sync: Improve handling of bad UniquePos (retry)

sync/internal_api/public/base/unique_position.cc

Index: sync/internal_api/public/base/unique_position.cc
diff --git a/sync/internal_api/public/base/unique_position.cc b/sync/internal_api/public/base/unique_position.cc
index 40bab6e175d1de51ede96aa020a4667158140d42..2d41614999c332fc81d371d2a2f6d0d4ae5fa9bf 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());