FindPython*: Add capability to control virtual env handling.

Fixes: #19097

Tests/FindPython/VirtualEnv/CMakeLists.txt

@@ -0,0 +1,42 @@
+cmake_minimum_required(VERSION 3.1)
+
+project(TestVirtualEnv LANGUAGES NONE)
+
+include(CTest)
+
+find_package(Python3 REQUIRED COMPONENTS Interpreter)
+if (NOT Python3_FOUND)
+  message (FATAL_ERROR "Fail to found Python 3")
+endif()
+
+set (Python3_VIRTUAL_ENV "${CMAKE_CURRENT_BINARY_DIR}/py3venv")
+
+execute_process (COMMAND "${Python3_EXECUTABLE}" -m venv "${Python3_VIRTUAL_ENV}"
+                 RESULT_VARIABLE result
+                 OUTPUT_VARIABLE outputs
+                 ERROR_VARIABLE outputs)
+if (result)
+  message (FATAL_ERROR "Fail to create virtual environment: ${outputs}")
+endif()
+
+add_test(NAME FindPython3.VirtualEnvDefault
+         COMMAND "${CMAKE_COMMAND}" -E env --unset=PYTHONHOME
+                                           "VIRTUAL_ENV=${Python3_VIRTUAL_ENV}"
+                 "${CMAKE_COMMAND}" "-DPYTHON3_VIRTUAL_ENV=${Python3_VIRTUAL_ENV}"
+                 -P "${CMAKE_CURRENT_LIST_DIR}/VirtualEnvDefault.cmake")
+
+add_test(NAME FindPython3.VirtualEnvOnly
+         COMMAND "${CMAKE_COMMAND}" -E env --unset=PYTHONHOME
+                                           "VIRTUAL_ENV=${Python3_VIRTUAL_ENV}"
+                 "${CMAKE_COMMAND}" "-DPYTHON3_VIRTUAL_ENV=${Python3_VIRTUAL_ENV}"
+                 -P "${CMAKE_CURRENT_LIST_DIR}/VirtualEnvOnly.cmake")
+add_test(NAME FindPython3.UnsetVirtualEnvOnly
+         COMMAND "${CMAKE_COMMAND}" -E env --unset=PYTHONHOME
+                                           --unset=VIRTUAL_ENV
+                 "${CMAKE_COMMAND}" -P "${CMAKE_CURRENT_LIST_DIR}/VirtualEnvOnly.cmake")
+
+add_test(NAME FindPython3.VirtualEnvStandard
+         COMMAND "${CMAKE_COMMAND}" -E env --unset=PYTHONHOME
+                                           "VIRTUAL_ENV=${Python3_VIRTUAL_ENV}"
+                 "${CMAKE_COMMAND}" "-DPYTHON3_VIRTUAL_ENV=${Python3_VIRTUAL_ENV}"
+                 -P "${CMAKE_CURRENT_LIST_DIR}/VirtualEnvStandard.cmake")

Tests/FindPython/VirtualEnv/VirtualEnvDefault.cmake

@@ -0,0 +1,6 @@
+
+find_package (Python3 REQUIRED)
+
+if (NOT Python3_EXECUTABLE MATCHES "^${PYTHON3_VIRTUAL_ENV}/.+")
+  message (FATAL_ERROR "Fail to use virtual environment")
+endif()

Tests/FindPython/VirtualEnv/VirtualEnvOnly.cmake

@@ -0,0 +1,16 @@
+
+#
+# Virtual environment is defined for python3
+# Trying to find a python2 using only virtual environment
+# It is expecting to fail if a virtual environment is active and to success otherwise.
+#
+set (Python2_FIND_VIRTUALENV ONLY)
+find_package (Python2 QUIET)
+
+if (PYTHON3_VIRTUAL_ENV AND Python2_FOUND)
+  message (FATAL_ERROR "Python2 unexpectedly found.")
+endif()
+
+if (NOT PYTHON3_VIRTUAL_ENV AND NOT Python2_FOUND)
+  message (FATAL_ERROR "Fail to find Python2.")
+endif()

Tests/FindPython/VirtualEnv/VirtualEnvStandard.cmake

@@ -0,0 +1,7 @@
+
+set (Python3_FIND_VIRTUALENV STANDARD)
+find_package (Python3 REQUIRED)
+
+if (Python3_EXECUTABLE MATCHES "^${PYTHON3_VIRTUAL_ENV}/.+")
+  message (FATAL_ERROR "Python3 virtual env unexpectedly found.")
+endif()