job-search/internal/store/store.go

// Package store persists opportunities and their OCDS releases in SQLite using
// sqlx and hand-written SQL. The SQL is in queries.go; this file holds the
// logic. Requires building with -tags sqlite_fts5.
package store

import (
	"context"
	"crypto/rand"
	"crypto/sha256"
	"database/sql"
	"encoding/hex"
	"errors"
	"fmt"
	"strings"
	"time"
	"unicode"

	"github.com/jmoiron/sqlx"
	"github.com/mattn/go-sqlite3"

	"git.avsdev.uk/Go/job-search/internal/types"
)

// Errors returned by the store.
var (
	// ErrNotFound is returned when a requested opportunity does not exist.
	ErrNotFound = errors.New("store: not found")
	// ErrUnknownBatch is returned by ListChangedOpportunities for a hash
	// that names no change batch.
	ErrUnknownBatch = errors.New("store: unknown change batch")
	// ErrNeedsMigration is returned by Open when the database has pending
	// migrations. Apply them with the migrate command, or pass
	// WithAutoMigrate.
	ErrNeedsMigration = errors.New("store: database needs migrating")
	// ErrSchemaTooNew is returned by Open when the database has migrations
	// this build doesn't know about, i.e. it was migrated by a newer build.
	ErrSchemaTooNew = errors.New(
		"store: database schema is newer than this build",
	)
	// ErrWouldEmptyDatabase is returned by StepDown when the migration would
	// drop all of the tables and thus empty the database.
	ErrWouldEmptyDatabase = errors.New(
		"store: migration cancelled; database would be emptied by undoing all migrations",
	)
)

// DefaultListLimit is used when OpportunityFilter.Limit is zero or negative.
const DefaultListLimit = 50

// Store is the opportunity database. Open it with Open; it is safe for
// concurrent use.
type Store struct {
	db *sqlx.DB
	// changeBatchRetention is how long change batches are kept; 0 or less
	// means for ever (see WithChangeBatchRetention).
	changeBatchRetention time.Duration
}

// SaveMultipleResult reports what SaveOpportunities changed.
type SaveMultipleResult struct {
	// NewOpportunities is the number of opportunities that were not already
	// stored.
	NewOpportunities int
	// NewReleases is the number of releases that were not already stored.
	NewReleases int
	// UpdatedOpportunities is incremented for each opportunity row and its
	// classifications that are updated (when every release in the save was older
	// than the one the stored opportunity was last updated from). It does not
	// include new opportunities.
	UpdatedOpportunities int
	// OpportunityResults is a slice of the results of each opportunity saved.
	OpportunityResults []SaveResult
	// AddedClassifications are the classification codes that no stored
	// opportunity had before the save and at least one has after it, ordered
	// by code; nil if there are none.
	AddedClassifications []types.ClassificationCode
	// RemovedClassificationIDs are the ids of the classification codes that
	// a stored opportunity had before the save and none has after it, in
	// code order; nil if there are none.
	RemovedClassificationIDs []int64
	// ChangeBatch is the hash of the change batch recording the
	// opportunities the save stored for the first time or gave new releases
	// (see ListChangedOpportunities); "" if there are none.
	ChangeBatch string
}

// SaveResult reports what SaveOpportunity changed.
type SaveResult struct {
	// NewOpportunity is true if the opportunity was not already stored.
	NewOpportunity bool
	// OpportunityUpdated is true if the opportunity row and its classifications
	// were written. It is false when every release in the save was older than the
	// one the stored opportunity was last updated from.
	OpportunityUpdated bool
	// NewReleases is the number of releases that were not already stored.
	NewReleases int
}

// MarkFilter filters ListOpportunities on one of an opportunity's marks.
type MarkFilter int

// Values of MarkFilter.
const (
	// MarkAny does not filter. It is the zero value.
	MarkAny MarkFilter = iota
	// MarkSet matches opportunities with the mark set.
	MarkSet
	// MarkUnset matches opportunities with the mark unset, including those
	// with no marks stored.
	MarkUnset
)

// OpportunityFilter selects opportunities for ListOpportunities. Zero-valued
// fields do not filter.
type OpportunityFilter struct {
	Status string
	// ClassificationCode matches opportunities with a classification of this code
	// in any scheme.
	ClassificationCode string
	// Title is free text matched against opportunity titles: every word must
	// appear, and each word also matches as a prefix ("cloud migr" matches
	// "Cloud migration services").
	Title string
	// ReleasedFrom (inclusive) and ReleasedBefore (exclusive) bound
	// Opportunity.LatestReleaseDate.
	ReleasedFrom   time.Time
	ReleasedBefore time.Time
	// Each mark filter matches Opportunity.Marks' field of the same name.
	Hidden       MarkFilter
	Skipped      MarkFilter
	Interested   MarkFilter
	BidSubmitted MarkFilter
	Won          MarkFilter
	Lost         MarkFilter
	// Limit defaults to DefaultListLimit. Results are ordered newest
	// LatestReleaseDate first.
	Limit  int
	Offset int
}

func (f OpportunityFilter) limit() int {
	if f.Limit <= 0 {
		return DefaultListLimit
	}
	return f.Limit
}

// prepareOpportunity validates opportunity and normalises it in place before
// saving: child rows get the opportunity's UUID and their release id, times
// are converted to UTC (so stored timestamps compare correctly as text), and
// LatestReleaseDate becomes the newest release date in the save.
func prepareOpportunity(opportunity *types.Opportunity) error {
	if opportunity == nil {
		return errors.New("store: nil opportunity")
	}
	if opportunity.UUID == "" {
		return errors.New("store: opportunity has no UUID")
	}

	opportunity.Marks.UUID = opportunity.UUID
	if err := opportunity.Marks.Validate(); err != nil {
		return fmt.Errorf("opportunity %s: %w", opportunity.UUID, err)
	}

	latest := opportunity.LatestReleaseDate
	for i := range opportunity.Releases {
		release := &opportunity.Releases[i]
		if release.ReleaseID == "" {
			return fmt.Errorf(
				"store: opportunity %s: release %d has no release id",
				opportunity.UUID,
				i,
			)
		}
		release.UUID = opportunity.UUID
		release.ReleaseDate = release.ReleaseDate.UTC()
		if release.ReleaseDate.After(latest) {
			latest = release.ReleaseDate
		}
		for tagIndex := range release.Tags {
			release.Tags[tagIndex].UUID = opportunity.UUID
			release.Tags[tagIndex].ReleaseID = release.ReleaseID
		}
	}

	if latest.IsZero() {
		return fmt.Errorf(
			"store: opportunity %s has no release date", opportunity.UUID,
		)
	}

	opportunity.LatestReleaseDate = latest.UTC()
	opportunity.StartDate = utcPtr(opportunity.StartDate)
	opportunity.EndDate = utcPtr(opportunity.EndDate)

	return nil
}

func utcPtr(when *time.Time) *time.Time {
	if when == nil {
		return nil
	}
	utc := when.UTC()
	return &utc
}

// ftsQuery turns free text into an FTS5 query that requires every word, each
// as a prefix match. Words are split the same way the unicode61 tokenizer
// splits them and each is quoted, so user input can never be interpreted as
// FTS5 syntax (operators, column filters, unbalanced quotes). It returns ""
// when the text contains no words.
func ftsQuery(text string) string {
	words := strings.FieldsFunc(text, func(char rune) bool {
		return !unicode.IsLetter(char) && !unicode.IsNumber(char)
	})
	for i, word := range words {
		words[i] = `"` + word + `"*`
	}
	return strings.Join(words, " ")
}

// Open opens the database at path. The schema must be exactly at this
// build's latest migration: Open returns ErrNeedsMigration if migrations are
// pending (run the migrate command first), or ErrSchemaTooNew if the database
// was migrated by a newer build. WithAutoMigrate applies pending migrations
// instead.
//
// A database file that doesn't exist is created, and then needs migrating.
func Open(ctx context.Context, path string, opts ...Option) (*Store, error) {
	var settings options
	for _, option := range opts {
		option(&settings)
	}

	sqldb, err := OpenDB(path)
	if err != nil {
		return nil, err
	}

	if err := sqldb.PingContext(ctx); err != nil {
		return nil, errors.Join(err, sqldb.Close())
	}

	if err := prepareSchema(ctx, sqldb, settings); err != nil {
		return nil, errors.Join(err, sqldb.Close())
	}

	return &Store{
		db:                   sqlx.NewDb(sqldb, "sqlite3"),
		changeBatchRetention: settings.changeBatchRetention,
	}, nil
}

// Close closes the database.
func (s *Store) Close() error { return s.db.Close() }

// Reads select whole rows ("SELECT *", "t.*", "c.*"). sqlx rejects a result
// column with no matching field, so a column added by a migration makes the
// reads fail loudly ("missing destination name ...") until it is mapped,
// instead of being silently ignored.
//
// opportunityRow gives the opportunities table's internal id column a home, so
// it stays out of types.Opportunity. sqlx maps the embedded Opportunity's
// fields as if declared here. The other tables have no internal columns and
// read straight into internal/types.
type opportunityRow struct {
	ID int64 `db:"id"`
	types.Opportunity
}

// changeBatch is a row of opportunity_change_batches: a batch of changed
// opportunities, named to clients by its Hash. ID and CreatedAt are internal
// to the store.
type changeBatch struct {
	ID        int64     `db:"id"`
	Hash      string    `db:"hash"`
	CreatedAt time.Time `db:"created_at"`
}

// SaveOpportunity stores opportunity and its releases in one transaction.
//
// Releases (and their tags) are write-once: ones already stored are skipped.
// The opportunity row and its classifications are replaced only if the
// opportunity's newest release is at least as new as the release the stored
// opportunity came from, so a late-arriving older release never undoes a
// newer one.
//
// opportunity.Marks are stored only for a new opportunity; existing marks are
// kept (use SetMarks to change them).
//
// A new opportunity, or one given new releases, is recorded in a change
// batch of its own (see ListChangedOpportunities).
//
// opportunity is normalised in place (see prepareOpportunity), and when it is
// written its classifications' IDs are filled in.
func (s *Store) SaveOpportunity(
	ctx context.Context, opportunity *types.Opportunity,
) (SaveResult, error) {
	if err := prepareOpportunity(opportunity); err != nil {
		return SaveResult{}, err
	}

	tx, err := s.db.BeginTxx(ctx, nil)
	if err != nil {
		return SaveResult{}, err
	}
	defer tx.Rollback() //nolint:errcheck // no-op after Commit

	result, _, err := saveOpportunity(ctx, tx, opportunity)
	if err != nil {
		return SaveResult{}, err
	}

	_, err = s.recordChangeBatch(
		ctx,
		tx,
		changedUUIDs([]*types.Opportunity{opportunity}, []SaveResult{result}),
	)
	if err != nil {
		return SaveResult{}, err
	}

	if err := tx.Commit(); err != nil {
		return SaveResult{}, err
	}

	return result, nil
}

// SaveOpportunities stores each of opportunities and its releases, all in one
// transaction, as SaveOpportunity does for one.
//
// Releases (and their tags) are write-once: ones already stored are skipped.
// An opportunity row and its classifications are replaced only if the
// opportunity's newest release is at least as new as the release the stored
// opportunity came from, so a late-arriving older release never undoes a
// newer one.
//
// Marks are stored only for a new opportunity; existing marks are kept (use
// SetMarks to change them).
//
// Each opportunity is normalised in place (see prepareOpportunity), and when
// it is written its classifications' IDs are filled in. The result reports
// the classification codes the save brought into or out of use (see
// ListClassifications), and the hash of the change batch recording the
// opportunities that were new or given new releases (see
// ListChangedOpportunities).
func (s *Store) SaveOpportunities(
	ctx context.Context, opportunities []*types.Opportunity,
) (SaveMultipleResult, error) {
	for _, opportunity := range opportunities {
		if err := prepareOpportunity(opportunity); err != nil {
			return SaveMultipleResult{}, err
		}
	}

	tx, err := s.db.BeginTxx(ctx, nil)
	if err != nil {
		return SaveMultipleResult{}, err
	}
	defer tx.Rollback() //nolint:errcheck // no-op after Commit

	before, err := classificationsInUse(ctx, tx)
	if err != nil {
		return SaveMultipleResult{}, err
	}

	total := SaveMultipleResult{
		OpportunityResults: make([]SaveResult, len(opportunities)),
	}
	for i, opportunity := range opportunities {
		result, isNew, err := saveOpportunity(ctx, tx, opportunity)
		if err != nil {
			return SaveMultipleResult{}, err
		}
		total.OpportunityResults[i] = result
		total.NewReleases += result.NewReleases
		switch {
		case isNew:
			total.NewOpportunities++
		case result.OpportunityUpdated:
			total.UpdatedOpportunities++
		}
	}

	after, err := classificationsInUse(ctx, tx)
	if err != nil {
		return SaveMultipleResult{}, err
	}
	added, removed := classificationChanges(before, after)
	total.AddedClassifications = added
	total.RemovedClassificationIDs = removed

	total.ChangeBatch, err = s.recordChangeBatch(
		ctx, tx, changedUUIDs(opportunities, total.OpportunityResults),
	)
	if err != nil {
		return SaveMultipleResult{}, err
	}

	if err := tx.Commit(); err != nil {
		return SaveMultipleResult{}, err
	}

	return total, nil
}

// classificationsInUse returns the classification codes at least one
// opportunity has, ordered by code, as tx sees them.
func classificationsInUse(
	ctx context.Context, tx *sqlx.Tx,
) ([]types.ClassificationCode, error) {
	var classifications []types.ClassificationCode
	err := tx.SelectContext(ctx, &classifications, selectClassificationsInUse)
	if err != nil {
		return nil, fmt.Errorf("listing classifications: %w", err)
	}

	return classifications, nil
}

// classificationChanges compares the classification codes in use before and
// after a save: it returns the codes in after but not before, and the ids of
// those in before but not after, each in the order they're given.
func classificationChanges(
	before, after []types.ClassificationCode,
) ([]types.ClassificationCode, []int64) {
	wasInUse := make(map[int64]bool, len(before))
	for _, classification := range before {
		wasInUse[classification.ID] = true
	}
	isInUse := make(map[int64]bool, len(after))
	for _, classification := range after {
		isInUse[classification.ID] = true
	}

	var added []types.ClassificationCode
	for _, classification := range after {
		if !wasInUse[classification.ID] {
			added = append(added, classification)
		}
	}
	var removed []int64
	for _, classification := range before {
		if !isInUse[classification.ID] {
			removed = append(removed, classification.ID)
		}
	}

	return added, removed
}

// changedUUIDs returns the UUIDs of the opportunities that results, from
// saving them, show were new or given new releases. An opportunity whose row
// was only rewritten, such as one fetched again from its newest stored
// release, hasn't changed.
func changedUUIDs(
	opportunities []*types.Opportunity, results []SaveResult,
) []string {
	var uuids []string
	for i, result := range results {
		if result.NewOpportunity || result.NewReleases > 0 {
			uuids = append(uuids, opportunities[i].UUID)
		}
	}

	return uuids
}

// recordChangeBatch records a change batch of the opportunities uuids inside
// tx and returns its hash, or "" without recording anything if uuids is
// empty. Batches older than the store's retention are deleted (see
// WithChangeBatchRetention).
//
// The batch's created_at is the current time, or just after the newest
// batch's if that isn't later (the clock went back, or two batches came
// within its resolution), so batches are ordered by created_at. tx holds the
// write lock from its start (see dsn), so no other batch can be recorded in
// between.
func (s *Store) recordChangeBatch(
	ctx context.Context, tx *sqlx.Tx, uuids []string,
) (string, error) {
	if len(uuids) == 0 {
		return "", nil
	}

	var latest changeBatch
	err := tx.GetContext(ctx, &latest, selectLatestChangeBatch)
	if err != nil && !errors.Is(err, sql.ErrNoRows) {
		return "", fmt.Errorf("reading the latest change batch: %w", err)
	}

	createdAt := time.Now().UTC()
	if !createdAt.After(latest.CreatedAt) {
		createdAt = latest.CreatedAt.Add(time.Nanosecond)
	}
	hash := newBatchHash(createdAt)

	var batchID int64
	err = tx.GetContext(ctx, &batchID, insertChangeBatch, hash, createdAt)
	if err != nil {
		return "", fmt.Errorf("recording a change batch: %w", err)
	}

	for _, uuid := range uuids {
		_, err = tx.ExecContext(ctx, insertChangeBatchOpportunity, batchID, uuid)
		if err != nil {
			return "", fmt.Errorf(
				"recording %s in a change batch: %w", uuid, err,
			)
		}
	}

	if s.changeBatchRetention > 0 {
		cutoff := createdAt.Add(-s.changeBatchRetention)
		_, err = tx.ExecContext(ctx, deleteChangeBatchesBefore, cutoff)
		if err != nil {
			return "", fmt.Errorf("deleting old change batches: %w", err)
		}
	}

	return hash, nil
}

// newBatchHash returns the hash of a change batch created at createdAt: the
// hex SHA-256 hash of random bytes and the time. The random bytes make it
// unguessable, so a client can only name a batch it was given.
func newBatchHash(createdAt time.Time) string {
	random := make([]byte, 32)
	rand.Read(random) // crypto/rand.Read never returns an error

	hash := sha256.Sum256(
		append(random, createdAt.Format(time.RFC3339Nano)...),
	)
	return hex.EncodeToString(hash[:])
}

// saveOpportunity does the work of SaveOpportunity and SaveOpportunities for
// one prepared opportunity inside tx. isNew reports that the opportunity was
// not already stored.
func saveOpportunity(
	ctx context.Context, tx *sqlx.Tx, opportunity *types.Opportunity,
) (SaveResult, bool, error) {
	var (
		result SaveResult
		stored time.Time
	)
	err := tx.GetContext(
		ctx, &stored, selectOpportunityLatestReleaseDate, opportunity.UUID,
	)
	switch {
	case errors.Is(err, sql.ErrNoRows):
		result.NewOpportunity = true
		result.OpportunityUpdated = true
	case err != nil:
		return SaveResult{}, false, fmt.Errorf(
			"reading opportunity %s: %w", opportunity.UUID, err,
		)
	default:
		result.OpportunityUpdated = !opportunity.LatestReleaseDate.Before(stored)
	}

	if result.OpportunityUpdated {
		if err := writeOpportunity(ctx, tx, opportunity); err != nil {
			return SaveResult{}, false, fmt.Errorf(
				"writing opportunity %s: %w", opportunity.UUID, err,
			)
		}
	}

	for _, release := range opportunity.Releases {
		inserted, err := insertReleaseWithTags(ctx, tx, release)
		if err != nil {
			return SaveResult{}, false, err
		}
		if inserted {
			result.NewReleases++
		}
	}

	return result, result.NewOpportunity, nil
}

// insertReleaseWithTags stores release and its tags unless it is already
// stored, and reports whether it was inserted.
func insertReleaseWithTags(
	ctx context.Context, tx *sqlx.Tx, release types.OpportunityRelease,
) (bool, error) {
	result, err := tx.NamedExecContext(ctx, insertRelease, release)
	if err != nil {
		return false, fmt.Errorf(
			"inserting release %s/%s: %w",
			release.UUID,
			release.ReleaseID,
			err,
		)
	}

	if affected, err := result.RowsAffected(); err != nil || affected == 0 {
		return false, err // affected == 0: already stored
	}

	for _, tag := range release.Tags {
		if _, err := tx.NamedExecContext(ctx, insertReleaseTag, tag); err != nil {
			return false, fmt.Errorf(
				"inserting tag for %s/%s: %w",
				release.UUID,
				release.ReleaseID,
				err,
			)
		}
	}

	return true, nil
}

// writeOpportunity upserts opportunity's row, gives it a marks row if it has
// none (existing marks are kept), and replaces its classifications, filling in
// their IDs.
func writeOpportunity(
	ctx context.Context, tx *sqlx.Tx, opportunity *types.Opportunity,
) (err error) {
	_, err = tx.NamedExecContext(ctx, upsertOpportunity, opportunity)
	if err != nil {
		return err
	}

	_, err = tx.NamedExecContext(ctx, insertOpportunityMarks, opportunity.Marks)
	if err != nil {
		return err
	}

	_, err = tx.ExecContext(
		ctx, deleteOpportunityClassifications, opportunity.UUID,
	)
	if err != nil {
		return err
	}

	statement, err := tx.PrepareNamedContext(
		ctx, upsertClassificationCodeReturningID,
	)
	if err != nil {
		return err
	}
	defer func() {
		err = errors.Join(err, statement.Close())
	}()

	for i := range opportunity.Classifications {
		classification := &opportunity.Classifications[i]

		err = statement.GetContext(ctx, &classification.ID, classification)
		if err != nil {
			return err
		}

		_, err = tx.ExecContext(
			ctx, insertOpportunityClassification, opportunity.UUID, classification.ID,
		)
		if err != nil {
			return err
		}
	}

	return nil
}

// SetMarks replaces the marks of the opportunity marks.UUID, or returns
// ErrNotFound. Marks that fail marks.Validate (e.g. types.ErrWonAndLost) are
// not stored. The opportunity is recorded in a change batch of its own (see
// ListChangedOpportunities), so a client catching up sees its new marks.
func (s *Store) SetMarks(
	ctx context.Context, marks types.OpportunityMarks,
) error {
	if marks.UUID == "" {
		return errors.New("store: marks have no UUID")
	}
	if err := marks.Validate(); err != nil {
		return fmt.Errorf("opportunity %s: %w", marks.UUID, err)
	}

	tx, err := s.db.BeginTxx(ctx, nil)
	if err != nil {
		return err
	}
	defer tx.Rollback() //nolint:errcheck // no-op after Commit

	_, err = tx.NamedExecContext(ctx, upsertOpportunityMarks, marks)
	if isForeignKeyError(err) {
		return ErrNotFound
	}
	if err != nil {
		return err
	}

	_, err = s.recordChangeBatch(ctx, tx, []string{marks.UUID})
	if err != nil {
		return err
	}

	return tx.Commit()
}

// isForeignKeyError reports whether err is SQLite rejecting a write that
// breaks a foreign key, e.g. one referring to a missing opportunity.
func isForeignKeyError(err error) bool {
	var sqliteErr sqlite3.Error
	return errors.As(err, &sqliteErr) &&
		sqliteErr.ExtendedCode == sqlite3.ErrConstraintForeignKey
}

// GetOpportunity returns the opportunity with its classifications, marks and
// all of its releases (with tags), or ErrNotFound.
func (s *Store) GetOpportunity(
	ctx context.Context, uuid string,
) (*types.Opportunity, error) {
	var row opportunityRow
	err := s.db.GetContext(ctx, &row, selectOpportunityByUUID, uuid)
	if errors.Is(err, sql.ErrNoRows) {
		return nil, ErrNotFound
	}
	if err != nil {
		return nil, err
	}

	opportunities := []types.Opportunity{row.Opportunity}
	if err := s.loadClassifications(ctx, opportunities); err != nil {
		return nil, err
	}
	if err := s.loadMarks(ctx, opportunities); err != nil {
		return nil, err
	}

	if opportunities[0].Releases, err = s.ListReleases(ctx, uuid); err != nil {
		return nil, err
	}

	return &opportunities[0], nil
}

// ListOpportunities returns opportunities matching filter, with their
// classifications and marks but without releases (use ListReleases or
// GetOpportunity for those).
func (s *Store) ListOpportunities(
	ctx context.Context, filter OpportunityFilter,
) ([]types.Opportunity, error) {
	query, args := listOpportunitiesQuery(filter)
	var rows []opportunityRow
	if err := s.db.SelectContext(ctx, &rows, query, args...); err != nil {
		return nil, err
	}

	return s.opportunitiesFromRows(ctx, rows)
}

// LatestChangeBatch returns the hash of the newest change batch, or "" if
// none has been recorded. Read before listing opportunities, it is the batch
// to give ListChangedOpportunities later to get what changed since.
func (s *Store) LatestChangeBatch(ctx context.Context) (string, error) {
	var latest changeBatch
	err := s.db.GetContext(ctx, &latest, selectLatestChangeBatch)
	if errors.Is(err, sql.ErrNoRows) {
		return "", nil
	}
	if err != nil {
		return "", fmt.Errorf("reading the latest change batch: %w", err)
	}

	return latest.Hash, nil
}

// ListChangedOpportunities returns the opportunities recorded in the change
// batches created after the batch with the hash after, once each, as
// ListOpportunities returns them but unpaged, and the hash of the newest of
// those batches, to give it next time. With no newer batches, it returns no
// opportunities and after. An unknown hash is ErrUnknownBatch.
//
// Batches recorded while it runs are left for next time, so none is missed:
// only those up to the newest batch when it starts are read.
func (s *Store) ListChangedOpportunities(
	ctx context.Context, after string,
) ([]types.Opportunity, string, error) {
	var since changeBatch
	err := s.db.GetContext(ctx, &since, selectChangeBatchByHash, after)
	if errors.Is(err, sql.ErrNoRows) {
		return nil, "", ErrUnknownBatch
	}
	if err != nil {
		return nil, "", fmt.Errorf("reading change batch %s: %w", after, err)
	}

	var latest changeBatch
	err = s.db.GetContext(ctx, &latest, selectLatestChangeBatch)
	if err != nil {
		return nil, "", fmt.Errorf("reading the latest change batch: %w", err)
	}

	var rows []opportunityRow
	err = s.db.SelectContext(
		ctx,
		&rows,
		selectOpportunitiesChangedBetween,
		since.CreatedAt,
		latest.CreatedAt,
	)
	if err != nil {
		return nil, "", fmt.Errorf("listing changed opportunities: %w", err)
	}

	opportunities, err := s.opportunitiesFromRows(ctx, rows)
	if err != nil {
		return nil, "", err
	}

	return opportunities, latest.Hash, nil
}

// opportunitiesFromRows returns the opportunities in rows, in order, with
// their classifications and marks. The result is never nil.
func (s *Store) opportunitiesFromRows(
	ctx context.Context, rows []opportunityRow,
) ([]types.Opportunity, error) {
	opportunities := make([]types.Opportunity, len(rows))
	for i, row := range rows {
		opportunities[i] = row.Opportunity
	}

	if err := s.loadClassifications(ctx, opportunities); err != nil {
		return nil, err
	}
	if err := s.loadMarks(ctx, opportunities); err != nil {
		return nil, err
	}

	return opportunities, nil
}

// listOpportunitiesQuery assembles the ListOpportunities query from the
// fragments in queries.go, with one argument per filter that is set (none for
// mark filters).
func listOpportunitiesQuery(filter OpportunityFilter) (string, []any) {
	var where []string
	var args []any
	add := func(clause string, argument any) {
		where = append(where, clause)
		args = append(args, argument)
	}

	if filter.Status != "" {
		add(filterOpportunityStatus, filter.Status)
	}
	if filter.ClassificationCode != "" {
		add(filterOpportunityClassificationCode, filter.ClassificationCode)
	}
	if match := ftsQuery(filter.Title); match != "" {
		add(filterOpportunityTitleMatch, match)
	}
	if !filter.ReleasedFrom.IsZero() {
		add(filterOpportunityReleasedFrom, filter.ReleasedFrom.UTC())
	}
	if !filter.ReleasedBefore.IsZero() {
		add(filterOpportunityReleasedBefore, filter.ReleasedBefore.UTC())
	}

	marks := []struct {
		filter MarkFilter
		clause string
	}{
		{filter: filter.Hidden, clause: filterOpportunityMarkHidden},
		{filter: filter.Skipped, clause: filterOpportunityMarkSkipped},
		{filter: filter.Interested, clause: filterOpportunityMarkInterested},
		{filter: filter.BidSubmitted, clause: filterOpportunityMarkBidSubmitted},
		{filter: filter.Won, clause: filterOpportunityMarkWon},
		{filter: filter.Lost, clause: filterOpportunityMarkLost},
	}
	for _, mark := range marks {
		switch mark.filter {
		case MarkSet:
			where = append(where, mark.clause)
		case MarkUnset:
			where = append(where, "NOT "+mark.clause)
		}
	}

	query := listOpportunitiesSelect
	if len(where) > 0 {
		query += "\nWHERE " + strings.Join(where, "\n\tAND ")
	}
	query += listOpportunitiesOrderAndPage

	return query, append(args, filter.limit(), filter.Offset)
}

// loadClassifications fills in Classifications for all opportunities with one
// query.
func (s *Store) loadClassifications(
	ctx context.Context, opportunities []types.Opportunity,
) error {
	if len(opportunities) == 0 {
		return nil
	}

	uuids := make([]string, len(opportunities))
	byUUID := make(map[string]*types.Opportunity, len(opportunities))
	for i := range opportunities {
		uuids[i] = opportunities[i].UUID
		byUUID[opportunities[i].UUID] = &opportunities[i]
	}

	query, args, err := sqlx.In(selectClassificationsForOpportunities, uuids)
	if err != nil {
		return err
	}

	// opportunity_uuid says which opportunity each row belongs to.
	var rows []struct {
		OpportunityUUID string `db:"opportunity_uuid"`
		types.ClassificationCode
	}
	err = s.db.SelectContext(ctx, &rows, s.db.Rebind(query), args...)
	if err != nil {
		return err
	}

	for _, classificationRow := range rows {
		opportunity := byUUID[classificationRow.OpportunityUUID]
		opportunity.Classifications = append(
			opportunity.Classifications, classificationRow.ClassificationCode,
		)
	}

	return nil
}

// loadMarks fills in Marks for all opportunities with one query. An
// opportunity with no marks row gets zero marks carrying its UUID.
func (s *Store) loadMarks(
	ctx context.Context, opportunities []types.Opportunity,
) error {
	if len(opportunities) == 0 {
		return nil
	}

	uuids := make([]string, len(opportunities))
	byUUID := make(map[string]*types.Opportunity, len(opportunities))
	for i := range opportunities {
		uuids[i] = opportunities[i].UUID
		byUUID[opportunities[i].UUID] = &opportunities[i]
		opportunities[i].Marks = types.OpportunityMarks{
			UUID: opportunities[i].UUID,
		}
	}

	query, args, err := sqlx.In(selectMarksForOpportunities, uuids)
	if err != nil {
		return err
	}

	var rows []types.OpportunityMarks
	err = s.db.SelectContext(ctx, &rows, s.db.Rebind(query), args...)
	if err != nil {
		return err
	}

	for _, row := range rows {
		byUUID[row.UUID].Marks = row
	}

	return nil
}

// ListReleases returns a opportunity's releases with their tags, oldest first.
func (s *Store) ListReleases(
	ctx context.Context, uuid string,
) ([]types.OpportunityRelease, error) {
	releases := []types.OpportunityRelease{}
	err := s.db.SelectContext(ctx, &releases, selectReleasesByOpportunity, uuid)
	if err != nil {
		return nil, err
	}

	byID := make(map[string]*types.OpportunityRelease, len(releases))
	for i := range releases {
		byID[releases[i].ReleaseID] = &releases[i]
	}

	var tags []types.OpportunityReleaseTag
	err = s.db.SelectContext(ctx, &tags, selectReleaseTagsByOpportunity, uuid)
	if err != nil {
		return nil, err
	}

	for _, tag := range tags {
		if release := byID[tag.ReleaseID]; release != nil {
			release.Tags = append(release.Tags, tag)
		}
	}

	return releases, nil
}

// ListClassifications returns every classification code that at least one
// stored opportunity has, ordered by code. A code no opportunity has any more
// isn't listed. The result is never nil.
func (s *Store) ListClassifications(
	ctx context.Context,
) ([]types.ClassificationCode, error) {
	classifications := []types.ClassificationCode{}
	err := s.db.SelectContext(ctx, &classifications, selectClassificationsInUse)
	if err != nil {
		return nil, fmt.Errorf("listing classifications: %w", err)
	}

	return classifications, nil
}

// LatestReleaseDate returns the date of the newest release stored for the
// opportunities from source, or the zero time if none are stored, so a
// fetcher can carry on from where it left off.
func (s *Store) LatestReleaseDate(
	ctx context.Context, source string,
) (time.Time, error) {
	var latest time.Time
	err := s.db.GetContext(ctx, &latest, selectSourceLatestReleaseDate, source)
	if errors.Is(err, sql.ErrNoRows) {
		return time.Time{}, nil
	}
	if err != nil {
		return time.Time{}, fmt.Errorf(
			"reading the latest release date of %s: %w", source, err,
		)
	}

	return latest, nil
}

// ReleaseExists reports whether a release is already stored, so a fetcher
// can skip converting it.
func (s *Store) ReleaseExists(
	ctx context.Context, uuid, releaseID string,
) (bool, error) {
	var exists bool
	err := s.db.GetContext(ctx, &exists, selectReleaseExists, uuid, releaseID)
	return exists, err
}