Bug #1734702 “perldoc shows lots of escapes” : Bugs : perl package : Ubuntu

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janl (janl) wrote on 2017-11-27:

#1

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Launchpad Janitor (janitor) wrote on 2018-01-08:

#2

Status changed to 'Confirmed' because the bug affects multiple users.

Changed in perl (Ubuntu):
status:	New → Confirmed

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Eliah Kagan (degeneracypressure) wrote on 2018-01-08:

#3

Text displayed in the terminal when I ran "perldoc -D perlsyn" Edit (906 bytes, text/plain)

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This bug affects my Lubuntu 17.10 (64-bit) system, which has version 5.26.0-8ubuntu1 of the perl and perl-doc packages. I noticed the bug immediately upon checking for it after Zanna (https://launchpad.net/~rose-anna-bleasdale / https://askubuntu.com/users/527764/zanna) reported experiencing it in Ubuntu MATE 17.10. Until this bug is fixed, I suggest the workaround of setting the PAGER environment variable to "less" so that the proper configuration for less is used; other full workarounds are also effective (see below).

The problem does not appear to depend in any way on what sort of terminal emulator is used. I tried in LXTerminal (TERM=xterm), XTerm (TERM=xterm), a virtual console (TERM=linux), and via SSH (TERM=xterm). Zanna tried in MATE Terminal (TERM=xterm), GNOME Terminal (TERM=xterm-256color), Konsole (TERM=xterm-256color), Tilda (TERM=xterm-256color), XTerm (TERM=xterm), and a virtual console (TERM=linux). See https://chat.stackexchange.com/transcript/message/42130009#42130009 and subsequent messages for details. It occurred for us both no matter what terminals we used. I believe this is the same bug as reported here.

In case it is helpful, I have attached a file showing the debugging output produced by running "perldoc -D perlsyn" on my Lubuntu 17.10 system. Note the presence of this line:

About to call Pod::Perldoc::ToTerm->pager_configuration("/usr/bin/sensible-pager")

I suspect that the problem is that the necessary configuration for less to permit terminal escape sequences--specifically, that the -R option either be passed to it or present in the LESS environment variable--is not satisfied when the pager configuration for sensible-pager is used, even when sensible-pager runs less, as it does by default in Ubuntu.

I further suspect that many, or perhaps all, 17.10 systems have this bug in their default configuration (though it would only be noticed if users install and run perldoc). This strongly resembles https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=735134. This bug may be considered a regression, in that the problem had previously occurred and was then prevented by the addition of this code in Perldoc.pm, which is present in 16.04 but removed in 17.10 (see below for details):

    # fix visible escape codes in ToTerm output
    # https://bugs.debian.org/758689
    local $ENV{LESS} = defined $ENV{LESS} ? "$ENV{LESS} -R" : "-R";

The simplest workaround for this bug that I am aware of is to pass the -t flag to perldoc. As in the workaround described in the bug report here, the resulting formatting is readable but not as fancy as usual.

A *full* workaround is also possible, and is only slightly more complicated. Telling perldoc explicitly that you want "less" by defining the PAGER environment variable works:

PAGER=less perldoc perlsyn

Otherwise, it finds sensible-pager first (see above), and appears not to place -R in LESS (see below), because it does not know that the sensible-pager shell script runs less.

The problem may also be avoided by ensuring that an environment variable with the name LESS and a value that specifies the R option is present. For example, this works:

LESS=-R perldoc perlsyn

Or thi...

This bug affects my Lubuntu 17.10 (64-bit) system, which has version 5.26.0-8ubuntu1 of the perl and perl-doc packages. I noticed the bug immediately upon checking for it after Zanna (https://launchpad.net/~rose-anna-bleasdale / https://askubuntu.com/users/527764/zanna) reported experiencing it in Ubuntu MATE 17.10. Until this bug is fixed, I suggest the workaround of setting the PAGER environment variable to "less" so that the proper configuration for less is used; other full workarounds are also effective (see below).

The problem does not appear to depend in any way on what sort of terminal emulator is used. I tried in LXTerminal (TERM=xterm), XTerm (TERM=xterm), a virtual console (TERM=linux), and via SSH (TERM=xterm). Zanna tried in MATE Terminal (TERM=xterm), GNOME Terminal (TERM=xterm-256color), Konsole (TERM=xterm-256color), Tilda (TERM=xterm-256color), XTerm (TERM=xterm), and a virtual console (TERM=linux). See https://chat.stackexchange.com/transcript/message/42130009#42130009 and subsequent messages for details. It occurred for us both no matter what terminals we used. I believe this is the same bug as reported here.

In case it is helpful, I have attached a file showing the debugging output produced by running "perldoc -D perlsyn" on my Lubuntu 17.10 system. Note the presence of this line:

About to call Pod::Perldoc::ToTerm->pager_configuration("/usr/bin/sensible-pager")

I suspect that the problem is that the necessary configuration for less to permit terminal escape sequences--specifically, that the -R option either be passed to it or present in the LESS environment variable--is not satisfied when the pager configuration for sensible-pager is used, even when sensible-pager runs less, as it does by default in Ubuntu.

I further suspect that many, or perhaps all, 17.10 systems have this bug in their default configuration (though it would only be noticed if users install and run perldoc). This strongly resembles https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=735134. This bug may be considered a regression, in that the problem had previously occurred and was then prevented by the addition of this code in Perldoc.pm, which is present in 16.04 but removed in 17.10 (see below for details):

# fix visible escape codes in ToTerm output
    # https://bugs.debian.org/758689
    local $ENV{LESS} = defined $ENV{LESS} ? "$ENV{LESS} -R" : "-R";

The simplest workaround for this bug that I am aware of is to pass the -t flag to perldoc. As in the workaround described in the bug report here, the resulting formatting is readable but not as fancy as usual.

A *full* workaround is also possible, and is only slightly more complicated. Telling perldoc explicitly that you want "less" by defining the PAGER environment variable works:

PAGER=less perldoc perlsyn

Otherwise, it finds sensible-pager first (see above), and appears not to place -R in LESS (see below), because it does not know that the sensible-pager shell script runs less.

The problem may also be avoided by ensuring that an environment variable with the name LESS and a value that specifies the R option is present. For example, this works:

LESS=-R perldoc perlsyn

Or this may be preferred, because if you have a LESS environment variable with other options, they will still be used (and if not, it has the same effect as above):

LESS="$LESS -R" perldoc perlsyn

Or you can pipe to less and pass -R:

perldoc perlsyn | less -R

If these workarounds do not work for you (i.e., for the original author of this bug report), then I might be mistaken in my belief that the bug I am experiencing is the same as yours.

This bug does not appear to affect Ubuntu 16.04 LTS. It does not happen on my Lubuntu 16.04 LTS (64-bit) system. Ubuntu 16.04 LTS has "/usr/share/perl/5.22/Pod/Perldoc.pm" which uses this loop to open the document in a pager:

foreach my $pager (@pagers) {
        $self->aside("About to try calling $pager $output\n");
        if ($self->is_vms) {
            last if system("$pager $output") == 0;
        } else {
            # fix visible escape codes in ToTerm output
            # bugs.debian.org/758689
            local $ENV{LESS} = defined $ENV{LESS} ? "$ENV{LESS} -R" : "-R";
            last if system("$pager \"$output\"") == 0;
        }
    }

Ubuntu 17.10 has this corresponding loop, which no longer directly attempts to modify the LESS environment variable in order to prevent Debian Bug 758689 from occurring:

foreach my $pager (@pagers) {
        $self->aside("About to try calling $pager $output\n");
        if ($self->is_vms) {
            last if system("$pager $output") == 0;
        } elsif($self->is_amigaos) {
            last if system($pager, $output) == 0;
        } else {
            my $formatter = $self->{'formatter_class'};
            if ( $formatter->can('pager_configuration') ) {
            $self->aside("About to call $formatter" . "->pager_configuration(\"$pager\")\n");
            $formatter->pager_configuration($pager, $self);
            }
            last if system("$pager \"$output\"") == 0;
        }
    }

Ubuntu
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perldoc shows lots of escapes

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Ubuntuperl package

perldoc shows lots of escapes

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Ubuntu
perl package