Added database.change_batch_retention, deleting old change batches

Each save that records a batch also deletes, in the same
transaction, the batches older than database.change_batch_retention
(30 days by default; 0 or less keeps them for ever), with their
records of opportunities. A client still holding a deleted batch
gets 410 Gone from ?changed_after= and reloads the whole list, as
for any unknown batch.

internal/store/store.go

@@ -51,6 +51,9 @@ const DefaultListLimit = 50
 // 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.
@@ -243,7 +246,8 @@ func Open(ctx context.Context, path string, opts ...Option) (*Store, error) {
 	}
 
 	return &Store{
-		db: sqlx.NewDb(sqldb, "sqlite3"),
+		db:                   sqlx.NewDb(sqldb, "sqlite3"),
+		changeBatchRetention: settings.changeBatchRetention,
 	}, nil
 }
 
@@ -307,7 +311,7 @@ func (s *Store) SaveOpportunity(
 		return SaveResult{}, err
 	}
 
-	_, err = recordChangeBatch(
+	_, err = s.recordChangeBatch(
 		ctx,
 		tx,
 		changedUUIDs([]*types.Opportunity{opportunity}, []SaveResult{result}),
@@ -387,7 +391,7 @@ func (s *Store) SaveOpportunities(
 	total.AddedClassifications = added
 	total.RemovedClassificationIDs = removed
 
-	total.ChangeBatch, err = recordChangeBatch(
+	total.ChangeBatch, err = s.recordChangeBatch(
 		ctx, tx, changedUUIDs(opportunities, total.OpportunityResults),
 	)
 	if err != nil {
@@ -465,14 +469,15 @@ func changedUUIDs(
 
 // recordChangeBatch records a change batch of the opportunities uuids inside
 // tx and returns its hash, or "" without recording anything if uuids is
-// empty.
+// 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 recordChangeBatch(
+func (s *Store) recordChangeBatch(
 	ctx context.Context, tx *sqlx.Tx, uuids []string,
 ) (string, error) {
 	if len(uuids) == 0 {
@@ -506,6 +511,14 @@ func recordChangeBatch(
 		}
 	}
 
+	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
 }
 
@@ -681,7 +694,7 @@ func (s *Store) SetMarks(
 		return err
 	}
 
-	_, err = recordChangeBatch(ctx, tx, []string{marks.UUID})
+	_, err = s.recordChangeBatch(ctx, tx, []string{marks.UUID})
 	if err != nil {
 		return err
 	}