Re: Alpine: useless-if-before-free: Exec format error

[Bruno Haible]

Hi Paul,

> One quibble: those long comments are 
> probably more trouble than they're worth.

I heavily disagree: This piece of code needs to consider
  - the distinction between POSIX:2017 and earlier versions,
  - execlp/execvp vs. execl/execv,
  - GuixSD vs. traditional file system layouts,
  - perl documentation,
  - perl source code (toke.c).

> I suggest we just cite perlrun's man page

No, that man page represents only a small part of the entire picture.
Future maintainers should not have to wade through a mail thread of 12 mails
in order to understand a piece of code.

> I found that at least one important detail 
> was missing: the "#! -*-perl-*-" line is not just there for Emacs, but is 
> also 
> there for Perl.

Oh, indeed! So, both this line and the '-x' option are essential for perl.
I'm updating the comment:

--- a/build-aux/useless-if-before-free
+++ b/build-aux/useless-if-before-free
@@ -6,12 +6,12 @@
 # or execvp() fails with ENOEXEC if it is a script that does not start
 # with a #! line.  The script interpreter mentioned in the #! line has
 # to be /bin/sh, because on GuixSD systems that is the only program that
-# has a fixed file name.  The second line is for editing this file in
-# Emacs.  The next two lines below are valid code in both sh and perl.
-# When executed by sh, they re-execute the script through the perl
-# program found in $PATH.  The '-x' option is essential; without it,
-# perl would re-execute the script through /bin/sh.  When executed by
-# perl, the next two lines are a no-op.
+# has a fixed file name.  The second line is essential for perl and is
+# also useful for editing this file in Emacs.  The next two lines below
+# are valid code in both sh and perl.  When executed by sh, they re-execute
+# the script through the perl program found in $PATH.  The '-x' option
+# is essential as well; without it, perl would re-execute the script
+# through /bin/sh.  When executed by  perl, the next two lines are a no-op.
 eval 'exec perl -wSx "$0" "$@"'
      if 0;
 

> I found that they got in the way of my understanding the code

Do you mean, we should highlight the comment about the program itself?
Like this:

#!/bin/sh
#! -*-perl-*-
# This is a prologue that allows to run a perl script as an executable
# on systems that are compliant to a POSIX version before POSIX:2017.
# On such systems, the usual invocation of an executable through execlp()
# or execvp() fails with ENOEXEC if it is a script that does not start
# with a #! line.  The script interpreter mentioned in the #! line has
# to be /bin/sh, because on GuixSD systems that is the only program that
# has a fixed file name.  The second line is essential for perl and is
# also useful for editing this file in Emacs.  The next two lines below
# are valid code in both sh and perl.  When executed by sh, they re-execute
# the script through the perl program found in $PATH.  The '-x' option
# is essential as well; without it, perl would re-execute the script
# through /bin/sh.  When executed by  perl, the next two lines are a no-op.
eval 'exec perl -wSx "$0" "$@"'
     if 0;

# ╭───────────────────────────────────────────────────────────────────────╮
# │ Detect instances of "if (p) free (p);".                               │
# │ Likewise "if (p != 0)", "if (0 != p)", or with NULL; and with braces. │
# ╰───────────────────────────────────────────────────────────────────────╯

my $VERSION = '2018-03-07 03:47'; # UTC
...

Bruno