package volsnap

import (
	"context"
	"encoding/json"
	"fmt"
	"log/slog"
	"os"
	"path/filepath"
	"strings"

	"github.com/google/uuid"

	"github.com/alexei/tinyforge/internal/store"
)

// Lifecycle is the deploy-side seam Engine.Restore needs but volsnap must not
// import directly (it would couple the snapshot package to the deployer/docker
// packages). The composition root supplies an adapter over the Deployer +
// Docker client via Engine.SetLifecycle.
type Lifecycle interface {
	// Lock acquires the per-workload deploy lock (C1) and returns the release
	// func. Held by Restore across stop→swap→redeploy.
	Lock(workloadID string) func()
	// StopContainers stops every running container for the workload (C4 quiesce)
	// and returns the image tag the newest running container was on, so the
	// redeploy can bring the SAME version back up ("" ⇒ source default tag).
	StopContainers(ctx context.Context, workloadID string) (runningTag string, err error)
	// Redeploy re-dispatches the workload's current config WITHOUT re-acquiring
	// the per-workload lock (the caller holds it). reference pins the image tag.
	Redeploy(ctx context.Context, w store.Workload, reference string) error
}

// SetLifecycle wires the deploy-side seam. Pass nil to leave restore disabled.
func (e *Engine) SetLifecycle(lc Lifecycle) { e.lifecycle = lc }

// restoreJournal is the on-disk write-ahead record of an in-flight restore.
// Written before the first destructive rename and deleted on completion; the
// startup RecoverInterruptedRestores sweep replays it after a crash.
type restoreJournal struct {
	SnapshotID string          `json:"snapshot_id"`
	WorkloadID string          `json:"workload_id"`
	Volumes    []journalVolume `json:"volumes"`
}

type journalVolume struct {
	Live    string `json:"live"`
	Old     string `json:"old"`
	Tmp     string `json:"tmp"`
	Swapped bool   `json:"swapped"`
	HadOld  bool   `json:"had_old"`
}

// staged pairs a resolved volume with its per-restore staging dirs.
type staged struct {
	rv  resolvedVol
	tmp string
	old string
}

// Restore overwrites the workload's live host-bind volumes with a snapshot's
// contents and brings the app back up. It is the single, engine-owned entry
// point for the data-loss-sensitive restore flow (image-source workloads only).
//
// Ordering is deliberate and crash-aware:
//
//	pre-flight (re-resolve all volumes C3, size + per-fs disk check C5) — abort
//	  here touches nothing
//	→ Lock (C1)  → re-validate workload  → StopContainers (C4 quiesce)
//	→ extract ALL volumes to sibling .tmp staging dirs (reads the source archive
//	  fully BEFORE the next step can prune it; shrinks the later destructive
//	  window to pure renames — R3)
//	→ capture a pre-restore snapshot (durable escape hatch, after quiesce,
//	  before any destructive rename — folded suggestion)
//	→ write the restore journal (R3 crash recovery)
//	→ swap each volume atomically (rename live→.old, .tmp→live — C2)
//	→ Redeploy (C4 — image containers are recreated, never reused)
//	→ remove .old + journal, emit audit event
//
// Engine.Restore holds NO e.mu (R1): per-workload serialization is the
// Lifecycle lock; e.Create takes its own e.mu for the pre-restore archive
// write, so calling it here cannot self-deadlock.
func (e *Engine) Restore(ctx context.Context, snapshotID, workloadID string) error {
	if e.lifecycle == nil {
		return fmt.Errorf("restore: lifecycle not configured")
	}

	snap, err := e.store.GetVolumeSnapshot(snapshotID)
	if err != nil {
		return err
	}
	if snap.WorkloadID != workloadID {
		return fmt.Errorf("snapshot %s does not belong to workload %s", snapshotID, workloadID)
	}
	w, err := e.store.GetWorkloadByID(workloadID)
	if err != nil {
		return err
	}
	if w.SourceKind != "image" {
		return fmt.Errorf("restore is only supported for image-source workloads")
	}
	settings, err := e.store.GetSettings()
	if err != nil {
		return fmt.Errorf("load settings: %w", err)
	}

	manifest, err := parseManifest(snap)
	if err != nil {
		return err
	}
	resolved, err := preflightResolve(e.store, w, settings, manifest) // C3 all-or-nothing
	if err != nil {
		return fmt.Errorf("pre-flight: %w", err)
	}
	archivePath, err := e.FilePath(snap)
	if err != nil {
		return err
	}
	perIndex, _, err := archiveUncompressedSize(archivePath, maxRestoreUncompressedBytes)
	if err != nil {
		return fmt.Errorf("size snapshot: %w", err)
	}
	if err := checkDiskSpace(resolved, perIndex); err != nil { // C5
		return err
	}

	// ── past pre-flight: take the per-workload lock and quiesce ──────────────
	unlock := e.lifecycle.Lock(workloadID)
	defer unlock()

	// A teardown may have won the lock and deleted the workload while we waited.
	if _, err := e.store.GetWorkloadByID(workloadID); err != nil {
		return fmt.Errorf("workload disappeared before restore: %w", err)
	}

	tag, err := e.lifecycle.StopContainers(ctx, workloadID) // C4 stop
	if err != nil {
		return fmt.Errorf("stop containers: %w", err)
	}

	// Extract every volume to its staging dir FIRST. This reads the source
	// archive fully before the pre-restore capture below can prune it, and
	// leaves only pure renames for the destructive phase (R3).
	token := uuid.New().String()[:8]
	stagedVols := make([]staged, 0, len(resolved))
	for _, rv := range resolved {
		tmp, old := stagingDirs(rv.LivePath, token, rv.Index)
		if _, exErr := safeExtractIndex(archivePath, rv.Index, tmp, maxRestoreUncompressedBytes); exErr != nil {
			cleanupStaging(stagedVols)
			_ = os.RemoveAll(tmp)
			// Nothing swapped yet — bring the app back up on its original data.
			e.redeployAfterAbort(ctx, w, tag)
			return fmt.Errorf("extract volume %q: %w", rv.Target, exErr)
		}
		stagedVols = append(stagedVols, staged{rv: rv, tmp: tmp, old: old})
	}

	// Durable pre-restore snapshot (escape hatch). Quiesced (after stop), and
	// the source archive is already fully extracted so a prune here is harmless.
	// Best-effort, matching the DB-restore precedent: a failure is logged but
	// does not abort — the .old dirs + journal are the in-operation safety net.
	if _, err := e.Create(w, settings, "pre-restore"); err != nil {
		slog.Warn("restore: pre-restore snapshot failed (continuing)",
			"workload", workloadID, "error", err)
	}

	// Journal before the first destructive rename so a crash can be recovered.
	jr := restoreJournal{SnapshotID: snapshotID, WorkloadID: workloadID}
	for _, sv := range stagedVols {
		jr.Volumes = append(jr.Volumes, journalVolume{Live: sv.rv.LivePath, Old: sv.old, Tmp: sv.tmp})
	}
	if err := e.writeJournal(jr); err != nil {
		cleanupStaging(stagedVols)
		e.redeployAfterAbort(ctx, w, tag)
		return fmt.Errorf("write restore journal: %w", err)
	}

	// ── destructive phase: pure atomic renames ──────────────────────────────
	done := make([]swap, 0, len(stagedVols))
	for i, sv := range stagedVols {
		hadOld, swErr := swapVolumeDir(sv.rv.LivePath, sv.tmp, sv.old)
		if swErr != nil {
			rollbackSwaps(done)               // restore already-swapped volumes
			cleanupStagingFrom(stagedVols, i) // drop remaining un-swapped tmp/old
			e.removeJournal(workloadID)
			e.redeployAfterAbort(ctx, w, tag)
			return fmt.Errorf("swap volume %q: %w", sv.rv.Target, swErr)
		}
		done = append(done, swap{live: sv.rv.LivePath, old: sv.old, tmp: sv.tmp, hadOld: hadOld})
		jr.Volumes[i].Swapped = true
		jr.Volumes[i].HadOld = hadOld
		_ = e.writeJournal(jr) // progress checkpoint (best-effort)
	}

	// Bring the app back up against the restored data (C4 — recreate, redeploy).
	if err := e.lifecycle.Redeploy(ctx, w, tag); err != nil {
		// The data IS restored; only the app failed to come back. Do NOT roll
		// back the volumes — surface the redeploy error so the operator retries
		// a deploy. Clean up the .old set-asides and the journal.
		cleanupOld(done)
		e.removeJournal(workloadID)
		return fmt.Errorf("redeploy after restore: %w", err)
	}

	cleanupOld(done)
	e.removeJournal(workloadID)
	e.emitRestoreEvent(workloadID, snapshotID, len(done))
	slog.Info("volume snapshot restored", "workload", workloadID, "snapshot", snapshotID, "volumes", len(done))
	return nil
}

// redeployAfterAbort re-dispatches after an aborted restore so a stopped app
// does not stay down. Best-effort: the error is logged, not returned (the
// restore failure is the primary error the caller surfaces).
func (e *Engine) redeployAfterAbort(ctx context.Context, w store.Workload, tag string) {
	if err := e.lifecycle.Redeploy(ctx, w, tag); err != nil {
		slog.Warn("restore: redeploy after abort failed", "workload", w.ID, "error", err)
	}
}

// RecoverInterruptedRestores replays restore journals left by a crash mid-
// restore, mirroring CleanOrphans (run once at startup, before serving). For
// each volume: a completed swap keeps the restored live dir and drops the set-
// aside original; an incomplete swap that left live missing is reverted from
// .old; stray staging dirs are removed. Returns the number of journals handled.
func (e *Engine) RecoverInterruptedRestores() (int, error) {
	e.mu.Lock()
	defer e.mu.Unlock()

	entries, err := os.ReadDir(e.snapDir)
	if err != nil {
		return 0, fmt.Errorf("read snapshot dir: %w", err)
	}
	recovered := 0
	for _, ent := range entries {
		name := ent.Name()
		if ent.IsDir() || !strings.HasPrefix(name, "restore-") || !strings.HasSuffix(name, ".json") {
			continue
		}
		path := filepath.Join(e.snapDir, name)
		data, rerr := os.ReadFile(path)
		if rerr != nil {
			slog.Warn("restore recovery: read journal", "file", name, "error", rerr)
			continue
		}
		var jr restoreJournal
		if jerr := json.Unmarshal(data, &jr); jerr != nil {
			slog.Warn("restore recovery: parse journal", "file", name, "error", jerr)
			continue
		}
		slog.Warn("restore recovery: replaying interrupted restore",
			"workload", jr.WorkloadID, "snapshot", jr.SnapshotID, "volumes", len(jr.Volumes))
		for _, v := range jr.Volumes {
			recoverVolume(v)
		}
		if rmErr := os.Remove(path); rmErr != nil {
			slog.Warn("restore recovery: remove journal", "file", name, "error", rmErr)
		}
		recovered++
	}
	return recovered, nil
}

// recoverVolume reconciles a single volume's on-disk state from its journal
// entry after a crash. Each branch leaves the live dir intact (either restored
// or original) and removes staging leftovers.
func recoverVolume(v journalVolume) {
	if v.Swapped {
		// Swap completed: live already holds restored data. Drop the set-aside.
		_ = os.RemoveAll(v.Old)
		_ = os.RemoveAll(v.Tmp)
		return
	}
	if _, err := os.Lstat(v.Live); os.IsNotExist(err) {
		if _, oerr := os.Lstat(v.Old); oerr == nil {
			// Crashed mid-rename (live→old done, tmp→live not): revert.
			_ = os.Rename(v.Old, v.Live)
		}
	} else {
		// live is intact (original). Any .old is a dangling partial copy.
		_ = os.RemoveAll(v.Old)
	}
	_ = os.RemoveAll(v.Tmp)
}

// ── journal + cleanup helpers ───────────────────────────────────────────────

func (e *Engine) journalPath(workloadID string) string {
	// workloadID is a server-generated id (loaded from the DB before we get
	// here). filepath.Base defends against any separator sneaking into the name.
	return filepath.Join(e.snapDir, "restore-"+filepath.Base(workloadID)+".json")
}

func (e *Engine) writeJournal(jr restoreJournal) error {
	data, err := json.Marshal(jr)
	if err != nil {
		return fmt.Errorf("encode journal: %w", err)
	}
	// Write atomically (tmp + rename): a torn journal would silently disable the
	// recovery sweep (RecoverInterruptedRestores skips unparseable journals), so
	// a crash mid-write must never leave a half-written WAL on disk. The .tmp
	// suffix is ignored by the recovery scan (it matches *.json only).
	final := e.journalPath(jr.WorkloadID)
	tmp := final + ".tmp"
	if err := os.WriteFile(tmp, data, 0o600); err != nil {
		return fmt.Errorf("write journal: %w", err)
	}
	if err := os.Rename(tmp, final); err != nil {
		_ = os.Remove(tmp)
		return fmt.Errorf("commit journal: %w", err)
	}
	return nil
}

func (e *Engine) removeJournal(workloadID string) {
	if err := os.Remove(e.journalPath(workloadID)); err != nil && !os.IsNotExist(err) {
		slog.Warn("restore: remove journal", "workload", workloadID, "error", err)
	}
}

func (e *Engine) emitRestoreEvent(workloadID, snapshotID string, volumes int) {
	meta, _ := json.Marshal(map[string]any{"snapshot_id": snapshotID, "volumes": volumes})
	if _, err := e.store.InsertEvent(store.EventLog{
		Source:     "volsnap",
		WorkloadID: workloadID,
		Severity:   "info",
		Message:    "volume snapshot restored",
		Metadata:   string(meta),
	}); err != nil {
		slog.Warn("restore: record event", "workload", workloadID, "error", err)
	}
}

// cleanupStaging removes the tmp + old staging dirs for every staged volume
// (used when aborting before the swap phase).
func cleanupStaging(sv []staged) {
	for _, s := range sv {
		_ = os.RemoveAll(s.tmp)
		_ = os.RemoveAll(s.old)
	}
}

// cleanupStagingFrom removes staging dirs from index `from` onward (the volumes
// not yet swapped when a swap failed).
func cleanupStagingFrom(sv []staged, from int) {
	for i := from; i < len(sv); i++ {
		_ = os.RemoveAll(sv[i].tmp)
		_ = os.RemoveAll(sv[i].old)
	}
}

// cleanupOld removes the .old set-aside dirs after a successful (or data-
// committed) restore to reclaim disk; the pre-restore snapshot is the durable
// rollback target.
func cleanupOld(done []swap) {
	for _, s := range done {
		_ = os.RemoveAll(s.old)
	}
}

// checkDiskSpace verifies each target filesystem has room for the volumes that
// will be staged on it (C5). Peak usage co-locates the live copy (renamed
// aside, no new space) and the extracted copy (new space ≈ uncompressed size),
// so the new allocation per filesystem is the sum of its volumes' extracted
// sizes plus headroom. The estimate is a lower bound (see archiveUncompressedSize);
// a mid-extract ENOSPC is still caught and rolled back.
func checkDiskSpace(resolved []resolvedVol, perIndex map[int]int64) error {
	needByParent := map[string]int64{}
	for _, rv := range resolved {
		needByParent[filepath.Dir(rv.LivePath)] += perIndex[rv.Index]
	}
	for parent, need := range needByParent {
		probe := firstExistingAncestor(parent)
		free, err := freeDiskBytes(probe)
		if err != nil {
			return fmt.Errorf("check disk space at %s: %w", probe, err)
		}
		if int64(free) < need+diskFreeHeadroomBytes {
			return fmt.Errorf("insufficient disk space at %s: need ~%d bytes, have %d",
				parent, need+diskFreeHeadroomBytes, free)
		}
	}
	return nil
}

// firstExistingAncestor walks up p until it finds a path that exists, so the
// free-space probe has a real filesystem to stat even when the volume dir (or
// its parent) hasn't been created yet.
func firstExistingAncestor(p string) string {
	for {
		if _, err := os.Stat(p); err == nil {
			return p
		}
		parent := filepath.Dir(p)
		if parent == p {
			return p
		}
		p = parent
	}
}