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QUIP: 5
Title: Choosing a Branch
Author: Jędrzej Nowacki, Friedemann Kleint
Status: Draft
Type: Process
Created: 2016-12-05
Post-History: http://lists.qt-project.org/pipermail/development/2017-January/028248.html

========================================
Choosing a Branch
========================================

Motivation
----------

This QUIP offers guidelines on the right branch to submit a change to. It
originated from the
`QtCS2016 Managing Qt Branches <http://wiki.qt.io/QtCS2016_Managing_Qt_Branches>`_
session.

Note 1: The QUIP applies to the normal course of development. Close to
feature-freezes, branching and releases, the release team may impose
tighter control.

Note 2: The guidelines apply to the Qt frameworks only. Other products (like
Qt Creator) may choose a different approach.

Goal of the release branches (5.X.Y)
------------------------------------

- Preparation of the code base for a release undisturbed by activities in
  the stable branches

Goal of the long term support (LTS) branches (5.6 at the time of writing)
-------------------------------------------------------------------------

- Support for 3 years after its initial release, extending the stable phase
- Release roughly every 6 Months or when appropriate
- Support slower moving OSes
- Fix bugs in applications that were already shipped bundling the LTS and
  continue to build against the LTS
- Have a low regression risk
- Add new minor platforms - if and only if changes are minimal, like
  win8 => win8.1
- Add support for newer toolchains - if and only if changes are small, like
  adding MSVC 2015
- Documentation clarifications

We divide the LTS into 3 periods:

*stable*
    Initially, the LTS branch is handled like any stable branch.

*strict*
    This period starts when the subsequent stable branch is created (for the
    5.6 LTS, this would have been 5.7). Submitting to the LTS branch directly
    is then no longer possible; changes must be submitted to the stable branch
    and undergo some testing first before being cherry-picked into the LTS
    branch. Upgrading the Qt libraries (within LTS) must not put installed
    applications at risk of breaking. Breaking existing bug workarounds is
    therefore not allowed, even if the bug is fixed.

*very strict*
   (third year) We believe that there are no more P1 and P0 bugs.

Goal of the stable branches (5.x)
---------------------------------

- Bug fixes
- Performance fixes
- Documentation fixes
- As a special case, pre-release stabilization immediately after branching
  off the dev branch

Goal of the dev branch:
-----------------------

- New feature development (adding new API, classes, libraries or modules)
- Refactoring
- Risky changes, requiring longer testing period (moc, qmake...)
- Completely new platforms (WinRT)

Approach
--------

The table below lists typical code changes, and indicates the branch the
change should be pushed to. In all cases, we need to estimate the impact of
the change. For example, a fix for a performance problem may end up in a
different branch depending on: how many users are affected, how big is the
improvement, and what is the risk of regressions. The table shows the most
stable branch (i.e. right-most), for every case. If a change is risky or it
has lower impact, then it should be placed in a less stable (i.e. further to
the left) branch.

+----------------------------+-----+--------+----------------------+---------+---------------------+
|                            |     |        |            LTS       |         |                     |
| Reason for Change          | Dev | Stable +--------+-------------+ Release | Notes               |
|                            |     |        | Strict | Very Strict |         |                     |
+----------------------------+-----+--------+--------+-------------+---------+---------------------+
| Binary compatibility break |     |        |        |             |    x    |                     |
+----------------------------+-----+--------+--------+-------------+---------+---------------------+
| Source compatibility break |     |        |        |             |    x    |                     |
+----------------------------+-----+--------+--------+-------------+---------+---------------------+
| API review finding         |     |        |        |             |    x    | Initial release     |
|                            |     |        |        |             |         | only                |
+----------------------------+-----+--------+--------+-------------+---------+---------------------+
| RTA/package test finding   |     |        |        |             |    x    |                     |
+----------------------------+-----+--------+--------+-------------+---------+---------------------+
| Security                   |     |        |        |             |    x    |                     |
+----------------------------+-----+--------+--------+-------------+---------+---------------------+
| 3rd party update           |     |        |        |    x        |         | Very Strict only    |
|                            |     |        |        |             |         | security/critical   |
+----------------------------+-----+--------+--------+-------------+---------+---------------------+
| Test improvement           |     |        |        |    x        |         |                     |
| (reduce flakiness)         |     |        |        |             |         |                     |
+----------------------------+-----+--------+--------+-------------+---------+---------------------+
| Adapt to minor OS/         |     |        |        |    x        |         |                     |
| Compiler updates           |     |        |        |             |         |                     |
+----------------------------+-----+--------+--------+-------------+---------+---------------------+
| Memory leak                |     |        |    x   |             |         |                     |
+----------------------------+-----+--------+--------+-------------+---------+---------------------+
| Fix to feature added       |     |        |    x   |             |         |                     |
| in LTS                     |     |        |        |             |         |                     |
+----------------------------+-----+--------+--------+-------------+---------+---------------------+
| Undefined behavior         |     |        |    x   |             |         |                     |
+----------------------------+-----+--------+--------+-------------+---------+---------------------+
| Something that makes       |     |        |    x   |             |         |                     |
| existing apps unstable     |     |        |        |             |         |                     |
+----------------------------+-----+--------+--------+-------------+---------+---------------------+
| Performance: significant   |     |        |    x   |             |         |                     |
| fix improving O()          |     |        |        |             |         |                     |
+----------------------------+-----+--------+--------+-------------+---------+---------------------+
| Performance: Issue         |     |   x    |        |             |         |                     |
| detected by profiling      |     |        |        |             |         |                     |
+----------------------------+-----+--------+--------+-------------+---------+---------------------+
| Performance: Issue related |  x  |        |        |             |         |                     |
| to best practices (like    |     |        |        |             |         |                     |
| usage of reserve())        |     |        |        |             |         |                     |
+----------------------------+-----+--------+--------+-------------+---------+---------------------+
| Fix compiler warnings      |     |        |    x   |             |         |                     |
+----------------------------+-----+--------+--------+-------------+---------+---------------------+
| Suppression of false       |     |   x    |        |             |         | e.g. static code    |
| positives raised by tools  |     |        |        |             |         | analysers           |
+----------------------------+-----+--------+--------+-------------+---------+---------------------+
| Documentation: small fixes |     |        |        |    x        |         |                     |
| (links, typos, warnings)   |     |        |        |             |         |                     |
+----------------------------+-----+--------+--------+-------------+---------+---------------------+
| Documentation: refactoring |     |   x    |        |             |         |                     |
+----------------------------+-----+--------+--------+-------------+---------+---------------------+
| Bugs: Easily triggered     |     |        |        |    x        |         |                     |
| crash                      |     |        |        |             |         |                     |
+----------------------------+-----+--------+--------+-------------+---------+---------------------+
| Bugs: Hard to reproduce    |     |        |    x   |             |         |                     |
| crash                      |     |        |        |             |         |                     |
+----------------------------+-----+--------+--------+-------------+---------+---------------------+
| Bugs: Regression           |     |        |    x   |             |         |                     |
+----------------------------+-----+--------+--------+-------------+---------+---------------------+
| Bugs: Hard-to-reproduce    |     |   x    |        |             |         |                     |
| regression noticed some    |     |        |        |             |         |                     |
| time after release         |     |        |        |             |         |                     |
+----------------------------+-----+--------+--------+-------------+---------+---------------------+
| Bugs: Other                |     |   x    |        |             |         |                     |
+----------------------------+-----+--------+--------+-------------+---------+---------------------+
| New modules                |  x  |        |        |             |         |                     |
+----------------------------+-----+--------+--------+-------------+---------+---------------------+
| New features               |  x  |        |        |             |         |                     |
+----------------------------+-----+--------+--------+-------------+---------+---------------------+
| Major build system changes |  x  |        |        |             |         |                     |
+----------------------------+-----+--------+--------+-------------+---------+---------------------+
| Refactoring: tests         |     |   x    |        |             |         |                     |
+----------------------------+-----+--------+--------+-------------+---------+---------------------+
| Refactoring: non-test code |  x  |        |        |             |         |                     |
+----------------------------+-----+--------+--------+-------------+---------+---------------------+
| Refactoring: build system  |  x  |        |        |             |         |                     |
+----------------------------+-----+--------+--------+-------------+---------+---------------------+
| Enabling new compiler      |  x  |        |        |             |         |                     |
| features                   |     |        |        |             |         |                     |
+----------------------------+-----+--------+--------+-------------+---------+---------------------+
| Using new C++              |  x  |        |        |             |         | This includes       |
| features                   |     |        |        |             |         | mass updates        |
+----------------------------+-----+--------+--------+-------------+---------+---------------------+

Glossary
--------

RTA
    Release testing automation

References
----------

- `Branch Guidelines <http://wiki.qt.io/Branch_Guidelines>`_
- `JIRA-Priorities <https://wiki.qt.io/JIRA-Priorities>`_