sync: Improve handling of bad UniquePositions

Makes the client assign a valid position to incoming bookmarks if the
server has not populated the required fields.  This code should never be
triggered unless there is a bug in the server.  This risks reordering
users' bookmarks, but that's probably preferable to a crash.

Detects bookmarks that do not have valid position information during
database load.  If these corrupted bookmarks are detected, the entire
database is declared to be corrupt.  Sync will then re-download
all of the user's data, which should fix the problem.

BUG=367247

Review URL: https://codereview.chromium.org/278153002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@270308 0039d316-1c4b-4281-b951-d872f2087c98

2 files changed, 14 insertions, 1 deletions

diff --git a/sync/internal_api/public/base/unique_position.cc b/sync/internal_api/public/base/unique_position.cc
index 40bab6e..2d41614 100644
--- a/sync/internal_api/public/base/unique_position.cc
+++ b/sync/internal_api/public/base/unique_position.cc
@@ -6,6 +6,7 @@
 
 #include "base/basictypes.h"
 #include "base/logging.h"
+#include "base/rand_util.h"
 #include "base/stl_util.h"
 #include "base/strings/string_number_conversions.h"
 #include "sync/protocol/unique_position.pb.h"
@@ -21,7 +22,8 @@ bool UniquePosition::IsValidSuffix(const std::string& suffix) {
   // The suffix must be exactly the specified length, otherwise unique suffixes
   // are not sufficient to guarantee unique positions (because prefix + suffix
   // == p + refixsuffix).
-  return suffix.length() == kSuffixLength;
+  return suffix.length() == kSuffixLength
+      && suffix[kSuffixLength-1] != 0;
 }
 
 // static.
@@ -36,6 +38,13 @@ bool UniquePosition::IsValidBytes(const std::string& bytes) {
 }
 
 // static.
+std::string UniquePosition::RandomSuffix() {
+  // Users random data for all but the last byte.  The last byte must not be
+  // zero.  We arbitrarily set it to 0x7f.
+  return base::RandBytesAsString(kSuffixLength - 1) + "\x7f";
+}
+
+// static.
 UniquePosition UniquePosition::CreateInvalid() {
   UniquePosition pos;
   DCHECK(!pos.IsValid());
diff --git a/sync/internal_api/public/base/unique_position.h b/sync/internal_api/public/base/unique_position.h
index eee5324..b844b82 100644
--- a/sync/internal_api/public/base/unique_position.h
+++ b/sync/internal_api/public/base/unique_position.h
@@ -46,6 +46,10 @@ class SYNC_EXPORT_PRIVATE UniquePosition {
   static bool IsValidSuffix(const std::string& suffix);
   static bool IsValidBytes(const std::string& bytes);
 
+  // Returns a valid, but mostly random suffix.
+  // Avoid using this; it can lead to inconsistent sort orderings if misused.
+  static std::string RandomSuffix();
+
   // Returns an invalid position.
   static UniquePosition CreateInvalid();