package app

import (
	"bytes"
	"context"
	"crypto/rand"
	"encoding/hex"
	"encoding/json"
	"errors"
	"fmt"
	"os"
	"os/exec"
	"path/filepath"
	"sort"
	"strings"
	"sync"
	"syscall"
	"time"

	"bee/audit/internal/platform"
)

const (
	blackboxMarkerName        = ".bee-blackbox"
	blackboxDiscoverInterval  = 2 * time.Second
	blackboxMinFlushPeriod    = 1 * time.Second
	blackboxMaxFlushPeriod    = 30 * time.Second
	blackboxRecoveryFastCount = 5

	// blackboxKickFileName is a marker under exportDir that SAT job
	// execution touches after each job finishes (see platform.SetJobBoundaryHook
	// wiring in New()). Workers poll its mtime so a just-finished job's
	// output reaches removable media promptly instead of waiting out
	// whatever the current adaptive flush period happens to be.
	blackboxKickFileName = ".blackbox-kick"

	// blackboxSyncBracketTimeout bounds how long a syncBracket-marked SAT job
	// (see platform.SetSyncBracketHook) blocks waiting for blackbox to catch
	// up before/after the actual load step. Generous relative to the normal
	// sub-second kick-to-sync latency, but bounded so a genuinely stuck or
	// unplugged target can't hang the diagnostic indefinitely.
	blackboxSyncBracketTimeout = 20 * time.Second
)

// blackboxKickPollInterval is how often an idle worker checks the kick file
// for a pending out-of-band sync request. A package var (not a const) so
// tests can shrink it instead of waiting out the real interval.
var blackboxKickPollInterval = 250 * time.Millisecond

// requestBlackboxSync touches the kick file under exportDir, signalling any
// running blackbox worker to sync on its next poll instead of waiting out
// its current flush period. Best-effort: a failure here just means the next
// scheduled sync picks up the data instead of an early one.
func requestBlackboxSync(exportDir string) {
	path := filepath.Join(exportDir, blackboxKickFileName)
	now := blackboxNow()
	if err := os.Chtimes(path, now, now); err == nil {
		return
	}
	_ = os.WriteFile(path, []byte(now.Format(time.RFC3339Nano)+"\n"), 0644)
}

// blackboxSyncWaitPollInterval is how often requestBlackboxSyncAndWait
// re-reads the state file while waiting for a sync to catch up. A package
// var so tests can shrink it.
var blackboxSyncWaitPollInterval = 200 * time.Millisecond

// requestBlackboxSyncAndWait kicks every enrolled blackbox target and blocks
// until each one has completed a sync cycle that started at or after this
// call, or until timeout elapses. Use this (instead of the fire-and-forget
// requestBlackboxSync) around an event where losing the data to a crash
// before it reaches removable media would matter — e.g. bracket a risky load
// step so its own SAT job directory (and the fact that it started at all) is
// durable on the flash drive before the load runs, and durable again once it
// finishes.
//
// Returns nil once caught up. Returns an error naming what didn't catch up
// (timeout, or a target stuck "degraded") — callers should treat that as
// best-effort informational (log it) rather than fail the job over it: a
// blackbox problem should not block the diagnostic the operator actually
// asked for.
func requestBlackboxSyncAndWait(exportDir, statePath string, timeout time.Duration) error {
	requestedAt := blackboxNow()
	requestBlackboxSync(exportDir)

	deadline := time.Now().Add(timeout)
	for {
		state, err := ReadBlackboxState(statePath)
		if err == nil {
			if ok, pending := blackboxStateCaughtUp(state, requestedAt); ok {
				return nil
			} else if time.Now().After(deadline) {
				return fmt.Errorf("blackbox sync did not catch up within %s (pending: %s)", timeout, pending)
			}
		} else if time.Now().After(deadline) {
			return fmt.Errorf("blackbox sync wait: could not read state after %s: %w", timeout, err)
		}
		time.Sleep(blackboxSyncWaitPollInterval)
	}
}

// blackboxStateCaughtUp reports whether every non-degraded enrolled target
// has synced at or after requestedAt. No targets enrolled counts as caught
// up (nothing to wait for — e.g. no removable media plugged in). A target
// stuck "degraded" (mount/copy failing) is skipped rather than waited on
// forever; its enrollment ID is named in the returned pending string so
// callers can log which target is the reason a real wait timed out.
func blackboxStateCaughtUp(state BlackboxState, requestedAt time.Time) (bool, string) {
	for _, t := range state.Targets {
		if t.Status == "degraded" {
			continue
		}
		syncedAt, err := time.Parse(time.RFC3339Nano, t.LastSyncAtUTC)
		if err != nil || syncedAt.Before(requestedAt) {
			return false, t.EnrollmentID
		}
	}
	return true, ""
}

// blackboxKickModTime returns the kick file's mtime, or the zero Time if it
// doesn't exist yet (nothing has requested a sync since boot).
func blackboxKickModTime(exportDir string) time.Time {
	info, err := os.Stat(filepath.Join(exportDir, blackboxKickFileName))
	if err != nil {
		return time.Time{}
	}
	return info.ModTime()
}

var DefaultBlackboxStatePath = DefaultExportDir + "/blackbox-state.json"

var (
	blackboxExecCommand = exec.Command
	blackboxNow         = func() time.Time { return time.Now().UTC() }
)

type BlackboxMarker struct {
	Version      int    `json:"version"`
	EnrollmentID string `json:"enrollment_id"`
	CreatedAtUTC string `json:"created_at_utc"`
	Host         string `json:"host,omitempty"`
}

type BlackboxTargetStatus struct {
	EnrollmentID      string                   `json:"enrollment_id"`
	Device            string                   `json:"device"`
	FS                platform.RemovableTarget `json:"fs"`
	BootFolder        string                   `json:"boot_folder"`
	Status            string                   `json:"status"`
	LastSyncAtUTC     string                   `json:"last_sync_at_utc,omitempty"`
	LastCycleDuration string                   `json:"last_cycle_duration,omitempty"`
	FlushPeriod       string                   `json:"flush_period"`
	LastError         string                   `json:"last_error,omitempty"`
	Mountpoint        string                   `json:"mountpoint,omitempty"`
}

type BlackboxState struct {
	Status           string                 `json:"status"`
	BootStartedAtUTC string                 `json:"boot_started_at_utc"`
	BootFolder       string                 `json:"boot_folder"`
	UpdatedAtUTC     string                 `json:"updated_at_utc"`
	Targets          []BlackboxTargetStatus `json:"targets"`
}

type blackboxRuntime struct {
	exportDir   string
	statePath   string
	system      *platform.System
	bootStarted time.Time
	bootFolder  string

	mu      sync.Mutex
	workers map[string]*blackboxWorker
}

type discoveredBlackboxTarget struct {
	marker       BlackboxMarker
	target       platform.RemovableTarget
	seenMount    string
	mountedByBee bool
}

type blackboxWorker struct {
	runtime      *blackboxRuntime
	enrollmentID string

	mu           sync.Mutex
	target       platform.RemovableTarget
	marker       BlackboxMarker
	mountpoint   string
	mountedByBee bool
	status       string
	lastSyncAt   time.Time
	lastDuration time.Duration
	flushPeriod  time.Duration
	lastError    string
	fastCycles   int
	lastKickSeen time.Time
	stopCh       chan struct{}
	stoppedCh    chan struct{}

	// syncCycleFunc defaults to w.syncCycle; overridable in tests so run()'s
	// wait/wake-on-kick logic can be exercised without a real removable-media
	// mount and copy.
	syncCycleFunc func() error
}

func RunBlackbox(ctx context.Context, exportDir, statePath string, system *platform.System) error {
	exportDir = strings.TrimSpace(exportDir)
	if exportDir == "" {
		exportDir = DefaultExportDir
	}
	statePath = strings.TrimSpace(statePath)
	if statePath == "" {
		statePath = DefaultBlackboxStatePath
	}
	if system == nil {
		system = platform.New()
	}
	bootStarted, err := bootStartedAtUTC()
	if err != nil {
		bootStarted = blackboxNow()
	}
	rt := &blackboxRuntime{
		exportDir:   exportDir,
		statePath:   statePath,
		system:      system,
		bootStarted: bootStarted,
		bootFolder:  SupportBundleBaseName(bootStarted),
		workers:     make(map[string]*blackboxWorker),
	}
	_ = os.MkdirAll(filepath.Dir(statePath), 0755)
	rt.persistState()
	ticker := time.NewTicker(blackboxDiscoverInterval)
	defer ticker.Stop()
	for {
		rt.reconcile()
		select {
		case <-ctx.Done():
			rt.stopAll()
			return ctx.Err()
		case <-ticker.C:
		}
	}
}

func ReadBlackboxState(path string) (BlackboxState, error) {
	path = strings.TrimSpace(path)
	if path == "" {
		path = DefaultBlackboxStatePath
	}
	raw, err := os.ReadFile(path)
	if err != nil {
		return BlackboxState{}, err
	}
	var state BlackboxState
	if err := json.Unmarshal(raw, &state); err != nil {
		return BlackboxState{}, err
	}
	return state, nil
}

func EnableBlackboxTarget(target platform.RemovableTarget) (BlackboxMarker, error) {
	target = sanitizeRemovableTarget(target)
	if target.Device == "" {
		return BlackboxMarker{}, fmt.Errorf("device is required")
	}
	mountpoint, mountedByBee, err := ensureMountedTarget(target, "marker")
	if err != nil {
		return BlackboxMarker{}, err
	}
	defer func() {
		if mountedByBee {
			_ = unmountTarget(mountpoint)
		}
	}()

	marker, _, err := readBlackboxMarker(mountpoint)
	if err != nil && !errors.Is(err, os.ErrNotExist) {
		return BlackboxMarker{}, err
	}
	if marker.EnrollmentID == "" {
		marker = BlackboxMarker{
			Version:      1,
			EnrollmentID: newBlackboxEnrollmentID(),
			CreatedAtUTC: blackboxNow().Format(time.RFC3339),
			Host:         hostnameOr("unknown"),
		}
	}
	if err := writeBlackboxMarker(mountpoint, marker); err != nil {
		return BlackboxMarker{}, err
	}
	return marker, nil
}

func DisableBlackboxTarget(device, enrollmentID string) error {
	device = strings.TrimSpace(device)
	enrollmentID = strings.TrimSpace(enrollmentID)
	if device == "" && enrollmentID == "" {
		return fmt.Errorf("device or enrollment_id is required")
	}
	system := platform.New()
	targets, err := system.ListRemovableTargets()
	if err != nil {
		return err
	}
	for _, target := range targets {
		target = sanitizeRemovableTarget(target)
		mountpoint, mountedByBee, mountErr := ensureMountedTarget(target, "marker")
		if mountErr != nil {
			continue
		}
		remove := false
		marker, _, err := readBlackboxMarker(mountpoint)
		if err == nil {
			if enrollmentID != "" && marker.EnrollmentID == enrollmentID {
				remove = true
			}
			if device != "" && target.Device == device {
				remove = true
			}
		}
		if remove {
			err = os.Remove(filepath.Join(mountpoint, blackboxMarkerName))
		}
		if mountedByBee {
			_ = unmountTarget(mountpoint)
		}
		if remove {
			return err
		}
	}
	return os.ErrNotExist
}

func (rt *blackboxRuntime) reconcile() {
	discovered, _ := rt.discoverMarkedTargets(rt.trackedDevices())

	rt.mu.Lock()
	defer rt.mu.Unlock()

	seen := make(map[string]struct{}, len(discovered))
	for _, found := range discovered {
		seen[found.marker.EnrollmentID] = struct{}{}
		worker, ok := rt.workers[found.marker.EnrollmentID]
		if !ok {
			worker = newBlackboxWorker(rt, found)
			rt.workers[found.marker.EnrollmentID] = worker
			go worker.run()
			continue
		}
		worker.update(found)
	}
	for id, worker := range rt.workers {
		if _, ok := seen[id]; ok {
			continue
		}
		worker.stop()
		delete(rt.workers, id)
	}
	rt.persistStateLocked()
}

func (rt *blackboxRuntime) stopAll() {
	rt.mu.Lock()
	workers := make([]*blackboxWorker, 0, len(rt.workers))
	for _, worker := range rt.workers {
		workers = append(workers, worker)
	}
	rt.workers = map[string]*blackboxWorker{}
	rt.persistStateLocked()
	rt.mu.Unlock()
	for _, worker := range workers {
		worker.stop()
	}
}

// trackedDevices returns the already-enrolled targets, keyed by device path,
// for every worker currently running. Used by discoverMarkedTargets to skip
// re-mounting devices a worker already owns.
func (rt *blackboxRuntime) trackedDevices() map[string]discoveredBlackboxTarget {
	rt.mu.Lock()
	defer rt.mu.Unlock()
	known := make(map[string]discoveredBlackboxTarget, len(rt.workers))
	for _, worker := range rt.workers {
		worker.mu.Lock()
		if worker.target.Device != "" {
			known[worker.target.Device] = discoveredBlackboxTarget{
				marker: worker.marker,
				target: worker.target,
			}
		}
		worker.mu.Unlock()
	}
	return known
}

// discoverMarkedTargets probes removable media for the bee-blackbox marker.
// known holds devices a worker is already running against (see
// trackedDevices) — those are reported back as-is, without mounting, since
// mounting/unmounting the same device on every discovery tick (every
// blackboxDiscoverInterval) fights the worker's own mount for the device and
// was observed to slow its actual sync cycle by an order of magnitude on
// FUSE-backed filesystems (NTFS via ntfs-3g). Only devices with no running
// worker get probed.
func (rt *blackboxRuntime) discoverMarkedTargets(known map[string]discoveredBlackboxTarget) ([]discoveredBlackboxTarget, error) {
	targets, err := rt.system.ListRemovableTargets()
	if err != nil {
		return nil, err
	}
	var out []discoveredBlackboxTarget
	for _, rawTarget := range targets {
		target := sanitizeRemovableTarget(rawTarget)
		if target.Device == "" {
			continue
		}
		if cached, ok := known[target.Device]; ok {
			cached.target = target
			out = append(out, cached)
			continue
		}
		mountpoint, mountedByBee, err := ensureMountedTarget(target, "probe")
		if err != nil {
			continue
		}
		marker, ok, err := readBlackboxMarker(mountpoint)
		if mountedByBee && !ok {
			_ = unmountTarget(mountpoint)
		}
		if err != nil || !ok || marker.EnrollmentID == "" {
			continue
		}
		if mountedByBee {
			_ = unmountTarget(mountpoint)
		}
		out = append(out, discoveredBlackboxTarget{
			marker:       marker,
			target:       target,
			seenMount:    mountpoint,
			mountedByBee: mountedByBee,
		})
	}
	sort.Slice(out, func(i, j int) bool {
		return out[i].marker.EnrollmentID < out[j].marker.EnrollmentID
	})
	return out, nil
}

func newBlackboxWorker(rt *blackboxRuntime, found discoveredBlackboxTarget) *blackboxWorker {
	w := &blackboxWorker{
		runtime:      rt,
		enrollmentID: found.marker.EnrollmentID,
		target:       found.target,
		marker:       found.marker,
		flushPeriod:  blackboxMinFlushPeriod,
		status:       "running",
		stopCh:       make(chan struct{}),
		stoppedCh:    make(chan struct{}),
	}
	w.syncCycleFunc = w.syncCycle
	return w
}

func (w *blackboxWorker) run() {
	defer close(w.stoppedCh)
	kickPoll := time.NewTicker(blackboxKickPollInterval)
	defer kickPoll.Stop()

	for {
		start := time.Now()
		err := w.syncCycleFunc()
		duration := time.Since(start)
		w.finishCycle(duration, err)
		w.lastKickSeen = blackboxKickModTime(w.runtime.exportDir)

		wait := w.currentFlushPeriod()
		timer := time.NewTimer(wait)
	waitLoop:
		for {
			select {
			case <-w.stopCh:
				timer.Stop()
				w.cleanup()
				return
			case <-timer.C:
				break waitLoop
			case <-kickPoll.C:
				if mtime := blackboxKickModTime(w.runtime.exportDir); mtime.After(w.lastKickSeen) {
					timer.Stop()
					break waitLoop
				}
			}
		}
	}
}

func (w *blackboxWorker) update(found discoveredBlackboxTarget) {
	w.mu.Lock()
	defer w.mu.Unlock()
	w.target = found.target
	w.marker = found.marker
}

func (w *blackboxWorker) stop() {
	select {
	case <-w.stopCh:
	default:
		close(w.stopCh)
	}
	<-w.stoppedCh
}

func (w *blackboxWorker) currentFlushPeriod() time.Duration {
	w.mu.Lock()
	defer w.mu.Unlock()
	return w.flushPeriod
}

func (w *blackboxWorker) finishCycle(duration time.Duration, err error) {
	w.mu.Lock()
	w.lastDuration = duration
	if err != nil {
		w.status = "degraded"
		w.lastError = err.Error()
		w.fastCycles = 0
		w.flushPeriod = adjustFlushPeriod(w.flushPeriod, duration, false, 0)
	} else {
		w.status = "running"
		w.lastSyncAt = blackboxNow()
		w.lastError = ""
		if duration <= w.flushPeriod/2 {
			w.fastCycles++
		} else {
			w.fastCycles = 0
		}
		w.flushPeriod = adjustFlushPeriod(w.flushPeriod, duration, true, w.fastCycles)
	}
	w.mu.Unlock()
	// persistState must be called without w.mu held: it acquires rt.mu then
	// each worker.mu inside persistStateLocked, so holding w.mu here would
	// cause a deadlock (w.mu → rt.mu → w.mu).
	w.runtime.persistState()
}

func adjustFlushPeriod(current, duration time.Duration, success bool, fastCycles int) time.Duration {
	if current <= 0 {
		current = blackboxMinFlushPeriod
	}
	if duration <= 0 {
		duration = current
	}
	next := current
	if duration > current {
		growA := time.Duration(float64(current) * 1.25)
		growB := time.Duration(float64(duration) * 1.25)
		if growB > growA {
			next = growB
		} else {
			next = growA
		}
	}
	if success && fastCycles >= blackboxRecoveryFastCount {
		next = time.Duration(float64(current) * 0.9)
	}
	if next < blackboxMinFlushPeriod {
		next = blackboxMinFlushPeriod
	}
	if next > blackboxMaxFlushPeriod {
		next = blackboxMaxFlushPeriod
	}
	return next
}

func (w *blackboxWorker) syncCycle() error {
	target, marker := w.snapshotTarget()
	mountpoint, mountedByBee, err := ensureMountedTarget(target, marker.EnrollmentID)
	if err != nil {
		return err
	}
	w.recordMountpoint(mountpoint, mountedByBee)

	root := filepath.Join(mountpoint, w.runtime.bootFolder)
	if err := os.MkdirAll(root, 0755); err != nil {
		return err
	}
	if err := categorizeExportTree(w.runtime.exportDir, root); err != nil {
		return err
	}
	// Same doc pair the support bundle ships at its root — a blackbox
	// capture on removable media has no manifest.txt/support-bundle
	// equivalent to explain its layout, so without this an agent handed
	// only the media would have nothing pointing it at README.md.
	if err := writeBundleDocs(root); err != nil {
		return err
	}
	if err := w.captureSnapshots(root); err != nil {
		return err
	}
	return syncFilesystem(root)
}

func (w *blackboxWorker) cleanup() {
	w.mu.Lock()
	mountpoint := w.mountpoint
	mountedByBee := w.mountedByBee
	w.mu.Unlock()
	if mountedByBee && mountpoint != "" {
		_ = unmountTarget(mountpoint)
	}
}

func (w *blackboxWorker) snapshotTarget() (platform.RemovableTarget, BlackboxMarker) {
	w.mu.Lock()
	defer w.mu.Unlock()
	return w.target, w.marker
}

func (w *blackboxWorker) recordMountpoint(mountpoint string, mountedByBee bool) {
	w.mu.Lock()
	defer w.mu.Unlock()
	w.mountpoint = mountpoint
	w.mountedByBee = mountedByBee
}

func (w *blackboxWorker) captureSnapshots(root string) error {
	if err := captureCommandAtomic(filepath.Join(root, "tasks", "_services", "combined.journal.log"), "journalctl", "--no-pager", "--since", w.runtime.bootStarted.Format(time.RFC3339)); err != nil {
		return err
	}
	for _, svc := range supportBundleServices {
		dir := filepath.Join(root, serviceBundleDir(svc))
		if err := captureCommandAtomic(filepath.Join(dir, svc+".journal.log"), "journalctl", "--no-pager", "-u", svc, "--since", w.runtime.bootStarted.Format(time.RFC3339)); err != nil {
			return err
		}
		if err := captureCommandAtomic(filepath.Join(dir, svc+".status.txt"), "systemctl", "status", svc, "--no-pager"); err != nil {
			return err
		}
	}
	if err := captureCommandAtomic(filepath.Join(root, "livecd", "host", "dmesg.txt"), "dmesg"); err != nil {
		return err
	}
	for _, item := range supportBundleOptionalFiles {
		if err := copyFileIfChanged(item.src, filepath.Join(root, item.name)); err != nil && !errors.Is(err, os.ErrNotExist) {
			return err
		}
	}
	return nil
}

func (rt *blackboxRuntime) persistState() {
	rt.mu.Lock()
	defer rt.mu.Unlock()
	rt.persistStateLocked()
}

func (rt *blackboxRuntime) persistStateLocked() {
	state := BlackboxState{
		Status:           "disabled",
		BootStartedAtUTC: rt.bootStarted.Format(time.RFC3339),
		BootFolder:       rt.bootFolder,
		UpdatedAtUTC:     blackboxNow().Format(time.RFC3339),
		Targets:          make([]BlackboxTargetStatus, 0, len(rt.workers)),
	}
	if len(rt.workers) > 0 {
		state.Status = "running"
	}
	for _, worker := range rt.workers {
		worker.mu.Lock()
		targetState := BlackboxTargetStatus{
			EnrollmentID: worker.enrollmentID,
			Device:       worker.target.Device,
			FS:           worker.target,
			BootFolder:   rt.bootFolder,
			Status:       worker.status,
			FlushPeriod:  worker.flushPeriod.String(),
			LastError:    worker.lastError,
			Mountpoint:   worker.mountpoint,
		}
		if !worker.lastSyncAt.IsZero() {
			// Nanosecond precision, not RFC3339's default seconds — a sync
			// that lands in the same wall-clock second as a kick request
			// must still compare as "after" it (see blackboxStateCaughtUp),
			// which second-only precision can get wrong.
			targetState.LastSyncAtUTC = worker.lastSyncAt.Format(time.RFC3339Nano)
		}
		if worker.lastDuration > 0 {
			targetState.LastCycleDuration = worker.lastDuration.String()
		}
		if worker.status == "degraded" {
			state.Status = "degraded"
		}
		worker.mu.Unlock()
		state.Targets = append(state.Targets, targetState)
	}
	sort.Slice(state.Targets, func(i, j int) bool {
		return state.Targets[i].EnrollmentID < state.Targets[j].EnrollmentID
	})
	_ = writeJSONAtomic(rt.statePath, state)
}

func bootStartedAtUTC() (time.Time, error) {
	raw, err := os.ReadFile("/proc/stat")
	if err != nil {
		return time.Time{}, err
	}
	for _, line := range strings.Split(string(raw), "\n") {
		line = strings.TrimSpace(line)
		if !strings.HasPrefix(line, "btime ") {
			continue
		}
		parts := strings.Fields(line)
		if len(parts) != 2 {
			break
		}
		sec, err := time.ParseDuration(parts[1] + "s")
		if err != nil {
			break
		}
		return time.Unix(int64(sec/time.Second), 0).UTC(), nil
	}
	return time.Time{}, fmt.Errorf("boot time not found")
}

func newBlackboxEnrollmentID() string {
	var buf [8]byte
	if _, err := rand.Read(buf[:]); err != nil {
		return fmt.Sprintf("bb-%d", time.Now().UnixNano())
	}
	return "bb-" + hex.EncodeToString(buf[:])
}

func sanitizeRemovableTarget(target platform.RemovableTarget) platform.RemovableTarget {
	target.Device = strings.TrimSpace(target.Device)
	target.FSType = strings.TrimSpace(target.FSType)
	target.Size = strings.TrimSpace(target.Size)
	target.Label = strings.TrimSpace(target.Label)
	target.Model = strings.TrimSpace(target.Model)
	target.Mountpoint = strings.TrimSpace(target.Mountpoint)
	return target
}

func ensureMountedTarget(target platform.RemovableTarget, suffix string) (mountpoint string, mountedByBee bool, retErr error) {
	target = sanitizeRemovableTarget(target)
	if target.Mountpoint != "" {
		if err := ensureWritableBlackboxMountpoint(target.Mountpoint); err == nil {
			return target.Mountpoint, false, nil
		}
	}
	mountpoint = filepath.Join("/tmp", "bee-blackbox-"+sanitizeFilename(suffix))
	if err := os.MkdirAll(mountpoint, 0755); err != nil {
		return "", false, err
	}
	// -o sync makes every write to the target synchronous at the VFS layer
	// (no page-cache write-back to lose on a hard reset) instead of relying
	// solely on the explicit syscall.Sync() calls in writeFileAtomic/
	// unmountTarget to flush it after the fact. Those explicit syncs stay in
	// place as a fallback (e.g. for target.Mountpoint above, an
	// already-mounted filesystem we don't control the options of) — with -o
	// sync already doing the work, they become a fast no-op most of the time
	// instead of the primary durability mechanism.
	if raw, err := blackboxExecCommand("mount", "-o", "sync", target.Device, mountpoint).CombinedOutput(); err != nil {
		return "", false, formatBlackboxMountTargetError(target, string(raw), err)
	}
	if err := ensureWritableBlackboxMountpoint(mountpoint); err != nil {
		_ = unmountTarget(mountpoint)
		return "", false, err
	}
	return mountpoint, true, nil
}

func unmountTarget(mountpoint string) error {
	syscall.Sync()
	raw, err := blackboxExecCommand("umount", mountpoint).CombinedOutput()
	if err != nil {
		msg := strings.TrimSpace(string(raw))
		if msg == "" {
			return err
		}
		return fmt.Errorf("%s: %w", msg, err)
	}
	return nil
}

func readBlackboxMarker(mountpoint string) (BlackboxMarker, bool, error) {
	raw, err := os.ReadFile(filepath.Join(mountpoint, blackboxMarkerName))
	if err != nil {
		if errors.Is(err, os.ErrNotExist) {
			return BlackboxMarker{}, false, os.ErrNotExist
		}
		return BlackboxMarker{}, false, err
	}
	var marker BlackboxMarker
	if err := json.Unmarshal(raw, &marker); err != nil {
		return BlackboxMarker{}, false, err
	}
	return marker, true, nil
}

func writeBlackboxMarker(mountpoint string, marker BlackboxMarker) error {
	if marker.Version == 0 {
		marker.Version = 1
	}
	return writeJSONAtomic(filepath.Join(mountpoint, blackboxMarkerName), marker)
}

func copyFileIfChanged(src, dst string) error {
	info, err := os.Stat(src)
	if err != nil {
		return err
	}
	if info.IsDir() {
		return os.MkdirAll(dst, info.Mode().Perm())
	}
	srcData, err := os.ReadFile(src)
	if err != nil {
		return err
	}
	if dstData, err := os.ReadFile(dst); err == nil && bytes.Equal(dstData, srcData) {
		return nil
	}
	return writeFileAtomic(dst, srcData, info.Mode().Perm())
}

func captureCommandAtomic(dst string, name string, args ...string) error {
	raw, err := blackboxExecCommand(name, args...).CombinedOutput()
	if len(raw) == 0 {
		if err != nil {
			raw = []byte(err.Error() + "\n")
		} else {
			raw = []byte("no output\n")
		}
	}
	return writeFileAtomic(dst, raw, 0644)
}

func writeJSONAtomic(path string, v any) error {
	raw, err := json.MarshalIndent(v, "", "  ")
	if err != nil {
		return err
	}
	raw = append(raw, '\n')
	return writeFileAtomic(path, raw, 0644)
}

func writeFileAtomic(path string, data []byte, perm os.FileMode) error {
	if err := os.MkdirAll(filepath.Dir(path), 0755); err != nil {
		return err
	}
	if existing, err := os.ReadFile(path); err == nil && bytes.Equal(existing, data) {
		return nil
	}
	tmp := path + ".tmp"
	f, err := os.OpenFile(tmp, os.O_CREATE|os.O_TRUNC|os.O_WRONLY, perm)
	if err != nil {
		return err
	}
	if _, err := f.Write(data); err != nil {
		_ = f.Close()
		return err
	}
	if err := f.Sync(); err != nil {
		_ = f.Close()
		return err
	}
	if err := f.Close(); err != nil {
		return err
	}
	if err := os.Rename(tmp, path); err != nil {
		return err
	}
	return syncFilesystem(filepath.Dir(path))
}

// syncFilesystem flushes pending writes to durable storage. Uses the sync(2)
// syscall directly rather than spawning /bin/sync — writeFileAtomic calls
// this once per copied file (syncDirectoryTree can touch hundreds of files
// per cycle), and syscall.Sync() does the same flush without a fork+exec per
// call. path is unused (sync(2) always flushes system-wide; kept as a
// parameter for call-site clarity about which tree the caller cares about).
func syncFilesystem(path string) error {
	syscall.Sync()
	return nil
}

func ensureWritableBlackboxMountpoint(mountpoint string) error {
	probe, err := os.CreateTemp(mountpoint, ".bee-blackbox-write-test-*")
	if err != nil {
		return fmt.Errorf("target filesystem is not writable: %w", err)
	}
	name := probe.Name()
	if closeErr := probe.Close(); closeErr != nil {
		_ = os.Remove(name)
		return closeErr
	}
	if err := os.Remove(name); err != nil {
		return err
	}
	return nil
}

func formatBlackboxMountTargetError(target platform.RemovableTarget, raw string, err error) error {
	msg := strings.TrimSpace(raw)
	fstype := strings.ToLower(strings.TrimSpace(target.FSType))
	if fstype == "exfat" && strings.Contains(strings.ToLower(msg), "unknown filesystem type 'exfat'") {
		return fmt.Errorf("mount %s: exFAT support is missing in this ISO build: %w", target.Device, err)
	}
	if msg == "" {
		return err
	}
	return fmt.Errorf("%s: %w", msg, err)
}