting after superblock
    let mut offset = SUPER_BLOCK_SIZE as i64;
    let header_size = NEEDLE_HEADER_SIZE;
    let mut entries: Vec<(NeedleId, Offset, Size)> = Vec::new();

    while offset + header_size as i64 <= total_data_size {
        // Read needle header (cookie + needle_id + size = 16 bytes).
        // A read failure is NOT the end of the data — every offset in
        // range maps into the shards, so an error means a truncated or
        // unreadable shard. Publishing the entries collected so far as
        // a successful .ecx would hand out a silently incomplete
        // recovery index; propagate instead. (The scan still ends
        // normally on the zero-cookie tail below.)
        let mut header_buf = [0u8; NEEDLE_HEADER_SIZE];
        if let Err(e) = read_from_data_shards(
            &shards,
            &mut header_buf,
            offset as u64,
            data_shards,
            locate_shard_size,
            large_block,
            small_block,
        ) {
            for s in &mut shards {
                s.close();
            }
            return Err(io::Error::new(
                e.kind(),
                format!("scan needle header at offset {}: {}", offset, e),
            ));
        }

        let cookie = Cookie::from_bytes(&header_buf[..COOKIE_SIZE]);
        let needle_id = NeedleId::from_bytes(&header_buf[COOKIE_SIZE..COOKIE_SIZE + NEEDLE_ID_SIZE]);
        let size = Size::from_bytes(&header_buf[COOKIE_SIZE + NEEDLE_ID_SIZE..header_size]);

        // Validate: stop if we hit zero cookie+id (end of data)
        if cookie.0 == 0 && needle_id.0 == 0 {
            break;
        }

        // Validate size is reasonable
        if size.0 < 0 && !size.is_deleted() {
            break;
        }

        let actual_size = get_actual_size(size, version);
        if actual_size <= 0 || offset + actual_size > total_data_size {
            break;
        }

        entries.push((needle_id, Offset::from_actual_offset(offset), size));

        // Advance to next needle (aligned to NEEDLE_PADDING_SIZE)
        offset += actual_size;
        let padding_rem = offset % NEEDLE_PADDING_SIZE as i64;
        if padding_rem != 0 {
            offset += NEEDLE_PADDING_SIZE as i64 - padding_rem;
        }
    }

    for shard in &mut shards {
        shard.close();
    }

    // Sort by NeedleId, then by offset (later entries override earlier)
    entries.sort_by_key(|&(key, offset, _)| (key, offset.to_actual_offset()));

    // Deduplicate: keep latest entry per needle_id
    entries.reverse();
    entries.dedup_by_key(|entry| entry.0);
    entries.reverse();

    // Write sorted .ecx
    let mut ecx_file = File::create(&ecx_path)?;
    for &(key, offset, size) in &entries {
        idx::write_index_entry(&mut ecx_file, key, offset, size)?;
    }
    ecx_file.sync_all()?;

    Ok(())
}

/// Read bytes from EC data shards at a logical offset in the .dat file,
/// resolving the shard/offset through the volume's block layout via
/// locate_data — the same mapping the read path uses.
#[allow(clippy::too_many_arguments)]
fn read_from_data_shards(
    shards: &[EcVolumeShard],
    buf: &mut [u8],
    logical_offset: u64,
    data_shards: usize,
    locate_shard_size: i64,
    large_block_size: i64,
    small_block_size: i64,
) -> io::Result<()> {
    let intervals = crate::storage::erasure_coding::ec_locate::locate_data(
        logical_offset as i64,
        Size(buf.len() as i32),
        locate_shard_size,
        data_shards as u32,
        large_block_size,
        small_block_size,
    );
    let mut bytes_read = 0usize;
    for interval in &intervals {
        let (shard_id, shard_offset) =
            interval.to_shard_id_and_offset(data_shards as u32, large_block_size, small_block_size);
        if shard_id as usize >= data_shards {
            return Err(io::Error::new(
                io::ErrorKind::InvalidInput,
                "shard index out of range",
            ));
        }
        let to_read = interval.size as usize;
        let dest = &mut buf[bytes_read..bytes_read + to_read];
        // Exact-read semantics: read_at may legally return fewer bytes
        // than requested, and treating a short read as complete leaves
        // the tail of `dest` as whatever the buffer held before. Loop
        // until filled; zero bytes inside the mapped range means the
        // shard is truncated — an error, not an end.
        let mut filled = 0usize;
        while filled < to_read {
            let n = shards[shard_id as usize]
                .read_at(&mut dest[filled..], shard_offset as u64 + filled as u64)?;
            if n == 0 {
                return Err(io::Error::new(
                    io::ErrorKind::UnexpectedEof,
                    format!(
                        "short read from data shard {}: {} of {} bytes at offset {}",
                        shard_id, filled, to_read, shard_offset
                    ),
                ));
            }
            filled += n;
        }
        bytes_read += to_read;
    }
    if bytes_read != buf.len() {
        return Err(io::Error::new(
            io::ErrorKind::UnexpectedEof,
            "short read from data shards",
        ));
    }
    Ok(())
}

/// Buffer size for one encode sub-batch per shard, mirroring Go's 256KB
/// bufferSize in WriteEcFiles. A block is processed in block_size/buffer_size
/// sub-batches, so memory stays at total_shards * 256KB no matter how large
/// the uniform block is.
const ENCODE_BUFFER_SIZE: usize = 256 * 1024;

/// Encode the .dat file data into shard files.
///
/// Uses a two-phase approach matching Go's ec_encoder.go:
/// 1. Process as many large blocks as possible
/// 2. Process remaining data with small blocks
///
/// `buffer_size` must divide both block sizes.
#[allow(clippy::too_many_arguments)]
pub(crate) fn encode_dat_file(
    dat_file: &File,
    dat_size: i64,
    rs: &ReedSolomon,
    shards: &mut [EcVolumeShard],
    builders: &mut [ShardChecksumBuilder],
    data_shards: usize,
    parity_shards: usize,
    buffer_size: usize,
    large_block_size: usize,
    small_block_size: usize,
) -> io::Result<()> {
    let total_shards = data_shards + parity_shards;
    let mut buffers: Vec<Vec<u8>> = (0..total_shards)
        .map(|_| vec![0u8; buffer_size])
        .collect();

    let mut remaining = dat_size;
    let mut offset: u64 = 0;

    // Phase 1: process whole large-block rows while enough data remains
    let large_row_size = large_block_size * data_shards;

    while remaining >= large_row_size as i64 {
        encode_data(
            dat_file,
            offset,
            large_block_size,
            rs,
            &mut buffers,
            shards,
            builders,
            data_shards,
        )?;
        offset += large_row_size as u64;
        remaining -= large_row_size as i64;
    }

    // Phase 2: process remaining data with small blocks
    let small_row_size = small_block_size * data_shards;

    while remaining > 0 {
        let to_process = remaining.min(small_row_size as i64);
        encode_data(
            dat_file,
            offset,
            small_block_size,
            rs,
            &mut buffers,
            shards,
            builders,
            data_shards,
        )?;
        offset += to_process as u64;
        remaining -= to_process;
    }

    Ok(())
}

/// Encode one row of blocks, streaming it in ENCODE_BUFFER_SIZE sub-batches so
/// arbitrarily large blocks never require block-sized allocations. Mirrors
/// Go's encodeData.
#[allow(clippy::too_many_arguments)]
fn encode_data(
    dat_file: &File,
    row_offset: u64,
    block_size: usize,
    rs: &ReedSolomon,
    buffers: &mut [Vec<u8>],
    shards: &mut [EcVolumeShard],
    builders: &mut [ShardChecksumBuilder],
    data_shards: usize,
) -> io::Result<()> {
    let buffer_size = buffers[0].len();
    if block_size % buffer_size != 0 {
        return Err(io::Error::new(
            io::ErrorKind::InvalidInput,
            format!(
                "unexpected block size {} buffer size {}",
                block_size, buffer_size
            ),
        ));
    }
    let batch_count = block_size / buffer_size;
    for b in 0..batch_count {
        encode_one_batch(
            dat_file,
            row_offset + (b * buffer_size) as u64,
            block_size,
            rs,
            buffers,
            shards,
            builders,
            data_shards,
        )?;
    }
    Ok(())
}

/// Encode one sub-batch: the same buffer-sized slice of every shard's block in
/// this row. Mirrors Go's encodeDataOneBatch.
#[allow(clippy::too_many_arguments)]
fn encode_one_batch(
    dat_file: &File,
    offset: u64,
    block_size: usize,
    rs: &ReedSolomon,
    buffers: &mut [Vec<u8>],
    shards: &mut [EcVolumeShard],
    builders: &mut [ShardChecksumBuilder],
    data_shards: usize,
) -> io::Result<()> {
    // Read data shards from the .dat file, zero-filling past EOF — the buffers
    // are reused across batches, so the tail must be cleared explicitly.
    for i in 0..data_shards {
        let read_offset = offset + (i * block_size) as u64;
        let n = read_at_most(dat_file, &mut buffers[i], read_offset)?;
        for b in buffers[i][n..].iter_mut() {
            *b = 0;
        }
    }

    // Encode parity shards
    rs.encode(&mut *buffers).map_err(|e| {
        io::Error::new(
            io::ErrorKind::Other,
            format!("reed-solomon encode: {:?}", e),
        )
    })?;

    // Write all shard buffers to files and feed the same bytes to each
    // shard's bitrot checksum builder, keeping covered_size == on-disk length.
    for (i, buf) in buffers.iter().enumerate() {
        shards[i].write_all(buf)?;
        builders[i].write(buf);
    }

    Ok(())
}

/// Read into `buf` at `offset` until it is full or EOF; returns bytes read.
fn read_at_most(dat_file: &File, buf: &mut [u8], offset: u64) -> io::Result<usize> {
    let mut n = 0;
    while n < buf.len() {
        #[cfg(unix)]
        let r = {
            use std::os::unix::fs::FileExt;
            dat_file.read_at(&mut buf[n..], offset + n as u64)?
        };
        #[cfg(not(unix))]
        let r = {
            let mut f = dat_file.try_clone()?;
            f.seek(SeekFrom::Start(offset + n as u64))?;
            f.read(&mut buf[n..])?
        };
        if r == 0 {
            break;
        }
        n += r;
    }
    Ok(n)
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::storage::needle::needle::Needle;
    use crate::storage::needle_map::NeedleMapKind;
    use crate::storage::volume::Volume;
    use tempfile::TempDir;

    #[test]
    fn test_ec_encode_decode_round_trip() {
        let tmp = TempDir::new().unwrap();
        let dir = tmp.path().to_str().unwrap();

        // Create a volume with some data
        let mut v = Volume::new(
            dir,
            dir,
            "",
            VolumeId(1),
            NeedleMapKind::InMemory,
            None,
            None,
            0,
            Version::current(),
        )
        .unwrap();

        for i in 1..=5 {
            let data = format!("test data for needle {}", i);
            let mut n = Needle {
                id: NeedleId(i),
                cookie: Cookie(i as u32),
                data: data.as_bytes().to_vec(),
                data_size: data.len() as u32,
                ..Needle::default()
            };
            v.write_needle(&mut n, true, false).unwrap();
        }
        v.sync_to_disk().unwrap();
        v.close();

        // Encode to EC shards
        let data_shards = 10;
        let parity_shards = 4;
        let total_shards = data_shards + parity_shards;
        write_ec_files(dir, dir, "", VolumeId(1), data_shards, parity_shards).unwrap();

        // Verify shard files exist
        for i in 0..total_shards {
            let path = format!("{}/{}.ec{:02}", dir, 1, i);
            assert!(
                std::path::Path::new(&path).exists(),
                "shard file {} should exist",
                path
            );
        }

        // Verify .ecx exists
        let ecx_path = format!("{}/1.ecx", dir);
        assert!(std::path::Path::new(&ecx_path).exists());
    }

    fn make_volume_with_needles(n: u64) -> TempDir {
        let tmp = TempDir::new().unwrap();
        let dir = tmp.path().to_str().unwrap();
        let mut v = Volume::new(
            dir,
            dir,
            "",
            VolumeId(1),
            NeedleMapKind::InMemory,
            None,
            None,
            0,
            Version::current(),
        )
        .unwrap();
        for i in 1..=n {
            // Larger payloads so encoded shards span multiple bitrot blocks
            // would require huge data; small payloads are fine for correctness.
            let data = format!("test data for needle {} {}", i, "x".repeat(64));
            let mut needle = Needle {
                id: NeedleId(i),
                cookie: Cookie(i as u32),
                data: data.as_bytes().to_vec(),
                data_size: data.len() as u32,
                ..Needle::default()
            };
            v.write_needle(&mut needle, true, false).unwrap();
        }
        v.sync_to_disk().unwrap();
        v.close();
        tmp
    }

    /// Encode-time capture writes a valid generation-0 `.ecsum` sidecar whose
    /// recorded checksums match the actual on-disk shards.
    #[test]
    fn test_encode_writes_valid_bitrot_sidecar() {
        use crate::storage::erasure_coding::ec_bitrot;

        let tmp = make_volume_with_needles(5);
        let dir = tmp.path().to_str().unwrap();
        write_ec_files(dir, dir, "", VolumeId(1), 10, 4).unwrap();

        let base = format!("{}/1", dir);
        let sidecar_path = ec_bitrot::bitrot_sidecar_path(&base, 0);
        assert!(
            std::path::Path::new(&sidecar_path).exists(),
            "generation-0 .ecsum sidecar should exist after encode"
        );

        let prot = ec_bitrot::load_bitrot_sidecar(&sidecar_path).unwrap();
        ec_bitrot::validate_manifest(&prot, 10, 4).unwrap();
        assert_eq!(prot.generation, 0);
        assert_eq!(prot.shards.len(), 14);
        assert_eq!(prot.encode_uuid.len(), 16);
        assert_eq!(prot.block_size, ec_bitrot::DEFAULT_BITROT_BLOCK_SIZE as u32);

        // Every shard's recorded covered_size must equal its on-disk length and
        // its block CRCs must verify clean.
        let bs = prot.block_size as i64;
        for entry in &prot.shards {
            let path = format!("{}.ec{:02}", base, entry.shard_id);
            let on_disk = std::fs::metadata(&path).unwrap().len() as i64;
            assert_eq!(
                entry.covered_size, on_disk,
                "covered_size must equal on-disk length for shard {}",
                entry.shard_id
            );
            let mm = ec_bitrot::verify_shard_file_blocks(&path, entry, bs).unwrap();
            assert!(
                mm.is_empty(),
                "shard {} should verify clean, got mismatches {:?}",
                entry.shard_id,
                mm
            );
        }
    }

    // encode_sample_volume writes a small volume and EC-encodes it, returning
    // the dir path so a test can drop/truncate shards and rebuild.
    fn encode_sample_volume(tmp: &TempDir) -> String {
        let dir = tmp.path().to_str().unwrap().to_string();
        let mut v = Volume::new(
            &dir,
            &dir,
            "",
            VolumeId(1),
            NeedleMapKind::InMemory,
            None,
            None,
            0,
            Version::current(),
        )
        .unwrap();
        for i in 1..=20 {
            let data = format!("test data for needle {} padded with bytes", i).repeat(64);
            let mut n = Needle {
                id: NeedleId(i),
                cookie: Cookie(i as u32),
                data: data.as_bytes().to_vec(),
                data_size: data.len() as u32,
                ..Needle::default()
            };
            v.write_needle(&mut n, true, false).unwrap();
        }
        v.sync_to_disk().unwrap();
        v.close();
        write_ec_files(&dir, &dir, "", VolumeId(1), 10, 4).unwrap();
        dir
    }

    // A truncated/corrupt input shard must abort rebuild_ec_files with an error
    // rather than reconstructing over a zero-padded tail and publishing a
    // truncated shard as restored redundancy.
    #[test]
    fn test_rebuild_ec_files_short_read_input_errors() {
        let tmp = TempDir::new().unwrap();
        let dir = encode_sample_volume(&tmp);

        // Truncate a present input shard to half its size.
        let victim = format!("{}/1.ec03", dir);
        let full = std::fs::metadata(&victim).unwrap().len();
        assert!(full > 0, "encoded shard should be non-empty");
        let f = std::fs::OpenOptions::new().write(true).open(&victim).unwrap();
        f.set_len(full / 2).unwrap();
        drop(f);

        // Rebuild a different (genuinely missing) shard.
        std::fs::remove_file(format!("{}/1.ec07", dir)).unwrap();
        let res = rebuild_ec_files(&dir, "", VolumeId(1), &[7], 10, 4, &[]);
        assert!(res.is_err(), "truncated input shard must abort the rebuild");
    }

    // Happy path: dropping a shard and rebuilding it from the rest must succeed
    // and recreate the shard file (the short-read guard must not false-positive).
    #[test]
    fn test_rebuild_ec_files_happy_path() {
        let tmp = TempDir::new().unwrap();
        let dir = encode_sample_volume(&tmp);

        let dropped = format!("{}/1.ec07", dir);
        std::fs::remove_file(&dropped).unwrap();
        rebuild_ec_files(&dir, "", VolumeId(1), &[7], 10, 4, &[]).unwrap();
        assert!(
            std::path::Path::new(&dropped).exists(),
            "rebuilt shard .ec07 should exist"
        );
        assert!(
            std::fs::metadata(&dropped).unwrap().len() > 0,
            "rebuilt shard should be non-empty"
        );
    }

    // Multi-disk rebuild: shards for the same volume are split across two
    // directories (simulating a volume server with two disk locations). The
    // shard to rebuild is in the primary dir; some of the input shards needed
    // for RS reconstruction exist only in the secondary dir. Passing the
    // secondary dir via `additional_dirs` must allow the rebuild to succeed
    // where it would previously return "missing non-rebuild shard N".
    #[test]
    fn test_rebuild_ec_files_multi_disk() {
        let tmp_primary = TempDir::new().unwrap();
        let tmp_secondary = TempDir::new().unwrap();
        let primary = tmp_primary.path().to_str().unwrap().to_string();
        let secondary = tmp_secondary.path().to_str().unwrap().to_string();

        // Encode into primary dir first (all 14 shards land there).
        let dir = encode_sample_volume(&tmp_primary);
        assert_eq!(dir, primary);

        // Simulate a multi-disk layout: move shards 0, 4, 8 to the secondary
        // dir, as if the master had placed them on a different disk.
        let moved_shards: &[u8] = &[0, 4, 8];
        for &shard_id in moved_shards {
            let src = format!("{}/1.ec{:02}", primary, shard_id);
            let dst = format!("{}/1.ec{:02}", secondary, shard_id);
            std::fs::rename(&src, &dst).unwrap();
        }

        // Remove shard 7 from primary — this is the shard we want to rebuild.
        let missing_path = format!("{}/1.ec07", primary);
        std::fs::remove_file(&missing_path).unwrap();

        // Without additional_dirs the rebuild must fail: shards 0, 4, 8 are
        // not in primary and shard 7 cannot be reconstructed without them.
        let res = rebuild_ec_files(&primary, "", VolumeId(1), &[7], 10, 4, &[]);
        assert!(
            res.is_err(),
            "rebuild without additional_dirs must fail when input shards are on another disk"
        );
        assert!(
            !std::path::Path::new(&missing_path).exists(),
            "failed rebuild must not leave a partial shard file behind"
        );

        // With additional_dirs pointing at the secondary, the rebuild must succeed.
        rebuild_ec_files(
            &primary,
            "",
            VolumeId(1),
            &[7],
            10,
            4,
            &[secondary.as_str()],
        )
        .unwrap();

        assert!(
            std::path::Path::new(&missing_path).exists(),
            "rebuilt shard .ec07 should exist in primary dir"
        );
        assert!(
            std::fs::metadata(&missing_path).unwrap().len() > 0,
            "rebuilt shard must be non-empty"
        );
    }

    // Multi-disk .ecx rebuild: data shards (0-9) needed to reconstruct the
    // logical .dat content are split across two directories, as on a
    // multi-disk volume server. Passing the secondary dir via
    // `additional_dirs` must allow rebuild_ecx_file to find them and
    // succeed where it would previously fail with
    // "cannot open data shard for ecx rebuild".
    #[test]
    fn test_rebuild_ecx_file_multi_disk() {
        let tmp_primary = TempDir::new().unwrap();
        let tmp_secondary = TempDir::new().unwrap();
        let primary = tmp_primary.path().to_str().unwrap().to_string();
        let secondary = tmp_secondary.path().to_str().unwrap().to_string();

        let dir = encode_sample_volume(&tmp_primary);
        assert_eq!(dir, primary);

        // Move some data shards (0-9) to the secondary dir, simulating a
        // multi-disk layout where not all data shards landed on the same disk.
        let moved_shards: &[u8] = &[1, 3, 6];
        for &shard_id in moved_shards {
            let src = format!("{}/1.ec{:02}", primary, shard_id);
            let dst = format!("{}/1.ec{:02}", secondary, shard_id);
            std::fs::rename(&src, &dst).unwrap();
        }

        // Delete the existing .ecx to force a rebuild.
        let ecx_path = format!("{}/1.ecx", primary);
        std::fs::remove_file(&ecx_path).unwrap();

        // Without additional_dirs, rebuild must fail: shards 1, 3, 6 are not
        // in primary and the full logical .dat content can't be reconstructed.
        let res = rebuild_ecx_file(&primary, "", VolumeId(1), 10, 0, 0, &[]);
        assert!(
            res.is_err(),
            "ecx rebuild without additional_dirs must fail when data shards are on another disk"
        );
        assert!(
            !std::path::Path::new(&ecx_path).exists(),
            "failed ecx rebuild must not leave a partial .ecx file behind"
        );

        // With additional_dirs pointing at the secondary, the rebuild must succeed.
        rebuild_ecx_file(&primary, "", VolumeId(1), 10, 0, 0, &[secondary.as_str()]).unwrap();

        assert!(
            std::path::Path::new(&ecx_path).exists(),
            "rebuilt .ecx should exist in primary dir"
        );
        assert!(
            std::fs::metadata(&ecx_path).unwrap().len() > 0,
            "rebuilt .ecx must be non-empty"
        );
    }

    // A uniform-layout volume (block size > 1MiB) must have its .ecx rebuilt
    // through the recorded geometry; the legacy 1MiB mapping would scan
    // garbage past the first block boundary.
    #[test]
    fn test_rebuild_ecx_file_uniform_layout() {
        use crate::storage::needle_map::NeedleMapKind;
        use crate::storage::volume::Volume;
        let tmp = TempDir::new().unwrap();
        let dir = tmp.path().to_str().unwrap().to_string();
        let mut v = Volume::new(
            &dir,
            &dir,
            "",
            VolumeId(2),
            NeedleMapKind::InMemory,
            None,
            None,
            0,
            Version::current(),
        )
        .unwrap();
        for i in 1u64..=12 {
            let data: Vec<u8> = (0..2 << 20)
                .map(|b| ((b as u64).wrapping_mul(2654435761).wrapping_add(i) >> 8) as u8)
                .collect();
            let mut n = Needle {
                id: NeedleId(i),
                cookie: Cookie(i as u32),
                data: data.clone(),
                data_size: data.len() as u32,
                ..Needle::default()
            };
            v.write_needle(&mut n, true, false).unwrap();
        }
        v.sync_to_disk().unwrap();
        v.close();
        let block_size = write_ec_files(&dir, &dir, "", VolumeId(2), 10, 4).unwrap();
        assert!(
            block_size > ERASURE_CODING_SMALL_BLOCK_SIZE as i64,
            "fixture must diverge from the legacy layout"
        );

        let ecx_path = format!("{}/2.ecx", dir);
        let canonical = std::fs::read(&ecx_path).unwrap();
        std::fs::remove_file(&ecx_path).unwrap();

        rebuild_ecx_file(&dir, "", VolumeId(2), 10, block_size, 0, &[]).unwrap();
        let rebuilt = std::fs::read(&ecx_path).unwrap();
        assert_eq!(canonical, rebuilt, "rebuilt .ecx must match the encode-time .ecx");
    }

    // A truncated data shard must FAIL the .ecx rebuild, not publish the
    // entries scanned so far as a successful (silently incomplete) index.
    #[test]
    fn test_rebuild_ecx_file_fails_on_truncated_shard() {
        use crate::storage::needle_map::NeedleMapKind;
        use crate::storage::volume::Volume;
        let tmp = TempDir::new().unwrap();
        let dir = tmp.path().to_str().unwrap().to_string();
        let mut v = Volume::new(
            &dir,
            &dir,
            "",
            VolumeId(3),
            NeedleMapKind::InMemory,
            None,
            None,
            0,
            Version::current(),
        )
        .unwrap();
        for i in 1u64..=12 {
            let data: Vec<u8> = (0..2 << 20)
                .map(|b| ((b as u64).wrapping_mul(2654435761).wrapping_add(i) >> 8) as u8)
                .collect();
            let mut n = Needle {
                id: NeedleId(i),
                cookie: Cookie(i as u32),
                data: data.clone(),
                data_size: data.len() as u32,
                ..Needle::default()
            };
            v.write_needle(&mut n, true, false).unwrap();
        }
        v.sync_to_disk().unwrap();
        v.close();
        let block_size = write_ec_files(&dir, &dir, "", VolumeId(3), 10, 4).unwrap();

        let ecx_path = format!("{}/3.ecx", dir);
        std::fs::remove_file(&ecx_path).unwrap();

        // Truncate shard 0 to just the superblock: the scan's very first
        // needle-header read (offset SUPER_BLOCK_SIZE, shard 0 under the
        // uniform layout) lands in the missing region. The pre-fix code
        // broke the scan there and published an EMPTY .ecx as success.
        let shard_path = format!("{}/3.ec00", dir);
        let f = std::fs::OpenOptions::new()
            .write(true)
            .open(&shard_path)
            .unwrap();
        f.set_len(crate::storage::super_block::SUPER_BLOCK_SIZE as u64)
            .unwrap();
        drop(f);

        let res = rebuild_ecx_file(&dir, "", VolumeId(3), 10, block_size, 0, &[]);
        assert!(res.is_err(), "rebuild over a truncated shard must fail");
        assert!(
            !std::path::Path::new(&ecx_path).exists(),
            "a failed rebuild must not leave a partial .ecx behind"
        );
    }

    #[test]
    fn test_reed_solomon_basic() {
        let data_shards = 10;
        let parity_shards = 4;
        let total_shards = data_shards + parity_shards;
        let rs = ReedSolomon::new(data_shards, parity_shards).unwrap();
        let block_size = 1024;
        let mut shards: Vec<Vec<u8>> = (0..total_shards)
            .map(|i| {
                if i < data_shards {
                    vec![(i as u8).wrapping_mul(7); block_size]
                } else {
                    vec![0u8; block_size]
                }
            })
            .collect();

        // Encode
        rs.encode(&mut shards).unwrap();

        // Verify parity is non-zero (at least some)
        let parity_nonzero: bool = shards[data_shards..]
            .iter()
            .any(|s| s.iter().any(|&b| b != 0));
        assert!(parity_nonzero);

        // Simulate losing 4 shards and reconstructing
        let original_0 = shards[0].clone();
        let original_1 = shards[1].clone();

        let mut shard_opts: Vec<Option<Vec<u8>>> = shards.into_iter().map(Some).collect();
        shard_opts[0] = None;
        shard_opts[1] = None;
        shard_opts[2] = None;
        shard_opts[3] = None;

        rs.reconstruct(&mut shard_opts).unwrap();

        assert_eq!(shard_opts[0].as_ref().unwrap(), &original_0);
        assert_eq!(shard_opts[1].as_ref().unwrap(), &original_1);
    }

    /// EC encode must read .idx from a separate index directory when configured.
    #[test]
    fn test_ec_encode_with_separate_idx_dir() {
        let dat_tmp = TempDir::new().unwrap();
        let idx_tmp = TempDir::new().unwrap();
        let dat_dir = dat_tmp.path().to_str().unwrap();
        let idx_dir = idx_tmp.path().to_str().unwrap();

        // Create a volume with separate data and index directories
        let mut v = Volume::new(
            dat_dir,
            idx_dir,
            "",
            VolumeId(1),
            NeedleMapKind::InMemory,
            None,
            None,
            0,
            Version::current(),
        )
        .unwrap();

        for i in 1..=5 {
            let data = format!("needle {} payload", i);
            let mut n = Needle {
                id: NeedleId(i),
                cookie: Cookie(i as u32),
                data: data.as_bytes().to_vec(),
                data_size: data.len() as u32,
                ..Needle::default()
            };
            v.write_needle(&mut n, true, false).unwrap();
        }
        v.sync_to_disk().unwrap();
        v.close();

        // Verify .dat is in data dir, .idx is in idx dir
        assert!(std::path::Path::new(&format!("{}/1.dat", dat_dir)).exists());
        assert!(!std::path::Path::new(&format!("{}/1.idx", dat_dir)).exists());
        assert!(std::path::Path::new(&format!("{}/1.idx", idx_dir)).exists());
        assert!(!std::path::Path::new(&format!("{}/1.dat", idx_dir)).exists());

        // EC encode with separate idx dir
        let data_shards = 10;
        let parity_shards = 4;
        let total_shards = data_shards + parity_shards;
        write_ec_files(
            dat_dir,
            idx_dir,
            "",
            VolumeId(1),
            data_shards,
            parity_shards,
        )
        .unwrap();

        // Verify all 14 shard files in data dir
        for i in 0..total_shards {
            let path = format!("{}/1.ec{:02}", dat_dir, i);
            assert!(
                std::path::Path::new(&path).exists(),
                "shard {} should exist in data dir",
                path
            );
        }

        // Verify .ecx in data dir (not idx dir)
        assert!(std::path::Path::new(&format!("{}/1.ecx", dat_dir)).exists());
        assert!(!std::path::Path::new(&format!("{}/1.ecx", idx_dir)).exists());

        // Verify no shard files leaked into idx dir
        for i in 0..total_shards {
            let path = format!("{}/1.ec{:02}", idx_dir, i);
            assert!(
                !std::path::Path::new(&path).exists(),
                "shard {} should NOT exist in idx dir",
                path
            );
        }
    }

    /// EC encode should fail gracefully when .idx is only in the data dir
    /// but we pass a wrong idx_dir. This guards against regressions where
    /// write_ec_files ignores the idx_dir parameter.
    #[test]
    fn test_ec_encode_fails_with_wrong_idx_dir() {
        let dat_tmp = TempDir::new().unwrap();
        let idx_tmp = TempDir::new().unwrap();
        let wrong_tmp = TempDir::new().unwrap();
        let dat_dir = dat_tmp.path().to_str().unwrap();
        let idx_dir = idx_tmp.path().to_str().unwrap();
        let wrong_dir = wrong_tmp.path().to_str().unwrap();

        let mut v = Volume::new(
            dat_dir,
            idx_dir,
            "",
            VolumeId(1),
            NeedleMapKind::InMemory,
            None,
            None,
            0,
            Version::current(),
        )
        .unwrap();

        let mut n = Needle {
            id: NeedleId(1),
            cookie: Cookie(1),
            data: b"hello".to_vec(),
            data_size: 5,
            ..Needle::default()
        };
        v.write_needle(&mut n, true, false).unwrap();
        v.sync_to_disk().unwrap();
        v.close();

        // Should fail: .idx is in idx_dir, not wrong_dir
        let result = write_ec_files(dat_dir, wrong_dir, "", VolumeId(1), 10, 4);
        assert!(
            result.is_err(),
            "should fail when idx_dir doesn't contain .idx"
        );
    }
}