docs/contributing.adoc

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[[contributing]]
= Contributing to Apache Kudu
:author: Kudu Team
:imagesdir: ./images
:icons: font
:toc:
:toclevels: 3
:doctype: book
:backend: html5
:sectlinks:
:experimental:

== Contributing Patches Using Gerrit

The Kudu team uses Gerrit for code review, rather than Github pull requests. Typically,
you pull from Github but push to Gerrit, and Gerrit is used to review code and merge
it into Github.

See the link:https://www.mediawiki.org/wiki/Gerrit/Tutorial[Gerrit Tutorial]
for an overview of using Gerrit for code review.

=== Initial Setup for Gerrit

. Sign in to link:http://gerrit.cloudera.org:8080[Gerrit] using your Github username.

. Go to link:http://gerrit.cloudera.org:8080/#/settings/[Settings]. Update your name
and email address on the *Contact Information* page, and upload a SSH public key.
If you do not update your name, it will show up as "Anonymous Coward" in Gerrit reviews.

. If you have not done so, clone the main Kudu repository. By default, the main remote
is called `origin`. When you fetch or pull, you will do so from `origin`.
+
[source,bash]
----
git clone https://github.com/apache/kudu
----

. Change to the new `kudu` directory.

. Add a `gerrit` remote. In the following command, substitute <username> with your
Github username.
+
[source,bash]
----
git remote add gerrit ssh://<username>@gerrit.cloudera.org:29418/kudu
----

. Run the following command to install the
Gerrit `commit-msg` hook. Use the following command, replacing `<username>` with your
Github username.
+
----
gitdir=$(git rev-parse --git-dir); scp -p -P 29418 <username>@gerrit.cloudera.org:hooks/commit-msg ${gitdir}/hooks/
----

. Be sure you have set the Kudu repository to use `pull --rebase` by default. You
can use the following two commands, assuming you have only ever checked out `master`
so far:
+
----
git config branch.autosetuprebase always
git config branch.master.rebase true
----
+
If for some reason you had already checked out branches other than `master`, substitute
`master` for the other branch names in the second command above.

=== Submitting Patches

To submit a patch, first commit your change (using a descriptive multi-line
commit message if possible), then push the request to the `gerrit` remote. For instance, to push a change
to the `master` branch:
----
git push gerrit HEAD:refs/for/master --no-thin
----

or to push a change to the `gh-pages` branch (to update the website):
----
git push gerrit HEAD:refs/for/gh-pages --no-thin
----

TIP: While preparing a patch for review, it's a good idea to follow
link:https://git-scm.com/book/en/v2/Distributed-Git-Contributing-to-a-Project#_commit_guidelines[generic git commit guidelines and good practices].

NOTE: The `--no-thin` argument is a workaround to prevent an error in Gerrit. See
https://code.google.com/p/gerrit/issues/detail?id=1582.

TIP: Consider creating Git aliases for the above commands. Gerrit also includes
a command-line tool called
link:https://www.mediawiki.org/wiki/Gerrit/Tutorial#Installing_git-review[git-review],
which you may find helpful.

Gerrit will add a change ID to your commit message and will create a Gerrit review,
whose URL will be emitted as part of the push reply. If desired, you can send a message
to the `kudu-dev` mailing list, explaining your patch and requesting review.

After getting feedback, you can update or amend your commit, (for instance, using
a command like `git commit --amend`) while leaving the Change
ID intact. Push your change to Gerrit again, and this will create a new patch set
in Gerrit and notify all reviewers about the change.

When your code has been reviewed and is ready to be merged into the Kudu code base,
a Kudu committer will merge it using Gerrit. You can discard your local branch.

=== Abandoning a Review

If your patch is not accepted or you decide to pull it from consideration, you can
use the Gerrit UI to *Abandon* the patch. It will still show in Gerrit's history,
but will not be listed as a pending review.

=== Reviewing Patches In Gerrit

You can view a unified or side-by-side diff of changes in Gerrit using the web UI.
To leave a comment, click the relevant line number or highlight the relevant part
of the line, and type 'c' to bring up a comment box. To submit your comments and/or
your review status, go up to the top level of the review and click *Reply*. You can
add additional top-level comments here, and submit them.

To check out code from a Gerrit review, click *Download* and paste the relevant Git
commands into your Git client. You can then update the commit and push to Gerrit to
submit a patch to the review, even if you were not the original reviewer.

Gerrit allows you to vote on a review. A vote of `+2` from at least one committer
(besides the submitter) is required before the patch can be merged.

== Code Style

Get familiar with these guidelines so that your contributions can be reviewed and
integrated quickly and easily.

In general, Kudu follows the
link:https://google.github.io/styleguide/cppguide.html[Google {cpp} Style Guide],
with the following exceptions:

=== Notes on {cpp} 11

Kudu uses {cpp} 11. Check out this handy guide to {cpp} 11 move semantics and rvalue
references: https://www.chromium.org/rvalue-references

We aim to follow most of the same guidelines, such as, where possible, migrating
away from `foo.Pass()` in favor of `std::move(foo)`.

=== Limitations on `boost` Use

`boost` classes from header-only libraries can be used in cases where a suitable
replacement does not exist in the Kudu code base. However:
* Do not introduce dependencies on `boost` classes not already depended on. We
  are actively trying to remove `boost` from the Kudu code base, and new
  dependencies make that work more difficult.
* Do not use `boost` in any public headers for the Kudu {cpp} client, because
  `boost` commonly breaks backward compatibility, and passing data between two
  `boost` versions (one by the user, one by Kudu) causes serious issues.

=== Line length

The Kudu team allows line lengths of 100 characters per line, rather than Google's standard of 80. Try to
keep under 80 where possible, but you can spill over to 100 or so if necessary.

=== Pointers

.Smart Pointers and Singly-Owned Pointers

Generally, most objects should have clear "single-owner" semantics.
Most of the time, singly-owned objects can be wrapped in a `gscoped_ptr<>`
or in a `unique_ptr` which ensures deletion on scope
exit and prevents accidental copying.
`gscoped_ptr` is similar to {cpp}11's `unique_ptr` in that it has a `release`
method and also provides emulated `move` semantics (see _gscoped_ptr.h_ for
example usage).

If an object is singly owned, but referenced from multiple places, such as when
the pointed-to object is known to be valid at least as long as the pointer itself,
associate a comment with the constructor which takes and stores the raw pointer,
as in the following example.

[source,c++]
----
  // 'blah' must remain valid for the lifetime of this class
  MyClass(const Blah* blah) :
    blah_(blah) {
  }
----

If you use raw pointers within STL collections or inside of vectors and other containers,
associate a comment with the container, which explains the ownership
semantics (owned or un-owned). Use utility code from _gutil/stl_util.h_, such as
`STLDeleteElements` or `ElementDeleter`, to ease handling of deletion of the
contained elements.

WARNING: Using `std::auto_ptr` is strictly disallowed because of its difficult and
bug-prone semantics. Besides, `std::auto_ptr` is declared deprecated
since {cpp}11.

.Smart Pointers for Multiply-Owned Pointers:

Although single ownership is ideal, sometimes it is not possible, particularly
when multiple threads are in play and the lifetimes of the pointers are not
clearly defined. In these cases, you can use either `std::shared_ptr` or
Kudu's own `scoped_refptr` from _gutil/ref_counted.hpp_. Each of these mechanisms
relies on reference counting to automatically delete the referent once no more
pointers remain. The key difference between these two types of pointers is that
`scoped_refptr` requires that the object extend a `RefCounted` base class, and
stores its reference count inside the object storage itself, while `shared_ptr`
maintains a separate reference count on the heap.

The pros and cons are:

.`shared_ptr`
[none]
* icon:plus-circle[role="green",alt="pro"] can be used with any type of object, without the
object deriving from a special base class
* icon:plus-circle[role="green",alt="pro"] part of the standard library and familiar to most
{cpp} developers
* icon:plus-circle[role="green",alt="pro"] supports the `weak_ptr` use cases:
 ** a temporary ownership when an object needs to be accessed only if it exists
 ** break circular references of `shared_ptr`, if any exists due to aggregation
* icon:minus-circle[role="green",alt="pro"] you can convert from the
`shared_ptr` into the `weak_ptr` and back
* icon:plus-circle[role="green",alt="pro"] if creating an instance with
`std::make_shared<>()` only one allocation is made (since {cpp}11;
a non-binding requirement in the Standard, though)
* icon:minus-circle[role="red",alt="con"] if creating a new object with
`shared_ptr<T> p(new T)` requires two allocations (one to create the ref count,
and one to create the object)
* icon:minus-circle[role="red",alt="con"] the ref count may not be near the object on the heap,
so extra cache misses may be incurred on access
* icon:minus-circle[role="red",alt="con"] the `shared_ptr` instance itself requires 16 bytes
(pointer to the ref count and pointer to the object)


.`scoped_refptr`
[none]
* icon:plus-circle[pro, role="green"] only requires a single allocation, and ref count
is on the same cache line as the object
* icon:plus-circle[pro, role="green"] the pointer only requires 8 bytes (since
the ref count is within the object)
* icon:plus-circle[pro, role="green"] you can manually increase or decrease
reference counts when more control is required
* icon:plus-circle[pro, role="green"] you can convert from a raw pointer back
to a `scoped_refptr` safely without worrying about double freeing
* icon:plus-circle[pro, role="green"] since we control the implementation, we
can implement features, such as debug builds that capture the stack trace of every
referent to help debug leaks.
* icon:minus-circle[con, role="red"] the referred-to object must inherit
from `RefCounted`
* icon:minus-circle[con, role="red"] does not support the `weak_ptr` use cases

Since `scoped_refptr` is generally faster and smaller, try to use it
rather than `shared_ptr` in new code. Existing code uses `shared_ptr`
in many places. When interfacing with that code, you can continue to use `shared_ptr`.

=== Function Binding and Callbacks

Existing code uses `boost::bind` and `boost::function` for function binding and
callbacks. For new code, use the `Callback` and `Bind` classes in `gutil` instead.
While less full-featured (`Bind` doesn't support argument
place holders, wrapped function pointers, or function objects), they provide
more options by the way of argument lifecycle management. For example, a
bound argument whose class extends `RefCounted` will be incremented during `Bind`
and decremented when the `Callback` goes out of scope.

See the large file comment in _gutil/callback.h_ for more details, and
_util/callback_bind-test.cc_ for examples.

=== `CMake` Style Guide

`CMake` allows commands in lower, upper, or mixed case. To keep
the CMake files consistent, please use the following guidelines:

- *built-in commands* in lowercase
----
add_subdirectory(some/path)
----
- *built-in arguments* in uppercase
----
message(STATUS "message goes here")
----
- *custom commands or macros* in uppercase
----
ADD_KUDU_TEST(some-test)
----

=== GFlags

Kudu uses gflags for both command-line and file-based configuration. Use these guidelines
to add a new gflag. All new gflags must conform to these
guidelines. Existing non-conformant ones will be made conformant in time.

.Name

The gflag's name conveys a lot of information, so choose a good name. The name
will propagate into other systems, such as the link:configuration_reference.html[Configuration
Reference].
- The different parts of a multi-word name should be separated by underscores.
  For example, `fs_data_dirs`.
- The name should be prefixed with the context that it affects. For example,
  `webserver_num_worker_threads` and `cfile_default_block_size`. Context can be
  difficult to define, so bear in mind that this prefix will be
  used to group similar gflags together. If the gflag affects the entire
  process, it should not be prefixed.
- If the gflag is for a quantity, the name should be suffixed with the units.
  For example, `tablet_copy_idle_timeout_ms`.
- Where possible, use short names. This will save time for those entering
  command line options by hand.
- The name is part of Kudu's compatibility contract, and should not change
  without very good reason.

.Default value

Choosing a default value is generally simple, but like the name, it propagates
into other systems.
- The default value is part of Kudu's compatibility contract, and should not
  change without very good reason.

.Description

The gflag's description should supplement the name and provide additional
context and information. Like the name, the description propagates into other
systems.
- The description may include multiple sentences. Each should begin with a
  capital letter, end with a period, and begin one space after the previous.
- The description should NOT include the gflag's type or default value; they are
  provided out-of-band.
- The description should be in the third person. Do not use words like `you`.
- A gflag description can be changed freely; it is not expected to remain the
  same across Kudu releases.

.Tags

Kudu's gflag tagging mechanism adds machine-readable context to each gflag, for
use in consuming systems such as documentation or management tools. See the large block
comment in _flag_tags.h_ for guidelines.

.Miscellaneous

- Avoid creating multiple gflags for the same logical parameter. For
  example, many Kudu binaries need to configure a WAL directory. Rather than
  creating `foo_wal_dir` and `bar_wal_dir` gflags, better to have a single
  `kudu_wal_dir` gflag for use universally.

== Testing

All new code should have tests.::
  Add new tests either in existing files, or create new test files as necessary.

All bug fixes should have tests.::
  It's OK to fix a bug without adding a
  new test if it's triggered by an existing test case. For example, if a
  race shows up when running a multi-threaded system test after 20
  minutes or so, it's worth trying to make a more targeted test case to
  trigger the bug. But if that's hard to do, the existing system test
  should be enough.

Tests should run quickly (< 1s).::
  If you want to write a time-intensive
  test, make the runtime dependent on `KuduTest#AllowSlowTests`, which is
  enabled via the `KUDU_ALLOW_SLOW_TESTS` environment variable and is
  used by Jenkins test execution.

Tests which run a number of iterations of some task should use a `gflags` command-line argument for the number of iterations.::
 This is handy for writing quick stress tests or performance tests.

Commits which may affect performance should include before/after `perf-stat(1)` output.::
  This will show performance improvement or non-regression.
  Performance-sensitive code should include some test case which can be used as a
  targeted benchmark.


== Documentation
See link:style_guide.html[Documentation Style Guide] for guidelines about contributing
to the official Kudu documentation.