AU: Don't send no-op operations in the delta payload.

delta_performer.cc

 // Copyright (c) 2010 The Chromium OS Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "update_engine/delta_performer.h"
 
 #include <endian.h>
 #include <errno.h>
 
 #include <algorithm>
@@ skipping 101 matching lines @@
 
 }  // namespace {}
 
 // Returns true if |op| is idempotent -- i.e., if we can interrupt it and repeat
 // it safely. Returns false otherwise.
 bool DeltaPerformer::IsIdempotentOperation(
     const DeltaArchiveManifest_InstallOperation& op) {
   if (op.src_extents_size() == 0) {
     return true;
   }
-  // TODO(adlr): detect other types of idempotent operations. For example,
-  // a MOVE may move a block onto itself.
-
-  // When in doubt, it's safe to declare an op non-idempotent.
+  // When in doubt, it's safe to declare an op non-idempotent. Note that we
+  // could detect other types of idempotent operations here such as a MOVE that
+  // moves blocks onto themselves. However, we rely on the server to not send
+  // such operations at all.
   ExtentRanges src_ranges;
   src_ranges.AddRepeatedExtents(op.src_extents());
   const uint64_t block_count = src_ranges.blocks();
   src_ranges.SubtractRepeatedExtents(op.dst_extents());
   return block_count == src_ranges.blocks();
 }
 
 int DeltaPerformer::Open(const char* path, int flags, mode_t mode) {
   int err;
   if (OpenFile(path, &fd_, &err))
@@ skipping 580 matching lines @@
   if (prefs_->GetInt64(kPrefsResumedUpdateFailures, &resumed_update_failures)) {
     resumed_update_failures++;
   } else {
     resumed_update_failures = 1;
   }
   prefs_->SetInt64(kPrefsResumedUpdateFailures, resumed_update_failures);
   return true;
 }
 
 }  // namespace chromeos_update_engine