Just a Theory

Tonight we had a Portland Perl Mongers Perl 5.10 code sprint. This was the first time I've ever worked with the Perl 5 core code, and while things weren't exactly intuitive, it wasn't bad to get started, either. I already had a clone of the Perl Git repository, so here's what I did to get started.

Here's how to clone the Git repository, and then check out the maint-5.10 branch and build and test Perl:

git clone git://perl5.git.perl.org/perl.git
cd perl
git checkout -b maint-5.10 origin/maint-5.10
sh Configure -Ode \
 -DDEBUGGING \
 -Dprefix=/usr/local/perl/blead \
 -Dusedevel \
 -Duseithreads \
 -Dccflags='-I/usr/local/include' -Dldflags='-L/usr/local/lib' \
 -Dlibpth='/usr/local/lib /usr/lib' \
 -Uversiononly \
 -Uinstallusrbinperl $@
make
make test

I had two test failures:

#   at ../lib/ExtUtils/CBuilder/t/00-have-compiler.t line 39.
#          got: '0'
#     expected: '1'
# Looks like you failed 1 test of 4.
FAILED at test 4

And:

#   at pod/pod2usage2.t line 235.
# Got:
# #Usage:
# #    This is a test for CPAN#33020
# #
# #Usage:
# #    And this will be also printed.
# #
# 

So the first thing I want to do is fix those failures. It took a bit of fiddling to figure out how to get it to run a single test. On the advice of Paul Fenwick and Schwern, I tried:

./perl -Ilib lib/ExtUtils/CBuilder/t/00-have-compiler.t

But then the test passed! So something has to be different when running under make test. With help from Duke Leto, I finally figured out the proper incantation:

make test TEST_FILES=../lib/ExtUtils/CBuilder/t/00-have-compiler.t

So the TEST_FILES option tells make test what tests to run, relative to the t/ directory. Shortly thereafter, Schwern told me about:

cd t
./perl TEST ../lib/ExtUtils/CBuilder/t/00-have-compiler.t

Which is nice, because it does a lot less work.

Now I was ready to figure out the ExtUtils::CBuilder bug and fix it. This is good, I thought, because ExtUtils::CBuilder is a Perl module, and while my C is teh suck, my Perl skillz are mad. So I do some poking around, and finally pinned down the failure to the have_compiler() method in ExtUtils::CBuilder::Base. A bit more poking and I had the error printed out:

# error building /tmp/compilet-1529033816.o from '/tmp/compilet-1529033816.c'
at ../../ExtUtils/CBuilder/Base.pm line 110.

Well fuck, that sure looks like a C problem. But then I added some more debugging code to see what command it's actually running, and from where. So I added this code:

use Cwd;
print STDERR "# CWD: ", getcwd, $/;
print STDERR "# ", join( ' ', @cc, @flags), $/;

And then I got this output:

# CWD: /Users/david/dev/perl/lib/ExtUtils/CBuilder
# ./perl -e1 -- -I/Users/david/dev/perl/perl -c -fno-common -DPERL_DARWIN -no-cpp-precomp -fno-strict-aliasing -pipe -fstack-protector -I/usr/local/include -O3 -o ./compilet-892348855.o

And of course, there's no perl in lib/ExtUtils/CBuilder. Why does it think there is? The test file has this code:

$b->{config}{cc} = "$^X -e1 --";
$b->{config}{ld} = "$^X -e1 --";

Well, that cinches it. $^X is the path to the currently-executing Perl, but in core--for me at least--that's a relative path. The test changes directories away from that path, and so it can't find it. So I just needed to get it to use the proper Perl. Frankly, this test should fail on every platform that runs it, so I'm not sure how it got committed to the maint-5.10 branch, but here we are.

So I moved up use File::Spec; and assigned File::Spec->rel2abs($^X) to a variable before the chdir, and used that variable for the cc mock. And that was it, the test passed. Yay!

Now it was time to submit a patch to p5p. I committed the fix to my local clone, and then, with help from Jacob, generated a patch with git format-patch -1. The -1 means just a single commit, which is all I'd committed. This generated a file, named 0001-Fixed-failing-test-for-ExtUtils-CBuilder.patch. I edited it to add a comment to the effect that the patch is targeted at the maint-5.10 branch.

Next Jacob helped me get git configured to send mail through my gmail account. I just added these lines to ~/.gitconfig:

[sendemail]
        smtpserver = smtp.gmail.com
        smtpserverport = 587
        smtpuser = justatheory
        smtppass = idontthinkso!
        smtpencryption = tls

I then sent the patch with:

git send-email --to perl5-porters@perl.org 0001-Fixed-failing-test-for-ExtUtils-CBuilder.patch

And that was it! You can see the result here. I started to fiddle with the pod2usage failure, but it was a bit obscure for me. Perhaps next time.

And speaking of next time, I'm forking the Perl repository on GitHub, and will probably start to use my own repository for stuff (if it can be kept up to date; RJBS points out that it's currently way behind the core repo). I'll likely also create a topic branch before fixing a bug, and then send the patch from there. That way, when it's committed to the remote branch in core, I won't have to rebase my local copy: I can just delete the topic branch. This is a nice way to track individual bug fixes that I've worked on, while letting the maint-5.10 branch just track the core repository, with no local changes.