package backup

import (
	"context"
	"fmt"
	"os"
	"path/filepath"
	"sort"
	"strings"
	"time"

	"gitea.dooplex.hu/admin/felhom-controller/internal/settings"
)

// ── R-87 — the box proves its own off-site copy still HOLDS something ────────────────────────────
//
// WHAT THIS ANSWERS THAT NOTHING ELSE DOES. `restic check --read-data-subset=100%` (R-359/R-399) runs
// weekly and proves the stored bytes are the bytes we stored. **It cannot tell us we stored the wrong
// thing.** A hollow recovery unit — no database dump, no volume tar — backs up cleanly, checks
// cleanly at full depth, restores cleanly, and gives the customer nothing back. R-403 measured that
// shape on this fleet on 2026-08-31: 120 082 104 B became 7 036 B in one nightly run, recorded as a
// success. Until this job, no tier and no cadence asked the question.
//
// WHAT IT DELIBERATELY DOES NOT ANSWER, said here because the green tick will be read as covering it
// otherwise: **it does not prove a restore puts data back into a running app.** It restores to a
// throwaway directory, looks, and deletes. Putting data back is drill work, and the spike said so —
// of the five restore-path defects human drills found in the six days to 2026-08-31, an unattended
// scratch restore would have caught ONE (R-356). `07-backup-architecture.md` §8 matrix row 4 does not
// move on the strength of this job.
//
// THE SHAPE IS `CheckOffboxIntegrity`'S, deliberately — same file conventions, same guards, same
// three-outcome honesty — because a second scheduled off-site job that reasons differently about the
// single-writer flag is how the two come to disagree about repository safety.
//
// ── THE HAZARD, AND WHY THIS JOB TAKES A FLAG THAT ITS RESTORE DOES NOT ─────────────────────────
//
// `resticStep` self-heals a crash lock by running `unlock --remove-all`, and its own doc comment
// records that this is safe only because *"the in-process single-flight mutex (held by every caller
// of this method) proves no sibling operation is live"*.
//
// `offbox_integrity.go`'s header states that invariant as universal — *"Every off-site operation
// takes acquireRunning for exactly that reason"* — and **it is not true today**:
// `RestoreOffboxScratch` does not take it (R-408, measured 2026-08-31; `restore_wizard.go` records
// the same fact independently and the UI works around it with a separate display flag). This job
// therefore takes the flag ITSELF rather than waiting for R-408 to be fixed, and SKIPS rather than
// waits — a skipped proof simply runs tomorrow, whereas waiting would pin a nightly backup behind it.
//
// ── AND IT NEVER WRITES TO THE REPOSITORY, FOR REAL ────────────────────────────────────────────
//
// R-95's constraint is that the off-site credential can delete, so a scheduled job must not be able
// to write. The spike established what "read-only" actually costs here, by observation rather than by
// reading the code:
//
//   - `restic restore` in 0.14.0 takes NO repository lock (measured on demo-hp with a lock sampler
//     that was positively controlled first: it saw a lock appear and vanish across a real `check`, and
//     zero across two restores);
//   - `restic check` DOES take one — so the comment claiming the check never writes is wrong (R-407);
//   - the PRODUCT's restore path writes anyway, because `unlockStale` runs `restic unlock` — a delete
//     verb against `locks/` — before every restore (`offbox_restore.go`).
//
// So this job runs its restore with `--no-lock` and WITHOUT `unlockStale`, through `m.runner()`
// rather than `resticStep`, so the `unlock --remove-all` escalation is not merely unlikely but
// unreachable. `TestR87_NeverWritesToTheRepository` asserts that as a NON-EFFECT — the argv, not the
// absence of an error. Using the seam rather than replacing `resticStep` is REUSE.md's own rule:
// replacing it would hide the escalation from exactly the assertion that must see it.
//
// `restic restore --verify` is NOT used as a correctness check and must never be: the spike measured
// it passing a byte-level corruption with size and mtime preserved, on a 213 MB tree, in 131 ms. It
// is a size-and-mtime reconciliation. The correctness question is answered by `JudgeRestoredUnit`.

// proofRestoreTimeout bounds one proof restore.
//
// CHOSEN from measurement, not inherited: on demo-hp 2026-08-31 a unit restore took 2.3–4.0 s per app
// regardless of size (185 KB → 2.25 s, 213 MB → 3.20 s — the cost is per-snapshot round-trip plus
// roughly 1 s per 200 MB), and all eight apps back to back took 25 s. Ten minutes is ~150x the
// slowest single app measured, so the failure mode of a wedged SFTP mount is a released flag and a
// retry tomorrow, never a box whose backups stop because a proof never returned. It is deliberately
// well under `integrityCheckTimeout` (30 min): this job downloads ONE app's unit, not the store.
const proofRestoreTimeout = 10 * time.Minute

// ProofResult is what one nightly proof did, so the log, the event, the report and any future debug
// endpoint state the same facts from one place.
//
// Skipped is a first-class outcome and NOT a failure — a proof that yielded to a running backup did
// the right thing. NoSnapshot is separated from a failure for the same reason R-185 separated "I
// could not look" from "I looked and it is broken": an app with no off-site snapshot yet is a
// staleness question, which R-339 and the hub's tier deadline already own, and alarming here would be
// a second alarm for a fact that already has one.
type ProofResult struct {
	RanAt      time.Time
	Duration   time.Duration
	Stack      string
	SnapshotID string
	Judgement  UnitProofResult

	Skipped    bool
	SkipReason string
	NoSnapshot bool
	// Err is a restore/plumbing failure — NOT a verdict about the backup. It never becomes the
	// "intact but empty" alarm, because a download that did not finish has seen nothing.
	Err error
	// CannotRun (R-414) is set when the proof could not START on this box at all — today the only
	// cause is that there is nowhere to put a scratch.
	//
	// IT IS SEPARATE FROM Err ON PURPOSE. An Err is transient: a download that failed says nothing
	// about the backup and nothing about the box, so it is right that it records nothing and retries.
	// This is a STANDING PROPERTY of the machine — it will be just as true tomorrow — and leaving it
	// unrecorded is what R-414 measured: `demo-felhom` failed this way every night with only a WARN,
	// and because nothing was written, `last_proof_result` stayed ABSENT — which is also what a
	// controller older than v0.231.0 sends. The hub could not tell "cannot run here" from "too old to
	// have the feature". That is the StatsKnown trap one level up, and this field is what closes it.
	CannotRun bool
	CannotWhy string
}

// Verdict renders the outcome for the persisted record and the wire. "" is reserved for "no verdict
// was reached", which is what a skip, a missing snapshot and a restore error all are.
func (r ProofResult) Verdict() string {
	if r.CannotRun {
		// A recorded, distinguishable state — never "" , which the hub reads as "not recorded".
		return ProofResultCannotRun
	}
	if r.Skipped || r.NoSnapshot || r.Err != nil {
		return ""
	}
	return string(r.Judgement.Verdict)
}

// ProofResultCannotRun is the persisted value for R-414's outcome. It is deliberately NOT one of the
// UnitProofVerdict values: those judge a UNIT, and here no unit was ever looked at.
const ProofResultCannotRun = "cannot_run"

// ProveOffboxUnit runs ONE nightly proof: pick the due app, restore its newest snapshot read-only to
// a throwaway directory, judge it, delete the copy, and record WHICH SNAPSHOT was proved.
//
// The order below is not arrangeable. The first three guards run before anything is downloaded, and
// the scratch is removed on EVERY path including failure — a failed judgement that leaves a copy
// behind is a slow disk-filler and, worse, a copy nobody knows the provenance of.
func (m *Manager) ProveOffboxUnit(ctx context.Context) ProofResult {
	res := ProofResult{RanAt: time.Now()}

	if !m.OffboxConfigured() {
		// A box with no off-site tier has nothing to prove. Silent, and not an alarm.
		res.Skipped, res.SkipReason = true, "no off-site target configured"
		return res
	}
	if err := m.acquireRunning(); err != nil {
		res.Skipped, res.SkipReason = true, "a backup or restore is already running"
		m.logger.Printf("[INFO] [offbox] proof: skipped — %s; due-ness is NOT advanced, so this retries on the next run", res.SkipReason)
		return res
	}
	defer m.releaseRunning()

	stack, id, unitPath, ok := m.nextProofTarget(ctx)
	if !ok {
		res.NoSnapshot = true
		m.logger.Printf("[INFO] [offbox] proof: nothing due — every deployed app's newest off-site snapshot has already been proved, or none has one yet")
		return res
	}
	res.Stack, res.SnapshotID = stack, id

	scratch, nsRoot, derr := m.offboxProofScratchDir(stack)
	if derr != nil {
		// R-414: a standing property of this box, not a transient failure — so it is RECORDED, not
		// merely warned about. See ProofResult.CannotRun.
		res.CannotRun, res.CannotWhy = true, derr.Error()
		m.logger.Printf("[WARN] [offbox] proof: %s has nowhere to restore to, and this box will fail the same way every night until a data drive is registered: %v", stack, derr)
		return res
	}
	// The headroom gate runs BEFORE the download, through the same helper and the same Hungarian
	// wording the customer's restore uses (`unitOnlyHeadroom`). A gate after the download is not a
	// gate.
	if herr := unitOnlyHeadroom(m.offboxFree()(nsRoot)); herr != nil {
		res.Err = herr
		m.logger.Printf("[WARN] [offbox] proof: %s refused before any download: %v", stack, herr)
		return res
	}

	// Whatever happens from here, the copy goes. Registered before the restore so an early return
	// cannot leak one, and it removes a stale copy from an interrupted previous run on the way in.
	m.removeProofScratch(stack, scratch)
	defer m.removeProofScratch(stack, scratch)

	start := time.Now()
	if rerr := m.restoreUnitReadOnly(ctx, stack, id, unitPath, scratch); rerr != nil {
		res.Duration = time.Since(start)
		res.Err = rerr
		m.logger.Printf("[ERROR] [offbox] proof: %s could not be restored for proving (nothing is concluded about the backup): %v", stack, rerr)
		return res
	}
	res.Duration = time.Since(start)

	// The unit sits inside the scratch at its own absolute snapshot path.
	res.Judgement = JudgeRestoredUnit(filepath.Join(scratch, strings.TrimPrefix(unitPath, string(filepath.Separator))))
	switch res.Judgement.Verdict {
	case UnitProofPass:
		m.logger.Printf("[INFO] [offbox] proof: %s PASSED on snapshot %s in %s — the backup holds what this app should have",
			stack, id, res.Duration.Round(time.Millisecond))
	case UnitProofCannotJudge:
		m.logger.Printf("[WARN] [offbox] proof: %s on snapshot %s could NOT be judged (%s) — recorded as such, never as a pass",
			stack, id, res.Judgement.Reason)
	default:
		m.logger.Printf("[ERROR] [offbox] proof: %s on snapshot %s is READABLE AND EMPTY (%s%s) — the store is not damaged; the backup does not contain this app's data",
			stack, id, res.Judgement.Reason, missingSuffix(res.Judgement.Missing))
	}
	return res
}

// missingSuffix renders the Missing list for a log line without ever printing a path outside the unit.
func missingSuffix(missing []string) string {
	if len(missing) == 0 {
		return ""
	}
	return ": " + strings.Join(missing, ", ")
}

// RecordProofVerdict persists the outcome and advances due-ness. The caller owns WHEN a verdict
// counts, exactly as `RecordIntegrityVerdict` does.
//
// A SKIP, A MISSING SNAPSHOT AND A RESTORE ERROR DO NOT REACH A VERDICT and must not advance
// due-ness — tomorrow tries again. A `cannot_judge` DOES advance it: re-downloading the same
// unjudgeable snapshot every night is load with no new information, the outcome is recorded where a
// surface can read it, and a new snapshot makes the app due again by itself.
func (m *Manager) RecordProofVerdict(res ProofResult) {
	v := res.Verdict()
	if v == "" {
		return
	}
	if err := m.settings.UpdateOffboxStatus(func(o *settings.OffboxTarget) {
		if v == ProofResultCannotRun {
			// RECORDED so the hub can see the state, but due-ness is deliberately NOT advanced: no
			// snapshot was proved, and marking one proved would stop the app ever being retried once a
			// drive is finally registered. A status report, not a proof.
			o.LastProofRun = res.RanAt.UTC().Format(time.RFC3339)
			o.LastProofStack = res.Stack
			o.LastProofSnapshot = ""
			o.LastProofResult = v
			o.LastProofReason = res.CannotWhy
			return
		}
		if o.ProvedSnapshots == nil {
			o.ProvedSnapshots = map[string]string{}
		}
		o.ProvedSnapshots[res.Stack] = res.SnapshotID
		o.LastProofRun = res.RanAt.UTC().Format(time.RFC3339)
		o.LastProofStack = res.Stack
		o.LastProofSnapshot = res.SnapshotID
		o.LastProofResult = v
		o.LastProofReason = string(res.Judgement.Reason)
	}); err != nil {
		m.logger.Printf("[ERROR] [offbox] proof: could not persist the outcome: %v — the proof RAN and its verdict for %s on %s was %q, but due-ness did not advance, so it will run again tomorrow",
			err, res.Stack, res.SnapshotID, v)
	}
}

// nextProofTarget picks ONE app: the deployed app whose newest off-site snapshot has not been proved.
//
// DUE-NESS IS PER SNAPSHOT, NOT PER CLOCK (R-86's model, 07 §3). An app is due when the ID of its
// newest snapshot differs from the ID last proved for it. A timestamp would re-prove the same
// snapshot forever and say nothing about the newest one — which is the defect R-341 recorded in a
// different costume.
//
// Apps are considered in a stable sorted order and the FIRST due one wins, so eight apps are covered
// in eight nights and the rotation cannot starve one: an app stops being due only once its newest
// snapshot has actually been proved.
func (m *Manager) nextProofTarget(ctx context.Context) (stack, id, unitPath string, ok bool) {
	if m.stackProvider == nil {
		return "", "", "", false
	}
	proved := map[string]string{}
	if t := m.settings.GetOffboxTarget(); t != nil && t.ProvedSnapshots != nil {
		proved = t.ProvedSnapshots
	}
	var names []string
	for _, st := range m.stackProvider.ListDeployedStacks() {
		names = append(names, st.Name)
	}
	sort.Strings(names)
	for _, name := range names {
		if !isSafeStackName(name) {
			continue
		}
		sid, paths, err := m.offboxLatestSnapshot(ctx, name)
		if err != nil || strings.TrimSpace(sid) == "" {
			// No snapshot yet for this app is NOT a failure of this test — staleness belongs to
			// R-339 and the hub's tier deadline. Logged, never alarmed.
			continue
		}
		if proved[name] == sid {
			continue // already proved on exactly this snapshot
		}
		up := offboxUnitPathOf(paths, name)
		if up == "" {
			// The snapshot exists and carries no recovery unit. That IS the strongest form of
			// "nothing recoverable", so it is a target rather than a skip — the judgement below will
			// find no manifest and fail closed.
			m.logger.Printf("[WARN] [offbox] proof: %s snapshot %s lists no recovery-unit path", name, sid)
			continue
		}
		return name, sid, up, true
	}
	return "", "", "", false
}

// restoreUnitReadOnly downloads ONE recovery unit into scratch WITHOUT writing anything to the
// repository.
//
// Three differences from `RestoreOffboxScratch`, each deliberate and each named:
//   - `--no-lock`, so restic does not create a lock file (it does not for `restore` in 0.14.0 anyway
//     — measured — but the flag makes that a guarantee rather than an observed behaviour of one
//     version);
//   - no `unlockStale`, so no delete verb is issued against `locks/`;
//   - `m.runner()` rather than `resticStep`, so the `unlock --remove-all` escalation is unreachable
//     rather than merely unlikely.
//
// It deliberately does NOT write the R-358 completion marker: that marker certifies a scratch for
// PLACEMENT into a live app, and a proof copy must never be placeable. Its absence is what keeps this
// directory inert even if the two roots were ever confused.
func (m *Manager) restoreUnitReadOnly(ctx context.Context, stack, id, unitPath, scratch string) error {
	if err := os.MkdirAll(scratch, 0o755); err != nil {
		return fmt.Errorf("proof scratch dir: %w", err)
	}
	t := m.settings.GetOffboxTarget()
	base, env := m.offboxBaseArgs(t)
	rctx, cancel := context.WithTimeout(ctx, proofRestoreTimeout)
	defer cancel()
	args := append(append([]string{}, base...),
		"restore", id, "--target", scratch, "--include", unitPath, "--no-lock")
	out, err := m.runner()(rctx, env, args...)
	if err != nil {
		return fmt.Errorf("offbox proof restore %s: %w: %s", stack, err, truncate(out))
	}
	return nil
}

// removeProofScratch deletes a proof copy, refusing loudly if the path is not strictly inside a
// proof root — the same prefix instinct as `DeleteOffsiteRestoreCopy`, because reaching here with an
// out-of-sandbox path means a helper above is wrong and a best-effort skip would hide that.
func (m *Manager) removeProofScratch(stack, scratch string) {
	clean := filepath.Clean(scratch)
	// R-414: the accepted roots must include the SYSTEM DATA PATH, because that is now where a
	// unit-only scratch lands on a box with no registered drive.
	//
	// CAUGHT BY LIVE VALIDATION ON demo-felhom, 2026-09-01, and it was a leak I introduced with the
	// fallback itself: `offsiteRestoreDriveRoots` is built from registered drives and schedulable
	// paths, so on a driveless box it is EMPTY — the scratch resolved fine and then this refused to
	// delete it ("refusing to remove … it is not inside a proof root"). Every nightly proof would have
	// left a copy behind, growing forever, on exactly the boxes the fallback exists for. The unit
	// tests did not see it because they register a drive.
	roots := append(m.offsiteRestoreDriveRoots(), strings.TrimSpace(m.cfg.Paths.SystemDataPath))
	for _, drive := range roots {
		if strings.TrimSpace(drive) == "" {
			continue
		}
		root := filepath.Clean(m.offsiteProofRootFor(drive)) + string(filepath.Separator)
		if strings.HasPrefix(clean+string(filepath.Separator), root) {
			if err := os.RemoveAll(clean); err != nil {
				m.logger.Printf("[WARN] [offbox] proof: could not remove the proof copy %s: %v", clean, err)
			}
			return
		}
	}
	m.logger.Printf("[WARN] [offbox] proof: refusing to remove %s — it is not inside a proof root", clean)
}