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[perl #57288] PerlIO::via::QuotedPrint not flushing

# New Ticket Created by  Kevin Ryde 
# Please include the string:  [perl #57288]
# in the subject line of all future correspondence about this issue. 
# <URL: https://rt.perl.org/rt3/Ticket/Display.html?id=57288 >
When a PerlIO::via::QuotedPrint layer is pushed on an opened file,
autoflush or an explicit ->flush() doesn't send output to the file
immediately.
The program qp.pl below shows the problem.  I hoped the OUT->flush would
make the "hello" appear in /tmp/foo immediately, but that doesn't
happen, not until after the program exits after the sleep.
I guess QuotedPrint doesn't supply a FLUSH routine, and the default for
PerlIO::via in that case is to do nothing, so a top-level flush doesn't
reach the :perlio+:unix full-buffered output layer below.
Several other PerlIO::via output modules on cpan behave the same,
I guess having used QuotedPrint as a model.  I wonder if either
- The PerlIO::via docs could advise every output layer to supply a FLUSH
  function calling $fh->flush on the sublayer.
- PerlIO::via could make such a "propagated" flush the default if you
  don't supply a FLUSH routine.
The latter could make life easier for straightforward non-buffering
transformer layers.  The only question would be whether anyone has
omitted a FLUSH func deliberately wanting not to call flush on the
sublayer.  That'd seem a bit unlikely to me, or only applicable to
something that was designed to be at the bottom of the stack anyway
(an output capture of some sort say) ...
---
Flags:
    category=library
    severity=wishlist
---
Site configuration information for perl 5.10.0:
Configured by Debian Project at Thu May  8 13:32:42 UTC 2008.
Summary of my perl5 (revision 5 version 10 subversion 0) configuration:
  Platform:
    osname=linux, osvers=2.6.24.4, archname=i486-linux-gnu-thread-multi
    uname='linux ninsei 2.6.24.4 #1 smp preempt fri apr 18 15:36:09 pdt 2008 i686 gnulinux '
    config_args='-Dusethreads -Duselargefiles -Dccflags=-DDEBIAN -Dcccdlflags=-fPIC -Darchname=i486-linux-gnu -Dprefix=/usr -Dprivlib=/usr/share/perl/5.10 -Darchlib=/usr/lib/perl/5.10 -Dvendorprefix=/usr -Dvendorlib=/usr/share/perl5 -Dvendorarch=/usr/lib/perl5 -Dsiteprefix=/usr/local -Dsitelib=/usr/local/share/perl/5.10.0 -Dsitearch=/usr/local/lib/perl/5.10.0 -Dman1dir=/usr/share/man/man1 -Dman3dir=/usr/share/man/man3 -Dsiteman1dir=/usr/local/man/man1 -Dsiteman3dir=/usr/local/man/man3 -Dman1ext=1 -Dman3ext=3perl -Dpager=/usr/bin/sensible-pager -Uafs -Ud_csh -Ud_ualarm -Uusesfio -Uusenm -DDEBUGGING=-g -Doptimize=-O2 -Duseshrplib -Dlibperl=libperl.so.5.10.0 -Dd_dosuid -des'
    hint=recommended, useposix=true, d_sigaction=define
    useithreads=define, usemultiplicity=define
    useperlio=define, d_sfio=undef, uselargefiles=define, usesocks=undef
    use64bitint=undef, use64bitall=undef, uselongdouble=undef
    usemymalloc=n, bincompat5005=undef
  Compiler:
    cc='cc', ccflags ='-D_REENTRANT -D_GNU_SOURCE -DDEBIAN -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 -fno-strict-aliasing -pipe -I/usr/local/include'
    ccversion='', gccversion='4.2.3 (Debian 4.2.3-5)', gccosandvers=''
    intsize=4, longsize=4, ptrsize=4, doublesize=8, byteorder=1234
    d_longlong=define, longlongsize=8, d_longdbl=define, longdblsize=12
    ivtype='long', ivsize=4, nvtype='double', nvsize=8, Off_t='off_t', lseeksize=8
    alignbytes=4, prototype=define
  Linker and Libraries:
    ld='cc', ldflags =' -L/usr/local/lib'
    libpth=/usr/local/lib /lib /usr/lib /usr/lib64
    libs=-lgdbm -lgdbm_compat -ldb -ldl -lm -lpthread -lc -lcrypt
    perllibs=-ldl -lm -lpthread -lc -lcrypt
    libc=/lib/libc-2.7.so, so=so, useshrplib=true, libperl=libperl.so.5.10.0
    gnulibc_version='2.7'
  Dynamic Linking:
    dlsrc=dl_dlopen.xs, dlext=so, d_dlsymun=undef, ccdlflags='-Wl,-E'
    cccdlflags='-fPIC', lddlflags='-shared -O2 -g -L/usr/local/lib'

• [perl #57288] PerlIO::via::QuotedPrint not flushing by Kevin Ryde