mapfile and unset: Does this actually work?

[Bart Schaefer <schaefer@brasslantern.com>]

Doc says:

mapfile
     This associative array takes as keys the names of files; the
     resulting value is the content of the file.  The value is treated
     identically to any other text coming from a parameter.  The value
     may also be assigned to, in which case the file in question is
     written (whether or not it originally existed); or an element may
     be unset, which will delete the file in question.

However:

% touch removable
% zmodload zsh/mapfile
% : $mapfile[removable] ; unset 'mapfile[removable]'
% ls -l removable
-rw-rw-r-- 1 schaefer schaefer 0 Jan 21 17:17 removable

I tried several variations of this and I can't get mapfile to remove anything.

Looking at the code:

static const struct gsu_hash mapfiles_gsu =
{ hashgetfn, setpmmapfiles, stdunsetfn };

There is an unsetpmmapfile function but it doesn't seem to be used
anywhere.  Even after recompiling with that in place of stdunsetfn
there is some strange behavior:

% zmodload zsh/mapfile
% echo 'text' > removable
% ls -l removable
-rw-rw-r-- 1 schaefer schaefer 5 Jan 21 17:42 removable
% noglob unset mapfile[removable]
% ls -l removable
ls: cannot access 'removable': No such file or directory

(so far so good, but)

% echo 'text' > removable
% () { local +h mapfile; unset 'mapfile[removable]' }
% ls -l removable
-rw-rw-r-- 1 schaefer schaefer 0 Jan 21 17:43 removable
% noglob unset mapfile[removable]
% ls -l removable
-rw-rw-r-- 1 schaefer schaefer 0 Jan 21 17:43 removable

Using "unset" on the local has somehow made that file inaccessible, it
just gets truncated instead of removed.  I think this is because:

% echo ${+mapfile[removable]}
0

The hash element for that file is still present but tagged PM_UNSET?

I started playing with this because I was concerned that having a
local mapfile go out of scope would cause files to be removed, but
even with unsetpmmapfile that fortunately doesn't occur.