add stale_file_handle error category

Summary:
I want to test for this condition in a later diff.

(Note: this ignores all push blocking failures!)

Reviewed By: chadaustin, strager

Differential Revision: D9652042

fbshipit-source-id: 919fc592f940bb20693d98b6fa074a4f3cd50932

error_category.cpp

@@ -79,6 +79,11 @@ bool error_category::equivalent(const std::error_code& code, int condition)
     case error_code::is_a_directory:
       return code == make_error_code(std::errc::is_a_directory);
 
+#ifdef ESTALE
+    case error_code::stale_file_handle:
+      return code == std::error_code(ESTALE, std::generic_category());
+#endif
+
     case error_code::too_many_symbolic_link_levels:
       // POSIX says open with O_NOFOLLOW should set errno to ELOOP if the path
       // is a symlink. However, FreeBSD (which ironically originated O_NOFOLLOW)

watchman_error_category.h

@@ -24,6 +24,7 @@ enum class error_code {
   timed_out,
   not_a_symlink,
   is_a_directory,
+  stale_file_handle,
 };
 
 // An error category implementation that is present for comparison purposes