[oe] Commit and Patch message guidelines - third draft.

[Mark Hatle]

This is the third draft of the guidelines... All previous feedback has been
incorporated...

It is the first of the drafts being sent to the OpenEmbedded-devel mailing list
for comments.  The intention of the TSC is to use the following guidelines to
help increase the quality of the commit and patches within Open Embedded.
Please review the following guidelines and let me know if you have any
questions, comments or concerns with them.

----

This set of guidelines is intended to help both the developer and reviewers of
changes determine reasonable patch and change commit messages. Often when
working with the code, you forget that not everyone is as familiar with the
problem and/or fix as you are. Often the next person in the code doesn't
understand what or why something is done so they quickly look at patch and
commit messages. Unless these messages are clear it will be difficult to
understand the relevance of a given change and how future changes may impact
previous decisions.

Both patches and commit messages require the same attention and basic details,
however the purposes of the messages are slightly different. A patch needs to
describe a specific change that fixes an individual problem in a component,
while a commit message describes one or more changes to the overall project or
system.

By following these guidelines we will have a better record of the problems and
solutions made over the course of development. It will also help establish a
clear provenance of all of the code and changes within the development.

General Information
-------------------

While specific to the Linux kernel development, the following could also be
considered a general guide for any Open Source development:

http://ldn.linuxfoundation.org/book/how-participate-linux-community

Many of the guidelines in this document are related to the items in that
information.

Pay particular attention to section 5.3 that talks about patch preparation.
They key thing to remember is to break up your changes into logical sections.
Otherwise you run the risk of not being able to even explain the purpose of a
change in the patch headers!

Patch and Commit Headers
------------------------
There seems to always be a question or two surrounding what a person
should put in a patch header, or commit message.

The general rules always apply, those being that there is a single line
short log or summary of the change (think of this as the subject when the patch
is e-mailed), and then the more detailed long log, and closure with tag lines
like the "Signed-off-by:" or "Integrated-by:"

See the examples below for examples and details.

New development
---------------

A minimal patch or commit message would be of the format:

   Short log / Statement of what needed to be changed.

   (Optional pointers to external resources, such as defect tracking)

   The intent of your change.

   (Optional, if it's not clear from above) how your change resolves the
   issues in the first part.

   Tag line(s) at the end.

For example:

   foobar: Adjusted the foo setting in bar

   When using foobar on systems with less than a gigabyte of RAM common
   usage patterns often result in an Out-of-memory condition causing
   slowdowns and unexpected application termination.

   Low-memory systems should continue to function without running into
   memory-starvation conditions with minimal cost to systems with more
   available memory.  High-memory systems will be less able to use the
   full extent of the system, a dynamically tunable option may be best,
   long-term.

   The foo setting in bar was decreased from X to X-50% in order to
   ensure we don't exhaust all system memory with foobar threads.

   Signed-off-by: Joe Developer <joe.developer at example.com>

The minimal commit is good for new code development and simple changes.

The single short log message indicates what needed to be changed. It should
begin with an indicator as to the primary item changed by this patch,
followed by a short summary of the change. In the above case we're indicating
that we've changed the "foobar" item, by "adjusting the foo setting in bar".

Optionally, you may include pointers to defects this change corrects. Unless
the defect format is specified by the component you are modifying, it is
suggested that you use a full URL to specify the reference to the defect
information.

You must then have a full description of the change. Specifying the intent of
your change and if necessary how the change resolves the issue.

As mentioned above this is intended to answer the "what were you thinking"
questions down the road and to know what other impacts are likely to
result of the change needs to be reverted. It also allows for better
solutions if someone comes along later and agrees with paragraph 1
(the problem statement) and either disagrees with paragraph 2
(the design) or paragraph 3 (the implementation).

Finally one or more tag lines should exist. Each developer responsible
for working on the patch is responsible for adding a Signed-off-by: tag line.

It is not acceptable to have an empty or non-existent header, or just a single
line message. The summary and description is required for all changes.

Importing from elsewhere
-----------------------------
If you are importing work from somewhere else, the minimum commit message is not
enough. It does not clearly establish the provenance of the code. The following
specified the guidelines required for importing code from elsewhere.

By default you should keep the original authors summary and commit message,
along with any Signed-off-by: information. If the original change that was
imported does not have a summary and/or commit message from the original author,
it is still your responsibility to add them to the patch. Just as if you wrote
the code, you should be able to clearly explain what the change does. It is also
necessary to document the original author of the change. You should indicate the
original author by simply stating "written by" or "posted to the ... mailing
list by". Only the original author can commit using a Signed-off-by: tag line.

It is also required that the origin of the work be fully documented. The origin
should be specified as part of the commit message in a way that an average
developer would be able to track down the original code. URLs should reference
original authoritative sources and not mirrors.

If changes were required to the patch, they should be documented as well.

Finally a tag line should be added to indicate that you worked with the patch.
The "Integrated-by:" tag line should be used to indicate that you did not need
to modify the patch, it was integrated unchanged. The "Signed-off-by:" tag line
should only be used when changes to the original patch were necessary.

For example, if you were to pull in the patch from the above example unmodified:

   foobar: Adjusted the foo setting in bar

   When using foobar on systems with less than a gigabyte of RAM common
   usage patterns often result in an Out-of-memory condition causing
   slowdowns and unexpected application termination.

   Low-memory systems should continue to function without running into
   memory-starvation conditions with minimal cost to systems with more
   available memory.  High-memory systems will be less able to use the
   full extent of the system, a dynamically tunable option may be best,
   long-term.

   The foo setting in bar was decreased from X to X-50% in order to
   ensure we don't exhaust all system memory with foobar threads.

   Signed-off-by: Joe Developer <joe.developer at example.com>

   The foobar recipe was imported from the OpenEmbedded git server
   (git://git.openembedded.org/openembedded) as of commit id
   b65a0e0c84cf489bfa00d6aa6c48abc5a237100f.

   Integrated-by: Your Name <your.name at openembedded.org>

The above example shows a clear way for a developer to find the original source
of the change. It indicates that the open embedded developer simply integrated
the change into the system without making any further modifications.

However in the same example, if the author did have to make some changes to the
original patch it would look like this:

   foobar: Adjusted the foo setting in bar

   When using foobar on systems with less than a gigabyte of RAM common
   usage patterns often result in an Out-of-memory condition causing
   slowdowns and unexpected application termination.

   Low-memory systems should continue to function without running into
   memory-starvation conditions with minimal cost to systems with more
   available memory.  High-memory systems will be less able to use the
   full extent of the system, a dynamically tunable option may be best,
   long-term.

   The foo setting in bar was decreased from X to X-50% in order to
   ensure we don't exhaust all system memory with foobar threads.

   Signed-off-by: Joe Developer <joe.developer at example.com>

   The foobar recipe was imported from the OpenEmbedded git server
   (git://git.openembedded.org/openembedded) as of commit id
   b65a0e0c84cf489bfa00d6aa6c48abc5a237100f.

   It was necessary to change the memory threshold from 50% to 42%
   due to the constraints of the implementation of bar in OpenEmbedded.

   Signed-off-by: Your Name <your.name at openembedded.org>


Common Errors in Patch and Commit messages
------------------------------------------

- Don't have one huge patch, split your change into logical subparts. It's
easier to track down problems afterwards with a binary search and also people
can then cherrypick changes into things like stable branches.

- Don't simply translate your change into English for a commit log. The log
"Change compare from zero to one" is bad because it describes only the code
change in the patch; we can see that from reading the patch itself. Let the code
tell the story of the mechanics of the change (as much as possible), and let
your comment tell the other details -- i.e. what the problem was, how it
manifested itself (symptoms), and if necessary, the justification for why the
fix was done in manner that it was.

- Don't start your commit log with "This patch..." -- we already know it is a patch.

- Don't repeat your short log in the long log. If you really really don't have
anything new and informational to add in as a long log, then don't put a long
log at all. This should be uncommon -- i.e. the only acceptable cases for no
long log would be something like "Fix spelling mistakes in Documentation/README"
or similar.

- Don't put absolute paths to source files that are specific to your site, i.e.
if you get a compile error on "fs/exec.c" then tidy up the parts of the compile
output to just show that portion. We don't need to see that it was
/home/wally/src/git/oe-core/meta/classes/insane.bbclass or similar - that full
path has no value to others.

- Always use the most significant ramification of the change in the words of
your subject/shortlog. For example, don't say "fix compile warning in foo" when
the compiler warning was really telling us that we were dereferencing complete
garbage off in the weeds that could in theory cause an OOPS under some
circumstances. When people are choosing commits for backports to stable or
distro kernels, the shortlog will be what they use for an initial sorting
selection. If they see "Fix possible OOPS in...." then these people will look
closer, whereas they most likely will skip over simple warning fixes.

- Don't put in the full 20 or more lines of a backtrace when really it is just
the last 5 or so function calls that are relevant to the crash/fix. If the entry
point, or start of the trace is relevant (i.e. a syscall or similar), you can
leave that, and then replace a chunk of the intermediate calls in the middle
with a single [...]

- Don't include timestamps or other unnecessary information, unless they are
relevant to the situation (i.e. indicating an unacceptable delay in a driver
initialization etc.)

- Don't use links to temporary resources like pastebin and friends. The commit
message may be read long after this resource timed out.

- Don't reference mirrors, but instead always reference original authoritative
sources.

- Avoid punctuation in the short log.