Use of uninitialized value $this in <HANDLE>

[toddr]

Transferred from RT - https://rt.cpan.org/Ticket/Display.html?id=125436

IO::File started throwing warnings on getline(), if there wasn't writing to file before.

$ perl5.18.2 -Mwarnings -MIO::File -e 'my $h; my $f=IO::File->new(\$h,"+>:"); $f->getline();'
$ 
$ perl5.24.3 -Mwarnings -MIO::File -e 'my $h; my $f=IO::File->new(\$h,"+>:"); $f->getline();'
Use of uninitialized value $this in <HANDLE> at -e line 1.
Use of uninitialized value $this in <HANDLE> at -e line 1.
Use of uninitialized value $this in <HANDLE> at -e line 1.
Use of uninitialized value $this in <HANDLE> at -e line 1.
Use of uninitialized value $this in <HANDLE> at -e line 1.
Use of uninitialized value $this in <HANDLE> at -e line 1.
Use of uninitialized value $this in <HANDLE> at -e line 1.
$ 
$ perl5.24.3 -Mwarnings -MIO::File -e 'my $h; my $f=IO::File->new(\$h,"+>:"); $f->write("a"); $f->getline();'
$

Commenting this line fixing the warning https://metacpan.org/source/TODDR/IO-1.39/lib/IO/Handle.pm#L435

[themage]

After a bit of time trying to reproduce it, I think in the original report there is a backslash () missing in IO::File->new:

$ perl5.24.3 -Mwarnings -MIO::File -e 'my $h; my $f=IO::File->new(\$h,"+>:"); $f->write("a"); $f->getline();'

[toddr]

markdown magic. I added quoting and now it reads right.

[tonycoz]

I see warnings even in 5.18.2, just not as many:

$ ~/perl/5.18.2/bin/perl -Mwarnings -MIO::File -e 'my $h; open my $f, "+>", \$h; $f->getline();'
Use of uninitialized value $this in <HANDLE> at -e line 1.
Use of uninitialized value $this in <HANDLE> at -e line 1.
Use of uninitialized value $this in <HANDLE> at -e line 1.

It doesn't happen in 5.14:

$ ~/perl/5.14.2-debug/bin/perl -Mwarnings -MIO::File -e 'my $h; open my $f, "+>", \$h; $f->getline();'

I expect this was introduced in 986a805 which made getline() act as if it was in the caller's scope, and the re-work for that in 986a805 hasn't changed that.

The warnings are generated when the various hooks PerlIO::scalar are called by the implementation of sv_gets() (called by do_readline(), called by pp_readline), those hooks have no way to tell when that readline operation starts and ends so we can't really limit the number of warnings produced to one per readline().

The warning could be considered reasonable, but I think it's way over punishing to report it many times per operation and reporting it on a readline() (aka getline()) could be confusing, since the handle might be passed in from some other code.

So I think PerlIO::scalar should be modified to prevent those warnings.

It might be reasonable to have warning on open, but the PerlIOScalar_pushed() handler carefully avoids warning on an undef SV and it probably not worthwhile changing that.

One other issue from reviewing the code while checking out the warning sources - the various functions PerlIOScalar_get_base(), PerlIOScalar_get_ptr(), PerlIOScalar_get_cnt), PerlIOScalar_set_ptrcnt() all directly or indirectly call get magic, if the underlying magic ends up returning different SVs those values will end up being inconsistent at best.

PerlIOScalar_read() avoids this for the read operation, but that doesn't help for sv_gets() or anything else that peeks at the buffer.

Unfortunately I don't see a way to make that safe in the general case.