Allow the Package Registry to be disabled (#14849)

When a project is packaged for redistribution the local package
registries should not be updated or consulted.  They are for developers.

Add variables to disable use of package registries globally:

* CMAKE_EXPORT_NO_PACKAGE_REGISTRY that disables the export(PACKAGE)
  command
* CMAKE_FIND_PACKAGE_NO_PACKAGE_REGISTRY that disables the User Package
  Registry in all the find_package calls.
* CMAKE_FIND_PACKAGE_NO_SYSTEM_PACKAGE_REGISTRY that disables the
  System Package Registry in all the find_package calls.

Update documentation and unit tests.

Help/command/export.rst

@@ -51,4 +51,7 @@ projects find and use a package from the current project's build tree
 without help from the user.  Note that the entry in the package
 registry that this command creates works only in conjunction with a
 package configuration file (<name>Config.cmake) that works with the
-build tree.
+build tree. In some cases, for example for packaging and for system
+wide installations, it is not desirable to write the user package
+registry. If the :variable:`CMAKE_EXPORT_NO_PACKAGE_REGISTRY` variable
+is enabled, the ``export(PACKAGE)`` command will do nothing.

Help/command/find_package.rst

@@ -272,7 +272,9 @@ enabled.
    (This step is implemented only on Windows.)
 
 6. Search paths stored in the CMake :ref:`User Package Registry`.
-   This can be skipped if ``NO_CMAKE_PACKAGE_REGISTRY`` is passed.
+   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.
 
@@ -285,7 +287,9 @@ enabled.
      CMAKE_SYSTEM_APPBUNDLE_PATH
 
 8. Search paths stored in the CMake :ref:`System Package Registry`.
-   This can be skipped if ``NO_CMAKE_SYSTEM_PACKAGE_REGISTRY`` is passed.
+   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.
 

Help/manual/cmake-packages.7.rst

@@ -564,6 +564,21 @@ containing the package configuration file.
 
 There is no system package registry on non-Windows platforms.
 
+.. _`Disabling the Package Registry`:
+
+Disabling the Package Registry
+------------------------------
+
+In some cases using the Package Registries is not desirable. CMake
+allows to disable them using the following variables:
+
+ * :variable:`CMAKE_EXPORT_NO_PACKAGE_REGISTRY` disables the
+   :command:`export(PACKAGE)` command.
+ * :variable:`CMAKE_FIND_PACKAGE_NO_PACKAGE_REGISTRY` disables the
+   User Package Registry in all the :command:`find_package` calls.
+ * :variable:`CMAKE_FIND_PACKAGE_NO_SYSTEM_PACKAGE_REGISTRY` disables
+   the System Package Registry in all the :command:`find_package` calls.
+
 Package Registry Example
 ------------------------
 

Help/manual/cmake-variables.7.rst

@@ -109,10 +109,13 @@ Variables that Change Behavior
    /variable/CMAKE_DISABLE_FIND_PACKAGE_PackageName
    /variable/CMAKE_ERROR_DEPRECATED
    /variable/CMAKE_ERROR_ON_ABSOLUTE_INSTALL_DESTINATION
+   /variable/CMAKE_EXPORT_NO_PACKAGE_REGISTRY
    /variable/CMAKE_SYSROOT
    /variable/CMAKE_FIND_LIBRARY_PREFIXES
    /variable/CMAKE_FIND_LIBRARY_SUFFIXES
    /variable/CMAKE_FIND_NO_INSTALL_PREFIX
+   /variable/CMAKE_FIND_PACKAGE_NO_PACKAGE_REGISTRY
+   /variable/CMAKE_FIND_PACKAGE_NO_SYSTEM_PACKAGE_REGISTRY
    /variable/CMAKE_FIND_PACKAGE_WARN_NO_MODULE
    /variable/CMAKE_FIND_ROOT_PATH
    /variable/CMAKE_FIND_ROOT_PATH_MODE_INCLUDE

Help/variable/CMAKE_EXPORT_NO_PACKAGE_REGISTRY.rst

@@ -0,0 +1,11 @@
+CMAKE_EXPORT_NO_PACKAGE_REGISTRY
+--------------------------------
+
+Disable the :command:`export(PACKAGE)` command.
+
+In some cases, for example for packaging and for system wide
+installations, it is not desirable to write the user package registry.
+If the :variable:`CMAKE_EXPORT_NO_PACKAGE_REGISTRY` variable is enabled,
+the :command:`export(PACKAGE)` command will do nothing.
+
+See also :ref:`Disabling the Package Registry`.

Help/variable/CMAKE_FIND_PACKAGE_NO_PACKAGE_REGISTRY.rst

@@ -0,0 +1,13 @@
+CMAKE_FIND_PACKAGE_NO_PACKAGE_REGISTRY
+--------------------------------------
+
+Skip :ref:`User Package Registry` in :command:`find_package` calls.
+
+In some cases, for example to locate only system wide installations, it
+is not desirable to use the :ref:`User Package Registry` when searching
+for packages. If the :variable:`CMAKE_FIND_PACKAGE_NO_PACKAGE_REGISTRY`
+variable is enabled, all the :command:`find_package` commands will skip
+the :ref:`User Package Registry` as if they were called with the
+``NO_CMAKE_PACKAGE_REGISTRY`` argument.
+
+See also :ref:`Disabling the Package Registry`.

Help/variable/CMAKE_FIND_PACKAGE_NO_SYSTEM_PACKAGE_REGISTRY.rst

@@ -0,0 +1,13 @@
+CMAKE_FIND_PACKAGE_NO_SYSTEM_PACKAGE_REGISTRY
+---------------------------------------------
+
+Skip :ref:`System Package Registry` in :command:`find_package` calls.
+
+In some cases, it is not desirable to use the
+:ref:`System Package Registry` when searching for packages. If the
+:variable:`CMAKE_FIND_PACKAGE_NO_SYSTEM_PACKAGE_REGISTRY` variable is
+enabled, all the :command:`find_package` commands will skip
+the :ref:`System Package Registry` as if they were called with the
+``NO_CMAKE_SYSTEM_PACKAGE_REGISTRY`` argument.
+
+See also :ref:`Disabling the Package Registry`.