Every time a user tries to read a file, the driver calls valid_read in
the master object to check if the read should be allowed. The argu‐
ments are the filename, the name of the person making the read, and the
calling function name. If valid_read returns non-zero, the read is
allowed.
This apply is also consulted in the middle of compiling an object: each
#include directive checks the header path with valid_read (with <func>
set to "include") before opening it. Returning 0 denies the include and
fails that compile. Because the apply runs mid-compile, it must not
trigger another compile itself (e.g. by loading an object). A runtime
error thrown from valid_read (or anywhere else while a file is being
compiled) aborts that compilation cleanly; the driver fully unwinds its
compiler state and remains able to load other objects afterwards.