Add the semver documentation

This is a fork of the upstream semver documentation with an aim towards
being a PEP440 compliant implementation of the ideas.

Change-Id: I66a543fc08e0c1148a6db02dce207deabafed664

2 files changed, 331 insertions, 2 deletions

diff --git a/doc/source/index.rst b/doc/source/index.rst
index d05657b..4013f90 100644
--- a/doc/source/index.rst
+++ b/doc/source/index.rst
@@ -32,6 +32,8 @@ will be very similar to git describe. If you do, then we'll assume that's the
 version you are working towards, and will generate alpha version strings
 based on commits since last tag and the current git sha.
 
+The versions are expected to be compliant with :doc:`semver`.
+
 AUTHORS and ChangeLog
 ---------------------
 
@@ -191,13 +193,14 @@ installed for `pbr.config.drivers`, one called `plain` which maps to the
 `Plain` class in `pbr.cfg.driver` and one called `fancy` which maps to the
 `Fancy` class in `pbr.cfg.driver`.
 
-Packager Notes
-==============
+Additional Docs
+===============
 
 .. toctree::
    :maxdepth: 1
 
    packagers
+   semver
 
 Indices and tables
 ==================
diff --git a/doc/source/semver.rst b/doc/source/semver.rst
new file mode 100644
index 0000000..d7f8421
--- /dev/null
+++ b/doc/source/semver.rst
@@ -0,0 +1,326 @@
+Linux Compatible Semantic Versioning 3.0.0
+==========================================
+
+This is a fork of Semantic Versioning 2.0. The specific changes have to do
+with the format of pre-release and build labels, specifically to make them
+not confusing when co-existing with Linux Distribution packaging.
+Inspiration for the format of the pre-release and build labels came from
+Python's PEP440.
+
+Summary
+-------
+
+Given a version number MAJOR.MINOR.PATCH, increment the:
+
+#. MAJOR version when you make incompatible API changes,
+#. MINOR version when you add functionality in a backwards-compatible
+   manner, and
+#. PATCH version when you make backwards-compatible bug fixes.
+
+Additional labels for pre-release and build metadata are available as
+extensions to the MAJOR.MINOR.PATCH format.
+
+Introduction
+------------
+
+In the world of software management there exists a dread place called
+"dependency hell." The bigger your system grows and the more packages
+you integrate into your software, the more likely you are to find
+yourself, one day, in this pit of despair.
+
+In systems with many dependencies, releasing new package versions can
+quickly become a nightmare. If the dependency specifications are too
+tight, you are in danger of version lock (the inability to upgrade a
+package without having to release new versions of every dependent
+package). If dependencies are specified too loosely, you will inevitably
+be bitten by version promiscuity (assuming compatibility with more
+future versions than is reasonable). Dependency hell is where you are
+when version lock and/or version promiscuity prevent you from easily and
+safely moving your project forward.
+
+As a solution to this problem, I propose a simple set of rules and
+requirements that dictate how version numbers are assigned and
+incremented. These rules are based on but not necessarily limited to
+pre-existing widespread common practices in use in both closed and
+open-source software. For this system to work, you first need to declare
+a public API. This may consist of documentation or be enforced by the
+code itself. Regardless, it is important that this API be clear and
+precise. Once you identify your public API, you communicate changes to
+it with specific increments to your version number. Consider a version
+format of X.Y.Z (Major.Minor.Patch). Bug fixes not affecting the API
+increment the patch version, backwards compatible API additions/changes
+increment the minor version, and backwards incompatible API changes
+increment the major version.
+
+I call this system "Semantic Versioning." Under this scheme, version
+numbers and the way they change convey meaning about the underlying code
+and what has been modified from one version to the next.
+
+Linux Compatible Semantic Versioning is different from Semantic
+Versioning in that it does not employ the use of the hypen in ways that
+are ambiguous when used with or adjacent to software packaged with dpkg or
+rpm. Instead, it draws from PEP440's approach of indicating pre-releases
+with leading characters in the version segment.
+
+Semantic Versioning Specification (SemVer)
+------------------------------------------
+
+The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
+"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
+document are to be interpreted as described in `RFC
+2119 <http://tools.ietf.org/html/rfc2119>`__.
+
+#.  Software using Semantic Versioning MUST declare a public API. This
+    API could be declared in the code itself or exist strictly in
+    documentation. However it is done, it should be precise and
+    comprehensive.
+
+#.  A normal version number MUST take the form X.Y.Z where X, Y, and Z
+    are non-negative integers, and MUST NOT contain leading zeroes. X is
+    the major version, Y is the minor version, and Z is the patch
+    version. Each element MUST increase numerically. For instance: 1.9.0
+    -> 1.10.0 -> 1.11.0.
+
+#.  Once a versioned package has been released, the contents of that
+    version MUST NOT be modified. Any modifications MUST be released as
+    a new version.
+
+#.  Major version zero (0.y.z) is for initial development. Anything may
+    change at any time. The public API should not be considered stable.
+
+#.  Version 1.0.0 defines the public API. The way in which the version
+    number is incremented after this release is dependent on this public
+    API and how it changes.
+
+#.  Patch version Z (x.y.Z \| x > 0) MUST be incremented if only
+    backwards compatible bug fixes are introduced. A bug fix is defined
+    as an internal change that fixes incorrect behavior.
+
+#.  Minor version Y (x.Y.z \| x > 0) MUST be incremented if new,
+    backwards compatible functionality is introduced to the public API.
+    It MUST be incremented if any public API functionality is marked as
+    deprecated. It MAY be incremented if substantial new functionality
+    or improvements are introduced within the private code. It MAY
+    include patch level changes. Patch version MUST be reset to 0 when
+    minor version is incremented.
+
+#.  Major version X (X.y.z \| X > 0) MUST be incremented if any
+    backwards incompatible changes are introduced to the public API. It
+    MAY also include minor and patch level changes. Patch and minor
+    version MUST be reset to 0 when major version is incremented.
+
+#.  A pre-release version MAY be denoted by appending a dot
+    separated identifier immediately following the patch version.
+    The identifier MUST comprise only a, b, rc followed by non-negative
+    integer value. The identifier MUST NOT be empty.
+    Pre-release versions have a lower precedence than the associated normal
+    version. A pre-release version indicates that
+    the version is unstable and might not satisfy the intended
+    compatibility requirements as denoted by its associated normal
+    version. Examples: 1.0.0.a1, 1.0.0.b99, 1.0.0.rc1000.
+
+#.  A development version MAY be denoted by appending a dot separated
+    indentifier immediately following the patch version.
+    The identifier MUST comprise the string dev followed by non-negative
+    integer value. The identifier MUST NOT be empty. Development versions
+    have a lower precedence than the associated normal version. A development
+    version is a completely unsupported and conveys no API promises when
+    related to other versions. They are more useful as communication
+    vehicles between developers of a community, whereas pre-releases, while
+    potentially prone to break still, are intended for externally facing
+    communication of not-yet-released ideas. Example: 1.0.0.dev1.
+
+#.  git version metadata MAY be denoted by appending a dot separated
+    identifier immediately following a developement version.
+    The identifier MUST comprise the character g followed by a seven
+    character git short-sha. The sha MUST NOT be empty. git version
+    metadata MUST be ignored when determining version precedence. Thus
+    two versions that differ only in the git version, have the same
+    precedence. Example: 1.0.0.a1.g95a9beb.
+
+#.  Build metadata MAY be denoted by appending a plus sign and a series
+    of dot separated identifiers immediately following the patch or
+    pre-release version. Identifiers MUST comprise only ASCII
+    alphanumerics [0-9A-Za-z]. Identifiers MUST NOT be empty. Build
+    metadata MUST be ignored when determining version precedence. Thus
+    two versions that differ only in the build metadata, have the same
+    precedence. Examples: 1.0.0.a1+001, 1.0.0+20130313144700,
+    1.0.0.b1+exp.sha.5114f85.
+
+#.  Precedence refers to how versions are compared to each other when
+    ordered. Precedence MUST be calculated by separating the version
+    into major, minor, patch and pre-release identifiers in that order
+    (Build metadata does not figure into precedence). Precedence is
+    determined by the first difference when comparing each of these
+    identifiers from left to right as follows: Major, minor, and patch
+    versions are always compared numerically. Example: 1.0.0 < 2.0.0 <
+    2.1.0 < 2.1.1. When major, minor, and patch are equal, a pre-release
+    version has lower precedence than a normal version. Example:
+    1.0.0.a1 < 1.0.0. When major, minor, and patch are equal, a development
+    version as a lower precedence than a normal version and of a pre-release
+    version. Example: 1.0.0.dev1 < 1.0.0 and 1.0.0dev9 < 1.0.0a1.
+    Precedence for two pre-release or development versions with
+    the same major, minor, and patch version MUST be determined by
+    comparing the identifier to the right of the patch version as follows:
+    if the alpha portion matches, the numeric portion is compared in
+    numerical sort order. If the alpha portion does not match, the sort
+    order is dev < a < b < rc. Example: 1.0.0.dev8 < 1.0.0.dev9
+    1.0.0.a1 < 1.0.0.b2 < 1.0.0.rc1 < 1.0.0.
+
+Why Use Semantic Versioning?
+----------------------------
+
+This is not a new or revolutionary idea. In fact, you probably do
+something close to this already. The problem is that "close" isn't good
+enough. Without compliance to some sort of formal specification, version
+numbers are essentially useless for dependency management. By giving a
+name and clear definition to the above ideas, it becomes easy to
+communicate your intentions to the users of your software. Once these
+intentions are clear, flexible (but not too flexible) dependency
+specifications can finally be made.
+
+A simple example will demonstrate how Semantic Versioning can make
+dependency hell a thing of the past. Consider a library called
+"Firetruck." It requires a Semantically Versioned package named
+"Ladder." At the time that Firetruck is created, Ladder is at version
+3.1.0. Since Firetruck uses some functionality that was first introduced
+in 3.1.0, you can safely specify the Ladder dependency as greater than
+or equal to 3.1.0 but less than 4.0.0. Now, when Ladder version 3.1.1
+and 3.2.0 become available, you can release them to your package
+management system and know that they will be compatible with existing
+dependent software.
+
+As a responsible developer you will, of course, want to verify that any
+package upgrades function as advertised. The real world is a messy
+place; there's nothing we can do about that but be vigilant. What you
+can do is let Semantic Versioning provide you with a sane way to release
+and upgrade packages without having to roll new versions of dependent
+packages, saving you time and hassle.
+
+If all of this sounds desirable, all you need to do to start using
+Semantic Versioning is to declare that you are doing so and then follow
+the rules. Link to this website from your README so others know the
+rules and can benefit from them.
+
+FAQ
+---
+
+How should I deal with revisions in the 0.y.z initial development phase?
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+The simplest thing to do is start your initial development release at
+0.1.0 and then increment the minor version for each subsequent release.
+
+How do I know when to release 1.0.0?
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+If your software is being used in production, it should probably already
+be 1.0.0. If you have a stable API on which users have come to depend,
+you should be 1.0.0. If you're worrying a lot about backwards
+compatibility, you should probably already be 1.0.0.
+
+Doesn't this discourage rapid development and fast iteration?
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Major version zero is all about rapid development. If you're changing
+the API every day you should either still be in version 0.y.z or on a
+separate development branch working on the next major version.
+
+If even the tiniest backwards incompatible changes to the public API require a major version bump, won't I end up at version 42.0.0 very rapidly?
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+This is a question of responsible development and foresight.
+Incompatible changes should not be introduced lightly to software that
+has a lot of dependent code. The cost that must be incurred to upgrade
+can be significant. Having to bump major versions to release
+incompatible changes means you'll think through the impact of your
+changes, and evaluate the cost/benefit ratio involved.
+
+Documenting the entire public API is too much work!
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+It is your responsibility as a professional developer to properly
+document software that is intended for use by others. Managing software
+complexity is a hugely important part of keeping a project efficient,
+and that's hard to do if nobody knows how to use your software, or what
+methods are safe to call. In the long run, Semantic Versioning, and the
+insistence on a well defined public API can keep everyone and everything
+running smoothly.
+
+What do I do if I accidentally release a backwards incompatible change as a minor version?
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+As soon as you realize that you've broken the Semantic Versioning spec,
+fix the problem and release a new minor version that corrects the
+problem and restores backwards compatibility. Even under this
+circumstance, it is unacceptable to modify versioned releases. If it's
+appropriate, document the offending version and inform your users of the
+problem so that they are aware of the offending version.
+
+What should I do if I update my own dependencies without changing the public API?
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+That would be considered compatible since it does not affect the public
+API. Software that explicitly depends on the same dependencies as your
+package should have their own dependency specifications and the author
+will notice any conflicts. Determining whether the change is a patch
+level or minor level modification depends on whether you updated your
+dependencies in order to fix a bug or introduce new functionality. I
+would usually expect additional code for the latter instance, in which
+case it's obviously a minor level increment.
+
+What if I inadvertently alter the public API in a way that is not compliant with the version number change (i.e. the code incorrectly introduces a major breaking change in a patch release)
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Use your best judgment. If you have a huge audience that will be
+drastically impacted by changing the behavior back to what the public
+API intended, then it may be best to perform a major version release,
+even though the fix could strictly be considered a patch release.
+Remember, Semantic Versioning is all about conveying meaning by how the
+version number changes. If these changes are important to your users,
+use the version number to inform them.
+
+How should I handle deprecating functionality?
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Deprecating existing functionality is a normal part of software
+development and is often required to make forward progress. When you
+deprecate part of your public API, you should do two things: (1) update
+your documentation to let users know about the change, (2) issue a new
+minor release with the deprecation in place. Before you completely
+remove the functionality in a new major release there should be at least
+one minor release that contains the deprecation so that users can
+smoothly transition to the new API.
+
+Does SemVer have a size limit on the version string?
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+No, but use good judgment. A 255 character version string is probably
+overkill, for example. Also, specific systems may impose their own
+limits on the size of the string.
+
+About
+-----
+
+The Linux Compatible Semantic Versioning specification was modified by
+`Monty Taylor <http://inaugust.com>`__, member of `The Satori
+Group <http://satori-group.com>`__, co-founder of OpenStack and Free
+Software Hacker.
+
+It was based on The Semantic Versioning specification, which was
+authored by `Tom Preston-Werner <http://tom.preston-werner.com>`__,
+inventor of Gravatars and cofounder of GitHub, with inputs from `PEP
+440 <http://www.python.org/dev/peps/pep-0440/>`__ which was authored by
+`Nick Coughlan <http://www.boredomandlaziness.org>`__ who is a core
+Python developer and generally a great guy. I don't really know which
+things Nick invented or co-founded, and I'm not really sure why we'd
+need to list those here, but Tom did, so I figured coding style is
+usually about sticking to the style that was there before you showed up.
+
+If you'd like to leave feedback, please `open an issue on
+GitHub <https://github.com/emonty/semver/issues>`__.
+
+License
+-------
+
+Creative Commons - CC BY 3.0 http://creativecommons.org/licenses/by/3.0/