[Boost-build] Understanding generated files / targets.

Subject: [Boost-build] Understanding generated files / targets.
From: Tom (tabsoftwareconsulting_at_[hidden])
Date: 2014-07-27 14:42:58

Hi boost.build experts,

I am trying to generate a C++ source (or header) file containing
versioning information generated from 'git describe'.  This has become
necessary as we have moved to git and a branched development model and
modifying a version file manually is a non-starter (due to conflicts
on merg) in this development model.  We will be driving the version
number in binaries by tags in revision control.

This is challenging mainly in that the generated file depends on state
not known (or knowable) by Boost.Build as opposed to a normal file
transformation.  This results in either not generating the file when
it should be (a tag is checked out or a file is edited which do not
affect the code handling the version number, then the build is run) or
in rebuilding things when it is not actually necessary (which causes
all tests to be re-run for example).

I do also understand the 'notfile' rule and I don't think it applies.
I am actually generating a real file, it just doesn't depend on any
other sources.

I've created a ticket with a detailed Jamroot and examples to try to
illustrate what I don't understand.  I'll try to be more brief here.

1. Is there a way to do this easily?  I looked around online and did
   not see anything that seemed quite right, they all depend on a file
   not unseen state.

2. Might there be a bug in the implementation or approach to handling
   generated files / targets?  There are several examples that don't
   make sense to me, but I could be misunderstanding some more complex
   issues.

Thanks!
Tom

trac: https://svn.boost.org/trac/boost/ticket/10248