We have something like the following (target bodies omitted):

  <target name="checkout-sources" depends="other stuff">
    <!-- takes several minutes -->

  <target name="determine-release-number"
    depends="checkout-sources" unless="release.number">
    <!-- determines release.number from sources -->

  <target name="target1" depends="determine-release-number">
    <!-- needs release.number -->

  <target name="target2" depends="checkout-sources, target1">
    <!-- needs sources and target1 -->

The problem: "ant -Drelease.number=... target1" causes
checkout-sources (and its dependencies) to be executed although there
is no need for it, as release.number is already defined.

We can't put the unless="release.number" on checkout-sources (and its
dependencies) because it would break target2 when release.number is
predefined.

We can't make checkout-sources a macro (called both by target2 and by
determine-release-number) because of all its dependencies, and also
because an up-to-date check on it is highly non-trivial, so in the
target2 case it might end up being executed twice.

We _could_ duplicate the body and the dependencies of checkout-sources
into determine-release-number (and remove the latter's dependency on
checkout-sources), but I'd have to do that for quite a number of the
dependencies as well, as they contribute to the expensiveness of
checkout-sources. The result would be a lot of redundancy, effectively
duplicating a dependency subgraph.

Is there a way to solve the problem, within a single Ant instance,
without creating redundancies?

-- Niklas Matthies

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