allow jinja2 templates to be located in current conda environment

[ukoethe]

As requested, this is an update of PR #575. (I nonetheless keep #575 open for now so that we can continue our conversation there.)

[ukoethe]

Would someone from the core team please care to comment on this PR? It is very critical for my work. Thank you very much! @msarahan @ilanschnell

To clarify the PR, I repeat its rationale here:

Jinja templates are a powerful way to configure meta.yaml. Right now, conda build searches for these templates in two locations: <conda_root>/Lib/site-packages/conda_build/templates (for site-specific configurations) and $RECIPE_DIR (for package-specific configurations). I propose to add the current environment (as specified in the environment variable $CONDA_ENV_PATH $CONDA_DEFAULT_ENV) as an additional search location to support environment-specific configurations.

Use case: Suppose one wants to build a set of packages with exactly identical toolset settings (same compiler options, tool versions etc.). This can be achieved by performing the build in a suitably prepared conda environment. However, conda does not yet provide a portable query method for information about the current environment within meta.yaml. The present PR adds this capability. For example, let's say that a jinja-template toolset-info.yaml is placed into the environment's root directory upon installation of some toolset package. This template can now be loaded in meta.yaml via
{% import '$CONDA_ENV_PATH/toolset-info.yaml' as toolset %}

{% import '$CONDA_DEFAULT_ENV/toolset-info.yaml' as toolset %}

and later be accessed via variables like {{ toolset.name }} or {{ toolset.version }} etc.

[ukoethe]

I just noticed that the environment variable CONDA_ENV_PATH was renamed into CONDA_DEFAULT_ENV recently and updated the PR accordingly.

[ilanschnell]

There is no variable CONDA_ENV_PATH, only CONDA_ENVS_PATH (the environments location directory), which is separate from CONDA_DEFAULT_ENV. None of these have been renamed.

[ukoethe]

There is no variable CONDA_ENV_PATH, only CONDA_ENVS_PATH (the environments location directory), which is separate from CONDA_DEFAULT_ENV. None of these have been renamed.

Strange. It worked for me, and I'm quite certain that I never defined CONDA_ENV_PATH myself. Anyway, in the latest commit I use CONDA_DEFAULT_ENV instead.

[msarahan]

+1 from me. The travis build failure is unrelated. Missing OpenSSL package that we haven't built on python 3.3 yet.

[msarahan]

Discussed with @ilanschnell and @groutr - we think this is a very nice addition. Thanks!

[ukoethe]

Thanks for merging, @msarahan! See conda/conda#1589 (comment) for a usage example.