textmachine/platform/internal/pgstore/credits.go

package pgstore

import (
	"context"
	"errors"
	"fmt"
	"net"
	"strings"
	"time"

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

	"textmachine/platform/internal/money"
)

var (
	// ErrInsufficientCredit is a refusal, not a failure: the account has less than the run needs.
	ErrInsufficientCredit = errors.New("pgstore: insufficient credit")
	// ErrNoReservation means the hold this settlement refers to is not open.
	ErrNoReservation = errors.New("pgstore: no open reservation")
	// ErrDuplicateHold is a second hold on an attempt id that already has one. It is an error, not
	// a no-op: a hold that debits nothing reserves nothing while reporting that it did.
	ErrDuplicateHold = errors.New("pgstore: attempt already has a hold")
	// ErrNotOwner is a book that does not belong to the account being charged for it.
	ErrNotOwner = errors.New("pgstore: book belongs to another account")
	// ErrNoAccount separates "this account has nothing" from "this account does not exist" — the
	// difference between a balance of zero and a typo in an admin command.
	ErrNoAccount = errors.New("pgstore: no such account")
)

// Grant credits an account and reports whether this call is what credited it. The free tier is one
// of these and nothing more.
//
// (source, sourceID) is the idempotency key, scoped to the account by the schema. applied is false
// when the key was already spent: the caller must say so rather than print a success it did not
// cause.
func (s *Store) Grant(ctx context.Context, userID string, amount money.MicroUSD, source, sourceID, note string, now time.Time) (applied bool, err error) {
	if amount <= 0 {
		return false, fmt.Errorf("pgstore: grant must be positive, got %d", amount)
	}
	if source == "" || sourceID == "" {
		return false, errors.New("pgstore: grant needs an idempotency key")
	}
	err = s.inTx(ctx, func(tx pgx.Tx) error {
		applied, err = appendLedger(ctx, tx, userID, "grant", amount, source, sourceID, note, now)
		return err
	})
	return applied, err
}

// Adjust corrects a balance. A ledger row is never edited: the correction is another row, which is
// what keeps the sum reproducible. The note is mandatory, in the DDL as well as here.
//
// ⚠ "NEVER EDITED" IS THE DISCIPLINE OF THIS PACKAGE AND NOT A RULE OF THE SCHEMA, and saying which
// is the point: `credit_ledger` carries five constraints and every one of them is about the SHAPE of
// a row — the kind, the sign, the idempotency key, a non-empty source, a note on a correction — so a
// plain UPDATE or DELETE against the table is ACCEPTED by Postgres (measured; register row PD-397).
// What holds the invariant is that no statement in this package writes one, and that is checked
// rather than trusted: TestTheLedgerIsAppendOnlyInTheCodeThatWritesIt walks every SQL string the
// package can execute. What no rule here reaches is anything that goes around the package by
// construction — a data migration, an operator's psql, a future tool — and a trigger was measured
// and refused for reasons written down beside that test.
func (s *Store) Adjust(ctx context.Context, userID string, amount money.MicroUSD, source, sourceID, note string, now time.Time) (applied bool, err error) {
	if amount == 0 || note == "" {
		return false, errors.New("pgstore: an adjustment needs a non-zero amount and a reason")
	}
	if source == "" || sourceID == "" {
		return false, errors.New("pgstore: adjustment needs an idempotency key")
	}
	err = s.inTx(ctx, func(tx pgx.Tx) error {
		applied, err = appendLedger(ctx, tx, userID, "adjustment", amount, source, sourceID, note, now)
		return err
	})
	return applied, err
}

// Balance is what the account may still spend, read from the cache that every ledger write updates
// in its own transaction.
func (s *Store) Balance(ctx context.Context, userID string) (money.MicroUSD, error) {
	var v *int64
	err := s.pool.QueryRow(ctx, `
		select b.balance_micro_usd
		  from users u left join account_balances b on b.user_id = u.id
		 where u.id = $1`, userID).Scan(&v)
	if errors.Is(err, pgx.ErrNoRows) {
		return 0, ErrNoAccount
	}
	if err != nil {
		return 0, fmt.Errorf("pgstore: balance: %w", err)
	}
	if v == nil {
		return 0, nil // an account with no ledger rows has no credit, which is not an error
	}
	return money.MicroUSD(*v), nil
}

// Account is what an operator needs to see about one account's money.
type Account struct {
	Balance  money.MicroUSD
	Reserved money.MicroUSD
	// LedgerSum is recomputed from the rows. It exists to be COMPARED with Balance, and both are
	// read in one snapshot: reading them separately reports drift that a concurrent grant caused
	// between the two queries.
	LedgerSum money.MicroUSD
}

// ReadAccount returns the money view in a single consistent snapshot.
func (s *Store) ReadAccount(ctx context.Context, userID string) (Account, error) {
	var a Account
	const q = `
		select coalesce((select balance_micro_usd from account_balances where user_id = $1), 0),
		       coalesce((select sum(amount_micro_usd) from credit_ledger where user_id = $1), 0),
		       coalesce((select sum(amount_micro_usd) from reservations
		                  where user_id = $1 and state = 'open'), 0)
		  from users where id = $1`
	err := s.pool.QueryRow(ctx, q, userID).Scan(&a.Balance, &a.LedgerSum, &a.Reserved)
	if errors.Is(err, pgx.ErrNoRows) {
		return Account{}, ErrNoAccount
	}
	if err != nil {
		return Account{}, fmt.Errorf("pgstore: read account: %w", err)
	}
	return a, nil
}

// Reservation is an open hold as an operator sees it.
type Reservation struct {
	EngineRunID string
	BookID      string
	Amount      money.MicroUSD
	Ceiling     money.MicroUSD
	OpenedAt    time.Time
}

// OpenReservations lists holds that were taken and never closed. Without this they are money that
// is gone from the balance and invisible to everything that could give it back.
func (s *Store) OpenReservations(ctx context.Context, userID string) ([]Reservation, error) {
	const q = `
		select engine_run_id, book_id, amount_micro_usd, ceiling_micro_usd, opened_at
		  from reservations where user_id = $1 and state = 'open' order by opened_at`
	rows, err := s.pool.Query(ctx, q, userID)
	if err != nil {
		return nil, fmt.Errorf("pgstore: open reservations: %w", err)
	}
	defer rows.Close()
	var out []Reservation
	for rows.Next() {
		var r Reservation
		if err := rows.Scan(&r.EngineRunID, &r.BookID, &r.Amount, &r.Ceiling, &r.OpenedAt); err != nil {
			return nil, fmt.Errorf("pgstore: scan reservation: %w", err)
		}
		out = append(out, r)
	}
	return out, rows.Err()
}

// Hold reserves credit before a run is spawned. Together with the per-book ceiling handed to the
// engine it is the enforcement half of the money design: the hold makes the credit unavailable to
// the next run, and the engine stops itself at the ceiling, so an overspend is impossible even
// while the platform is blind. The event stream is freshness only.
//
// The ceiling to hand the engine is the amount held; read it back with OpenReservations.
func (s *Store) Hold(ctx context.Context, userID, bookID, engineRunID string, amount money.MicroUSD, now time.Time) error {
	return s.inTx(ctx, func(tx pgx.Tx) error {
		return holdTx(ctx, tx, userID, bookID, engineRunID, amount, now)
	})
}

// holdTx is Hold inside a caller's transaction. It exists because admitting a run writes the run
// row, its attempt, the hold and the queue entry together or not at all (StartRun): a hold in its
// own transaction is a hold that can outlive the thing it was taken for.
func holdTx(ctx context.Context, tx pgx.Tx, userID, bookID, engineRunID string, amount money.MicroUSD, now time.Time) error {
	if amount <= 0 {
		return fmt.Errorf("pgstore: hold must be positive, got %d", amount)
	}
	// account_balances is locked FIRST here and in every other operation. A path that locked
	// the reservation first would invert the order against this one and deadlock — measured,
	// not hypothetical.
	balance, err := lockBalance(ctx, tx, userID)
	if err != nil {
		return err
	}
	if balance < amount {
		return ErrInsufficientCredit
	}
	// The book must belong to the account being charged. The foreign key only proves the book
	// exists, which is not the same question.
	tag, err := tx.Exec(ctx, `
		insert into reservations (engine_run_id, user_id, book_id, amount_micro_usd, ceiling_micro_usd, state, opened_at)
		select $1, $2, $3, $4, $4, 'open', $5 from books where id = $3 and owner_id = $2`,
		engineRunID, userID, bookID, int64(amount), now)
	if err != nil {
		// A LIVE reservation on this attempt collides on the primary key, and that is the ordinary
		// way a duplicate hold happens — the declared ErrDuplicateHold below is reachable only in the
		// rarer shape where the reservation row was removed and the ledger key was not. Both are the
		// same fact to a caller, and a raw SQLSTATE is not something a worker can act on (PD-81).
		if isUnique(err, "reservations_pkey") {
			return ErrDuplicateHold
		}
		return fmt.Errorf("pgstore: open reservation: %w", err)
	}
	if tag.RowsAffected() == 0 {
		return ErrNotOwner
	}
	applied, err := appendLedger(ctx, tx, userID, "hold", -amount, "run", engineRunID, "", now)
	if err != nil {
		return err
	}
	if !applied {
		// The ledger already holds this attempt id, so nothing was debited. Reporting success
		// would spawn a run against credit that was never reserved.
		return ErrDuplicateHold
	}
	return nil
}

// Settle closes a reservation with what the attempt actually cost: the hold comes back and the real
// cost is charged, in one transaction. Settling twice is refused — the reservation is no longer
// open — which is what makes it safe on a retried path.
//
// A cost above the hold is CAPPED at the hold and the row says so. Spending more than was reserved
// means the engine's ceiling did not hold, and the account is not the place to absorb that.
func (s *Store) Settle(ctx context.Context, engineRunID string, spent money.MicroUSD, now time.Time) error {
	if spent < 0 {
		return fmt.Errorf("%w: got %d", ErrNegativeSpend, spent)
	}
	return s.inTx(ctx, func(tx pgx.Tx) error {
		userID, held, err := closeReservation(ctx, tx, engineRunID, "settled", now)
		if err != nil {
			return err
		}
		note := ""
		if spent > held {
			note = fmt.Sprintf("capped at the hold; the engine reported %s", spent.USD())
			spent = held
		}
		if err := releaseHold(ctx, tx, userID, engineRunID, held, now); err != nil {
			return err
		}
		// ⚠ THE FLAG IS READ, and this line used to discard it — the odd one of the three places that
		// call appendLedger. `holdTx` answers ErrDuplicateHold on a spent key and `releaseHold`
		// answers ErrReleaseKeySpent and rolls back (PD-97); here a spent `run_settle` key charged
		// NOTHING while the hold had already gone back whole, and the caller was told nil. That is the
		// account paying for a run it did not have.
		//
		// Reachability today is ZERO and the reason is worth writing down rather than trusting: the
		// reservation is closed under `state = 'open'`, so a second Settle on the same attempt gets
		// ErrNoReservation and never arrives here, and the key can only be spent if the reservation
		// was opened again on the same attempt id — which `holdTx` refuses for exactly this reason.
		// It is the out-of-band sequence PD-97 names, and the answer to it is the same as PD-97's.
		applied, err := appendLedger(ctx, tx, userID, "settlement", -spent, "run_settle", engineRunID, note, now)
		if err != nil {
			return err
		}
		if !applied {
			return fmt.Errorf("%w: %s", ErrSettlementKeySpent, engineRunID)
		}
		return nil
	})
}

// ErrSettlementKeySpent is a reservation whose settlement key has already been used. Like
// ErrReleaseKeySpent it is REPORTED rather than applied, so the transaction rolls back and the row
// stays for a human: the alternative is a settlement that returns the hold and charges nothing.
var ErrSettlementKeySpent = errors.New("pgstore: the settlement of this attempt was already posted")

// ErrNegativeSpend is a settlement asked for with a spend below zero — a figure no meter produces
// and one that would CREDIT an account for having run something.
//
// A sentinel and not a bare string, and that is this pack's correction rather than decoration: the
// guard was measured to be TRANSITIVE (register row PD-394 — deleting it left the whole battery
// green, because a negative spend writes a settlement row with a positive amount and
// `credit_ledger_sign` refuses it). A test asserting only "an error came back" therefore passes
// whether the guard exists or not, and the pack's own first pin did exactly that: the planted
// mutation survived it. With a name, "the guard fired" is a fact a test can assert and the schema
// cannot counterfeit.
var ErrNegativeSpend = errors.New("pgstore: spend cannot be negative")

// ErrReleaseKeySpent is a reservation whose release key has already been used. It means the money
// came back once and the reservation was opened again on the same attempt id — an out-of-band
// sequence, and the only one where closing the reservation would return nothing (PD-97). Reported
// instead of applied, so the transaction rolls back and the row stays open for a human.
var ErrReleaseKeySpent = errors.New("pgstore: the release of this attempt was already posted")

// releaseHold puts the reserved amount back and refuses to pretend it did when the key was spent.
func releaseHold(ctx context.Context, tx pgx.Tx, userID, engineRunID string, held money.MicroUSD, now time.Time) error {
	applied, err := appendLedger(ctx, tx, userID, "hold_release", held, "run_release", engineRunID, "", now)
	if err != nil {
		return err
	}
	if !applied {
		return fmt.Errorf("%w: %s", ErrReleaseKeySpent, engineRunID)
	}
	return nil
}

// Release gives a reservation back untouched: the run never started, or it cost nothing.
func (s *Store) Release(ctx context.Context, engineRunID string, now time.Time) error {
	return s.inTx(ctx, func(tx pgx.Tx) error {
		userID, held, err := closeReservation(ctx, tx, engineRunID, "released", now)
		if err != nil {
			return err
		}
		return releaseHold(ctx, tx, userID, engineRunID, held, now)
	})
}

// ErrAttemptSpawned is an attempt that has a unit, refused where the caller believed it had none.
var ErrAttemptSpawned = errors.New("pgstore: the attempt was spawned")

// ReleaseUnspawned is Release for the one caller that has to be sure: the reconciler settling an
// attempt it believes never started a process, whose hold therefore comes back WHOLE.
//
// The belief comes from a snapshot the sweep took at its start, and between that read and this write
// the queue worker can spawn the attempt, the engine can spend and the unit can exit — which is not
// a corner case but the ordinary shape of a fast run in a sweep with other runs ahead of it in the
// list. Measured: an attempt that spent $0.50 was charged $0.000000 and the whole hold went back, an
// underpayment nothing later looks for. So the belief is re-checked HERE, in the same transaction as
// the money, against the row rather than against the snapshot.
func (s *Store) ReleaseUnspawned(ctx context.Context, engineRunID string, attemptID int64, now time.Time) error {
	return s.inTx(ctx, func(tx pgx.Tx) error {
		var unit *string
		err := tx.QueryRow(ctx,
			`select unit_name from run_attempts where id = $1 for update`, attemptID).Scan(&unit)
		if errors.Is(err, pgx.ErrNoRows) {
			return ErrNoRun
		}
		if err != nil {
			return fmt.Errorf("pgstore: read attempt: %w", err)
		}
		if unit != nil {
			return fmt.Errorf("%w: %s", ErrAttemptSpawned, *unit)
		}
		userID, held, err := closeReservation(ctx, tx, engineRunID, "released", now)
		if err != nil {
			return err
		}
		return releaseHold(ctx, tx, userID, engineRunID, held, now)
	})
}

// lockBalance takes the account's row lock and returns the balance under it. Every money operation
// starts here, so they all take their locks in the same order.
func lockBalance(ctx context.Context, tx pgx.Tx, userID string) (money.MicroUSD, error) {
	var v int64
	err := tx.QueryRow(ctx, `select balance_micro_usd from account_balances where user_id = $1 for update`, userID).Scan(&v)
	if errors.Is(err, pgx.ErrNoRows) {
		// No balance row is TWO different facts, and answering both with "insufficient credit" told
		// an operator who mistyped an account id that the account was broke (PD-82). The extra query
		// runs only on this path, and only the nullable-side lock it replaces would have been free —
		// Postgres refuses FOR UPDATE on the nullable side of an outer join, so there is no one-query
		// form of this question.
		var exists bool
		switch err := tx.QueryRow(ctx, `select true from users where id = $1`, userID).Scan(&exists); {
		case errors.Is(err, pgx.ErrNoRows):
			return 0, ErrNoAccount
		case err != nil:
			return 0, fmt.Errorf("pgstore: read account: %w", err)
		}
		return 0, ErrInsufficientCredit // the account exists and has no ledger rows: no credit
	}
	if err != nil {
		return 0, fmt.Errorf("pgstore: read balance: %w", err)
	}
	return money.MicroUSD(v), nil
}

func closeReservation(ctx context.Context, tx pgx.Tx, engineRunID, state string, now time.Time) (string, money.MicroUSD, error) {
	// Read the owner unlocked, lock the balance, then close under the state guard. The guard is
	// what makes the unlocked read safe: a reservation closed by someone else in between makes the
	// update match nothing.
	var userID string
	err := tx.QueryRow(ctx, `select user_id from reservations where engine_run_id = $1`, engineRunID).Scan(&userID)
	if errors.Is(err, pgx.ErrNoRows) {
		return "", 0, ErrNoReservation
	}
	if err != nil {
		return "", 0, fmt.Errorf("pgstore: find reservation: %w", err)
	}
	if _, err := lockBalance(ctx, tx, userID); err != nil && !errors.Is(err, ErrInsufficientCredit) {
		return "", 0, err
	}
	var amount int64
	err = tx.QueryRow(ctx, `
		update reservations set state = $2, closed_at = $3
		 where engine_run_id = $1 and state = 'open'
		returning amount_micro_usd`, engineRunID, state, now).Scan(&amount)
	if errors.Is(err, pgx.ErrNoRows) {
		return "", 0, ErrNoReservation
	}
	if err != nil {
		return "", 0, fmt.Errorf("pgstore: close reservation: %w", err)
	}
	return userID, money.MicroUSD(amount), nil
}

// appendLedger writes one row and moves the cached balance with it, in the caller's transaction.
// The two are never written apart: a cache that can lag its source is a second answer about money.
// applied is false when the idempotency key was already spent.
//
// ⚠ Again the CODE and not the schema: `account_balances` has no relation to the ledger in the DDL
// and no bound at all, so a plain UPDATE sets a balance to any number including a false or a
// negative one (PD-397). What makes the two agree is that this function is their only writer — the
// property the migration's own comment concedes is a test's and not a constraint's — and the
// agreement is asserted end to end by TestCreditLifecycleKeepsTheCacheEqualToTheLedger.
func appendLedger(ctx context.Context, tx pgx.Tx, userID, kind string, amount money.MicroUSD, source, sourceID, note string, now time.Time) (bool, error) {
	tag, err := tx.Exec(ctx, `
		insert into credit_ledger (user_id, kind, amount_micro_usd, source, source_id, note, created_at)
		values ($1, $2, $3, $4, $5, $6, $7)
		on conflict (user_id, source, source_id) do nothing`,
		userID, kind, int64(amount), source, sourceID, note, now)
	if err != nil {
		// A typo in an account id is the commonest way an operator gets here, and Balance already
		// answers it with ErrNoAccount. Reporting the same fact as a raw constraint name reads as a
		// broken database (PD-56).
		var pg *pgconn.PgError
		if errors.As(err, &pg) && pg.ConstraintName == "credit_ledger_user_id_fkey" {
			return false, ErrNoAccount
		}
		return false, fmt.Errorf("pgstore: append ledger: %w", err)
	}
	if tag.RowsAffected() == 0 {
		return false, nil
	}
	if _, err := tx.Exec(ctx, `
		insert into account_balances (user_id, balance_micro_usd, updated_at)
		values ($1, $2, $3)
		on conflict (user_id) do update
		   set balance_micro_usd = account_balances.balance_micro_usd + excluded.balance_micro_usd,
		       updated_at = excluded.updated_at`,
		userID, int64(amount), now); err != nil {
		return false, fmt.Errorf("pgstore: update balance: %w", err)
	}
	return true, nil
}

// CreditHeldBy names ANOTHER book of this account whose run is holding the credit, or "".
//
// It is what turns a shrunken run scale from a mystery into a fact: a second book's scale is
// silently smaller — or gone — while the first book's hold is open, and until 0.3.0 nothing on the
// wire said so. One reason is enough because the platform has no other: a run in flight on the SAME
// book is refused by its own error, and nothing else lowers what another book may spend.
//
// It returns the AMOUNT those holds total as well: a hold is a debit when it is taken, so the
// balance already excludes it, and adding it back is the only way to answer "would the scale be
// longer without this hold" — which is what the contract's `blocked` actually claims.
func (s *Store) CreditHeldBy(ctx context.Context, userID, exceptBookID string) (string, money.MicroUSD, error) {
	var bookID string
	var held money.MicroUSD
	// ⚠ The book with the LARGEST hold, not the oldest. `blocked` is what the user is invited to act
	// on, and the sum above is what decided that the scale is shorter — so naming a book whose $0.03
	// hold was merely opened first, beside another holding $9.60, sends them to cancel a run that
	// frees nothing. Ties go to the older one, so the answer is stable between two reads.
	err := s.pool.QueryRow(ctx, `
		select coalesce((select book_id from reservations
		                  where user_id = $1 and state = 'open' and book_id <> $2
		                  group by book_id
		                  order by sum(amount_micro_usd) desc, min(opened_at) limit 1), ''),
		       coalesce((select sum(amount_micro_usd) from reservations
		                  where user_id = $1 and state = 'open' and book_id <> $2), 0)`,
		userID, exceptBookID).Scan(&bookID, &held)
	if err != nil {
		return "", 0, fmt.Errorf("pgstore: read the account's open holds: %w", err)
	}
	return bookID, held, nil
}

// lockBook takes the book's row lock, which is the FIRST lock of every transaction in this package
// that touches more than one of the tables below.
//
// The order is: books → runs → run_attempts → account_balances → reservations. It is written down
// here because it is not derivable from any one function — a transaction that takes two of these in
// the other order deadlocks with a transaction that takes them in this one, and Postgres resolves
// that by aborting a side, which costs a sweep or an API call rather than corrupting anything. That
// is exactly what happened while the materializer locked the attempt first and the reconciler locked
// the book first: 258 of 300 concurrent pairs aborted.
//
// A missing book is not an error here: the caller's own statements find nothing and decide what that
// means. Locking is not the place to invent a not-found policy.
func lockBook(ctx context.Context, tx pgx.Tx, bookID string) error {
	var id string
	err := tx.QueryRow(ctx, `select id from books where id = $1 for update`, bookID).Scan(&id)
	if err != nil && !errors.Is(err, pgx.ErrNoRows) {
		return fmt.Errorf("pgstore: lock book: %w", err)
	}
	return nil
}

// IsTransient reports whether an error means "run the same transaction again" rather than "this
// write must never be attempted again".
//
// It exists because the two used to be conflated: any error out of the materializer quarantined the
// attempt, so one aborted deadlock blinded the projection of a live, paying run permanently. The
// first version of this function knew only about deadlocks, and a re-check found the commoner half —
// a Postgres RESTART, which is a planned event on any managed database, arrives as 57P01 or as a
// broken connection, and left the same permanent blindness behind.
//
// Retrying is safe for every caller of this: the materializer is idempotent by its high-water mark,
// so a transaction that may or may not have committed is re-read and re-checked rather than
// re-applied.
//
//   - class 08 — connection exception, including 08006 "connection failure";
//   - 57P01/57P02/57P03 — the server is shutting down, crashed a sibling, or is not accepting
//     connections yet, which is precisely what a rolling restart looks like from here;
//   - 40001/40P01 — serialization failure and a deadlock Postgres broke;
//   - anything pgx itself marks safe to retry, and any net.Error: a reset connection is not a
//     journal this platform cannot read. File errors are *fs.PathError and do NOT satisfy net.Error,
//     so a malformed journal still stops the projection as it must.
func IsTransient(err error) bool {
	if err == nil {
		return false
	}
	var pg *pgconn.PgError
	if errors.As(err, &pg) {
		switch {
		case pg.Code == "40001", pg.Code == "40P01":
			return true
		case strings.HasPrefix(pg.Code, "08"), strings.HasPrefix(pg.Code, "57P"):
			return true
		}
		return false
	}
	if pgconn.SafeToRetry(err) {
		return true
	}
	var ne net.Error
	return errors.As(err, &ne)
}

func (s *Store) inTx(ctx context.Context, fn func(pgx.Tx) error) error {
	return s.tx(ctx, pgx.TxOptions{}, fn)
}

// inReadTx runs a read on ONE snapshot.
//
// READ COMMITTED takes a fresh snapshot per STATEMENT, so a guard read in one statement and the rows
// in the next can straddle a wholesale replacement: the delta guard passes against the old reset
// mark and the rows come back from the new state, which is the short list that looks complete
// (PD-163). Read-only and repeatable-read costs nothing here and cannot abort — only serializable
// does.
func (s *Store) inReadTx(ctx context.Context, fn func(pgx.Tx) error) error {
	return s.tx(ctx, pgx.TxOptions{IsoLevel: pgx.RepeatableRead, AccessMode: pgx.ReadOnly}, fn)
}

func (s *Store) tx(ctx context.Context, opts pgx.TxOptions, fn func(pgx.Tx) error) error {
	tx, err := s.pool.BeginTx(ctx, opts)
	if err != nil {
		return fmt.Errorf("pgstore: begin: %w", err)
	}
	defer func() { _ = tx.Rollback(ctx) }()
	if err := fn(tx); err != nil {
		return err
	}
	if err := tx.Commit(ctx); err != nil {
		return fmt.Errorf("pgstore: commit: %w", err)
	}
	return nil
}

// DeleteBook removes a book and the CLOSED reservations that referenced it. An OPEN one blocks the
// delete (the foreign key is RESTRICT), which is the point: removing a book with money reserved
// against it would leave the hold in the ledger with nothing left to release it.
//
// Closed reservations carry no financial fact the ledger does not already hold — they are
// operational state — so removing them with the book loses nothing.
func (s *Store) DeleteBook(ctx context.Context, bookID string) error {
	return s.inTx(ctx, func(tx pgx.Tx) error {
		// Book first, like everything else that touches two of these tables (lockBook). This one took
		// the reservations first, which is the inverted order — no cycle exists for it today, and that
		// is exactly the kind of reasoning an invariant written in one place is supposed to replace.
		if err := lockBook(ctx, tx, bookID); err != nil {
			return err
		}
		if _, err := tx.Exec(ctx,
			`delete from reservations where book_id = $1 and state <> 'open'`, bookID); err != nil {
			return fmt.Errorf("pgstore: clear reservations: %w", err)
		}
		if _, err := tx.Exec(ctx, `delete from books where id = $1`, bookID); err != nil {
			return fmt.Errorf("pgstore: delete book: %w", err)
		}
		return nil
	})
}