fix(auditbeat system/package): handle empty package.v1 bucket (#44296) (#44301)

Ensure the schema migration logic properly handles cases where the package.v1
bucket exists but is empty, avoiding assumptions about the presence of the
state_timestamp key.

Fixes #44294

(cherry picked from commit d7332ab)

Co-authored-by: Andrew Kroh <[email protected]>

Author: mergify[bot]

CHANGELOG.next.asciidoc

@@ -137,6 +137,7 @@ otherwise no tag is added. {issue}42208[42208] {pull}42403[42403]
 - hasher: Add a cached hasher for upcoming backend. {pull}41952[41952]
 - Split common tty definitions. {pull}42004[42004]
 - Fix potential data loss in add_session_metadata. {pull}42795[42795]
+- system/package: Fix an error that can occur while migrating the internal package database schema. {issue}44294[44294] {pull}44296[44296]
 
 *Auditbeat*
 

x-pack/auditbeat/module/system/package/package.go

@@ -758,11 +758,6 @@ func migrateDatastoreSchema(tx *bbolt.Tx) error {
 	log := logp.NewLogger(metricsetName)
 	log.Debugf("Migrating data from %v to %v bucket.", bucketNameV1, bucketNameV2)
 
-	var timestampGob []byte
-	if timestampGob = v1Bucket.Get([]byte(bucketKeyStateTimestamp)); len(timestampGob) == 0 {
-		return fmt.Errorf("error migrating %v data: no timestamp found", bucketNameV1)
-	}
-
 	var packages []*Package
 	if data := v1Bucket.Get([]byte(bucketKeyPackages)); len(data) > 0 {
 		dec := gob.NewDecoder(bytes.NewReader(data))
@@ -795,8 +790,11 @@ func migrateDatastoreSchema(tx *bbolt.Tx) error {
 		return fmt.Errorf("error migrating data: failed to create %v bucket: %w", bucketNameV2, err)
 	}
 
-	if err = v2Bucket.Put([]byte(bucketKeyStateTimestamp), timestampGob); err != nil {
-		return fmt.Errorf("error migrating data: failed to write %v to %v bucket: %w", bucketKeyStateTimestamp, bucketNameV2, err)
+	// Copy the gob encoded state timestamp from the v1 bucket to the v2 bucket.
+	if timestampGob := v1Bucket.Get([]byte(bucketKeyStateTimestamp)); timestampGob != nil {
+		if err = v2Bucket.Put([]byte(bucketKeyStateTimestamp), timestampGob); err != nil {
+			return fmt.Errorf("error migrating data: failed to write %v to %v bucket: %w", bucketKeyStateTimestamp, bucketNameV2, err)
+		}
 	}
 
 	builder, release := fbGetBuilder()

x-pack/auditbeat/module/system/package/package_test.go

@@ -241,6 +241,28 @@ func TestPackageDatabaseMigration(t *testing.T) {
 	}
 }
 
+// TestPackageDatabaseMigrationWithEmptyPackageV1Bucket verifies that an
+// empty package.v1 bucket can be migrated to the new schema without errors.
+//
+// This is a reproduction of https://github.com/elastic/beats/issues/44294.
+func TestPackageDatabaseMigrationWithEmptyPackageV1Bucket(t *testing.T) {
+	// Create empty package.v1 bucket.
+	dbPath := filepath.Join(t.TempDir(), "beat.db")
+	ds := datastore.New(dbPath, 0o600)
+
+	bucket, err := ds.OpenBucket("package.v1")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err = bucket.Close(); err != nil {
+		t.Fatal(err)
+	}
+
+	if err := ds.Update(migrateDatastoreSchema); err != nil {
+		t.Fatal(err)
+	}
+}
+
 func copyFile(old, new string) error {
 	o, err := os.Open(old)
 	if err != nil {