Keep foreign keys on for every database connection (closes #207)

SQLite keeps the foreign keys setting per connection, and the server
turned it on once with a PRAGMA. A query cancelled while it runs makes
the pool replace its only connection, and the new one ran without
foreign keys until a restart.

connect now adds `_pragma=foreign_keys(1)` to the database URL. The
driver runs it on every connection it opens. The server refuses to
start when DBURL sets foreign_keys itself, because the driver sorts
the pragmas and that one could win.

One test cancels a query and checks, with a TEMP table, that the
connection was replaced and that foreign keys are on. Another checks
the startup error for several spellings.

Model: opus-5-5

internal/db/db.go

@@ -5,9 +5,11 @@ import (
 	"context"
 	"database/sql"
 	"embed"
+	"errors"
 	"fmt"
 	"io/fs"
 	"log/slog"
+	"net/url"
 	"sort"
 	"strconv"
 	"strings"
@@ -22,6 +24,10 @@ import (
 
 const minMigrationParts = 2
 
+var errForeignKeysInDBURL = errors.New(
+	"DBURL must not set foreign_keys, which the server always turns on",
+)
+
 // SchemaFiles contains embedded SQL migration files.
 //
 //go:embed schema/*.sql
@@ -90,6 +96,33 @@ func (database *Database) connect(ctx context.Context) error {
 		dbURL = config.DefaultDBURL
 	}
 
+	// SQLite keeps the foreign keys setting per connection. The driver
+	// runs each _pragma parameter on every connection it opens. So the
+	// one added below also covers the connection the pool opens after
+	// a cancelled query breaks the old one. A foreign_keys pragma in
+	// DBURL could undo it, because the driver sorts the pragmas.
+	_, query, hasQuery := strings.Cut(dbURL, "?")
+
+	urlParams, err := url.ParseQuery(query)
+	if err != nil {
+		return fmt.Errorf("parse DBURL: %w", err)
+	}
+
+	for _, pragma := range urlParams["_pragma"] {
+		if strings.Contains(strings.ToLower(pragma), "foreign_keys") {
+			return fmt.Errorf(
+				"%w: _pragma=%s", errForeignKeysInDBURL, pragma,
+			)
+		}
+	}
+
+	separator := "?"
+	if hasQuery {
+		separator = "&"
+	}
+
+	dbURL += separator + "_pragma=foreign_keys(1)"
+
 	database.log.Info(
 		"connecting to database", "url", dbURL,
 	)
@@ -109,13 +142,6 @@ func (database *Database) connect(ctx context.Context) error {
 	database.conn = conn
 	database.log.Info("database connected")
 
-	_, err = database.conn.ExecContext(
-		ctx, "PRAGMA foreign_keys = ON",
-	)
-	if err != nil {
-		return fmt.Errorf("enable foreign keys: %w", err)
-	}
-
 	return database.runMigrations(ctx)
 }