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[perl5.git] / Porting / patching.pod

=head1 Name

patching.pod - Appropriate format for patches to the perl source tree

=head2 Where to get this document

The latest version of this document is available from
     http://perrin.dimensional.com/perl/perlpatch.html

=head2 How to contribute to this document

You may mail corrections, additions, and suggestions to me
at dgris@dimensional.com but the preferred method would be
to follow the instructions set forth in this document and 
submit a patch 8-).

=head1 Description

=head2 Why this document exists

As an open source project Perl relies on patches and contributions from
its users to continue functioning properly and to root out the inevitable
bugs.  But, some users are unsure as to the I<right> way to prepare a patch
and end up submitting seriously malformed patches.  This makes it very
difficult for the current maintainer to integrate said patches into their
distribution.  This document sets out usage guidelines for patches in an
attempt to make everybody's life easier.

=head2 Common problems

The most common problems appear to be patches being mangled by certain
mailers (I won't name names, but most of these seem to be originating on
boxes running a certain popular commercial operating system). Other problems
include patches not rooted in the appropriate place in the directory structure,
and patches not produced using standard utilities (such as diff).

=head1 Proper Patch Guidelines

=head2 What to patch

Generally speaking you should patch the latest development release
of perl.  The maintainers of the individual branches will see to it
that patches are picked up and applied as appropriate.

=head2 How to prepare your patch

=over 4

=item Creating your patch

First, back up the original files.  This can't be stressed enough,
back everything up _first_.

Also, please create patches against a clean distribution of the perl source.
This insures that everyone else can apply your patch without clobbering their
source tree.

=item diff

While individual tastes vary (and are not the point here) patches should
be created using either C<-u> or C<-c> arguments to diff.  These produce,
respectively, unified diffs (where the changed line appears immediately next
to the original) and context diffs (where several lines surrounding the changes
are included).  See the manpage for diff for more details.

Also, the preferred method for patching is -

C<diff [C<-c> | C<-u>] E<lt>old-fileE<gt> E<lt>new-fileE<gt>>

Note the order of files.

Also, if your patch is to the core (rather than to a module) it
is better to create it as a context diff as some machines have
broken patch utilities that choke on unified diffs.

GNU diff has many desirable features not provided by most vendor-supplied
diffs.  Some examples using GNU diff:

    # generate a patch for a newly added file
    % diff -u /dev/null new/file
    
    # generate a patch to remove a file (patch > v2.4 will remove it cleanly)
    % diff -u old/goner /dev/null
    
    # get additions, deletions along with everything else, recursively
    % diff -ruN olddir newdir
    
    # ignore whitespace
    % diff -bu a/file b/file
    
    # show function name in every hunk (safer, more informative)
    % diff -u -F '^[_a-zA-Z0-9]+ *(' old/file new/file


=item Directories

Patches should be generated from the source root directory, not from the
directory that the patched file resides in.  This insures that the maintainer
patches the proper file and avoids name collisions (especially common when trying
to apply patches to files that appear in both $src_root/ext/* and $src_root/lib/*).
It is better to diff the file in $src_root/ext than the file in $src_root/lib.

=item Filenames

The most usual convention when submitting patches for a single file is to make
your changes to a copy of the file with the same name as the original.  Rename
the original file in such a way that it is obvious what is being patched ($file~ or
$file.old seem to be popular).

If you are submitting patches that affect multiple files then you should backup
the entire directory tree (to $source_root.old/ for example).  This will allow
C<diff C<-c> E<lt>old-dirE<gt> E<lt>new-dirE<gt>> to create all the patches
at once.

=back

=head2 What to include in your patch

=over 4

=item Description of problem

The first thing you should include is a description of the problem that
the patch corrects.  If it is a code patch (rather than a documentation
patch) you should also include a small test case that illustrates the
bug.

=item Direction for application

You should include instructions on how to properly apply your patch.
These should include the files affected, any shell scripts or commands
that need to be run before or after application of the patch, and
the command line necessary for application.

=item If you have a code patch

If you are submitting a code patch there are several other things that
you need to do.

=over 4

=item Comments, Comments, Comments

Be sure to adequately comment your code.  While commenting every
line is unnecessary, anything that takes advantage of side effects of
operators, that creates changes that will be felt outside of the
function being patched, or that others may find confusing should
be documented.  If you are going to err, it is better to err on the
side of adding too many comments than too few.

=item Style

Please follow the indentation style and nesting style in use in the
block of code that you are patching.

=item Testsuite

When submitting a patch you should make every effort to also include
an addition to perl's regression tests to properly exercise your
patch.  Your testsuite additions should generally follow these
guidelines (courtesy of Gurusamy Sarathy (gsar@engin.umich.edu))-

	Know what you're testing.  Read the docs, and the source.
	Tend to fail, not succeed.
	Interpret results strictly.
	Use unrelated features (this will flush out bizarre interactions).
	Use non-standard idioms (otherwise you are not testing TIMTOWTDI).
	Avoid using hardcoded test numbers whenever possible (the 
          EXPECTED/GOT found in t/op/tie.t is much more maintainable, 
          and gives better failure reports).
	Give meaningful error messages when a test fails.
	Avoid using qx// and system() unless you are testing for them.  If you
	  do use them, make sure that you cover _all_ perl platforms.
	Unlink any temporary files you create.
	Promote unforeseen warnings to errors with $SIG{__WARN__}.
	Be sure to use the libraries and modules shipped with version 
          being tested, not those that were already installed.
	Add comments to the code explaining what you are testing for.
	Make updating the '1..42' string unnecessary.  Or make sure that 
          you update it.
	Test _all_ behaviors of a given operator, library, or function-
	  All optional arguments
	  Return values in various contexts (boolean, scalar, list, lvalue)
	  Use both global and lexical variables
	  Don't forget the exceptional, pathological cases.

=back

=item Test your patch

Apply your patch to a clean distribution, compile, and run the
regression test suite (you did remember to add one for your
patch, didn't you).

=back

=head2 An example patch creation

This should work for most patches-

      cp MANIFEST MANIFEST.old
      emacs MANIFEST
      (make changes)
      cd ..
      diff -c perl5.008_42/MANIFEST.old perl5.008_42/MANIFEST > mypatch
      (testing the patch:)
      mv perl5.008_42/MANIFEST perl5.008_42/MANIFEST.new
      cp perl5.008_42/MANIFEST.old perl5.008_42/MANIFEST
      patch -p < mypatch
      (should succeed)
      diff perl5.008_42/MANIFEST perl5.008_42/MANIFEST.new
      (should produce no output)

=head2 Submitting your patch

=over 4

=item Mailers

Please, please, please (get the point? 8-) don't use a mailer that
word wraps your patch or that MIME encodes it.  Both of these leave
the patch essentially worthless to the maintainer.

If you have no choice in mailers and no way to get your hands on a
better one there is, of course, a perl solution.  Just do this-

      perl -ne 'print pack("u*",$_)' patch > patch.uue

and post patch.uue with a note saying to unpack it using

      perl -ne 'print unpack("u*",$_)' patch.uue > patch

=item Subject lines for patches

The subject line on your patch should read

[PATCH]5.xxx_xx (Area) Description

where the x's are replaced by the appropriate version number,
area is a short keyword identifying what area of perl you are
patching, and description is a very brief summary of the
problem (don't forget this is an email header).

Examples-

[PATCH]5.004_04 (DOC) fix minor typos

[PATCH]5.004_99 (CORE) New warning for foo() when frobbing

[PATCH]5.005_42 (CONFIG) Added support for fribnatz 1.5

=item Where to send your patch

If your patch is for the perl core it should be sent perlbug@perl.org.
If it is a patch to a module that you downloaded from CPAN you should
submit your patch to that module's author.

=back

=head2 Applying a patch

=over 4

=item General notes on applying patches

The following are some general notes on applying a patch
to your perl distribution.

=over 4

=item patch C<-p>

It is generally easier to apply patches with the C<-p> argument to
patch.  This helps reconcile differing paths between the machine the
patch was created on and the machine on which it is being applied.

=item Cut and paste

_Never_ cut and paste a patch into your editor.  This usually clobbers
the tabs and confuses patch.

=item Hand editing patches

Avoid hand editing patches as this frequently screws up the whitespace
in the patch and confuses the patch program.

=back

=back

=head2 Final notes

If you follow these guidelines it will make everybody's life a little
easier.  You'll have the satisfaction of having contributed to perl,
others will have an easy time using your work, and it should be easier
for the maintainers to coordinate the occasionally large numbers of 
patches received.

Also, just because you're not a brilliant coder doesn't mean that you
can't contribute.  As valuable as code patches are there is always a
need for better documentation (especially considering the general
level of joy that most programmers feel when forced to sit down and
write docs).  If all you do is patch the documentation you have still
contributed more than the person who sent in an amazing new feature
that no one can use because no one understands the code (what I'm
getting at is that documentation is both the hardest part to do
(because everyone hates doing it) and the most valuable).

Mostly, when contributing patches, imagine that it is B<you> receiving
hundreds of patches and that it is B<your> responsibility to integrate
them into the source.  Obviously you'd want the patches to be as easy
to apply as possible.  Keep that in mind.  8-)

=head1 Last Modified

Last modified 21 January 1999 
Daniel Grisinger <dgris@dimensional.com>

=head1 Author and Copyright Information

Copyright (c) 1998 Daniel Grisinger

Adapted from a posting to perl5-porters by Tim Bunce (Tim.Bunce@ig.co.uk).

I'd like to thank the perl5-porters for their suggestions.