find_package: Use PackageName_ROOT variables as search prefixes

This feature was originally added by commit v3.9.0-rc1~71^2~2 (find_*:
Add a new PackageRoot search path group, 2017-05-03) and documented by
commit v3.9.0-rc1~71^2 (find_*: Add docs for PackageRoot search path
group, 2017-05-03).  However, we had to disable the feature and remove
the documentation in commit v3.9.1~2^2 (find_*: Disable the PACKAGE_ROOT
search path group for CMake 3.9, 2017-08-08) due to breaking projects
that used `PackageName_ROOT` variables themselves.

Add policy `CMP0074` to restore the `PackageName_ROOT` variable behavior
in a compatible way.  Also revise the stack of root paths to store the
paths themselves rather than the package names.  This way the policy can
be considered at the `find_package` call site instead of individual
`find_` calls inside a find module.

Co-Author: Chuck Atkins <chuck.atkins@kitware.com>
Issue: #17144

9 files changed, 81 insertions, 14 deletions

diff --git a/Help/command/FIND_XXX.txt b/Help/command/FIND_XXX.txt
index 7db221c..13ea8ab 100644
--- a/Help/command/FIND_XXX.txt
+++ b/Help/command/FIND_XXX.txt
@@ -16,6 +16,7 @@ The general signature is:
              [PATH_SUFFIXES suffix1 [suffix2 ...]]
              [DOC "cache documentation string"]
              [NO_DEFAULT_PATH]
+             [NO_PACKAGE_ROOT_PATH]
              [NO_CMAKE_PATH]
              [NO_CMAKE_ENVIRONMENT_PATH]
              [NO_SYSTEM_ENVIRONMENT_PATH]
@@ -60,6 +61,10 @@ If ``NO_DEFAULT_PATH`` is specified, then no additional paths are
 added to the search.
 If ``NO_DEFAULT_PATH`` is not specified, the search process is as follows:
 
+.. |FIND_PACKAGE_ROOT_PREFIX_PATH_XXX_SUBDIR| replace::
+   |prefix_XXX_SUBDIR| for each ``<prefix>`` in ``PackageName_ROOT`` if called
+   from within a find module
+
 .. |CMAKE_PREFIX_PATH_XXX_SUBDIR| replace::
    |prefix_XXX_SUBDIR| for each ``<prefix>`` in :variable:`CMAKE_PREFIX_PATH`
 
@@ -71,7 +76,19 @@ If ``NO_DEFAULT_PATH`` is not specified, the search process is as follows:
    |prefix_XXX_SUBDIR| for each ``<prefix>`` in
    :variable:`CMAKE_SYSTEM_PREFIX_PATH`
 
-1. Search paths specified in cmake-specific cache variables.
+1. If called from within a find module, search prefix paths unique to the
+   current package being found.  Specifically look in the ``PackageName_ROOT``
+   CMake and environment variables.  The package root variables are maintained
+   as a stack so if called from nested find modules, root paths from the
+   parent's find module will be searchd after paths from the current module,
+   i.e. ``CurrentPackage_ROOT``, ``ENV{CurrentPackage_ROOT}``,
+   ``ParentPackage_ROOT``, ``ENV{ParentPacakge_ROOT}``, etc.
+   This can be skipped if ``NO_PACKAGE_ROOT_PATH`` is passed.
+   See policy :policy:`CMP0074`.
+
+   * |FIND_PACKAGE_ROOT_PREFIX_PATH_XXX|
+
+2. Search paths specified in cmake-specific cache variables.
    These are intended to be used on the command line with a ``-DVAR=value``.
    The values are interpreted as :ref:`;-lists <CMake Language Lists>`.
    This can be skipped if ``NO_CMAKE_PATH`` is passed.
@@ -80,7 +97,7 @@ If ``NO_DEFAULT_PATH`` is not specified, the search process is as follows:
    * |CMAKE_XXX_PATH|
    * |CMAKE_XXX_MAC_PATH|
 
-2. Search paths specified in cmake-specific environment variables.
+3. Search paths specified in cmake-specific environment variables.
    These are intended to be set in the user's shell configuration,
    and therefore use the host's native path separator
    (``;`` on Windows and ``:`` on UNIX).
@@ -90,17 +107,17 @@ If ``NO_DEFAULT_PATH`` is not specified, the search process is as follows:
    * |CMAKE_XXX_PATH|
    * |CMAKE_XXX_MAC_PATH|
 
-3. Search the paths specified by the ``HINTS`` option.
+4. Search the paths specified by the ``HINTS`` option.
    These should be paths computed by system introspection, such as a
    hint provided by the location of another item already found.
    Hard-coded guesses should be specified with the ``PATHS`` option.
 
-4. Search the standard system environment variables.
+5. Search the standard system environment variables.
    This can be skipped if ``NO_SYSTEM_ENVIRONMENT_PATH`` is an argument.
 
    * |SYSTEM_ENVIRONMENT_PATH_XXX|
 
-5. Search cmake variables defined in the Platform files
+6. Search cmake variables defined in the Platform files
    for the current system.  This can be skipped if ``NO_CMAKE_SYSTEM_PATH``
    is passed.
 
@@ -108,7 +125,7 @@ If ``NO_DEFAULT_PATH`` is not specified, the search process is as follows:
    * |CMAKE_SYSTEM_XXX_PATH|
    * |CMAKE_SYSTEM_XXX_MAC_PATH|
 
-6. Search the paths specified by the PATHS option
+7. Search the paths specified by the PATHS option
    or in the short-hand version of the command.
    These are typically hard-coded guesses.
 
diff --git a/Help/command/find_file.rst b/Help/command/find_file.rst
index e56097b..2a14ad7 100644
--- a/Help/command/find_file.rst
+++ b/Help/command/find_file.rst
@@ -8,6 +8,9 @@ find_file
 .. |prefix_XXX_SUBDIR| replace:: ``<prefix>/include``
 .. |entry_XXX_SUBDIR| replace:: ``<entry>/include``
 
+.. |FIND_PACKAGE_ROOT_PREFIX_PATH_XXX| replace::
+   ``<prefix>/include/<arch>`` if :variable:`CMAKE_LIBRARY_ARCHITECTURE`
+   is set, and |FIND_PACKAGE_ROOT_PREFIX_PATH_XXX_SUBDIR|
 .. |CMAKE_PREFIX_PATH_XXX| replace::
    ``<prefix>/include/<arch>`` if :variable:`CMAKE_LIBRARY_ARCHITECTURE`
    is set, and |CMAKE_PREFIX_PATH_XXX_SUBDIR|
diff --git a/Help/command/find_library.rst b/Help/command/find_library.rst
index f774f17..0861d67 100644
--- a/Help/command/find_library.rst
+++ b/Help/command/find_library.rst
@@ -8,6 +8,9 @@ find_library
 .. |prefix_XXX_SUBDIR| replace:: ``<prefix>/lib``
 .. |entry_XXX_SUBDIR| replace:: ``<entry>/lib``
 
+.. |FIND_PACKAGE_ROOT_PREFIX_PATH_XXX| replace::
+   ``<prefix>/lib/<arch>`` if :variable:`CMAKE_LIBRARY_ARCHITECTURE` is set,
+   and |FIND_PACKAGE_ROOT_PREFIX_PATH_XXX_SUBDIR|
 .. |CMAKE_PREFIX_PATH_XXX| replace::
    ``<prefix>/lib/<arch>`` if :variable:`CMAKE_LIBRARY_ARCHITECTURE` is set,
    and |CMAKE_PREFIX_PATH_XXX_SUBDIR|
diff --git a/Help/command/find_package.rst b/Help/command/find_package.rst
index b2e70f2..89c5a7a 100644
--- a/Help/command/find_package.rst
+++ b/Help/command/find_package.rst
@@ -64,6 +64,7 @@ The complete Config mode command signature is::
                [PATHS path1 [path2 ... ]]
                [PATH_SUFFIXES suffix1 [suffix2 ...]]
                [NO_DEFAULT_PATH]
+               [NO_PACKAGE_ROOT_PATH]
                [NO_CMAKE_PATH]
                [NO_CMAKE_ENVIRONMENT_PATH]
                [NO_SYSTEM_ENVIRONMENT_PATH]
@@ -249,7 +250,14 @@ The set of installation prefixes is constructed using the following
 steps.  If ``NO_DEFAULT_PATH`` is specified all ``NO_*`` options are
 enabled.
 
-1. Search paths specified in cmake-specific cache variables.  These
+1. Search paths specified in the ``PackageName_ROOT`` CMake and environment
+   variables.  The package root variables are maintained as a stack so if
+   called from within a find module, root paths from the parent's find
+   module will also be searched after paths for the current package.
+   This can be skipped if ``NO_PACKAGE_ROOT_PATH`` is passed.
+   See policy :policy:`CMP0074`.
+
+2. Search paths specified in cmake-specific cache variables.  These
    are intended to be used on the command line with a ``-DVAR=value``.
    The values are interpreted as :ref:`;-lists <CMake Language Lists>`.
    This can be skipped if ``NO_CMAKE_PATH`` is passed::
@@ -258,7 +266,7 @@ enabled.
      CMAKE_FRAMEWORK_PATH
      CMAKE_APPBUNDLE_PATH
 
-2. Search paths specified in cmake-specific environment variables.
+3. Search paths specified in cmake-specific environment variables.
    These are intended to be set in the user's shell configuration,
    and therefore use the host's native path separator
    (``;`` on Windows and ``:`` on UNIX).
@@ -269,26 +277,26 @@ enabled.
      CMAKE_FRAMEWORK_PATH
      CMAKE_APPBUNDLE_PATH
 
-3. Search paths specified by the ``HINTS`` option.  These should be paths
+4. Search paths specified by the ``HINTS`` option.  These should be paths
    computed by system introspection, such as a hint provided by the
    location of another item already found.  Hard-coded guesses should
    be specified with the ``PATHS`` option.
 
-4. Search the standard system environment variables.  This can be
+5. Search the standard system environment variables.  This can be
    skipped if ``NO_SYSTEM_ENVIRONMENT_PATH`` is passed.  Path entries
    ending in ``/bin`` or ``/sbin`` are automatically converted to their
    parent directories::
 
      PATH
 
-5. Search paths stored in the CMake :ref:`User Package Registry`.
+6. Search paths stored in the CMake :ref:`User Package Registry`.
    This can be skipped if ``NO_CMAKE_PACKAGE_REGISTRY`` is passed or by
    setting the :variable:`CMAKE_FIND_PACKAGE_NO_PACKAGE_REGISTRY`
    to ``TRUE``.
    See the :manual:`cmake-packages(7)` manual for details on the user
    package registry.
 
-6. Search cmake variables defined in the Platform files for the
+7. Search cmake variables defined in the Platform files for the
    current system.  This can be skipped if ``NO_CMAKE_SYSTEM_PATH`` is
    passed::
 
@@ -296,14 +304,14 @@ enabled.
      CMAKE_SYSTEM_FRAMEWORK_PATH
      CMAKE_SYSTEM_APPBUNDLE_PATH
 
-7. Search paths stored in the CMake :ref:`System Package Registry`.
+8. Search paths stored in the CMake :ref:`System Package Registry`.
    This can be skipped if ``NO_CMAKE_SYSTEM_PACKAGE_REGISTRY`` is passed
    or by setting the
    :variable:`CMAKE_FIND_PACKAGE_NO_SYSTEM_PACKAGE_REGISTRY` to ``TRUE``.
    See the :manual:`cmake-packages(7)` manual for details on the system
    package registry.
 
-8. Search paths specified by the ``PATHS`` option.  These are typically
+9. Search paths specified by the ``PATHS`` option.  These are typically
    hard-coded guesses.
 
 .. |FIND_XXX| replace:: find_package
diff --git a/Help/command/find_path.rst b/Help/command/find_path.rst
index 76342d0..988a3fa 100644
--- a/Help/command/find_path.rst
+++ b/Help/command/find_path.rst
@@ -8,6 +8,9 @@ find_path
 .. |prefix_XXX_SUBDIR| replace:: ``<prefix>/include``
 .. |entry_XXX_SUBDIR| replace:: ``<entry>/include``
 
+.. |FIND_PACKAGE_ROOT_PREFIX_PATH_XXX| replace::
+   ``<prefix>/include/<arch>`` if :variable:`CMAKE_LIBRARY_ARCHITECTURE`
+   is set, and |FIND_PACKAGE_ROOT_PREFIX_PATH_XXX_SUBDIR|
 .. |CMAKE_PREFIX_PATH_XXX| replace::
    ``<prefix>/include/<arch>`` if :variable:`CMAKE_LIBRARY_ARCHITECTURE`
    is set, and |CMAKE_PREFIX_PATH_XXX_SUBDIR|
diff --git a/Help/command/find_program.rst b/Help/command/find_program.rst
index d3430c0..4f00773 100644
--- a/Help/command/find_program.rst
+++ b/Help/command/find_program.rst
@@ -8,6 +8,8 @@ find_program
 .. |prefix_XXX_SUBDIR| replace:: ``<prefix>/[s]bin``
 .. |entry_XXX_SUBDIR| replace:: ``<entry>/[s]bin``
 
+.. |FIND_PACKAGE_ROOT_PREFIX_PATH_XXX| replace::
+   |FIND_PACKAGE_ROOT_PREFIX_PATH_XXX_SUBDIR|
 .. |CMAKE_PREFIX_PATH_XXX| replace::
    |CMAKE_PREFIX_PATH_XXX_SUBDIR|
 .. |CMAKE_XXX_PATH| replace:: :variable:`CMAKE_PROGRAM_PATH`
diff --git a/Help/manual/cmake-policies.7.rst b/Help/manual/cmake-policies.7.rst
index 254656a..7e2255d 100644
--- a/Help/manual/cmake-policies.7.rst
+++ b/Help/manual/cmake-policies.7.rst
@@ -57,6 +57,7 @@ Policies Introduced by CMake 3.12
 .. toctree::
    :maxdepth: 1
 
+   CMP0074: find_package uses PackageName_ROOT variables. </policy/CMP0074>
    CMP0073: Do not produce legacy _LIB_DEPENDS cache entries. </policy/CMP0073>
 
 Policies Introduced by CMake 3.11
diff --git a/Help/policy/CMP0074.rst b/Help/policy/CMP0074.rst
new file mode 100644
index 0000000..ffac4a7
--- /dev/null
+++ b/Help/policy/CMP0074.rst
@@ -0,0 +1,22 @@
+CMP0074
+-------
+
+:command:`find_package` uses ``PackageName_ROOT`` variables.
+
+In CMake 3.12 and above the ``find_package(PackageName)`` command now searches
+a prefix specified by a ``PackageName_ROOT`` CMake or environment variable.
+Package roots are maintained as a stack so nested calls to all ``find_*``
+commands inside find modules also search the roots as prefixes.  This policy
+provides compatibility with projects that have not been updated to avoid using
+``PackageName_ROOT`` variables for other purposes.
+
+The ``OLD`` behavior for this policy is to ignore ``PackageName_ROOT``
+variables.  The ``NEW`` behavior for this policy is to use ``PackageName_ROOT``
+variables.
+
+This policy was introduced in CMake version 3.12.  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.
+
+.. include:: DEPRECATED.txt
diff --git a/Help/release/dev/find-package_root-restore.rst b/Help/release/dev/find-package_root-restore.rst
new file mode 100644
index 0000000..3e6b69e
--- /dev/null
+++ b/Help/release/dev/find-package_root-restore.rst
@@ -0,0 +1,8 @@
+find-package_root-restore
+-------------------------
+
+* The :command:`find_package` command now searches a prefix specified by
+  a ``PackageName_ROOT`` CMake or environment variable.  Package roots are
+  maintained as a stack so nested calls to all ``find_*`` commands inside
+  find modules also search the roots as prefixes.
+  See policy :policy:`CMP0074`.