CMake/Help/policy/CMP0114.rst

CMP0114
-------

.. versionadded:: 3.19

:module:`ExternalProject` step targets fully adopt their steps.

The :command:`ExternalProject_Add` ``STEP_TARGETS`` option, and the
:command:`ExternalProject_Add_StepTargets` function, can be used to
create build targets for individual steps of an external project.

In CMake 3.18 and below, step targets have some limitations:

* Step targets always depend on targets named by the
  :command:`ExternalProject_Add` ``DEPENDS`` option even though
  not all steps need them.  In order to allow step targets to be created
  without those dependencies, the :command:`ExternalProject_Add`
  ``INDEPENDENT_STEP_TARGETS`` option or the
  :command:`ExternalProject_Add_StepTargets` ``NO_DEPENDS`` option may
  be used.  However, adding such "independent" step targets makes sense
  only for specific steps such as ``download``, ``update``, and ``patch``
  because they do not need any of the external project's build dependencies.
  Furthermore, it does not make sense to create independent step targets
  for steps that depend on non-independent steps.  Such rules are not
  enforced, and projects that do not follow them can generate build systems
  with confusing and generator-specific behavior.

* Step targets hold copies of the custom commands implementing their
  steps that are separate from the copies in the primary target created
  by :command:`ExternalProject_Add`, and the primary target does not
  depend on the step targets.  In parallel builds that drive the primary
  target and step targets concurrently, multiple copies of the steps'
  commands may run concurrently and race each other.

  Also, prior to policy :policy:`CMP0113`, the step targets generated
  by :ref:`Makefile Generators` also contain all the custom commands
  on which their step depends.  This can lead to repeated execution of
  those steps even in serial builds.

In CMake 3.19 and above, the :module:`ExternalProject` module prefers
a revised design to address these problems:

* Each step is classified as "independent" if it does not depend
  on other targets named by the :command:`ExternalProject_Add` ``DEPENDS``.
  The predefined steps are automatically classified by default:

  * The ``download``, ``update``, and ``patch`` steps are independent.
  * The ``configure``, ``build``, ``test``, and ``install`` steps are not.

  For custom steps, the :command:`ExternalProject_Add_Step` command provides
  an ``INDEPENDENT`` option to mark them as independent.  It is an error to
  mark a step as independent if it depends on other steps that are not.  Note
  that this use of the term "independent" refers only to independence from
  external targets and is orthogonal to a step's dependencies on other steps.

* Step targets created by the :command:`ExternalProject_Add` ``STEP_TARGETS``
  option or the :command:`ExternalProject_Add_Step` function are now
  independent if and only if their steps are marked as independent.
  The :command:`ExternalProject_Add` ``INDEPENDENT_STEP_TARGETS`` option
  and :command:`ExternalProject_Add_StepTargets` ``NO_DEPENDS`` option
  are no longer allowed.

* Step targets, when created, are fully responsible for holding the
  custom commands implementing their steps.  The primary target created
  by :command:`ExternalProject_Add` depends on the step targets, and the
  step targets depend on each other.  The target-level dependencies match
  the file-level dependencies used by the custom commands for each step.

  When the :command:`ExternalProject_Add` ``UPDATE_DISCONNECTED`` or
  ``TEST_EXCLUDE_FROM_MAIN`` option is used, or the
  :command:`ExternalProject_Add_Step` ``EXCLUDE_FROM_MAIN`` option is used
  for a custom step, some step targets may be created automatically.
  These are needed to hold the steps commonly depended upon by the primary
  target and the disconnected step targets.

Policy ``CMP0114`` provides compatibility for projects that have not been
updated to expect the new behavior.  The ``OLD`` behavior for this policy
is to use the above-documented behavior from 3.18 and below.  The ``NEW``
behavior for this policy is to use the above-documented behavior preferred
by 3.19 and above.

This policy was introduced in CMake version 3.19.  CMake version
|release| warns when the policy is not set and uses ``OLD`` behavior.
Use the :command:`cmake_policy` command to set it to ``OLD`` or ``NEW``
explicitly.