build: Start using library versioning for various libraries

According to libtool three individual numbers stand for
CURRENT:REVISION:AGE, or C:R:A for short. The libtool
script typically tacks these three numbers onto the end
of the name of the .so file it creates. The formula for
calculating the file numbers on Linux and Solaris is

   /path/to/library/<library_name>.(C - A).(A).(R)

As you release new versions of your library, you will
update the library's C:R:A. Although the rules for changing
these version numbers can quickly become confusing, a few
simple tips should help keep you on track. The libtool
documentation goes into greater depth.

In essence, every time you make a change to the library and
release it, the C:R:A should change. A new library should start
with 0:0:0. Each time you change the public interface
(i.e., your installed header files), you should increment the
CURRENT number. This is called your interface number. The main
use of this interface number is to tag successive revisions
of your API.

The AGE number is how many consecutive versions of the API the
current implementation supports. Thus if the CURRENT library
API is the sixth published version of the interface and it is
also binary compatible with the fourth and fifth versions
(i.e., the last two), the C:R:A might be 6:0:2. When you break
binary compatibility, you need to set AGE to 0 and of course
increment CURRENT.

The REVISION marks a change in the source code of the library
that doesn't affect the interface-for example, a minor bug fix.
Anytime you increment CURRENT, you should set REVISION back to 0.

Change-Id: Id72e74c1642c804fea6f93ec109135c7c16f1810
BUG: 862082
Signed-off-by: Harshavardhana <harsha@harshavardhana.net>
Reviewed-on: http://review.gluster.org/5645
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Niels de Vos <ndevos@redhat.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>

1 files changed, 44 insertions, 0 deletions

diff --git a/doc/versioning.md b/doc/versioning.md
new file mode 100644
index 00000000000..10c1511b79b
--- /dev/null
+++ b/doc/versioning.md
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+Versioning
+==========
+
+### current
+
+The number of the current interface exported by the library. A current value
+of '1', means that you are calling the interface exported by this library
+interface 1.
+
+### revision
+
+The implementation number of the most recent interface exported by this library.
+In this case, a revision value of `0` means that this is the first
+implementation of the interface.
+
+If the next release of this library exports the same interface, but has a
+different implementation (perhaps some bugs have been fixed), the revision
+number will be higher, but current number will be the same. In that case, when
+given a choice, the library with the highest revision will always be used by
+the runtime loader.
+
+### age
+
+The number of previous additional interfaces supported by this library. If age
+were '2', then this library can be linked into executables which were built with
+a release of this library that exported the current interface number, current,
+or any of the previous two interfaces. By definition age must be less than or
+equal to current. At the outset, only the first ever interface is implemented,
+so age can only be `0'.
+
+For every release of the library `-version-info` argument needs to be set
+correctly depending on any interface changes you have made.
+
+This is quite straightforward when you understand what the three numbers mean:
+
+If you have changed any of the sources for this library, the revision number
+must be incremented. This is a new revision of the current interface. If the
+interface has changed, then current must be incremented, and revision reset
+to '0'.
+
+This is the first revision of a new interface. If the new interface is a
+superset of the previous interface (that is, if the previous interface has not
+been broken by the changes in this new release), then age must be incremented.
+This release is backwards compatible with the previous release.