stacked filetest ops stat unexpected “filename containing newline”

[mgkuhn]

Module: core

Description

The following MWE script unexpectedly outputs a warning about an “unsuccessful stat on filename containing newline”:

#!/usr/bin/perl
use 5.014;
use warnings;
use HTTP::Tiny;

HTTP::Tiny->new->head("https://www.perl.org/");
if ('a' =~ /^(.)$/) {
    if (-f -r $1) {
        say "a";
    }
}

A workaround is to replace -f -r $1 with -r $1 && -f _. This is in contradiction to the documentation in man perlfunc, which states that the former is just “a form of purely syntactic sugar” for the latter.

Steps to Reproduce

Run the above script on Ubuntu Linux (e.g., 20.04).

Actual behavior

$ ./bug.pl 
Unsuccessful stat on filename containing newline at ./bug.pl line 8.

Expected behavior

$ ./bug.pl 
$ touch a
$ ./bug.pl 
a

Perl should try to stat the filename a instead of any “filename containing newline”,
and output a if and only if file a exists and is readable.

Perl configuration

I encountered this behaviour on both Perl 5.30.0 as included in Ubuntu Linux 20.04 (x86_64), as well as Perl 5.22.1 as included in Ubuntu Linux 16.04 (x86_64).

[hvds]

The use of HTTP::Tiny appears to be a partial red herring; without it I see:

% ${PERL} -wle 'if ("a" =~ /^(.)$/) { if (-f -r $1) { print "ok" } }'
Use of uninitialized value $1 in -r at -e line 1.
% ${PERL} -wle 'if ("a" =~ /^(.)$/) { print "-$1-"; if (-f -r $1) { print "ok" } }'
-a-
% touch a ; ${PERL} -wle 'if ("a" =~ /^(.)$/) { print "-$1-"; if (-f -r $1) { print "ok" } }'
-a-
ok
% 

.. so somehow -f -r $1 is picking up something wrong for $1 if it hasn't been read first.

[hvds]

Hmm, it correctly reaches op.c:S_try_amagic_ftest, but in:

    if (!(PL_op->op_private & OPpFT_STACKING)) SvGETMAGIC(arg);

the first stacked opcode has OPpFT_STACKING, so we don't perform the getmagic.

We need a test that will be true on the first of a stack of filetests, and false on the rest; however this test will be true on the last of a stack, and false on the rest. Not sure how best to fix that.