From: ilmari@ilmari.org (Dagfinn Ilmari =?utf-8?Q?Manns=C3=A5ker?=)
To: Peter Eisentraut <peter.eisentraut@2ndquadrant.com>
Cc: Noah Misch <noah@leadboat.com>,
  pgsql-hackers <pgsql-hackers@postgresql.org>
Subject: Re: libpq environment variables in the server
References: <33383613-690e-6f1b-d5ba-4957ff40f6ce@2ndquadrant.com>
	<744962da-c27f-c992-4058-364463857ee0@2ndquadrant.com>
	<20190303080620.GA1750246@rfd.leadboat.com>
	<35af0500-5eea-d2f0-339e-07a457a5f6bc@2ndquadrant.com>
	<20190315040053.GB2607362@rfd.leadboat.com>
	<872a3a54-a169-acfc-322a-f6c5b8e89ece@2ndquadrant.com>
Mail-Copies-To: nobody
Date: Fri, 15 Mar 2019 10:06:29 +0000
In-Reply-To: <872a3a54-a169-acfc-322a-f6c5b8e89ece@2ndquadrant.com> (Peter
	Eisentraut's message of "Fri, 15 Mar 2019 09:15:24 +0100")
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Peter Eisentraut <peter.eisentraut@2ndquadrant.com> writes:

> On 2019-03-15 05:00, Noah Misch wrote:
>> I consider the following style more idiomatic:
>> 
>>  {
>>      local %ENV;
>>      delete $ENV{PGAPPNAME};
>>      ...
>>  }
>
> That doesn't work because the first line clears the entire environment.

The solution to that is to do 'local %ENV = %ENV;', to assign a copy of
the original to the localised variable.  This doesn't work on VMS,
because its concept of environment variables is quite different from
UNIX, but PostgreSQL doesn't support that anyway.

> What does work is
>
> {
>     delete local $ENV{PGAPPNAME};
>     ...
> }
>
> But that is documented as new in Perl 5.12.0, so we might not be able to
> use it.  It appears to work in the 5.8.9 I have lying around, so I'm
> confused.

It "works" as in it's not a syntax error, but it doesn't actually
localise the deletion. The following program:

    use strict;
    use warnings;
    use feature 'say';

    our %env = qw(foo bar baz bat);
    say "original:  ", join(", ", sort keys %env);
    {
        delete local $env{foo};
        say "localised: ", join(", ", sort keys %env);
    }
    say "restored?  ", join(", ", sort keys %env);

on 5.12 prints:

    original:  baz, foo
    localised: baz
    restored?  baz, foo

while on 5.10 it prints:

    original:  baz, foo
    localised: baz
    restored?  baz

BTW, https://perl.bot/ is handy for testing things like this on various
Perl versions you don't have lying around.

- ilmari
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