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autom4te: replace output file atomically (#110305)

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In 2003, Joey Hess reported the following bug against Debian's
autoconf package (see http://bugs.debian.org/221483):

    I noticed that if I ctrl-c autoconf, it can leave a partially
    written, executable configure script. I was lucky enough to
    get a configure script that exited with a shell parse error,
    but if I had been unlucky, it might have exited 0 without
    doing all the tests I expected it to do.  That would have
    sucked to ship to users.

    There are many ways to update a file in a way that is not
    prone to these problems, and I suggest that autoconf adopt
    one of them.

Ben Pfaff wrote a patch to make autom4te replace the output file
atomically; Debian has carried it since 2006.  He submitted it
to autoconf upstream in 2008 but it never went anywhere.

I (Zack) have dusted off the patch and made some minor improvements:
using File::Temp (with DIR set to the directory of the output file)
instead of a predictable temporary file name, and using
Autom4te::FileUtils::update_file instead of File::Copy::move.

I do not attempt to test the fix (the test would be inherently racey)
nor do I have autom4te delete the temp file if it crashes while the
file is being written (there is no way to do this with 100%
reliability and it strikes me as likely to cause more problems than it
solves).

Fixes our bug #110305.

* bin/autom4te.in (handle_output): When $output is to a regular or
  nonexistent file, write to a temporary file in the same directory
  and then rename it over $output after completion.
This commit is contained in:
Ben Pfaff 2020-11-10 09:42:58 -05:00 committed by Zack Weinberg
parent 29df8097dd
commit 1725c94714
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GPG Key ID: 384F8E68AC65B0D5
1 changed files with 30 additions and 10 deletions
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40
bin/autom4te.in
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@ -546,21 +546,38 @@ sub handle_output ($$)
foreach (sort keys %forbidden);
verb "allowed tokens: $allowed";
# Read the (cached) raw M4 output, produce the actual result. We
# have to use the 2nd arg to have Autom4te::XFile honor the third, but then
# stdout is to be handled by hand :(. Don't use fdopen as it means
# we will close STDOUT, which we already do in END.
my $out = new Autom4te::XFile;
# Read the (cached) raw M4 output, produce the actual result.
# If we are writing to a regular file, replace it atomically.
my $atomic_replace = 0;
my $out;
if ($output eq '-')
{
$out->open (">$output");
# Don't just make $out be STDOUT, because then we would close STDOUT,
# which we already do in END.
$out = new Autom4te::XFile ('>&STDOUT');
}
elsif (-e $output && ! -f $output)
{
$out = new Autom4te::XFile ($output, '>');
}
else
{
$out->open ($output, O_CREAT | O_WRONLY | O_TRUNC, oct ($mode));
my (undef, $outdir, undef) = fileparse ($output);
use File::Temp ();
$out = new File::Temp (UNLINK => 0, DIR => $outdir);
fatal "cannot create a file in $outdir: $!"
unless $out;
# File::Temp doesn't give us access to 3-arg open(2), unfortunately.
# In older Perls, implicit conversion of a File::Temp to its filename
# cannot be relied upon.
chmod (oct ($mode) & ~(umask), $out->filename)
or fatal "setting mode of " . $out->filename . ": $!";
$atomic_replace = 1;
}
fatal "cannot create $output: $!"
unless $out;
my $in = new Autom4te::XFile ($ocache . $req->id, "<");
my %prohibited;
@ -585,7 +602,8 @@ sub handle_output ($$)
foreach (split (/\W+/))
{
$prohibited{$_} = $.
if !/^$/ && /$forbidden/o && !/$allowed/o && ! exists $prohibited{$_};
if !/^$/ && /$forbidden/o && !/$allowed/o
&& ! exists $prohibited{$_};
}
# Performed *last*: the empty quadrigraph.
@ -595,6 +613,8 @@ sub handle_output ($$)
}
$out->close();
update_file ($out->filename, $output, $force)
if $atomic_replace;
# If no forbidden words, we're done.
return