What are the steps to reproduce the error?

Curious if this is T9043.

From the IRC @leonpalm had indicated that

The issue itself was quickly resolved. Was a file permissions problem.

Sounded like the location of the arcanist/libphutil may not have been writable by the user running arc upgrade

Yes, correct. Calling arc upgrade with sudo succeeded the second time around.

This error handling block is incorrect:

try {
  phutil_passthru('git pull --rebase');
} catch (Exception $ex) {
  phutil_passthru('git rebase --abort');
  throw $ex;
}

phutil_passthru() returns the error code (like exec_manual()); it does not throw on failure (like execx()).

It looks like I wrote this wrong in D2435. The use in the try assumes it throws while the use in the catch assumes it does not, so I think I just did this wrong.

Lol nice.