check backup directory exists before listing children

Summary:
InsertPathnameToSizeBytes() is called on shared/ and shared_checksum/ directories, which only exist for certain configurations. If we try to list a non-existent directory's contents, some Envs will dump an error message. Let's avoid this by checking whether the directory exists before listing its contents.
Closes https://github.com/facebook/rocksdb/pull/1895

Differential Revision: D4596301

Pulled By: ajkr

fbshipit-source-id: c809679

utilities/backupable/backupable_db.cc

@@ -1416,16 +1416,19 @@ Status BackupEngineImpl::InsertPathnameToSizeBytes(
     std::unordered_map<std::string, uint64_t>* result) {
   assert(result != nullptr);
   std::vector<Env::FileAttributes> files_attrs;
-  Status status = env->GetChildrenFileAttributes(dir, &files_attrs);
-  if (!status.ok()) {
-    return status;
+  Status status = env->FileExists(dir);
+  if (status.ok()) {
+    status = env->GetChildrenFileAttributes(dir, &files_attrs);
+  } else if (status.IsNotFound()) {
+    // Insert no entries can be considered success
+    status = Status::OK();
   }
   const bool slash_needed = dir.empty() || dir.back() != '/';
   for (const auto& file_attrs : files_attrs) {
     result->emplace(dir + (slash_needed ? "/" : "") + file_attrs.name,
                     file_attrs.size_bytes);
   }
-  return Status::OK();
+  return status;
 }
 
 Status BackupEngineImpl::GarbageCollect() {