package pgstore

import (
	"context"
	"errors"
	"fmt"
	"time"

	"github.com/jackc/pgx/v5"
	"github.com/jackc/pgx/v5/pgconn"

	"textmachine/platform/internal/money"
)

// Tx is a transaction of this store. Exported so a caller can join its own write to one — the queue
// does, because a run row and its queue entry must be written together — without every package in
// the zone importing pgx to name the type.
type Tx = pgx.Tx

var (
	// ErrRunInFlight is a second live run on one book. The database refuses it (the partial unique
	// index runs_one_live_per_book) rather than the API explaining it: the engine holds an EXCLUSIVE
	// lock on the project file, so two live runs is not a state anything downstream can represent.
	ErrRunInFlight = errors.New("pgstore: the book already has a live run")
	// ErrNoRun is a run that does not exist or does not belong to the caller.
	ErrNoRun = errors.New("pgstore: no such run")
)

// StartRunInput is one accepted run request.
type StartRunInput struct {
	UserID          string
	BookID          string
	VerifyBank      bool
	CeilingChapters int
	// Ceiling is what those chapters are worth and what is HELD before anything is spawned. The hold
	// and the ceiling handed to the engine are the same number by construction: the hold makes the
	// credit unavailable to any other run, and the engine stops itself there, so an overspend is
	// impossible even while the platform is blind (D39.100).
	Ceiling money.MicroUSD
	Now     time.Time
}

// StartedRun is what the caller needs after a successful start.
type StartedRun struct {
	Run
	AttemptID int64
	// JournalOffset is the size of the book's journal at the moment this attempt was admitted. The
	// journal is per BOOK and append-only, so an attempt's own lines begin after everything already
	// in it; without this the tailer would adopt the previous attempt's handshake.
	JournalOffset int64
}

// StartRun admits a run: the run row, its first attempt, the hold and the queue entry, all in ONE
// transaction.
//
// One transaction because each of them alone is a way to lose money or work. A hold without a run
// row is credit reserved for nothing; a run row without a hold is a run that spends credit nobody
// set aside; a queue entry without either is a worker spawning an engine against a book whose
// bookkeeping does not exist. research/25 names the first of those explicitly ("a hold can leak
// before the directory exists"), and the answer is not a compensating sweep but not splitting the
// write in the first place.
//
// enqueue is handed the transaction so the queue's own insert joins it. It is a callback rather than
// a second call because River owns its SQL and this package owns its own: neither reaches into the
// other, and the atomicity is still real.
func (s *Store) StartRun(ctx context.Context, in StartRunInput, journalOffset int64, enqueue func(context.Context, Tx, string) error) (StartedRun, error) {
	if in.CeilingChapters <= 0 {
		return StartedRun{}, fmt.Errorf("pgstore: a run needs a positive chapter ceiling, got %d", in.CeilingChapters)
	}
	out := StartedRun{JournalOffset: journalOffset}
	err := s.inTx(ctx, func(tx pgx.Tx) error {
		// The book first, before the hold — see lockBook for why the order is global and not local.
		// Here it also makes the race between two admissions of one book a queue instead of a
		// collision: the loser still gets ErrRunInFlight, it just gets it after waiting.
		if err := lockBook(ctx, tx, in.BookID); err != nil {
			return err
		}
		runID := newID("run")
		const insertRun = `
			insert into runs (id, book_id, status, verify_bank, ceiling_chapters, started_at, revision)
			select $1, $2, 'translating', $3, $4, $5, b.revision + 1
			  from books b where b.id = $2 and b.owner_id = $6
			returning id, book_id, revision, status, verify_bank, ceiling_chapters,
			          coalesce(paused_reason, ''), started_at, finished_at`
		err := tx.QueryRow(ctx, insertRun, runID, in.BookID, in.VerifyBank, in.CeilingChapters, in.Now, in.UserID).
			Scan(&out.ID, &out.BookID, &out.Revision, &out.Status, &out.VerifyBank, &out.CeilingChapters,
				&out.PausedReason, &out.StartedAt, &out.FinishedAt)
		if errors.Is(err, pgx.ErrNoRows) {
			return ErrNoBook // the book is missing, or it is not this account's
		}
		if isUnique(err, "runs_one_live_per_book") {
			return ErrRunInFlight
		}
		if err != nil {
			return fmt.Errorf("pgstore: insert run: %w", err)
		}
		if err := tx.QueryRow(ctx, `
			insert into run_attempts (run_id, attempt_no, started_at, last_offset)
			values ($1, 1, $2, $3) returning id`, out.ID, in.Now, journalOffset).Scan(&out.AttemptID); err != nil {
			return fmt.Errorf("pgstore: insert attempt: %w", err)
		}
		// The hold is taken here, BEFORE anything is spawned, and it is the enforcement half of the
		// money design rather than an accounting note.
		if err := holdTx(ctx, tx, in.UserID, in.BookID, engineRunKey(out.ID, 1), in.Ceiling, in.Now); err != nil {
			return err
		}
		if _, err := tx.Exec(ctx, `
			update books set status = 'translating',
			                 revision = `+nextRevisionOfThisBooksLibrary+` where id = $1`, in.BookID); err != nil {
			return fmt.Errorf("pgstore: mark book translating: %w", err)
		}
		if enqueue != nil {
			return enqueue(ctx, tx, out.ID)
		}
		return nil
	})
	if err != nil {
		return StartedRun{}, err
	}
	return out, nil
}

// engineRunKey is the reservation key of an attempt.
//
// ⚠ It is the PLATFORM's attempt identity, not the engine's `engine_run_id`: the engine mints its
// own id per invocation and the platform only learns it from the handshake, which arrives after the
// money has already been reserved. Reservations are keyed by attempt for exactly that reason — the
// hold has to exist before there is anything to key it by on the engine's side.
func engineRunKey(runID string, attempt int) string {
	return fmt.Sprintf("%s#%d", runID, attempt)
}

func isUnique(err error, constraint string) bool {
	var pg *pgconn.PgError
	return errors.As(err, &pg) && pg.Code == "23505" && pg.ConstraintName == constraint
}

// LiveRun is a run the reconciler has to make a decision about.
type LiveRun struct {
	RunID       string
	BookID      string
	UserID      string
	Workdir     string
	AttemptID   int64
	AttemptNo   int
	UnitName    string
	EngineRunID string
	Position    Position
	Quarantined bool
	VerifyBank  bool
	// CeilingChapters is the run's whole budget, in the unit the user chose; Ceiling is what THIS
	// attempt was allowed to spend. They differ after a restart, which gets what is left.
	CeilingChapters int
	Ceiling         money.MicroUSD
	// EngineBinary is the VERSIONED path this attempt was pinned to (unified backlog row 139). Read
	// back and USED — for the resume and for the repair channel — because a pin nothing reads is a
	// column, not a pin: the engine ships more often than a run finishes, and asking the CURRENT
	// binary about a book an older one is translating is a different question.
	EngineBinary string
	// SpendBaseline is what the BOOK had already cost when this attempt began. Nil means it was never
	// captured, which no current path produces (the spawn refuses without it).
	SpendBaseline *money.MicroUSD
	// CeilingArg is the limit a PREVIOUS claim of this attempt already handed the engine. Read back
	// because a retry must hand the same one: recomputing it from a counter that the first claim's own
	// engine has been moving gives that engine a second, larger limit and leaves the column that is
	// supposed to answer "what limit did that process have" describing neither. Zero means no claim
	// has been recorded (the column predates nothing else being able to tell).
	CeilingArg money.MicroUSD
	// PausedReason is what the stream already said about this run. A run that reported a ceiling halt
	// is `paused` however its process then ended (contract §BookStatus: never `failed`).
	PausedReason string
	// Status is the run's product status. The reconciler works on live runs, where it is
	// `translating`; the resume path reads a run that has already ended and decides by it.
	Status string
	// StopRequestedAt is when THIS platform asked the run to stop, and it is the only thing that can
	// tell a stop from a crash: the engine catches SIGTERM and exits 1, so the marker says
	// `exit-code/exited/1` for both (register row PD-152). Nil means nobody asked.
	StopRequestedAt *time.Time
	// StartedAt is the RUN's start; AttemptStartedAt is THIS attempt's. The reconciler's grace is
	// measured against the second: a restarted attempt inherits a start time hours old, and the
	// grace then expires before systemd has had a chance to create anything.
	StartedAt        time.Time
	AttemptStartedAt time.Time
}

// Position mirrors ingest.Position without importing it: this package owns the columns, and the
// dependency runs the other way.
type Position struct {
	Offset   int64
	LastSeq  int64
	LastHash []byte
}

// ListLiveRuns returns every run that has not finished, with its live attempt.
//
// This is the reconciler's source of truth, together with the book's directory — NOT systemd
// (research/25 §Опс). A transient unit does not survive a reboot and is unloaded the moment it
// exits, so asking systemd "what is running" answers a different question than "what did this
// platform promise a user".
func (s *Store) ListLiveRuns(ctx context.Context) ([]LiveRun, error) {
	return s.queryRuns(ctx, `
		  join run_attempts a on a.run_id = r.id and a.ended_at is null
		  left join reservations res on res.engine_run_id = r.id || '#' || a.attempt_no
		                            and res.state = 'open'
		 where r.finished_at is null
		 order by r.started_at`)
}

// runColumns is the reconciler's view of a run. Written once because the two queries that use it
// differ only in which runs they select, and a scan list copied twice is a scan list that drifts.
const runColumns = `
		select r.id, r.book_id, b.owner_id, b.workdir, r.verify_bank, r.ceiling_chapters,
		       coalesce(r.paused_reason, ''), r.started_at, r.status, r.stop_requested_at,
		       a.id, a.attempt_no, coalesce(a.unit_name, ''), coalesce(a.engine_run_id, ''),
		       a.last_offset, a.last_seq, a.last_line_sha256, a.quarantine_reason is not null,
		       coalesce(res.amount_micro_usd, 0), a.engine_binary, a.spend_baseline_micro_usd,
		       a.ceiling_arg_micro_usd, a.started_at
		  from runs r
		  join books b on b.id = r.book_id`

func (s *Store) queryRuns(ctx context.Context, tail string, args ...any) ([]LiveRun, error) {
	rows, err := s.pool.Query(ctx, runColumns+tail, args...)
	if err != nil {
		return nil, fmt.Errorf("pgstore: list runs: %w", err)
	}
	defer rows.Close()
	var out []LiveRun
	for rows.Next() {
		var l LiveRun
		var ceiling, ceilingArg int64
		var baseline *int64
		if err := rows.Scan(&l.RunID, &l.BookID, &l.UserID, &l.Workdir, &l.VerifyBank, &l.CeilingChapters,
			&l.PausedReason, &l.StartedAt, &l.Status, &l.StopRequestedAt,
			&l.AttemptID, &l.AttemptNo, &l.UnitName, &l.EngineRunID,
			&l.Position.Offset, &l.Position.LastSeq, &l.Position.LastHash, &l.Quarantined, &ceiling,
			&l.EngineBinary, &baseline, &ceilingArg, &l.AttemptStartedAt); err != nil {
			return nil, fmt.Errorf("pgstore: scan run: %w", err)
		}
		l.Ceiling = money.MicroUSD(ceiling)
		l.CeilingArg = money.MicroUSD(ceilingArg)
		if baseline != nil {
			v := money.MicroUSD(*baseline)
			l.SpendBaseline = &v
		}
		out = append(out, l)
	}
	return out, rows.Err()
}

// RunSpent is what a run has actually been charged so far, across all its attempts. Settlements are
// ledger rows keyed by attempt, so the sum is over the run's own key space and nothing else's.
func (s *Store) RunSpent(ctx context.Context, runID string) (money.MicroUSD, error) {
	var v int64
	err := s.pool.QueryRow(ctx, `
		select coalesce(-sum(amount_micro_usd), 0) from credit_ledger
		 where source = 'run_settle' and split_part(source_id, '#', 1) = $1`, runID).Scan(&v)
	if err != nil {
		return 0, fmt.Errorf("pgstore: read run spend: %w", err)
	}
	return money.MicroUSD(v), nil
}

// SpendBound is the UPPER bound on what an attempt can have cost: the smallest meter reading any
// LATER attempt of the same book recorded before it started.
//
// It exists because settlement reads a lifetime counter of the BOOK at the moment it retries, and
// that retry can happen after another run of the same book has already moved it. A settlement is
// allowed to defer — the engine has to be asked and asking can fail — and a deferred one is not
// blocked from being overtaken: the earlier run is finished, so nothing stops the account starting
// another. Measured: a run that cost $0.10 was charged $2.10, the difference being what its
// successor had spent by then, and the successor then paid that same amount again.
//
// A later attempt's baseline is exactly the right bound, because it was read BEFORE that attempt
// added anything and AFTER this one had stopped. Nil means no later attempt has been spawned, and
// then the counter has not been moved by anyone else.
func (s *Store) SpendBound(ctx context.Context, bookID string, attemptID int64) (*money.MicroUSD, error) {
	var v *int64
	err := s.pool.QueryRow(ctx, `
		select min(a.spend_baseline_micro_usd)
		  from run_attempts a join runs r on r.id = a.run_id
		 where r.book_id = $1 and a.id > $2 and a.spend_baseline_micro_usd is not null`,
		bookID, attemptID).Scan(&v)
	if err != nil {
		return nil, fmt.Errorf("pgstore: read spend bound: %w", err)
	}
	if v == nil {
		return nil, nil
	}
	bound := money.MicroUSD(*v)
	return &bound, nil
}

// ErrRunNotLive is a run that cannot be stopped because it is already over. Distinct from ErrNoRun,
// which is a run this account cannot see at all: the first is the contract's 409 and the second its
// 404, and answering the wrong one either tells a stranger that a run exists or tells an owner that
// theirs does not.
var ErrRunNotLive = errors.New("pgstore: the run is not live")

// ErrStopRequested is a restart refused because the run it would restart has been asked to stop. Not
// an error of the caller: the reconciler ends the run instead.
var ErrStopRequested = errors.New("pgstore: the run has been asked to stop")

// RequestStop records that THIS platform asked a run to stop, and hands back the unit to ask.
//
// The order is the whole design (migration 00014): the intent is COMMITTED before systemd is
// touched, so a platform that dies between the two still knows on its next sweep that the run it
// finds ended was stopped on purpose — and, just as important, that it must not restart it.
//
// Idempotent by coalesce: pressing stop twice is one intent with the FIRST timestamp, because the
// timestamp is evidence about which of the two events came first and a later one would erase that.
//
// The revision it answers with is the BOOK's — see ReadRun for why every run-carrying response uses
// one counter.
func (s *Store) RequestStop(ctx context.Context, userID, runID string, now time.Time) (Run, string, error) {
	const q = `
		update runs r set stop_requested_at = coalesce(r.stop_requested_at, $3)
		  from books b
		 where r.id = $1 and b.id = r.book_id and b.owner_id = $2 and r.finished_at is null
		returning r.id, r.book_id, b.revision, r.status, r.verify_bank, r.ceiling_chapters,
		          coalesce(r.paused_reason, ''), r.started_at, r.finished_at,
		          coalesce((select a.unit_name from run_attempts a
		                     where a.run_id = r.id and a.ended_at is null
		                     order by a.attempt_no desc limit 1), '')`
	var out Run
	var unit string
	err := s.pool.QueryRow(ctx, q, runID, userID, now).Scan(&out.ID, &out.BookID, &out.Revision,
		&out.Status, &out.VerifyBank, &out.CeilingChapters, &out.PausedReason, &out.StartedAt,
		&out.FinishedAt, &unit)
	if errors.Is(err, pgx.ErrNoRows) {
		// Nothing matched, and the two reasons need different answers. Asked separately and only on
		// this path, so the ordinary stop stays one round trip.
		return Run{}, "", s.whyNotLive(ctx, userID, runID)
	}
	if err != nil {
		return Run{}, "", fmt.Errorf("pgstore: request stop: %w", err)
	}
	return out, unit, nil
}

func (s *Store) whyNotLive(ctx context.Context, userID, runID string) error {
	var visible bool
	if err := s.pool.QueryRow(ctx, `
		select exists (select 1 from runs r join books b on b.id = r.book_id
		                where r.id = $1 and b.owner_id = $2)`, runID, userID).Scan(&visible); err != nil {
		return fmt.Errorf("pgstore: read run: %w", err)
	}
	if visible {
		return ErrRunNotLive
	}
	return ErrNoRun
}

// ReadRunForResume loads a run and its LAST attempt, whether or not either is still live.
//
// The reconciler's own list is deliberately not reusable here: it selects the attempt that has not
// ended, and every run this call is about has ended. What resume needs is the state the run stopped
// in and the attempt whose money and journal position it stopped at.
func (s *Store) ReadRunForResume(ctx context.Context, userID, runID string) (LiveRun, error) {
	rows, err := s.queryRuns(ctx, `
		  join run_attempts a on a.run_id = r.id
		                     and a.attempt_no = (select max(attempt_no) from run_attempts
		                                          where run_id = r.id)
		  left join reservations res on res.engine_run_id = r.id || '#' || a.attempt_no
		                            and res.state = 'open'
		 where r.id = $1 and b.owner_id = $2`, runID, userID)
	if err != nil {
		return LiveRun{}, err
	}
	if len(rows) == 0 {
		return LiveRun{}, ErrNoRun
	}
	return rows[0], nil
}

// ReadRun is the run row as the contract projects it, read under the caller's ownership.
//
// ⚠ The revision is the BOOK's, not the `runs.revision` column, and that is the contract rather than
// a shortcut: "the counter is PER BOOK — every book-scoped read and the id of every stream frame of
// that book's run carry the same number" (§Revision), and a client MUST DROP a read whose revision is
// below one it has applied. The book card already answers this way (httpapi getBook); a handle that
// answered from the run's own column would hand the client a number below the card's and the client,
// obeying the contract, would drop the answer to the button it just pressed.
func (s *Store) ReadRun(ctx context.Context, userID, runID string) (Run, error) {
	const q = `
		select r.id, r.book_id, b.revision, r.status, r.verify_bank, r.ceiling_chapters,
		       coalesce(r.paused_reason, ''), r.started_at, r.finished_at
		  from runs r join books b on b.id = r.book_id
		 where r.id = $1 and b.owner_id = $2`
	var out Run
	err := s.pool.QueryRow(ctx, q, runID, userID).Scan(&out.ID, &out.BookID, &out.Revision, &out.Status,
		&out.VerifyBank, &out.CeilingChapters, &out.PausedReason, &out.StartedAt, &out.FinishedAt)
	if errors.Is(err, pgx.ErrNoRows) {
		return Run{}, ErrNoRun
	}
	if err != nil {
		return Run{}, fmt.Errorf("pgstore: read run: %w", err)
	}
	return out, nil
}

// RestartInput is one interrupted attempt being replaced.
type RestartInput struct {
	RunID     string
	AttemptID int64
	UserID    string
	BookID    string
	// Ceiling is what is LEFT of the run's budget. A restart that reserved the full ceiling again
	// would let one run spend it twice.
	Ceiling money.MicroUSD
	Offset  int64
	// EngineBinary overrides the pinned path. Nil means INHERIT — which is the default, because a
	// resume is the same run continuing and row 139 lets another version in only on purpose.
	EngineBinary *string
	Now          time.Time
}

// RestartRun closes an interrupted attempt and opens the next one, with its own hold, in ONE
// transaction — the same reason StartRun is one transaction.
//
// The old attempt's reservation is NOT carried over: it was taken for a process that no longer
// exists, and it is closed by the settlement that runs before this.
func (s *Store) RestartRun(ctx context.Context, in RestartInput) (LiveRun, error) {
	var out LiveRun
	err := s.inTx(ctx, func(tx Tx) error {
		// Book first, as in every other transaction that touches both (lockBook). This one took the
		// attempt first and the materializer takes the book first, which is the pair that deadlocked.
		if err := lockBook(ctx, tx, in.BookID); err != nil {
			return err
		}
		// The RUN row second — the order lockBook writes down, which this transaction used to take
		// last, after the money. Taking it here also does the work below: what is read from the row is
		// read under the lock that will do the writing.
		var stopRequested, finished *time.Time
		if err := tx.QueryRow(ctx,
			`select stop_requested_at, finished_at from runs where id = $1 for update`, in.RunID).
			Scan(&stopRequested, &finished); err != nil {
			if errors.Is(err, pgx.ErrNoRows) {
				return ErrNoRun
			}
			return fmt.Errorf("pgstore: lock run: %w", err)
		}
		// A stop asked for on a run that is STILL LIVE outranks a restart, and the check belongs here
		// rather than in the caller's snapshot: the reconciler decides to restart, spends seconds
		// settling (a `tmctl status` call), and the user presses stop inside that window. Restarting
		// then cleared the fresh intent, opened a second attempt and took a new hold — the user had a
		// 202 for a stop that never happened and paid for the work they had just cancelled.
		//
		// On a FINISHED run the same column is history: a resume re-opens a run that was stopped, and
		// the intent belongs to the life that ended — it is cleared below with the rest of what said
		// the run was over.
		//
		// A stop arriving DURING this transaction is not lost either: it waits on this row lock and
		// lands on the re-opened run, which the next sweep then stops.
		if stopRequested != nil && finished == nil {
			return ErrStopRequested
		}
		// The previous attempt is closed if it is still open, and left exactly as it is if it is not.
		// Both callers arrive here: the reconciler replaces an attempt that was INTERRUPTED and is
		// still open, while a resume continues a run whose attempt already ended with a verdict of its
		// own — and overwriting that verdict would erase how the run the user stopped actually ended.
		//
		// What serializes two callers is no longer this row but the next one: `unique (run_id,
		// attempt_no)` lets exactly one of them insert attempt N+1, and the loser gets the same
		// ErrNoRun it always got.
		var attemptNo int
		if err := tx.QueryRow(ctx, `
			update run_attempts set ended_at = coalesce(ended_at, $2),
			                        exit_result = coalesce(exit_result, 'interrupted')
			 where id = $1 returning attempt_no`, in.AttemptID, in.Now).Scan(&attemptNo); err != nil {
			if errors.Is(err, pgx.ErrNoRows) {
				return ErrNoRun // the attempt is gone
			}
			return fmt.Errorf("pgstore: close interrupted attempt: %w", err)
		}
		next := attemptNo + 1
		var id int64
		var pinned string
		// The new attempt INHERITS the binary the run was pinned to (unified backlog row 139): the
		// engine ships more often than a run finishes, so resuming on whatever was deployed since is a
		// different program continuing someone else's work. Changing it is allowed, but only by saying
		// so — see runs.Config.AllowEngineVersionChange.
		if err := tx.QueryRow(ctx, `
			insert into run_attempts (run_id, attempt_no, started_at, last_offset, engine_binary)
			select $1, $2, $3, $4, coalesce($5, prev.engine_binary, '')
			  from run_attempts prev where prev.id = $6
			returning id, engine_binary`, in.RunID, next, in.Now, in.Offset, in.EngineBinary, in.AttemptID).
			Scan(&id, &pinned); err != nil {
			if isUnique(err, "run_attempts_run_id_attempt_no_key") {
				return ErrNoRun // another caller opened this attempt first
			}
			return fmt.Errorf("pgstore: open next attempt: %w", err)
		}
		if err := holdTx(ctx, tx, in.UserID, in.BookID, engineRunKey(in.RunID, next), in.Ceiling, in.Now); err != nil {
			return err
		}
		// finished_at, settled_at and stop_requested_at are CLEARED, and each for its own reason. The
		// reconciler's restart works on a run where all three are already null, so there it changes
		// nothing; a RESUME re-opens a run that ended, and leaving them would mean a live run the
		// one-live-per-book index does not see, money marked resolved that this attempt has not spent
		// yet, and a stop request from the previous life that would classify this attempt's ending as
		// a stop nobody asked for.
		if _, err := tx.Exec(ctx, `
			update runs set status = 'translating', paused_reason = null, finished_at = null,
			                settled_at = null, stop_requested_at = null,
			                revision = (select revision + 1 from books where id = runs.book_id)
			 where id = $1`, in.RunID); err != nil {
			// Clearing finished_at puts the run back under the one-live-run-per-book index, and the
			// book may already have a NEWER live run — nothing stops an account starting one after it
			// stopped this one. That is a conflict and not a failure: the whole transaction rolls back,
			// so the hold taken three lines above is undone with it, and the caller gets the same error
			// a second admission would have got.
			if isUnique(err, "runs_one_live_per_book") {
				return ErrRunInFlight
			}
			return fmt.Errorf("pgstore: reopen run: %w", err)
		}
		if _, err := tx.Exec(ctx, `
			update books set status = 'translating',
			                 revision = `+nextRevisionOfThisBooksLibrary+` where id = $1`,
			in.BookID); err != nil {
			return fmt.Errorf("pgstore: mark book translating: %w", err)
		}
		out = LiveRun{RunID: in.RunID, BookID: in.BookID, UserID: in.UserID, AttemptID: id,
			AttemptNo: next, Ceiling: in.Ceiling, StartedAt: in.Now, AttemptStartedAt: in.Now,
			EngineBinary: pinned, Position: Position{Offset: in.Offset}}
		return nil
	})
	if err != nil {
		return LiveRun{}, err
	}
	// The fields the caller needs to spawn but this transaction did not read.
	const q = `select b.workdir, r.verify_bank, r.ceiling_chapters from runs r
	             join books b on b.id = r.book_id where r.id = $1`
	if err := s.pool.QueryRow(ctx, q, in.RunID).Scan(&out.Workdir, &out.VerifyBank, &out.CeilingChapters); err != nil {
		return LiveRun{}, fmt.Errorf("pgstore: read restarted run: %w", err)
	}
	return out, nil
}

// ReleaseSpawnClaim gives an attempt back after a unit could NOT be created, so the next sweep
// retries this attempt instead of reading a recorded-but-absent unit as an interrupted run.
func (s *Store) ReleaseSpawnClaim(ctx context.Context, attemptID int64) error {
	_, err := s.pool.Exec(ctx,
		`update run_attempts set unit_name = null where id = $1`, attemptID)
	if err != nil {
		return fmt.Errorf("pgstore: release spawn claim: %w", err)
	}
	return nil
}

// AttemptReservationOpen reports whether an attempt's money is still reserved. The restart path asks
// before it opens a SECOND reservation: settling can legitimately fail (the engine's figure could not
// be read), and proceeding then holds the ceiling twice and strands the first hold where no sweep
// looks for it again.
func (s *Store) AttemptReservationOpen(ctx context.Context, runID string, attempt int) (bool, error) {
	var open bool
	err := s.pool.QueryRow(ctx, `
		select exists (select 1 from reservations where engine_run_id = $1 and state = 'open')`,
		engineRunKey(runID, attempt)).Scan(&open)
	if err != nil {
		return false, fmt.Errorf("pgstore: read reservation state: %w", err)
	}
	return open, nil
}

// PauseRun records a run that cannot go on, without pretending it failed.
func (s *Store) PauseRun(ctx context.Context, runID string, attemptID int64, reason string, now time.Time) error {
	if reason != PausedCreditExhausted {
		return fmt.Errorf("pgstore: %q is not a pause reason", reason)
	}
	return s.inTx(ctx, func(tx Tx) error {
		// Book first, as everywhere else that touches both (see FinishRun).
		var bookID string
		if err := tx.QueryRow(ctx,
			`select id from books where id = (select book_id from runs where id = $1) for update`,
			runID).Scan(&bookID); err != nil {
			if errors.Is(err, pgx.ErrNoRows) {
				return nil
			}
			return fmt.Errorf("pgstore: lock book: %w", err)
		}
		// A stop the user asked for outranks a pause, and the check belongs inside this TRANSACTION,
		// under the same locks: the reconciler decides to pause after settling — seconds of a
		// `tmctl status` call — and a stop landing inside that window would otherwise be answered with
		// `paused/credit_exhausted`, which
		// says the money ran out when what happened is that its owner stopped it.
		var stopRequested *time.Time
		if err := tx.QueryRow(ctx,
			`select stop_requested_at from runs where id = $1 for update`, runID).Scan(&stopRequested); err != nil {
			if errors.Is(err, pgx.ErrNoRows) {
				return nil
			}
			return fmt.Errorf("pgstore: lock run: %w", err)
		}
		if stopRequested != nil {
			return ErrStopRequested
		}
		// The attempt being closed must still be a LIVE attempt OF THIS RUN — the third path to carry
		// the guard `FinishRun` (PD-181) and `FinishUnspawnedStop` (FP5-2) already have, and it was
		// missing here. Without it a pass holding an old snapshot pauses a run whose attempt has since
		// been restarted: the run reads `finished`, the SECOND attempt's hold stays open and falls out
		// of both `ListLiveRuns` (the run is finished) and `UnsettledRuns` (the attempt is not), and an
		// engine keeps spending under a run its owner is told ran out of money. Found by cross-family
		// review of the acceptance dofix.
		if err := tx.QueryRow(ctx, `
			update runs set status = 'paused', paused_reason = $2, finished_at = $3,
			                revision = (select revision + 1 from books where id = runs.book_id)
			 where id = $1 and finished_at is null
			   and exists (select 1 from run_attempts a
			                where a.id = $4 and a.run_id = runs.id and a.ended_at is null)
			returning book_id`, runID, reason, now, attemptID).Scan(&bookID); err != nil {
			if errors.Is(err, pgx.ErrNoRows) {
				return nil
			}
			return fmt.Errorf("pgstore: pause run: %w", err)
		}
		if _, err := tx.Exec(ctx, `
			update run_attempts set ended_at = coalesce(ended_at, $3)
			 where id = $1 and run_id = $2`, attemptID, runID, now); err != nil {
			return fmt.Errorf("pgstore: end attempt: %w", err)
		}
		if _, err := tx.Exec(ctx, `
			update books set status = 'paused',
			                 revision = `+nextRevisionOfThisBooksLibrary+` where id = $1`, bookID); err != nil {
			return fmt.Errorf("pgstore: pause book: %w", err)
		}
		return nil
	})
}

// RunForSpawn is what the worker needs to spawn an attempt.
type RunForSpawn struct {
	LiveRun
	// AlreadySpawned is a unit name that was recorded for this attempt. The worker must NOT spawn a
	// second engine for it: a queue job can be retried after a platform restart, and the run it
	// refers to may well still be running.
	AlreadySpawned bool
}

// ReadRunForSpawn loads the live attempt of a run.
func (s *Store) ReadRunForSpawn(ctx context.Context, runID string) (RunForSpawn, error) {
	live, err := s.ListLiveRuns(ctx)
	if err != nil {
		return RunForSpawn{}, err
	}
	for _, l := range live {
		if l.RunID == runID {
			return RunForSpawn{LiveRun: l, AlreadySpawned: l.UnitName != ""}, nil
		}
	}
	return RunForSpawn{}, ErrNoRun
}

// SpawnRecord is what is written down about an attempt just before its unit is created.
type SpawnRecord struct {
	AttemptID int64
	Unit      string
	// Binary is the versioned engine path this attempt is pinned to (unified backlog row 139).
	Binary string
	// Ceiling is the attempt's own budget: the increment the user bought and the amount held.
	Ceiling money.MicroUSD
	// CeilingArg is what the engine is actually told, which is the book's cumulative cap and
	// therefore a different number from the second run of a book onwards (D39.122).
	CeilingArg money.MicroUSD
	// Baseline is where the book's lifetime meter stood before this attempt added to it.
	Baseline money.MicroUSD
}

// RecordSpawn claims the right to start this attempt, and writes down what is about to run: the
// unit, the binary version it is pinned to, the budget it carries and the ceiling it is given.
// Written BEFORE the unit is created, so a crash between the two leaves a record to reconcile rather
// than an unattributable process.
//
// claimed is false when the attempt already has a unit name, when the attempt has ENDED, or when its
// run is over. It is a COMPARE-AND-SET rather than a plain update because two callers legitimately
// reach here at once — the queue worker that was handed the run and the reconciler that found it
// unspawned — and the loser must not start a second engine. systemd would refuse the duplicate NAME,
// so that accident was survivable; surviving by someone else's uniqueness rule is not the same as
// being correct, and the day a resume changes the naming it stops holding.
//
// ⚠ The three conditions BESIDES the unit name are money, and they were not here until a run could be
// stopped before it ever spawned. The window: a worker sits inside `bookMeter` for the seconds a
// `tmctl status` takes, the user stops the run, the sweep ends it and gives the whole hold back
// (nothing was spawned) — and the worker then wakes up and creates a unit for a run that is finished
// and settled. That engine would spend against its own book cap with NO open reservation, and
// nothing would ever look at it: the reconciler lists runs by `finished_at is null` and settlements
// by an open reservation, so it is in neither list. The stop INTENT is asked about one step earlier
// for the same reason: it lands while the worker is inside that same `bookMeter`, before any sweep
// could have finished the run, and a claim granted then starts an engine for work its owner
// cancelled before it began. All three are asked in the statement that claims, because a check made
// before it is a check with a window after it.
func (s *Store) RecordSpawn(ctx context.Context, r SpawnRecord) (claimed bool, err error) {
	// What a PREVIOUS claim of this attempt decided is kept. The claim is given back when the unit
	// could not be created (ReleaseSpawnClaim), and "could not be created" is not the same as "was not
	// created": a systemd-run that was killed after it had already asked for the unit reports a failure
	// and leaves an engine running. The next claim would then overwrite the baseline with a meter that
	// engine has been moving, and the attempt would be billed for the difference from a figure that
	// already includes its own work — an underpayment nothing later looks for. Where the unit really
	// was not created the two values are identical, so keeping the first costs nothing.
	tag, err := s.pool.Exec(ctx, `
		update run_attempts
		   set unit_name = $2, engine_binary = $3, ceiling_micro_usd = $4,
		       ceiling_arg_micro_usd = case when spend_baseline_micro_usd is null
		                                    then $5 else ceiling_arg_micro_usd end,
		       spend_baseline_micro_usd = coalesce(spend_baseline_micro_usd, $6)
		 where id = $1 and unit_name is null and ended_at is null
		   and exists (select 1 from runs r where r.id = run_attempts.run_id
		                and r.finished_at is null and r.stop_requested_at is null)`,
		r.AttemptID, r.Unit, r.Binary, int64(r.Ceiling), int64(r.CeilingArg), int64(r.Baseline))
	if err != nil {
		return false, fmt.Errorf("pgstore: record spawn: %w", err)
	}
	return tag.RowsAffected() == 1, nil
}

// SaveCursor persists the tailer's position when nothing was materialized — a re-read of lines the
// cursor already covers still moves the byte hint, and losing that means re-reading them forever.
func (s *Store) SaveCursor(ctx context.Context, attemptID int64, p Position) error {
	_, err := s.pool.Exec(ctx, `
		update run_attempts set last_offset = $2 where id = $1 and last_offset < $2`, attemptID, p.Offset)
	if err != nil {
		return fmt.Errorf("pgstore: save cursor: %w", err)
	}
	return nil
}

// Quarantine stops materializing an attempt without touching the run.
//
// The engine is NOT stopped: it is spending money the account has already reserved, and our
// inability to read its journal is not a reason to throw that away. What stops is the projection —
// after this the run's state is only as fresh as the resync channel makes it, and the reason says
// so out loud rather than leaving a screen that quietly stopped moving.
func (s *Store) Quarantine(ctx context.Context, attemptID int64, reason string) error {
	_, err := s.pool.Exec(ctx,
		`update run_attempts set quarantine_reason = $2 where id = $1 and quarantine_reason is null`,
		attemptID, reason)
	if err != nil {
		return fmt.Errorf("pgstore: quarantine attempt: %w", err)
	}
	return nil
}