Check for validity of the cached expanded APK more thoroughly (#1987)

Instead of just checking for the presence of a single file, this ensures
that all active pieces of the expandapk structure are actually still
valid. It checks that all necessary files exist and checks that cached
file handles are actually the ones pointing to the files present in the
filesystem.

Author: markusthoemmes

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

@@ -1292,7 +1292,7 @@ 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{}