seaweedfs/weed/storage/store_ec.go

package storage

import (
	"bytes"
	"context"
	"errors"
	"fmt"
	"io"
	"os"
	"slices"
	"sync"
	"sync/atomic"
	"time"

	"github.com/klauspost/reedsolomon"
	"golang.org/x/sync/semaphore"

	"github.com/seaweedfs/seaweedfs/weed/glog"
	"github.com/seaweedfs/seaweedfs/weed/operation"
	"github.com/seaweedfs/seaweedfs/weed/pb"
	"github.com/seaweedfs/seaweedfs/weed/pb/master_pb"
	"github.com/seaweedfs/seaweedfs/weed/pb/volume_server_pb"
	"github.com/seaweedfs/seaweedfs/weed/stats"
	"github.com/seaweedfs/seaweedfs/weed/storage/erasure_coding"
	"github.com/seaweedfs/seaweedfs/weed/storage/needle"
	"github.com/seaweedfs/seaweedfs/weed/storage/types"
)

// errShardNotLocal indicates that the requested EC shard is simply not
// stored on this volume server. It is expected during normal reads when
// shards are spread across multiple servers, so callers should not log
// it as an error.
var errShardNotLocal = errors.New("ec shard not on this server")

// FindEcShardTargetLocation returns the disk that should receive a new
// shard / index file for (collection, vid). The selection order is:
//
//  0. a disk that already owns one of shardIds (in-memory claim),
//  1. a disk that already has the EC volume mounted (in-memory state),
//  2. a disk that owns the .ecx file on disk (volume not mounted yet),
//  3. any HDD with free space,
//  4. any disk with free space.
//
// Step 0 keeps the per-server invariant that a shard id is owned by at
// most one disk. Steps 1-4 only know the volume, and a multi-disk server
// legitimately mounts the same vid on several disks, so the free-count
// tie-break alone can send a re-copy of a shard the server already holds
// (a retried ec.balance / ec.rebuild move) to a sibling disk. Both disks
// then claim the same (vid, shard) and report it to the master from two
// disk ids, and which claimant serves reads or survives a later
// unmount/delete of the shard id becomes an accident of Locations order.
// Overwriting in place is what the caller meant, so an owning disk wins
// ahead of the space filters too — a re-copy of a shard already on that
// disk needs no new shard slot, and a genuinely full disk fails the
// write with ENOSPC rather than silently splitting the claim.
//
// Step 2 is the missing primitive that pinned subsequent shards to the
// first-shard disk during ec.rebuild. ec.rebuild only sets CopyEcxFile=true
// for the first shard, then relies on auto-select to land later shards on
// the same disk. Without an on-disk check, FindEcVolume returns nothing
// (no mount yet) and the fallback picks "any HDD with free space" — which
// can split shards from their index files across disks of the same node
// and lose them at startup. See issue #9212 and the orphan-shard
// reconciliation in #9244.
//
// dataShardCount is the data-shard count for this volume's EC layout (10
// for the OSS default, but custom ratios are supported via .vif). Callers
// pass it explicitly so this helper stays free of package-level constants
// — easier to mirror into builds that ship a different default ratio.
//
// Implementation walks s.Locations once and scores each disk by tier; the
// highest-tier disk wins, ties broken by free count. The earlier waterfall
// across four FindFreeLocation passes was equivalent but acquired
// volumesLock and ecVolumesLock RLocks (via VolumesLen / EcShardCount) up
// to four times per disk per call.
func (s *Store) FindEcShardTargetLocation(collection string, vid needle.VolumeId, dataShardCount int, shardIds ...erasure_coding.ShardId) *DiskLocation {
	const (
		tierAnyDisk = iota + 1
		tierHDD
		tierEcxOnDisk
		tierMounted
		tierOwnsShard
	)

	var (
		best      *DiskLocation
		bestTier  int
		bestOwned int
		bestFree  int32
	)
	for _, loc := range s.Locations {
		owned := ownedEcShardCount(loc, vid, shardIds)
		if loc.readOnlyMount {
			// A read-only owner still owns the shard: placing another copy
			// on a writable sibling would register it on two disks.
			if owned > 0 {
				return nil
			}
			continue
		}
		freeCount := ecFreeShardCount(loc, dataShardCount)
		if owned == 0 {
			if loc.isDiskSpaceLow.Load() {
				continue
			}
			if freeCount <= 0 {
				continue
			}
		}
		tier := tierAnyDisk
		if loc.DiskType == types.HardDriveType {
			tier = tierHDD
		}
		if loc.HasEcxFileOnDisk(collection, vid) {
			tier = tierEcxOnDisk
		}
		if _, mounted := loc.FindEcVolume(vid); mounted {
			tier = tierMounted
		}
		if owned > 0 {
			tier = tierOwnsShard
		}
		better := best == nil || tier > bestTier
		if !better && tier == bestTier {
			// owned only separates disks inside tierOwnsShard; it is 0
			// everywhere else, so this falls through to the free-count
			// tie-break for the other tiers.
			better = owned > bestOwned || (owned == bestOwned && freeCount > bestFree)
		}
		if better {
			best = loc
			bestTier = tier
			bestOwned = owned
			bestFree = freeCount
		}
	}
	return best
}

// EcShardOwnerDisks returns the distinct disks that already own one of
// shardIds for vid, in Locations order. More than one owner means the batch
// has no single correct destination — whichever disk receives it would
// duplicate a sibling disk's claim — so batch callers must split by owner
// (VolumeEcShardsCopy refuses such a batch instead of guessing).
func (s *Store) EcShardOwnerDisks(vid needle.VolumeId, shardIds []erasure_coding.ShardId) []*DiskLocation {
	var owners []*DiskLocation
	for _, loc := range s.Locations {
		if ownedEcShardCount(loc, vid, shardIds) > 0 {
			owners = append(owners, loc)
		}
	}
	return owners
}

// ownedEcShardCount reports how many of shardIds this disk already claims
// for vid, per the in-memory registration the read path and heartbeats use.
func ownedEcShardCount(loc *DiskLocation, vid needle.VolumeId, shardIds []erasure_coding.ShardId) int {
	if len(shardIds) == 0 {
		return 0
	}
	loc.ecVolumesLock.RLock()
	defer loc.ecVolumesLock.RUnlock()
	ecVolume, found := loc.ecVolumes[vid]
	if !found {
		return 0
	}
	owned := 0
	for _, shard := range ecVolume.Shards {
		for _, shardId := range shardIds {
			if shard.ShardId == shardId {
				owned++
				break
			}
		}
	}
	return owned
}

// ecFreeShardCount returns the free EC shard capacity of loc, expressed
// in shard slots (not volume-equivalent slots). dataShardCount is the
// data-shard count of the EC layout being placed — see
// FindEcShardTargetLocation's docstring for why it's a parameter.
//
// FindFreeLocation in store.go does the same math but divides by
// DataShardsCount at the end. That truncation can exclude a disk that
// still has room for several individual shards (e.g. MaxVolumeCount=1,
// EcShardCount=1, dataShardCount=10 → reports 0 despite 9 free shard
// slots), which in this helper would re-route subsequent shards off the
// .ecx-owning disk and re-introduce the orphan-shard layout #9212 is
// trying to prevent. So we keep the result in shard slots throughout.
//
// MaxVolumeCount == 0 is the "unlimited" sentinel used elsewhere in the
// store (see hasFreeDiskLocation). Reporting a synthetic large free
// count keeps unlimited disks eligible while still letting tie-breaks
// prefer the less-loaded one.
func ecFreeShardCount(loc *DiskLocation, dataShardCount int) int32 {
	if dataShardCount <= 0 {
		return 0
	}
	if loc.MaxVolumeCount <= 0 {
		const unlimitedFree = int32(1 << 30)
		used := int32(loc.VolumesLen())*int32(dataShardCount) + int32(loc.EcShardCount())
		if used >= unlimitedFree {
			return 1
		}
		return unlimitedFree - used
	}
	free := (loc.MaxVolumeCount - int32(loc.VolumesLen())) * int32(dataShardCount)
	free -= int32(loc.EcShardCount())
	if free < 0 {
		return 0
	}
	return free
}

func (s *Store) CollectErasureCodingHeartbeat() *master_pb.Heartbeat {
	var ecShardMessages []*master_pb.VolumeEcShardInformationMessage
	collectionEcShardSize := make(map[string]int64)
	for diskId, location := range s.Locations {
		location.ecVolumesLock.RLock()
		for _, ecShards := range location.ecVolumes {
			if _, _, quarantined := ecShards.GetIoErrorState(); quarantined {
				continue
			}
			ecShardMessages = append(ecShardMessages, ecShards.ToVolumeEcShardInformationMessage(uint32(diskId))...)

			for _, ecShard := range ecShards.Shards {
				collectionEcShardSize[ecShards.Collection] += ecShard.Size()
			}
		}
		location.ecVolumesLock.RUnlock()
	}

	for col, size := range collectionEcShardSize {
		stats.VolumeServerDiskSizeGauge.WithLabelValues(col, "ec").Set(float64(size))
	}

	for col := range s.reportedEcCollections {
		if _, stillHere := collectionEcShardSize[col]; !stillHere {
			stats.VolumeServerDiskSizeGauge.DeleteLabelValues(col, "ec")
		}
	}
	s.reportedEcCollections = make(map[string]struct{}, len(collectionEcShardSize))
	for col := range collectionEcShardSize {
		s.reportedEcCollections[col] = struct{}{}
	}

	return &master_pb.Heartbeat{
		EcShards:      ecShardMessages,
		HasNoEcShards: len(ecShardMessages) == 0,
	}

}

func (s *Store) MountEcShards(collection string, vid needle.VolumeId, shardId erasure_coding.ShardId, sourceDiskType string) error {
	ecVolume, diskId, err := s.LoadEcShardAcrossDisks(collection, vid, shardId)
	if err != nil {
		return err
	}
	glog.V(0).Infof("MountEcShards %d.%d on disk ID %d", vid, shardId, diskId)

	// Apply the orchestrator-supplied source disk type so the EC
	// volume reports under it instead of the location's. Empty means
	// "fall back to location's disk type" (#9423).
	if sourceDiskType != "" {
		ecVolume.SetDiskType(types.ToDiskType(sourceDiskType))
	}

	si := erasure_coding.NewShardsInfo()
	si.Set(erasure_coding.NewShardInfo(shardId, erasure_coding.ShardSize(ecVolume.ShardSize())))
	s.NewEcShardsChan <- &master_pb.VolumeEcShardInformationMessage{
		Id:          uint32(vid),
		Collection:  collection,
		EcIndexBits: uint32(si.Bitmap()),
		ShardSizes:  si.SizesInt64(),
		DiskType:    string(ecVolume.DiskType()),
		ExpireAtSec: ecVolume.ExpireAtSec,
		DiskId:      uint32(diskId),
		EncodeTsNs:  ecVolume.EncodeTsNs,
	}
	s.UpdateMaxVolumeIdSeen(vid)
	return nil
}

// LoadEcShardAcrossDisks mounts shardId on whichever local disk holds its
// file and returns the mounted volume plus the disk it landed on. It emits
// no heartbeat delta — callers that change the master's view do that
// themselves (MountEcShards), while same-server repoints that must not flip
// the master's registration use it directly.
func (s *Store) LoadEcShardAcrossDisks(collection string, vid needle.VolumeId, shardId erasure_coding.ShardId) (*erasure_coding.EcVolume, uint32, error) {
	// The .ecx index file may live on a different disk than the one
	// holding the .ec?? shard being mounted: ec.balance / ec.rebuild can
	// place the .ecx on one local disk while later distributing shards
	// across sibling disks of the same volume server. The per-disk
	// IdxDirectory used by LoadEcShard would ENOENT the .ecx, so look up
	// the .ecx owner across all DiskLocations once and route NewEcVolume
	// at the directory that actually has the file. A 0-byte .ecx is
	// treated as missing here (writeToFile can leave a stub on a failed
	// EC distribute) so we still scan the rest of the disks.
	ecxIdxDir, ecxFound := s.findEcxIdxDirForVolume(collection, vid)

	// Collect failures so an all-disks-fail return reports every disk we
	// tried rather than just the first one. Before this loop reordered
	// itself to keep going after the first non-ENOENT error, a single
	// shard-on-disk-without-.ecx situation would bail the loop and the
	// operator saw "cannot open ec volume index" naming exactly one disk
	// even when others held a valid index.
	type diskError struct {
		dir string
		err error
	}
	var failures []diskError

	for diskId, location := range s.Locations {
		idxDir := location.IdxDirectory
		if ecxFound {
			// Fast path: if findEcxIdxDirForVolume already pointed at
			// one of this disk's directories, the disk owns the .ecx
			// and the local IdxDirectory is the right answer — skip
			// the HasEcxFileOnDisk stat. Only fall back to the sibling
			// disk's idxDir when this disk's directories are neither.
			if location.IdxDirectory != ecxIdxDir && location.Directory != ecxIdxDir {
				if !location.HasEcxFileOnDisk(collection, vid) {
					idxDir = ecxIdxDir
				}
			}
		}
		ecVolume, err := location.loadEcShardWithIdxDir(collection, vid, shardId, idxDir)
		if err == nil {
			return ecVolume, uint32(diskId), nil
		}
		if errors.Is(err, os.ErrNotExist) {
			// Shard or index not on this disk; another disk may own it.
			continue
		}
		failures = append(failures, diskError{dir: location.Directory, err: err})
	}

	if len(failures) == 0 {
		// No disk had the shard or the index; this volume server is not
		// holding any artefacts for the requested shard. Name what we
		// scanned for so the operator can tell "no .ecx anywhere" apart
		// from "shard not on this server".
		if !ecxFound {
			return nil, 0, fmt.Errorf("MountEcShards %d.%d: no .ecx index found on any local disk", vid, shardId)
		}
		return nil, 0, fmt.Errorf("MountEcShards %d.%d not found on disk", vid, shardId)
	}
	// Some disks returned a real (non-ENOENT) error. Report them all so
	// the caller can see whether the failures cluster around one disk
	// (likely hardware) or are spread out (likely a config problem).
	var b []byte
	for i, f := range failures {
		if i > 0 {
			b = append(b, "; "...)
		}
		b = append(b, fmt.Sprintf("%s: %v", f.dir, f.err)...)
	}
	return nil, 0, fmt.Errorf("MountEcShards %d.%d load failures: %s", vid, shardId, string(b))
}

func (s *Store) UnmountEcShards(vid needle.VolumeId, shardId erasure_coding.ShardId, reqEncodeTsNs int64) error {
	// Walk every disk: a split-disk reconciled volume can mount the same vid on
	// more than one disk, so a first-match unmount would leave a sibling copy
	// mounted and heartbeating. Emit one deletion delta per disk.
	unmountedAny := false
	var lastErr error
	for diskId, location := range s.Locations {
		ecShard, found := location.FindEcShard(vid, shardId)
		if !found {
			continue
		}
		// Capture the encode generation before unloading so the deletion delta
		// carries it like the mount delta does.
		var encodeTsNs int64
		if ecVolume, ok := location.FindEcVolume(vid); ok {
			encodeTsNs = ecVolume.EncodeTsNs
		}
		// Generation fence: when the caller carries a generation (stale-worker
		// cleanup), only unmount a strictly-older generation; preserve a disk whose
		// generation is same-or-newer, 0, or unknown, so a stale run cannot unmount
		// a newer run's live shards. reqEncodeTsNs==0 (legacy/shell) unmounts all.
		if reqEncodeTsNs > 0 && !(encodeTsNs > 0 && encodeTsNs < reqEncodeTsNs) {
			glog.V(1).Infof("UnmountEcShards %d.%d disk_id:%d skipped: disk gen %d not older than request gen %d", vid, shardId, diskId, encodeTsNs, reqEncodeTsNs)
			continue
		}
		if deleted := location.UnloadEcShard(vid, shardId); deleted {
			si := erasure_coding.NewShardsInfo()
			si.Set(erasure_coding.NewShardInfo(shardId, 0))
			s.DeletedEcShardsChan <- &master_pb.VolumeEcShardInformationMessage{
				Id:          uint32(vid),
				Collection:  ecShard.Collection,
				EcIndexBits: si.Bitmap(),
				ShardSizes:  si.SizesInt64(),
				DiskType:    string(ecShard.DiskType),
				DiskId:      uint32(diskId),
				EncodeTsNs:  encodeTsNs,
			}
			glog.V(0).Infof("UnmountEcShards %d.%d disk_id:%d", vid, shardId, diskId)
			unmountedAny = true
		} else {
			lastErr = fmt.Errorf("UnmountEcShards %d.%d not found on disk %d", vid, shardId, diskId)
		}
	}

	// nil when no disk held the shard (idempotent re-unmount).
	if !unmountedAny {
		return lastErr
	}
	return nil
}

func (s *Store) findEcShard(vid needle.VolumeId, shardId erasure_coding.ShardId) (diskId uint32, shard *erasure_coding.EcVolumeShard, found bool) {
	for diskId, location := range s.Locations {
		if v, found := location.FindEcShard(vid, shardId); found {
			return uint32(diskId), v, found
		}
	}
	return 0, nil, false
}

// FindEcShard returns the shard if any DiskLocation on this server holds it,
// along with that disk's id.
func (s *Store) FindEcShard(vid needle.VolumeId, shardId erasure_coding.ShardId) (diskId uint32, shard *erasure_coding.EcVolumeShard, found bool) {
	return s.findEcShard(vid, shardId)
}

// FindEcVolumeWithShard returns the EcVolume on the disk that owns the given
// shard, plus the shard. The read guard must check the identity of the volume
// that owns the bytes served: on a multi-disk server one vid can hold shards
// from different encode runs across disks, so a first-match volume can differ.
func (s *Store) FindEcVolumeWithShard(vid needle.VolumeId, shardId erasure_coding.ShardId) (*erasure_coding.EcVolume, *erasure_coding.EcVolumeShard, bool) {
	for _, location := range s.Locations {
		if shard, found := location.FindEcShard(vid, shardId); found {
			if ev, ok := location.FindEcVolume(vid); ok {
				return ev, shard, true
			}
		}
	}
	return nil, nil, false
}

// EcMetadataDirs lists every directory on this server that could hold an EC
// volume's metadata — each disk's data and index directory. Startup mirroring
// gives each shard-bearing disk its own .ecx/.ecj/.vif, but the checksum
// sidecar is not mirrored and a repair delivers exactly one copy, so a runtime
// looking only at its own two directories cannot see it. Handing this list to
// the sidecar resolution keeps one authoritative copy reachable from every
// runtime rather than duplicating a file that is rewritten as shards are
// repaired and generations published.
func (s *Store) EcMetadataDirs() []string {
	var dirs []string
	appendDir := func(dir string) {
		if dir == "" {
			return
		}
		for _, existing := range dirs {
			if existing == dir {
				return
			}
		}
		dirs = append(dirs, dir)
	}
	for _, location := range s.Locations {
		appendDir(location.Directory)
		appendDir(location.IdxDirectory)
	}
	return dirs
}

// FindAllEcVolumes returns every per-disk *EcVolume the store maps for vid. A vid
// can mount on N disks as N distinct runtimes, and the first-match FindEcVolume
// hides the siblings — so anything that has to reach the whole volume, rather
// than any one runtime of it, iterates this instead. Order mirrors the
// deterministic s.Locations order; nil when no disk holds the vid.
func (s *Store) FindAllEcVolumes(vid needle.VolumeId) []*erasure_coding.EcVolume {
	var evs []*erasure_coding.EcVolume
	for _, location := range s.Locations {
		if ev, found := location.FindEcVolume(vid); found {
			evs = append(evs, ev)
		}
	}
	return evs
}

func (s *Store) FindEcVolume(vid needle.VolumeId) (*erasure_coding.EcVolume, bool) {
	for _, location := range s.Locations {
		if s, found := location.FindEcVolume(vid); found {
			return s, true
		}
	}
	return nil, false
}

// FindEcVolumeDiskIds returns every disk_id on this store that has an
// EcVolume entry for the given volume. Useful for diagnostic logging
// when a single FindEcVolume hit hides which disk is actually holding
// the mount (e.g., the ReceiveFile mounted-volume guard).
func (s *Store) FindEcVolumeDiskIds(vid needle.VolumeId) []uint32 {
	var ids []uint32
	for diskId, location := range s.Locations {
		if _, found := location.FindEcVolume(vid); found {
			ids = append(ids, uint32(diskId))
		}
	}
	return ids
}

// shardFiles is a list of shard files, which is used to return the shard locations
func (s *Store) CollectEcShards(vid needle.VolumeId, shardFileNames []string) (ecVolume *erasure_coding.EcVolume, found bool) {
	for _, location := range s.Locations {
		if s, foundShards := location.CollectEcShards(vid, shardFileNames); foundShards {
			ecVolume = s
			found = true
		}
	}
	return
}

func (s *Store) DestroyEcVolume(vid needle.VolumeId) {
	for _, location := range s.Locations {
		location.DestroyEcVolume(vid)
	}
}

// UnloadEcVolume drops any in-memory EcVolume for vid from every disk and closes
// its fds without deleting files, so a following unlink frees the inodes.
func (s *Store) UnloadEcVolume(vid needle.VolumeId) {
	for _, location := range s.Locations {
		location.unloadEcVolume(vid)
	}
}

func (s *Store) ReadEcShardNeedle(vid needle.VolumeId, n *needle.Needle, onReadSizeFn func(size types.Size)) (int, error) {
	for _, location := range s.Locations {
		if localEcVolume, found := location.FindEcVolume(vid); found {

			offset, size, intervals, err := localEcVolume.LocateEcShardNeedle(n.Id, localEcVolume.Version)
			if err != nil {
				return 0, fmt.Errorf("locate in local ec volume: %w", err)
			}
			if size.IsDeleted() {
				return 0, ErrorDeleted
			}

			if onReadSizeFn != nil {
				onReadSizeFn(size)
			}

			glog.V(3).Infof("read ec volume %d offset %d size %d intervals:%+v", vid, offset.ToActualOffset(), size, intervals)

			if len(intervals) > 1 {
				glog.V(3).Infof("ReadEcShardNeedle needle id %s intervals:%+v", n.String(), intervals)
			}
			// ReadBytes re-reads the id out of a possibly corrupt header, so the
			// requested id is kept aside for the rebuild path to recover under.
			needleId := n.Id
			bytes, isDeleted, err := s.readEcShardIntervals(needleId, localEcVolume, intervals)
			// A holder reporting the needle deleted is authoritative -- deletes are
			// never invented and never undone -- so answer that ahead of whatever
			// error the shards it could not gather produced.
			if isDeleted {
				return 0, ErrorDeleted
			}
			if err != nil {
				return 0, fmt.Errorf("ReadEcShardIntervals: %w", err)
			}

			err = n.ReadBytes(bytes, offset.ToActualOffset(), size, localEcVolume.Version)
			if err != nil {
				if rebuilt, isDeleted := s.rebuildEcShardNeedle(n, needleId, localEcVolume, intervals, offset.ToActualOffset(), size, bytes); isDeleted {
					return 0, ErrorDeleted
				} else if rebuilt != nil {
					return len(rebuilt), nil
				}
				return 0, fmt.Errorf("ec volume %d needle %s offset %d size %d: %w", vid, n.String(), offset.ToActualOffset(), size, err)
			}

			return len(bytes), nil
		}
	}
	return 0, fmt.Errorf("ec shard %d not found", vid)
}

var (
	// ecRecoverBudget bounds the interval-sized buffers EC recovery holds across
	// all concurrent reads: a peer that is slow to fail keeps a whole fan-out of
	// them alive for the gRPC timeout, and a burst of those walked servers into an
	// OOM. A var so a test can narrow it alongside ecRecoverSem.
	ecRecoverBudget int64 = 256 << 20
	ecRecoverSem          = semaphore.NewWeighted(ecRecoverBudget)
)

// ecIntervalReadConcurrency bounds the fan-out of a single needle read. Blocks
// that follow each other in the .dat live on different shards, so a needle
// spanning several of them costs one round trip per block when read in sequence.
const ecIntervalReadConcurrency = 8

func (s *Store) readEcShardIntervals(needleId types.NeedleId, ecVolume *erasure_coding.EcVolume, intervals []erasure_coding.Interval) (data []byte, is_deleted bool, err error) {
	if err = s.cachedLookupEcShardLocations(ecVolume); err != nil {
		return nil, false, fmt.Errorf("failed to locate shard via master grpc %s: %v", s.masterAddress(), err)
	}

	var totalSize int
	for _, interval := range intervals {
		totalSize += int(interval.Size)
	}
	data = make([]byte, totalSize)

	if len(intervals) <= 1 {
		for _, interval := range intervals {
			if is_deleted, err = s.readOneEcShardInterval(needleId, ecVolume, interval, data); err != nil {
				return nil, is_deleted, err
			}
		}
		return data, is_deleted, nil
	}

	errs := make([]error, len(intervals))
	var deleted atomic.Bool
	var wg sync.WaitGroup
	sem := make(chan struct{}, ecIntervalReadConcurrency)
	var pos int
	for i, interval := range intervals {
		buf := data[pos : pos+int(interval.Size)]
		pos += int(interval.Size)
		wg.Add(1)
		sem <- struct{}{}
		go func() {
			defer wg.Done()
			defer func() { <-sem }()
			isDeleted, e := s.readOneEcShardInterval(needleId, ecVolume, interval, buf)
			if isDeleted {
				deleted.Store(true)
			}
			errs[i] = e
		}()
	}
	wg.Wait()

	is_deleted = deleted.Load()
	for _, e := range errs {
		if e != nil {
			return nil, is_deleted, e
		}
	}
	return data, is_deleted, nil
}

// readOneEcShardInterval fills data, which must be interval.Size long.
func (s *Store) readOneEcShardInterval(needleId types.NeedleId, ecVolume *erasure_coding.EcVolume, interval erasure_coding.Interval, data []byte) (is_deleted bool, err error) {
	shardId, actualOffset := ecVolume.IntervalToShardIdAndOffset(interval)

	// try local read
	err = s.readLocalEcShardInterval(ecVolume, shardId, data, actualOffset)
	if err == nil {
		return
	}
	if errors.Is(err, errShardNotLocal) {
		// expected when shards are spread across servers; fall through to remote read
		glog.V(4).Infof("ec shard %d.%d not local, will try remote", ecVolume.VolumeId, shardId)
	} else {
		glog.V(0).Infof("read local ec shard %d.%d offset %d: %v", ecVolume.VolumeId, shardId, actualOffset, err)
	}

	ecVolume.ShardLocationsLock.RLock()
	sourceDataNodes, hasShardIdLocation := ecVolume.ShardLocations[shardId]
	ecVolume.ShardLocationsLock.RUnlock()

	// try reading directly
	if hasShardIdLocation {
		_, is_deleted, err = s.readRemoteEcShardInterval(sourceDataNodes, needleId, ecVolume.VolumeId, shardId, data, actualOffset, ecVolume.EncodeTsNs)
		if err == nil {
			return
		}
		glog.V(0).Infof("read remote ec shard %d.%d locations: %v", ecVolume.VolumeId, shardId, err)
		// Recovery below skips this very shard, so nothing else invalidates the
		// location that just failed -- and a shard that has moved would otherwise
		// be reconstructed on every read until the map's own window expires.
		markShardLocationsStale(ecVolume)
	}

	// try reading by recovering from other shards
	_, is_deleted, err = s.recoverOneRemoteEcShardInterval(needleId, ecVolume, shardId, data, actualOffset)
	if err == nil {
		return
	}
	glog.V(0).Infof("recover ec shard %d.%d : %v", ecVolume.VolumeId, shardId, err)

	return
}

// rebuildEcShardNeedle repairs a needle whose reassembled bytes failed to
// parse: a shard that served an interval can be silently corrupt, so each
// interval's source shard is a suspect. Rebuilding the suspect ranges from the
// surviving shards — with the suspects excluded from the gather, so a bad copy
// cannot poison its own reconstruction — produces the bytes Reed-Solomon says
// are right, and a successful needle parse arbitrates them correct. On success
// the parse has already hydrated n, every shard whose range changed is dropped
// from the location cache, and the repaired bytes are returned. Nil when no
// hypothesis parses (index or needle corrupt in all shards alike). A deleted
// report from a holder stops the search: deletes are never invented, so the
// needle is gone whatever the local index says.
func (s *Store) rebuildEcShardNeedle(n *needle.Needle, needleId types.NeedleId, ecVolume *erasure_coding.EcVolume, intervals []erasure_coding.Interval, offset int64, size types.Size, corrupted []byte) ([]byte, bool) {
	ecCtx := ecVolume.ECContext
	if ecCtx == nil {
		ecCtx = erasure_coding.NewDefaultECContext(ecVolume.Collection, ecVolume.VolumeId)
	}

	shardIds := make([]erasure_coding.ShardId, len(intervals))
	offsets := make([]int64, len(intervals))
	suspects := make(map[erasure_coding.ShardId]bool)
	for i, interval := range intervals {
		shardIds[i], offsets[i] = ecVolume.IntervalToShardIdAndOffset(interval)
		suspects[shardIds[i]] = true
	}

	try := func(exclude map[erasure_coding.ShardId]bool) ([]byte, bool) {
		candidate := make([]byte, len(corrupted))
		pos := 0
		for i, interval := range intervals {
			buf := candidate[pos : pos+int(interval.Size)]
			pos += int(interval.Size)
			if !exclude[shardIds[i]] {
				copy(buf, corrupted[pos-int(interval.Size):pos])
				continue
			}
			_, isDeleted, err := s.recoverEcShardInterval(needleId, ecVolume, shardIds[i], buf, offsets[i], exclude)
			if isDeleted {
				return nil, true
			}
			if err != nil {
				return nil, false
			}
		}
		// A candidate whose header decodes a different id than was asked for is
		// no proof the bytes are right, whatever the checksum said. Parse into a
		// scratch needle: a failed earlier parse can leave stale fields in n.
		check := &needle.Needle{Id: needleId}
		if check.ReadBytes(candidate, offset, size, ecVolume.Version) != nil || check.Id != needleId {
			return nil, false
		}
		// The parse verdict makes every differing range a confirmed corruption.
		pos = 0
		for i, interval := range intervals {
			if !bytes.Equal(candidate[pos:pos+int(interval.Size)], corrupted[pos:pos+int(interval.Size)]) {
				forgetShardId(ecVolume, shardIds[i])
			}
			pos += int(interval.Size)
		}
		*n = *check
		return candidate, false
	}

	// When parity still covers every suspect, one pass excludes them all and
	// repairs any subset of them being corrupt at once. Otherwise one suspect
	// is rebuilt at a time, which fixes a single corrupt suspect.
	if len(suspects) <= ecCtx.ParityShards {
		if candidate, isDeleted := try(suspects); isDeleted {
			return nil, true
		} else if candidate != nil {
			return candidate, false
		}
		// The gather behind a failed candidate can itself have read a second
		// corrupt shard as reconstruction input. With slack left in the parity
		// budget, retry while dropping one gathered input at a time.
		if len(suspects) < ecCtx.ParityShards {
			if candidate, isDeleted := s.rebuildEcNeedleFromDroppedInputs(n, needleId, ecVolume, ecCtx, intervals, shardIds, offsets, offset, size, corrupted, suspects); isDeleted {
				return nil, true
			} else if candidate != nil {
				return candidate, false
			}
		}
		if len(suspects) == 1 {
			return nil, false
		}
	}
	for shardId := range suspects {
		if candidate, isDeleted := try(map[erasure_coding.ShardId]bool{shardId: true}); isDeleted {
			return nil, true
		} else if candidate != nil {
			return candidate, false
		}
	}
	return nil, false
}

// rebuildEcNeedleFromDroppedInputs handles a rebuild whose every suspect range
// was recovered yet the candidate still failed to parse: the gather behind it
// can itself have read a second corrupt shard as reconstruction input. Each
// interval is gathered once — keeping every shard that answers rather than
// stopping at DataShards — then the decode is retried with one gathered input
// dropped at a time and a needle parse arbitrates. The hypotheses tried are
// the inputs the first decode consumed, the only shards whose corruption can
// explain the failed candidate. Nil when no dropped-input subset parses.
func (s *Store) rebuildEcNeedleFromDroppedInputs(n *needle.Needle, needleId types.NeedleId, ecVolume *erasure_coding.EcVolume, ecCtx *erasure_coding.ECContext, intervals []erasure_coding.Interval, shardIds []erasure_coding.ShardId, offsets []int64, offset int64, size types.Size, corrupted []byte, suspects map[erasure_coding.ShardId]bool) ([]byte, bool) {
	var totalSize int64
	for _, interval := range intervals {
		totalSize += int64(interval.Size)
	}
	weight := totalSize * int64(ecCtx.Total())
	if weight > ecRecoverBudget {
		weight = ecRecoverBudget
	}
	if err := ecRecoverSem.Acquire(context.Background(), weight); err != nil {
		return nil, false
	}
	defer ecRecoverSem.Release(weight)

	gathers := make([][][]byte, len(intervals))
	picked := make(map[erasure_coding.ShardId]bool)
	for i, interval := range intervals {
		shardIntervals, isDeleted := s.gatherEcShardIntervals(needleId, ecVolume, ecCtx, suspects, int(interval.Size), offsets[i], ecCtx.Total())
		if isDeleted {
			return nil, true
		}
		taken := 0
		for shardId := 0; shardId < ecCtx.Total() && taken < ecCtx.DataShards; shardId++ {
			if shardIntervals[shardId] != nil {
				picked[erasure_coding.ShardId(shardId)] = true
				taken++
			}
		}
		gathers[i] = shardIntervals
	}

	for drop := erasure_coding.ShardId(0); int(drop) < ecCtx.Total(); drop++ {
		if !picked[drop] {
			continue
		}
		candidate := make([]byte, len(corrupted))
		copy(candidate, corrupted)
		pos := 0
		ok := true
		for i, interval := range intervals {
			view := slices.Clone(gathers[i])
			view[drop] = nil
			if err := reconstructEcShardInterval(ecVolume, ecCtx, view, shardIds[i]); err != nil {
				ok = false
				break
			}
			copy(candidate[pos:pos+int(interval.Size)], view[shardIds[i]])
			pos += int(interval.Size)
		}
		if !ok {
			continue
		}
		check := &needle.Needle{Id: needleId}
		if check.ReadBytes(candidate, offset, size, ecVolume.Version) != nil || check.Id != needleId {
			continue
		}
		// The verdict convicts every differing range — and the dropped input,
		// without which the same decode kept failing to parse.
		pos = 0
		for i, interval := range intervals {
			if !bytes.Equal(candidate[pos:pos+int(interval.Size)], corrupted[pos:pos+int(interval.Size)]) {
				forgetShardId(ecVolume, shardIds[i])
			}
			pos += int(interval.Size)
		}
		forgetShardId(ecVolume, drop)
		*n = *check
		return candidate, false
	}
	return nil, false
}

func forgetShardId(ecVolume *erasure_coding.EcVolume, shardId erasure_coding.ShardId) {
	// failed to access the source data nodes, clear it up
	ecVolume.ShardLocationsLock.Lock()
	delete(ecVolume.ShardLocations, shardId)
	ecVolume.ShardLocationsStale = true
	ecVolume.ShardLocationsLock.Unlock()
}

// markShardLocationsStale flags the cached map for a prompt re-check after a
// read failed against one of its locations. Unlike forgetShardId it keeps the
// entry: a direct read is worth retrying, since a dead peer fails fast.
func markShardLocationsStale(ecVolume *erasure_coding.EcVolume) {
	ecVolume.ShardLocationsLock.Lock()
	ecVolume.ShardLocationsStale = true
	ecVolume.ShardLocationsLock.Unlock()
}

// ecShardLocationsTTL is how long a cached shard map is trusted. A complete map
// is trusted longest. One short of DataShards, or one a failed read has just
// invalidated, is re-checked promptly: until it is, every read of the dropped
// shard skips the direct fetch and pays for a Reed-Solomon recovery instead.
func ecShardLocationsTTL(shardCount int, stale bool, ecCtx *erasure_coding.ECContext) time.Duration {
	switch {
	case stale || shardCount < ecCtx.DataShards:
		return 11 * time.Second
	case shardCount == ecCtx.Total():
		return 37 * time.Minute
	default:
		return 7 * time.Minute
	}
}

func (s *Store) cachedLookupEcShardLocations(ecVolume *erasure_coding.EcVolume) (err error) {

	// Use the volume's own EC ratio so a custom-ratio volume (e.g. 9+3) is judged
	// complete/recoverable against its real data-shard count, not the build default.
	// In OSS the ratio is always 10+4, so this is a no-op.
	ecCtx := ecVolume.ECContext
	if ecCtx == nil {
		ecCtx = erasure_coding.NewDefaultECContext(ecVolume.Collection, ecVolume.VolumeId)
	}

	// Judge the map and consume its mark in one critical section, so a mark raised
	// from here on belongs to the next refresh rather than being cleared by this
	// one -- the read that raised it has disproved the map this lookup is about to
	// install. Recover goroutines mutate all three fields via forgetShardId, so an
	// unguarded read would race a concurrent map write besides.
	ecVolume.ShardLocationsLock.Lock()
	stale := ecVolume.ShardLocationsStale
	ttl := ecShardLocationsTTL(len(ecVolume.ShardLocations), stale, ecCtx)
	fresh := ecVolume.ShardLocationsRefreshTime.Add(ttl).After(time.Now())
	if !fresh {
		ecVolume.ShardLocationsStale = false
	}
	ecVolume.ShardLocationsLock.Unlock()
	if fresh {
		return nil
	}

	glog.V(3).Infof("lookup and cache ec volume %d locations", ecVolume.VolumeId)

	err = operation.WithMasterServerClient(context.Background(), false, s.masterAddress(), s.grpcDialOption, func(masterClient master_pb.SeaweedClient) error {
		req := &master_pb.LookupEcVolumeRequest{
			VolumeId: uint32(ecVolume.VolumeId),
		}
		resp, err := masterClient.LookupEcVolume(context.Background(), req)
		if err != nil {
			return fmt.Errorf("lookup ec volume %d: %v", ecVolume.VolumeId, err)
		}
		if len(resp.ShardIdLocations) < ecCtx.DataShards {
			return fmt.Errorf("only %d shards found but %d required", len(resp.ShardIdLocations), ecCtx.DataShards)
		}

		ecVolume.ShardLocationsLock.Lock()
		for _, shardIdLocations := range resp.ShardIdLocations {
			shardId := erasure_coding.ShardId(shardIdLocations.ShardId)
			delete(ecVolume.ShardLocations, shardId)
			for _, loc := range shardIdLocations.Locations {
				ecVolume.ShardLocations[shardId] = append(ecVolume.ShardLocations[shardId], pb.NewServerAddressFromLocation(loc))
			}
		}
		ecVolume.ShardLocationsRefreshTime = time.Now()
		ecVolume.ShardLocationsLock.Unlock()

		return nil
	})
	if err != nil {
		// The lookup this mark was consumed for did not answer, and the refresh time
		// is only advanced on success -- so without putting the mark back, a map a
		// read had already disproved would be trusted for its full window again.
		// Marking unconditionally is safe: where no mark was consumed the refresh
		// time is stale anyway, and the next call looks up whatever the mark says.
		markShardLocationsStale(ecVolume)
	}
	return
}

func (s *Store) readLocalEcShardInterval(ecVolume *erasure_coding.EcVolume, shardId erasure_coding.ShardId, buf []byte, offset int64) error {
	// Resolve the shard together with the EcVolume on the disk that owns it; the
	// shard may live on a sibling disk of this server.
	ownerVolume, shard, found := s.FindEcVolumeWithShard(ecVolume.VolumeId, shardId)
	if !found {
		return fmt.Errorf("shard %d for volume %d: %w", shardId, ecVolume.VolumeId, errShardNotLocal)
	}
	// Skip a local shard whose identity doesn't match the caller's index, so the
	// read recovers from the correct generation. Lenient only when the caller has
	// no identity (pre-upgrade): a known caller must not accept an unstamped local
	// shard, which would serve a stale pre-upgrade generation.
	if ecVolume.EncodeTsNs != 0 && ecVolume.EncodeTsNs != ownerVolume.EncodeTsNs {
		glog.V(1).Infof("skip local ec shard %d.%d from a different encode run: caller EncodeTsNs %d, local %d", ecVolume.VolumeId, shardId, ecVolume.EncodeTsNs, ownerVolume.EncodeTsNs)
		return fmt.Errorf("shard %d for volume %d: %w", shardId, ecVolume.VolumeId, errShardNotLocal)
	}

	readBytes, err := shard.ReadAt(buf, offset)
	if err != nil {
		ownerVolume.CheckReadWriteError(err)
		return fmt.Errorf("failed to read local EC shard %d for volume %d: %w", shardId, ecVolume.VolumeId, err)
	}
	ownerVolume.CheckReadWriteError(nil)
	if got, want := readBytes, len(buf); got != want {
		return fmt.Errorf("expected %d bytes for local EC shard %d on volume %d, got %d", want, shardId, ecVolume.VolumeId, got)
	}

	return nil
}

func (s *Store) readRemoteEcShardInterval(sourceDataNodes []pb.ServerAddress, needleId types.NeedleId, vid needle.VolumeId, shardId erasure_coding.ShardId, buf []byte, offset int64, expectedEncodeTsNs int64) (n int, is_deleted bool, err error) {

	if len(sourceDataNodes) == 0 {
		return 0, false, fmt.Errorf("failed to find ec shard %d.%d", vid, shardId)
	}

	for _, sourceDataNode := range sourceDataNodes {
		glog.V(3).Infof("read remote ec shard %d.%d from %s", vid, shardId, sourceDataNode)
		n, is_deleted, err = s.doReadRemoteEcShardInterval(sourceDataNode, needleId, vid, shardId, buf, offset, expectedEncodeTsNs)
		if err == nil {
			return
		}
		glog.V(1).Infof("read remote ec shard %d.%d from %s: %v", vid, shardId, sourceDataNode, err)
	}

	return
}

func (s *Store) doReadRemoteEcShardInterval(sourceDataNode pb.ServerAddress, needleId types.NeedleId, vid needle.VolumeId, shardId erasure_coding.ShardId, buf []byte, offset int64, expectedEncodeTsNs int64) (n int, is_deleted bool, err error) {

	err = operation.WithVolumeServerClient(false, sourceDataNode, s.grpcDialOption, func(client volume_server_pb.VolumeServerClient) error {

		// copy data slice
		shardReadClient, err := client.VolumeEcShardRead(context.Background(), &volume_server_pb.VolumeEcShardReadRequest{
			VolumeId:   uint32(vid),
			ShardId:    uint32(shardId),
			Offset:     offset,
			Size:       int64(len(buf)),
			FileKey:    uint64(needleId),
			EncodeTsNs: expectedEncodeTsNs,
		})
		if err != nil {
			return fmt.Errorf("failed to start reading ec shard %d.%d from %s: %v", vid, shardId, sourceDataNode, err)
		}

		for {
			resp, receiveErr := shardReadClient.Recv()
			if receiveErr == io.EOF {
				break
			}
			if receiveErr != nil {
				return fmt.Errorf("receiving ec shard %d.%d from %s: %v", vid, shardId, sourceDataNode, receiveErr)
			}
			// Validate the served shard's identity client-side, so the guard holds
			// even against a pre-upgrade server that ignored the request field (it
			// returns 0). A mismatch fails the read; the caller recovers from parity.
			if expectedEncodeTsNs != 0 && resp.EncodeTsNs != expectedEncodeTsNs {
				return fmt.Errorf("ec shard %d.%d from %s belongs to a different encode run (want %d, got %d)", vid, shardId, sourceDataNode, expectedEncodeTsNs, resp.EncodeTsNs)
			}
			if resp.IsDeleted {
				is_deleted = true
			}
			copy(buf[n:n+len(resp.Data)], resp.Data)
			n += len(resp.Data)
		}

		return nil
	})
	if err != nil {
		return 0, is_deleted, fmt.Errorf("read ec shard %d.%d from %s: %v", vid, shardId, sourceDataNode, err)
	}

	// A non-deleted interval must arrive whole: the server stamps EncodeTsNs only
	// on chunks that carry bytes, so a short or empty stream (e.g. immediate EOF
	// from a pre-upgrade or stale server) leaves the buffer partly zero-filled and
	// unvalidated. Reject it so the caller recovers from parity. The is_deleted
	// short-circuit legitimately returns n=0 with no data and is exempt, matching
	// readLocalEcShardInterval's got==len(buf) rule for the local path.
	if !is_deleted && n != len(buf) {
		return n, is_deleted, fmt.Errorf("short read ec shard %d.%d from %s: got %d want %d", vid, shardId, sourceDataNode, n, len(buf))
	}

	return
}

// gatherEcShardIntervals collects the same interval from every shard outside skip,
// returning one buffer per shard and nil where the shard could not be gathered. It stops
// once want of them are in hand: reconstruction consumes no more than DataShards, so
// callers pass that unless a retry needs the wider set.
func (s *Store) gatherEcShardIntervals(needleId types.NeedleId, ecVolume *erasure_coding.EcVolume, ecCtx *erasure_coding.ECContext, skip map[erasure_coding.ShardId]bool, size int, offset int64, want int) (shardIntervals [][]byte, isDeleted bool) {
	shardIntervals = make([][]byte, ecCtx.Total())

	// A shard this server already holds costs no round trip and no peer buffer,
	// so seed those before asking peers for the rest.
	available := 0
	for shardId := erasure_coding.ShardId(0); int(shardId) < ecCtx.Total() && available < want; shardId++ {
		if skip[shardId] {
			continue
		}
		if _, _, found := s.FindEcVolumeWithShard(ecVolume.VolumeId, shardId); !found {
			continue
		}
		data := make([]byte, size)
		if localErr := s.readLocalEcShardInterval(ecVolume, shardId, data, offset); localErr != nil {
			glog.V(3).Infof("recover: read local ec shard %d.%d: %v", ecVolume.VolumeId, shardId, localErr)
			continue
		}
		shardIntervals[shardId] = data
		available++
	}

	var candidates []erasure_coding.ShardId
	candidateLocations := make(map[erasure_coding.ShardId][]pb.ServerAddress)
	ecVolume.ShardLocationsLock.RLock()
	for shardId, locations := range ecVolume.ShardLocations {

		// skip a shard being rebuilt, one already seeded locally, or an empty shard
		if skip[shardId] || int(shardId) >= ecCtx.Total() || shardIntervals[shardId] != nil {
			continue
		}
		if len(locations) == 0 {
			glog.V(3).Infof("readRemoteEcShardInterval missing %d.%d from %+v", ecVolume.VolumeId, shardId, locations)
			continue
		}
		candidates = append(candidates, shardId)
		candidateLocations[shardId] = locations
	}
	ecVolume.ShardLocationsLock.RUnlock()

	// The recover goroutines run concurrently, so the deleted flag is collected
	// atomically and folded into the named return after they join, rather than each
	// goroutine writing the shared bool directly.
	var isDeletedFlag atomic.Bool

	// Reconstruction consumes DataShards buffers, so reading every remaining shard
	// holds a third more than that and asks a third more of peers that are, by
	// then, already struggling. Widen only when a wave falls short.
	for len(candidates) > 0 && available < want {
		wave := min(want-available, len(candidates))
		var wg sync.WaitGroup
		var fetched atomic.Int64
		for _, shardId := range candidates[:wave] {
			locations := candidateLocations[shardId]
			wg.Add(1)
			go func() {
				defer wg.Done()
				data := make([]byte, size)
				nRead, isDeleted, readErr := s.readRemoteEcShardInterval(locations, needleId, ecVolume.VolumeId, shardId, data, offset, ecVolume.EncodeTsNs)
				if readErr != nil {
					glog.V(3).Infof("recover: readRemoteEcShardInterval %d.%d %d bytes from %+v: %v", ecVolume.VolumeId, shardId, nRead, locations, readErr)
					forgetShardId(ecVolume, shardId)
				}
				if isDeleted {
					isDeletedFlag.Store(true)
				}
				if nRead == size {
					shardIntervals[shardId] = data
					fetched.Add(1)
				}
			}()
		}
		wg.Wait()
		candidates = candidates[wave:]
		available += int(fetched.Load())
		if isDeletedFlag.Load() {
			// every shard of a deleted needle answers deleted, so another wave cannot help
			break
		}
	}

	return shardIntervals, isDeletedFlag.Load()
}

// checkEcShardRebuildable rejects a parity target. ReconstructData rebuilds data shards
// only, so it leaves a parity slot nil and reports no error, and the caller would copy
// that out as a zero-filled buffer and report a successful read.
func checkEcShardRebuildable(ecVolume *erasure_coding.EcVolume, ecCtx *erasure_coding.ECContext, shardIdToRecover erasure_coding.ShardId) error {
	if int(shardIdToRecover) >= ecCtx.DataShards {
		return fmt.Errorf("cannot reconstruct shard %d.%d: only data shards can be rebuilt, %d of %d are parity",
			ecVolume.VolumeId, shardIdToRecover, ecCtx.ParityShards, ecCtx.Total())
	}
	return nil
}

// reconstructEcShardInterval rebuilds one shard's interval in place from the others.
func reconstructEcShardInterval(ecVolume *erasure_coding.EcVolume, ecCtx *erasure_coding.ECContext, shardIntervals [][]byte, shardIdToRecover erasure_coding.ShardId) error {
	if err := checkEcShardRebuildable(ecVolume, ecCtx, shardIdToRecover); err != nil {
		return err
	}

	enc, err := reedsolomon.New(ecCtx.DataShards, ecCtx.ParityShards)
	if err != nil {
		return fmt.Errorf("failed to create encoder: %w", err)
	}

	// Count and log available shards for diagnostics
	availableShards := make([]erasure_coding.ShardId, 0, ecCtx.Total())
	missingShards := make([]erasure_coding.ShardId, 0, ecCtx.ParityShards+1)
	for shardId := 0; shardId < ecCtx.Total(); shardId++ {
		if shardIntervals[shardId] != nil {
			availableShards = append(availableShards, erasure_coding.ShardId(shardId))
		} else {
			missingShards = append(missingShards, erasure_coding.ShardId(shardId))
		}
	}

	glog.V(3).Infof("recover ec shard %d.%d: %d shards available %v, %d missing %v",
		ecVolume.VolumeId, shardIdToRecover,
		len(availableShards), availableShards,
		len(missingShards), missingShards)

	if len(availableShards) < ecCtx.DataShards {
		return fmt.Errorf("cannot recover shard %d.%d: only %d shards available %v, need at least %d (missing: %v)",
			ecVolume.VolumeId, shardIdToRecover,
			len(availableShards), availableShards,
			ecCtx.DataShards, missingShards)
	}

	// Rebuild only what was asked for. ReconstructData rebuilds every missing data
	// shard, and a gather that stopped at DataShards can leave up to ParityShards of
	// them missing -- an interval-sized buffer and a decode each, discarded unread.
	// The mask is Total() long, not DataShards: reedsolomon documents both lengths
	// but indexes the short one past its end when a parity shard is absent, which
	// here it usually is.
	required := make([]bool, ecCtx.Total())
	required[shardIdToRecover] = true
	if err := enc.ReconstructSome(shardIntervals, required); err != nil {
		return fmt.Errorf("failed to reconstruct data for shard %d.%d with %d available shards %v: %w",
			ecVolume.VolumeId, shardIdToRecover, len(availableShards), availableShards, err)
	}

	return nil
}

func (s *Store) recoverOneRemoteEcShardInterval(needleId types.NeedleId, ecVolume *erasure_coding.EcVolume, shardIdToRecover erasure_coding.ShardId, buf []byte, offset int64) (n int, is_deleted bool, err error) {
	return s.recoverEcShardInterval(needleId, ecVolume, shardIdToRecover, buf, offset, map[erasure_coding.ShardId]bool{shardIdToRecover: true})
}

// recoverEcShardInterval rebuilds shardIdToRecover's interval from the shards
// outside exclude, which always contains shardIdToRecover itself.
func (s *Store) recoverEcShardInterval(needleId types.NeedleId, ecVolume *erasure_coding.EcVolume, shardIdToRecover erasure_coding.ShardId, buf []byte, offset int64, exclude map[erasure_coding.ShardId]bool) (n int, is_deleted bool, err error) {
	glog.V(3).Infof("recover ec shard %d.%d from other locations", ecVolume.VolumeId, shardIdToRecover)

	// Reconstruct with the volume's OWN EC ratio (loaded from its .vif), not the
	// build default, so a custom-ratio volume (e.g. 9+3) is decoded with the matrix
	// that actually produced its shards -- decoding it as 10+4 would corrupt the
	// recovered bytes. In OSS the ratio is always 10+4, so this is a no-op.
	ecCtx := ecVolume.ECContext
	if ecCtx == nil {
		ecCtx = erasure_coding.NewDefaultECContext(ecVolume.Collection, ecVolume.VolumeId)
	}
	// checked before the gather: a doomed target should not cost a fan-out, nor drop a
	// sibling's location through forgetShardId on the way to failing
	if err := checkEcShardRebuildable(ecVolume, ecCtx, shardIdToRecover); err != nil {
		return 0, false, err
	}

	// Charge the buffers this recovery is about to hold against the budget, so a
	// burst of them queues here rather than on the heap: DataShards gathered, plus
	// the one the rebuild allocates for the shard it recreates.
	weight := int64(len(buf)) * int64(ecCtx.DataShards+1)
	if weight > ecRecoverBudget {
		// An interval whose fan-out outgrows the whole budget takes all of it and
		// so runs alone, rather than blocking forever on an acquire that can never
		// succeed. The cap is then one such recovery, not a burst of them.
		weight = ecRecoverBudget
	}
	if err = ecRecoverSem.Acquire(context.Background(), weight); err != nil {
		return 0, false, err
	}
	defer ecRecoverSem.Release(weight)

	shardIntervals, is_deleted := s.gatherEcShardIntervals(needleId, ecVolume, ecCtx, exclude, len(buf), offset, ecCtx.DataShards)
	if err := reconstructEcShardInterval(ecVolume, ecCtx, shardIntervals, shardIdToRecover); err != nil {
		return 0, is_deleted, err
	}
	glog.V(4).Infof("recovered ec shard %d.%d from other locations", ecVolume.VolumeId, shardIdToRecover)

	copy(buf, shardIntervals[shardIdToRecover])

	return len(buf), is_deleted, nil
}

func (s *Store) EcVolumes() (ecVolumes []*erasure_coding.EcVolume) {
	for _, location := range s.Locations {
		location.ecVolumesLock.RLock()
		for _, v := range location.ecVolumes {
			ecVolumes = append(ecVolumes, v)
		}
		location.ecVolumesLock.RUnlock()
	}
	slices.SortFunc(ecVolumes, func(a, b *erasure_coding.EcVolume) int {
		return int(a.VolumeId) - int(b.VolumeId)
	})
	return ecVolumes
}