Get rid of ‘Not a format reference’

Get rid of ‘Not a format reference’

This commit:

commit 2dd78f96d61cc6382dc72214930c993567209597
Author: Jarkko Hietaniemi <jhi@iki.fi>
Date:   Sun Aug 6 01:33:55 2000 +0000

    Continue fixing the io warnings.  This also
    sort of fixes bug ID 20000802.003: the core dump
    is no more.  Whether the current behaviour is correct
    (giving a warning: "Not a format reference"), is another matter.

    p4raw-id: //depot/perl@6531

added a check to see whether the format GV’s name is null, and, if
so, it dies with ‘Not a format reference’.  Before that, that message
occurred only for lack of a GV.

The bug mentioned is now #3617, involving write(*STDOUT{IO}).  write
puts an implicit *{} around its argument.

*{$io} has historically been very buggy in its stringification, so
this patch seems to have been working around that bugginess, by fall-
ing back to the ‘Not a format reference’ error if the name couldn’t be
determined for ‘Undefined format "foo" called’.

*{$io} was fixed once and for all in 5.16.  It now stringifies as
*foopackage::__ANONIO__.

I don’t think producing a completetly different error based on the
name of the GV (whether it’s "foo" or "") is correct at all.  And the
patch that made it happen was just a fix for a crash that can’t hap-
pen any more.

So the only case that should produce ‘Not a format reference’ is that
in which there is no format GV (fgv).

I can prove that fgv is always set (see below), and has been at least
since 5.000, so that ‘Not a format reference’ actually could never
occur before 2dd78f96d61c.  (Actually, XS code could set PL_defoutgv
to null until the previous commit, but everything would start crashing
as a result, so it has never been done in practice.)

gv_efullname4 always returns a name, so checking SvPOK(tmpsv) is
redundant; checking whether the string buffer begins with a non-null
char is not even correct, as "\0foo" fails that test.

Proof that fgv is always set:

The current (prior to this commit) code in pp_enterwrite is like this:

    if (MAXARG == 0) {
	gv = PL_defoutgv;
	EXTEND(SP, 1);
    }
    else {
	gv = MUTABLE_GV(POPs);
	if (!gv)
	    gv = PL_defoutgv;
    }

If the stack value is null (which actually can’t happen), PL_defoutgv
is used.  PL_defoutgv can’t be null.

At this point, gv is set to something non-null.

    io = GvIO(gv);
    if (!io) {
	RETPUSHNO;
    }

Here we only set fgv to IoFMT_GV(io) if it is non-null.  Otherwise we
use gv, which we know is non-null.

    if (IoFMT_GV(io))
	fgv = IoFMT_GV(io);
    else
	fgv = gv;