Check for validity of the cached expanded APK more thoroughly

internal/tarfs/tarfs.go

@@ -262,6 +262,10 @@ func New(ra io.ReaderAt, size int64) (*FS, error) {
 	return fsys, nil
 }
 
+func (fsys *FS) UnderlyingReader() io.ReaderAt {
+	return fsys.ra
+}
+
 func (fsys *FS) Close() error {
 	if fsys == nil {
 		return nil

pkg/apk/apk/implementation.go

@@ -1281,7 +1281,7 @@ func (c *apkCache) get(ctx context.Context, a *APK, pkg InstallablePackage) (*ex
 		// If we find a value in the cache, we should check to make sure the tar file it references still exists.
 		// If it references a non-existent file, we should act as though this was a cache miss and expand the
 		// APK again.
-		if _, err := os.Stat(result.exp.TarFile); os.IsNotExist(err) {
+		if !result.exp.IsValid() {
 			newValue := sync.OnceValue(fn)
 			c.onces.Store(u, newValue)
 			result = newValue()

pkg/apk/apk/implementation_test.go

@@ -756,11 +756,13 @@ func TestFetchPackage(t *testing.T) {
 	t.Run("handle missing cache files when expanding APK", func(t *testing.T) {
 		tmpDir := t.TempDir()
 		a := prepLayout(t, tmpDir)
+
 		// Fill the cache
 		exp, err := a.expandPackage(ctx, pkg)
 		require.NoError(t, err, "unable to expand package")
 		_, err = os.Stat(exp.TarFile)
 		require.NoError(t, err, "unable to stat cached tar file")
+
 		// Delete the tar file from the cache
 		require.NoError(t, os.Remove(exp.TarFile), "unable to delete cached tar file")
 		_, err = os.Stat(exp.TarFile)
@@ -769,9 +771,22 @@ func TestFetchPackage(t *testing.T) {
 		// Expand the package again, this should re-populate the cache.
 		exp2, err := a.expandPackage(ctx, pkg)
 		require.NoError(t, err, "unable to expandPackage after deleting cached tar file")
+		_, err = os.Stat(exp2.TarFile)
+		require.NoError(t, err, "unable to stat cached tar file")
+
+		// Delete and recreate the tar file from the cache (changing its inodes)
+		bs, err := os.ReadFile(exp2.TarFile)
+		require.NoError(t, err, "unable to read cached tar file")
+		require.NoError(t, os.Remove(exp2.TarFile), "unable to delete cached tar file")
+		require.NoError(t, os.WriteFile(exp2.TarFile, bs, 0o644), "unable to recreate cached tar file")
+
+		// Ensure that the underlying reader is different (i.e. we re-read the file)
+		exp3, err := a.expandPackage(ctx, pkg)
+		require.NoError(t, err, "unable to expandPackage after deleting and recreating cached tar file")
+		require.NotEqual(t, exp2.TarFS.UnderlyingReader(), exp3.TarFS.UnderlyingReader())
 
 		// We should be able to read the APK contents
-		rc, err := exp2.APK()
+		rc, err := exp3.APK()
 		require.NoError(t, err, "unable to get reader for APK()")
 		_, err = io.ReadAll(rc)
 		require.NoError(t, err, "unable to read APK contents")

pkg/apk/expandapk/expandapk.go

@@ -209,6 +209,42 @@ func (m *multiReadCloser) Close() error {
 	return errors.Join(errs...)
 }
 
+// IsValid checks that the expanded APK is still valid by verifying that
+// the underlying files exist and that the file handles match the expected files.
+// Since this structure is heavily cached, this is useful to verify that the
+// cached data is still valid.
+func (a *APKExpanded) IsValid() bool {
+	if f, ok := a.TarFS.UnderlyingReader().(*os.File); ok {
+		// Verify that the file descriptor matches the expected file on disk.
+		fdInfo, err := f.Stat()
+		if err != nil {
+			return false
+		}
+
+		pathInfo, err := os.Stat(f.Name())
+		if err != nil {
+			return false
+		}
+
+		if !os.SameFile(fdInfo, pathInfo) {
+			return false
+		}
+	}
+
+	// Check that all the expected files exist.
+	files := []string{a.ControlFile, a.PackageFile, a.TarFile}
+	if a.SignatureFile != "" {
+		files = append(files, a.SignatureFile)
+	}
+	for _, file := range files {
+		if _, err := os.Stat(file); err != nil {
+			return false
+		}
+	}
+
+	return true
+}
+
 func (a *APKExpanded) Close() error {
 	errs := []error{}