Prevent SQLite busy errors

server/internal/store/sqlite.go

@@ -54,6 +54,20 @@ func OpenSQLite(ctx context.Context, path string) (*Store, error) {
 		return nil, err
 	}
 
+	// SQLite permits one writer at a time. Keep access serialized inside this
+	// process and wait briefly for locks held during startup or maintenance.
+	db.SetMaxOpenConns(1)
+	db.SetMaxIdleConns(1)
+	for _, stmt := range []string{
+		`PRAGMA busy_timeout = 5000`,
+		`PRAGMA journal_mode = WAL`,
+	} {
+		if _, err := db.ExecContext(ctx, stmt); err != nil {
+			_ = db.Close()
+			return nil, err
+		}
+	}
+
 	s := &Store{db: db}
 	if err := s.Migrate(ctx); err != nil {
 		_ = db.Close()

server/internal/store/sqlite_test.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"database/sql"
 	"path/filepath"
+	"sync"
 	"testing"
 
 	_ "modernc.org/sqlite"
@@ -63,3 +64,41 @@ VALUES ('rs_legacy', 'dev_legacy');
 		t.Fatalf("got target %q, want ssh", sessions[0].Target)
 	}
 }
+
+func TestSQLiteConcurrentAccessDoesNotReturnBusy(t *testing.T) {
+	ctx := context.Background()
+	st, err := OpenSQLite(ctx, filepath.Join(t.TempDir(), "concurrent.db"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer st.Close()
+
+	enrolled, err := st.EnrollDevice(ctx, "router", "OpenWrt")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	var wg sync.WaitGroup
+	errs := make(chan error, 80)
+	for range 40 {
+		wg.Add(2)
+		go func() {
+			defer wg.Done()
+			_, _, err := st.ListRemoteSessions(ctx, enrolled.DeviceID, RemoteSessionListOptions{})
+			errs <- err
+		}()
+		go func() {
+			defer wg.Done()
+			_, err := st.SaveHeartbeat(ctx, enrolled.DeviceID, []byte(`{"hostname":"router"}`), []byte(`{}`))
+			errs <- err
+		}()
+	}
+	wg.Wait()
+	close(errs)
+
+	for err := range errs {
+		if err != nil {
+			t.Fatal(err)
+		}
+	}
+}