sync: Improve handling of bad UniquePos (retry)

sync/syncable/directory_backing_store.cc

 // Copyright 2012 The Chromium 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 "sync/syncable/directory_backing_store.h"
 
 #include "build/build_config.h"
 
 #include <limits>
 
@@ skipping 105 matching lines @@
       return scoped_ptr<EntryKernel>();
     }
 
     kernel->mutable_ref(static_cast<UniquePositionField>(i)) =
         UniquePosition::FromProto(proto);
   }
   for (; i < ATTACHMENT_METADATA_FIELDS_END; ++i) {
     kernel->mutable_ref(static_cast<AttachmentMetadataField>(i)).ParseFromArray(
         statement->ColumnBlob(i), statement->ColumnByteLength(i));
   }
+
+  // Sanity check on positions.  We risk strange and rare crashes if our
+  // assumptions about unique position values are broken.
+  if (kernel->ShouldMaintainPosition() &&
+      !kernel->ref(UNIQUE_POSITION).IsValid()) {
+    DVLOG(1) << "Unpacked invalid position on an entity that should have a "
+             << "valid position.  Assuming the DB is corrupt.";
+    return scoped_ptr<EntryKernel>();
+  }
+
   return kernel.Pass();
 }
 
 namespace {
 
 string ComposeCreateTableColumnSpecs() {
   const ColumnSpec* begin = g_metas_columns;
   const ColumnSpec* end = g_metas_columns + arraysize(g_metas_columns);
   string query;
   query.reserve(kUpdateStatementBufferSize);
@@ skipping 1422 matching lines @@
   }
   query.append(" ) ");
   values.append(" )");
   query.append(values);
   save_statement->Assign(db_->GetUniqueStatement(
       base::StringPrintf(query.c_str(), "metas").c_str()));
 }
 
 }  // namespace syncable
 }  // namespace syncer