test(db): reapply DSN pragmas after OpenMemory deserializes its template

internal/db/db.go

@@ -9,6 +9,7 @@ import (
 	"errors"
 	"fmt"
 	"log/slog"
+	"net/url"
 	"os"
 	"path/filepath"
 	"sort"
@@ -88,6 +89,14 @@ func OpenMemory() (*sql.DB, error) {
 		db.Close()
 		return nil, fmt.Errorf("load memory template: %w", err)
 	}
+	// Some DSN pragmas (synchronous, cache_size) belong to the schema, not the
+	// connection, so Deserialize resets them to SQLite's defaults. Set them again.
+	for _, p := range connectionPragmas() {
+		if _, err := db.Exec("PRAGMA " + p); err != nil {
+			db.Close()
+			return nil, fmt.Errorf("reapply PRAGMA %s: %w", p, err)
+		}
+	}
 	return db, nil
 }
 
@@ -271,6 +280,13 @@ const connectionPragmaDSN = "?_pragma=foreign_keys(1)" +
 	"&_pragma=temp_store(2)" +
 	"&_pragma=cache_size(-16000)"
 
+// connectionPragmas returns the pragmas connectionPragmaDSN sets, in the
+// driver's name(value) form.
+func connectionPragmas() []string {
+	q, _ := url.ParseQuery(strings.TrimPrefix(connectionPragmaDSN, "?"))
+	return q["_pragma"]
+}
+
 func setPragmas(db *sql.DB) error {
 	pragmas := []string{
 		"PRAGMA journal_mode=WAL",

internal/db/openmemory_test.go

@@ -9,8 +9,8 @@ import (
 )
 
 // OpenMemory hands out copies of one migrated template; a write to one copy
-// must not show up in the next, and per-connection pragmas must survive the
-// copy.
+// must not show up in the next, and every pragma the DSN sets must still hold
+// on the copy.
 func TestOpenMemory_CopiesAreIndependent(t *testing.T) {
 	a, err := OpenMemory()
 	if err != nil {
@@ -33,9 +33,15 @@ func TestOpenMemory_CopiesAreIndependent(t *testing.T) {
 	if n != 0 {
 		t.Fatalf("row written to one OpenMemory database leaked into another")
 	}
-	var fk int
-	if err := b.QueryRow(`PRAGMA foreign_keys`).Scan(&fk); err != nil || fk != 1 {
-		t.Fatalf("foreign_keys = %d (err %v), want 1", fk, err)
+	for _, p := range connectionPragmas() {
+		name, want, _ := strings.Cut(strings.TrimSuffix(p, ")"), "(")
+		var got string
+		if err := b.QueryRow("PRAGMA " + name).Scan(&got); err != nil {
+			t.Fatal(err)
+		}
+		if got != want {
+			t.Errorf("PRAGMA %s = %s, want %s", name, got, want)
+		}
 	}
 	if got, want := len(versionSetForTest(t, b)), countMigrationFiles(t); got != want {
 		t.Fatalf("schema_migrations has %d versions, want %d", got, want)