build: ref-count policy FS lifecycle

Make memoized policy FS ref-counted so repeated get()/close() pairs don't
prematurely close shared handles.

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>

build/policy_loader.go

@@ -228,28 +228,48 @@ func normalizeLocalPolicyPath(name, contextDir string) string {
 
 type memoizedPolicyFS struct {
 	init    func() (fs.StatFS, func() error, error)
-	once    sync.Once
+	mu      sync.Mutex
+	loaded  bool
+	refs    int
 	fs      fs.StatFS
 	closeFn func() error
 	err     error
 }
 
 func (m *memoizedPolicyFS) get() (fs.StatFS, error) {
-	m.once.Do(func() {
-		if m.init == nil {
-			return
+	m.mu.Lock()
+	defer m.mu.Unlock()
+	if !m.loaded {
+		m.loaded = true
+		if m.init != nil {
+			m.fs, m.closeFn, m.err = m.init()
 		}
-		m.fs, m.closeFn, m.err = m.init()
-	})
+	}
 	if m.err != nil {
 		return nil, m.err
 	}
+	m.refs++
 	return m.fs, nil
 }
 
 func (m *memoizedPolicyFS) close() error {
-	if m.closeFn != nil {
-		return m.closeFn()
+	m.mu.Lock()
+	if m.refs > 0 {
+		m.refs--
+	}
+	if m.refs > 0 {
+		m.mu.Unlock()
+		return nil
+	}
+	closeFn := m.closeFn
+	m.fs = nil
+	m.closeFn = nil
+	m.err = nil
+	m.loaded = false
+	m.mu.Unlock()
+
+	if closeFn != nil {
+		return closeFn()
 	}
 	return nil
 }

build/policy_loader_test.go

@@ -0,0 +1,85 @@
+package build
+
+import (
+	"io/fs"
+	"testing"
+	"testing/fstest"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestMemoizedPolicyFSRefCountedClose(t *testing.T) {
+	var initCalls int
+	var closeCalls int
+
+	m := &memoizedPolicyFS{
+		init: func() (fs.StatFS, func() error, error) {
+			initCalls++
+			root := fstest.MapFS{
+				"policy.rego": &fstest.MapFile{Data: []byte("package docker\n")},
+			}
+			return root, func() error {
+				closeCalls++
+				return nil
+			}, nil
+		},
+	}
+
+	first, err := m.get()
+	require.NoError(t, err)
+	require.NotNil(t, first)
+	require.Equal(t, 1, initCalls)
+
+	second, err := m.get()
+	require.NoError(t, err)
+	require.NotNil(t, second)
+	require.Equal(t, 1, initCalls)
+
+	require.NoError(t, m.close())
+	require.Equal(t, 0, closeCalls)
+
+	third, err := m.get()
+	require.NoError(t, err)
+	require.NotNil(t, third)
+	require.Equal(t, 1, initCalls)
+
+	require.NoError(t, m.close())
+	require.Equal(t, 0, closeCalls)
+
+	require.NoError(t, m.close())
+	require.Equal(t, 1, closeCalls)
+}
+
+func TestMemoizedPolicyFSReinitializesAfterAllRefsClosed(t *testing.T) {
+	var initCalls int
+	var closeCalls int
+
+	m := &memoizedPolicyFS{
+		init: func() (fs.StatFS, func() error, error) {
+			initCalls++
+			root := fstest.MapFS{
+				"policy.rego": &fstest.MapFile{Data: []byte("package docker\n")},
+			}
+			return root, func() error {
+				closeCalls++
+				return nil
+			}, nil
+		},
+	}
+
+	first, err := m.get()
+	require.NoError(t, err)
+	require.NotNil(t, first)
+	require.Equal(t, 1, initCalls)
+
+	require.NoError(t, m.close())
+	require.Equal(t, 1, closeCalls)
+
+	second, err := m.get()
+	require.NoError(t, err)
+	require.NotNil(t, second)
+	require.Equal(t, 2, initCalls)
+
+	require.NoError(t, m.close())
+	require.Equal(t, 2, closeCalls)
+}