feat: add a method to compute the DB size of the state

pkg/state/impl/sqlite/size.go

@@ -0,0 +1,53 @@
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this
+// file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+package sqlite
+
+import (
+	"context"
+	"fmt"
+
+	"zombiezen.com/go/sqlite"
+
+	"github.com/cosi-project/state-sqlite/pkg/sqlitexx"
+)
+
+// DBSize returns the size in bytes of tables used by this package.
+//
+// It uses SQLite's dbstat virtual table to calculate the size of the
+// resources and events tables within the main database file (logical
+// table page usage), which does not include any separate WAL/SHM files.
+func (st *State) DBSize(ctx context.Context) (int64, error) {
+	conn, err := st.db.Take(ctx)
+	if err != nil {
+		return 0, fmt.Errorf("error taking connection for db size: %w", err)
+	}
+
+	defer st.db.Put(conn)
+
+	var size int64
+
+	q, err := sqlitexx.NewQuery(
+		conn,
+		`SELECT coalesce(SUM(pgsize), 0) AS total_size FROM dbstat WHERE name = $table1 OR name = $table2`,
+	)
+	if err != nil {
+		return 0, fmt.Errorf("preparing query for db size: %w", err)
+	}
+
+	if err = q.
+		BindString("$table1", st.options.TablePrefix+"resources").
+		BindString("$table2", st.options.TablePrefix+"events").
+		QueryRow(
+			func(stmt *sqlite.Stmt) error {
+				size = stmt.GetInt64("total_size")
+
+				return nil
+			},
+		); err != nil {
+		return 0, fmt.Errorf("failed to get db size: %w", err)
+	}
+
+	return size, nil
+}

pkg/state/impl/sqlite/size_test.go

@@ -0,0 +1,44 @@
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this
+// file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+package sqlite_test
+
+import (
+	"strconv"
+	"testing"
+
+	"github.com/cosi-project/runtime/pkg/state/conformance"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+
+	"github.com/cosi-project/state-sqlite/pkg/state/impl/sqlite"
+)
+
+func TestDBSizeEmpty(t *testing.T) {
+	t.Parallel()
+
+	withSqliteCore(t, func(st *sqlite.State) {
+		size, err := st.DBSize(t.Context())
+		require.NoError(t, err)
+		assert.Greater(t, size, int64(0), "even empty tables should have some overhead")
+	})
+}
+
+func TestDBSizeGrowsWithData(t *testing.T) {
+	t.Parallel()
+
+	withSqliteCore(t, func(st *sqlite.State) {
+		sizeBefore, err := st.DBSize(t.Context())
+		require.NoError(t, err)
+
+		for i := range 100 {
+			require.NoError(t, st.Create(t.Context(), conformance.NewPathResource("ns1", strconv.Itoa(i))))
+		}
+
+		sizeAfter, err := st.DBSize(t.Context())
+		require.NoError(t, err)
+
+		assert.Greater(t, sizeAfter, sizeBefore, "size should grow after inserting resources")
+	})
+}