plugins/bundle: use unique temporary files (#4786)

In order to use the feature to persist activated bundles to disk in a
cloud environment with shared storage, e.g. Kubernetes with the Amazon
EFS storage driver, each instance of OPA needs to either synchronize
their access to the temporary file using advisory file locks, or use
unique temporary files. If not, then the following situation may occur:

p1: open and trunc tmp file
p1: write to tmp file
p2: open and trunc tmp file
p1: rename tmp file to dst
p2: write to tmp file
p2: rename tmp file to dst

This may then lead to the persisted bundle being truncated or corrupted.

Here the approach of using unique temporary files is chosen because it
avoids the overhead of introducing file locks, and the additional
dependency since Go lacks any such mechanisms in the standard library.

This solution should avoid truncated or corrupt bundles as `rename()` is
guaranteed to be atomic, even in file systems like NFS.

Fixes: #4782

Signed-off-by: Fredrik Appelros <fredrik.appelros@sinch.com>

plugins/bundle/plugin.go

@@ -653,10 +653,9 @@ func (p *Plugin) configDelta(newConfig *Config) (map[string]*Source, map[string]
 func (p *Plugin) saveBundleToDisk(name string, raw io.Reader) error {
 
 	bundleDir := filepath.Join(p.bundlePersistPath, name)
-	tmpFile := filepath.Join(bundleDir, ".bundle.tar.gz.tmp")
 	bundleFile := filepath.Join(bundleDir, "bundle.tar.gz")
 
-	saveErr := saveCurrentBundleToDisk(bundleDir, ".bundle.tar.gz.tmp", raw)
+	tmpFile, saveErr := saveCurrentBundleToDisk(bundleDir, raw)
 	if saveErr != nil {
 		p.log(name).Error("Failed to save new bundle to disk: %v", saveErr)
 
@@ -674,26 +673,26 @@ func (p *Plugin) saveBundleToDisk(name string, raw io.Reader) error {
 	return os.Rename(tmpFile, bundleFile)
 }
 
-func saveCurrentBundleToDisk(path, filename string, raw io.Reader) error {
+func saveCurrentBundleToDisk(path string, raw io.Reader) (string, error) {
 	if _, err := os.Stat(path); os.IsNotExist(err) {
 		err = os.MkdirAll(path, os.ModePerm)
 		if err != nil {
-			return err
+			return "", err
 		}
 	}
 
 	if raw == nil {
-		return fmt.Errorf("no raw bundle bytes to persist to disk")
+		return "", fmt.Errorf("no raw bundle bytes to persist to disk")
 	}
 
-	dest, err := os.OpenFile(filepath.Join(path, filename), os.O_WRONLY|os.O_CREATE|os.O_TRUNC, 0644)
+	dest, err := os.CreateTemp(path, ".bundle.tar.gz.*.tmp")
 	if err != nil {
-		return err
+		return "", err
 	}
 	defer dest.Close()
 
 	_, err = io.Copy(dest, raw)
-	return err
+	return dest.Name(), err
 }
 
 func loadBundleFromDisk(path, name string, src *Source) (*bundle.Bundle, error) {

plugins/bundle/plugin_test.go

@@ -2530,16 +2530,16 @@ func TestSaveCurrentBundleToDisk(t *testing.T) {
 
 	defer os.RemoveAll(srcDir)
 
-	err = saveCurrentBundleToDisk(srcDir, "bundle.tar.gz", getTestRawBundle(t))
+	bundlePath, err := saveCurrentBundleToDisk(srcDir, getTestRawBundle(t))
 	if err != nil {
 		t.Fatalf("unexpected error %v", err)
 	}
 
-	if _, err := os.Stat(filepath.Join(srcDir, "bundle.tar.gz")); err != nil {
+	if _, err := os.Stat(bundlePath); err != nil {
 		t.Fatalf("unexpected error %v", err)
 	}
 
-	err = saveCurrentBundleToDisk(srcDir, "bundle.tar.gz", nil)
+	_, err = saveCurrentBundleToDisk(srcDir, nil)
 	if err == nil {
 		t.Fatal("expected error but got nil")
 	}