'comint-truncate-buffer + shell-resync-dirs = hang

[jrwats]

There is this odd bug that occurs when I'm running in an inferior
shell within emacs with 'comint-trucate-buffer added as to 'comint-
output-filter-functions, run a command that spits out enough input to
cause a buffer truncate, and then finally call M-x dirs (or M-x shell-
resync-dirs).  The command stays in the while loop defined in
shell.el:

(defun shell-resync-dirs ()
   ;;code...
   (while (not (looking-at regexp))
   ;;code...
)

This only happens when I run the shell with comint-truncate-buffer and
run enough commands such that the output is actually truncated.