[Sync] Erase sync DB when corrupted

sync/test/directory_backing_store_corruption_testing.cc

Index: sync/test/directory_backing_store_corruption_testing.cc
diff --git a/sync/test/directory_backing_store_corruption_testing.cc b/sync/test/directory_backing_store_corruption_testing.cc
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+++ b/sync/test/directory_backing_store_corruption_testing.cc
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+// Copyright 2015 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/test/directory_backing_store_corruption_testing.h"
+
+#include "base/files/file_util.h"
+#include "base/files/scoped_file.h"
+
+namespace syncer {
+namespace syncable {
+namespace corruption_testing {
+
+// This value needs to be large enough to force the underlying DB to be read
+// from disk before writing. The value *may* depend on the underlying DB page
+// size as well as the DB's cache_size PRAGMA.
+const int kNumEntriesRequiredForCorruption = 4000;
+
+bool CorruptDatabase(const base::FilePath& backing_file_path) {
+  // Corrupt the DB by write a bunch of zeros at the beginning.
+  //
+  // Because the file may already open for writing, it's important that we open
+  // it in a sharing-compatible way for platforms that have the concept of
+  // shared/exclusive file access (e.g. Windows).
+  base::ScopedFILE db_file(base::OpenFile(backing_file_path, "wb"));
+  if (!db_file.get())
+    return false;
+  const std::string zeros(4096, '\0');
+  const int num_written = fwrite(zeros.data(), zeros.size(), 1, db_file.get());
+  return num_written == 1U;
+}
+
+}  // namespace corruption_util
+}  // namespace syncable
+}  // namespace syncer