Many warnings reading empty in-memory file

[jimav]

If a read-write (open mode "+>") in-memory file is created and immediately read, many warnings appear.

Steps to reproduce

#!/usr/bin/perl
use strict; use warnings; use v5.20;

open XX, "+>", "/tmp/testfile" or die $!;
seek XX,0,0 or die $!;
my @XXbuf = <XX>;  # no warnings (as expected)
say "Read (@XXbuf) from XX";

open YY, "+>", \(my $string) or die $!;
#print YY "stuff";
seek YY,0,0 or die $!;
my @YYbuf = <YY>;  # ******* MANY "Use of uninitialized value in <HANDLE>"
say "Read (@YYbuf) from YY";

Expected behavior
Expecting a silent EOF indicator, like a regular file.

P.S. truncate does not seem to work on in-memory files, but over-writing does.

Perl configuration

Summary of my perl6 (revision 5 version 30 subversion 0) configuration:
  
  Platform:
    osname=linux
    osvers=4.19.0
    archname=x86_64-linux-gnu-thread-multi
    uname='linux localhost 4.19.0 #1 smp debian 4.19.0 x86_64 gnulinux '
    config_args='-Dmksymlinks -Dusethreads -Duselargefiles -Dcc=x86_64-linux-gnu-gcc -Dcpp=x86_64-linux-gnu-cpp -Dld=x86_64-linux-gnu-gcc -Dccflags=-DDEBIAN -Wdate-time -D_FORTIFY_SOURCE=2 -g -O2 -fdebug-prefix-map=/build/perl-Wfb2Cd/perl-5.30.0=. -fstack-protector-strong -Wformat -Werror=format-security -Dldflags= -Wl,-Bsymbolic-functions -Wl,-z,relro -Dlddlflags=-shared -Wl,-Bsymbolic-functions -Wl,-z,relro -Dcccdlflags=-fPIC -Darchname=x86_64-linux-gnu -Dprefix=/usr -Dprivlib=/usr/share/perl/5.30 -Darchlib=/usr/lib/x86_64-linux-gnu/perl/5.30 -Dvendorprefix=/usr -Dvendorlib=/usr/share/perl5 -Dvendorarch=/usr/lib/x86_64-linux-gnu/perl5/5.30 -Dsiteprefix=/usr/local -Dsitelib=/usr/local/share/perl/5.30.0 -Dsitearch=/usr/local/lib/x86_64-linux-gnu/perl/5.30.0 -Dman1dir=/usr/share/man/man1 -Dman3dir=/usr/share/man/man3 -Dsiteman1dir=/usr/local/man/man1 -Dsiteman3dir=/usr/local/man/man3 -Duse64bitint -Dman1ext=1 -Dman3ext=3perl -Dpager=/usr/bin/sensible-pager -Uafs -Ud_csh -Ud_ualarm -Uusesfio -Uusenm -Ui_libutil -Ui_xlocale -Uversiononly -DDEBUGGING=-g -Doptimize=-O2 -dEs -Duseshrplib -Dlibperl=libperl.so.5.30.0'
    hint=recommended
    useposix=true
    d_sigaction=define
    useithreads=define
    usemultiplicity=define
    use64bitint=define
    use64bitall=define
    uselongdouble=undef
    usemymalloc=n
    default_inc_excludes_dot=define
    bincompat5005=undef
  Compiler:
    cc='x86_64-linux-gnu-gcc'
    ccflags ='-D_REENTRANT -D_GNU_SOURCE -DDEBIAN -fwrapv -fno-strict-aliasing -pipe -I/usr/local/include -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64'
    optimize='-O2 -g'
    cppflags='-D_REENTRANT -D_GNU_SOURCE -DDEBIAN -fwrapv -fno-strict-aliasing -pipe -I/usr/local/include'
    ccversion=''
    gccversion='9.3.0'
    gccosandvers=''
    intsize=4
    longsize=8
    ptrsize=8
    doublesize=8
    byteorder=12345678
    doublekind=3
    d_longlong=define
    longlongsize=8
"/tmp/j7" 164L, 10427C                                     1,19          Top

[Leont]

If a read-write (open mode "+>") in-memory file is created and immediately read, many warnings appear.

Yeah, possibly PerlIO::scalar should convert an undefined value to an empty string.

P.S. truncate does not seem to work on in-memory files, but over-writing does.

That is an entirely separate issue. I'm not sure it should, our abstraction kind of breaks there.

[Grinnz]

In memory files are required to be byte strings. You'll also get issues if you try to open it to a variable that contains wide characters, or is a reference. Maybe coercion is justified in this specific undef case.

[jimav]

There's nothing in the string. Wide or octet.

[Grinnz]

Indeed, the problem is it is not a string. You get the same warnings for trying to print undef to a handle.

[Grinnz]

Somewhat poor example, sorry - basically what happened here is you told it to treat a scalar as a file, and that scalar's contents are undef, but it's impossible for a file's contents to be undef, so the abstraction doesn't sanely handle it.

[Grinnz]

Even more clearly: undef (the default state of a scalar value) and empty string are not the same, and you can avoid this by doing instead: open my $fh, "+>", \(my $string = '')

[jimav]

Okay, I understand. As you say open FH "+>", \(my $string="") makes it work as intended, without warnings.

I am happy if this ticket is closed as user error. Thank you for taking the time to explain.

[jkeenan]

Okay, I understand. As you say open FH "+>", \(my $string="") makes it work as intended, without warnings.

I am happy if this ticket is closed as user error. Thank you for taking the time to explain.

Closed per OP consent.

[demerphq]

Hi James I am going to reopen this and assign it to myself. I think this is a terrible user experience and to me it violates autovivification expectations. I think if you use an undef var in this context it should autoviv to "". We can close it again if the community doesn't agree once I have a patch.