AU: Try delta updates first, then full updates

src/platform/update_engine/delta_diff_generator.h

Index: src/platform/update_engine/delta_diff_generator.h
diff --git a/src/platform/update_engine/delta_diff_generator.h b/src/platform/update_engine/delta_diff_generator.h
index 2c355a42c5f941fb0822729fbbf7a15f15852113..5d422c34f424523d0e712624f2c86963303ef22a 100644
--- a/src/platform/update_engine/delta_diff_generator.h
+++ b/src/platform/update_engine/delta_diff_generator.h
@@ -7,6 +7,7 @@
 
 #include <sys/types.h>
 #include <sys/stat.h>
+#include <set>
 #include <string>
 #include <vector>
 #include "base/basictypes.h"
@@ -32,14 +33,19 @@ class DeltaDiffGenerator {
   // Takes a DeltaArchiveManifest as given from EncodeMetadataToProtoBuffer(),
   // fill in the missing fields (DeltaArchiveManifest_File_data_*), and
   // write the full delta out to the output file.
+  // Any paths in nondiff_paths will be included in full, rather than
+  // as a diff. This is useful for files that change during postinstall, since
+  // future updates can't depend on them having remaining unchanged.
   // Returns true on success.
   // If non-empty, the device at force_compress_dev_path will be compressed.
-  static bool EncodeDataToDeltaFile(DeltaArchiveManifest* archive,
-                                    const std::string& old_path,
-                                    const std::string& new_path,
-                                    const std::string& out_file,
-                                    const std::string& force_compress_dev_path);
-
+  static bool EncodeDataToDeltaFile(
+      DeltaArchiveManifest* archive,
+      const std::string& old_path,
+      const std::string& new_path,
+      const std::string& out_file,
+      const std::set<std::string>& nondiff_paths,
+      const std::string& force_compress_dev_path);
+                                    
  private:
   // These functions encode all the data about a file that's not already
   // stored in the DeltaArchiveManifest message into the vector 'out'.
@@ -58,9 +64,15 @@ class DeltaDiffGenerator {
   static bool EncodeFile(const std::string& old_dir,
                          const std::string& new_dir,
                          const std::string& file_name,
+                         const bool avoid_diff,
                          DeltaArchiveManifest_File_DataFormat* out_data_format,
-                         std::vector<char>* out);
+                         std::vector<char>* out,
+                         bool* no_change);
 
+  // nondiff_paths is passed in to EncodeDataToDeltaFile() with
+  // paths relative to the installed system (e.g. /etc/fstab), but
+  // WriteFileDiffsToDeltaFile requires always_full_target_paths to be
+  // the entire path of the new file.
   // If non-empty, the device at force_compress_dev_path will be compressed.
   static bool WriteFileDiffsToDeltaFile(
       DeltaArchiveManifest* archive,
@@ -70,6 +82,7 @@ class DeltaDiffGenerator {
       const std::string& new_path,
       FileWriter* out_file_writer,
       int* out_file_length,
+      std::set<std::string> always_full_target_paths,
       const std::string& force_compress_dev_path);
 
   // This should never be constructed