AU: Add support for signing of update payloads after they're generated.

delta_performer.h

Index: delta_performer.h
diff --git a/delta_performer.h b/delta_performer.h
index 8a9914d4c77627f612c4ed00f0cb4c351feb2f23..c693333a1ddcd71842027457d1187a2fa2a19e91 100644
--- a/delta_performer.h
+++ b/delta_performer.h
@@ -25,6 +25,12 @@ class PrefsInterface;
 
 class DeltaPerformer : public FileWriter {
  public:
+  enum MetadataParseResult {
+    kMetadataParseSuccess,
+    kMetadataParseError,
+    kMetadataParseInsufficientData,
+  };
+
   DeltaPerformer(PrefsInterface* prefs)
       : prefs_(prefs),
         fd_(-1),
@@ -100,6 +106,17 @@ class DeltaPerformer : public FileWriter {
   // success, false otherwise.
   static bool ResetUpdateProgress(PrefsInterface* prefs, bool quick);
 
+  // Attempts to parse the update metadata starting from the beginning of
+  // |payload| into |manifest|. On success, sets |metadata_size| to the total
+  // metadata bytes (including the delta magic and metadata size fields), and
+  // returns kMetadataParseSuccess. Returns kMetadataParseInsufficientData if
+  // more data is needed to parse the complete metadata. Returns
+  // kMetadataParseError if the metadata can't be parsed given the payload.
+  static MetadataParseResult ParsePayloadMetadata(
+      const std::vector<char>& payload,
+      DeltaArchiveManifest* manifest,
+      uint64_t* metadata_size);
+
   void set_current_kernel_hash(const std::vector<char>& hash) {
     current_kernel_hash_ = hash;
   }