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Re: [bug#67841] [PATCH] Clarify error messages for misuse of m4_warn and

From: Jacob Bachmeyer
Subject: Re: [bug#67841] [PATCH] Clarify error messages for misuse of m4_warn and --help for -W.
Date: Fri, 15 Dec 2023 18:08:00 -0600
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv: Gecko/20090807 MultiZilla/ SeaMonkey/1.1.17 Mnenhy/

Zack Weinberg wrote:

Since this touches code shared between Autoconf and Automake, I’m not
checking it in yet and I’m requesting comments from both sides of the
fence.  Also, there’s a perl 2.14ism in one place (s///a) which I need
to figure out how to make 2.6-compatible before it can land.

+sub report_bad_channel ($$)
+  my ($channel, $location) = @_;
+  my $message;
+  my $report_as = 'error';
+  # quotemeta is both too aggressive (e.g. it escapes '-') and
+  # too generous (it turns control characters into \ + themselves,
+  # not into symbolic escapes).
+  my $q_channel = $channel;
+  $q_channel =~ s/\\/\\\\/g;
+  $q_channel =~ s/([^\x20-\x7e])/"\\x".sprintf("%02x", ord($1))/aeg;
+  $q_channel =~ s/(?=[\"\$\'\@\`])/\\/g;
+  $q_channel = '"' . $q_channel . '"';


If I am reading perlre correctly, you should be able to simply drop the /a modifier because it has no effect on the pattern you have written, since you are using an explicit character class and are *not* using the /i modifier. (The documentation says that /a only affects the \d, \s, \w Perl character classes and the POSIX character classes.) Further, Perl's sprintf is indeed sprintf, so you should be able to rewrite that substitution as:

s/([^\x20-\x7e])/sprintf('\\x%02x', ord $1)/eg

This slightly simplifies the program (eliminating one concatenation), which is probably of trivial consequence, since this code is on an error path and not an inner loop. More importantly, including the "\\x" in the sprintf format string makes the intended output more clear. The single quotes are both a (very slight) performance optimization and serve to document that no direct interpolation is intended because this is a format string. Perl's handling of backslash in single quotes is fun: backslash escapes itself but will also be taken literally if it does not form an escape that is allowed in q[].

-- Jacob

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