build,win: accept Python 3 if 2 is not available

If there is no Python 2 available, use Python 3. This allows to test
running configure with Python 3.

PR-URL: #29236
Reviewed-By: Christian Clauss <cclauss@me.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>

tools/msvs/find_python.cmd

@@ -1,34 +1,62 @@
 @IF NOT DEFINED DEBUG_HELPER @ECHO OFF
 
-echo Looking for Python 2.x
+echo Looking for Python
 setlocal enabledelayedexpansion
 
-:: If python.exe is in %Path%, just validate
+:: To remove the preference for Python 2, but still support it, just remove
+:: the 5 blocks marked with "Python 2:" and the support warnings
+
+:: Python 2: If python.exe is in %Path%, use if it's Python 2
 FOR /F "delims=" %%a IN ('where python.exe 2^> NUL') DO (
   SET need_path=0
   SET p=%%~dpa
-  IF NOT ERRORLEVEL 1 GOTO :validate
+  CALL :validate-v2
+  IF NOT ERRORLEVEL 1 GOTO :found-python2
+  GOTO :done-path-v2
 )
+:done-path-v2
 
-:: Query the 3 locations mentioned in PEP 514 for a python2 InstallPath
+:: Python 2: Query the 3 locations mentioned in PEP 514 for a python2 InstallPath
 FOR %%K IN ( "HKCU\Software", "HKLM\SOFTWARE", "HKLM\Software\Wow6432Node") DO (
   SET need_path=1
   CALL :find-versions-v2 %%K
-  :: If validate returns 0 just jump to the end
-  IF NOT ERRORLEVEL 1 GOTO :validate
+  IF NOT ERRORLEVEL 1 CALL :validate-v2
+  IF NOT ERRORLEVEL 1 GOTO :found-python2
+)
+
+:: Use python.exe if in %PATH%
+set need_path=0
+for /f "delims=" %%a in ('where python.exe 2^> nul') do (
+  set p=%%~dpa
+  goto :found-python
+)
+
+:: Query the 3 locations mentioned in PEP 514 for a Python InstallPath
+set need_path=1
+for %%k in ( "HKCU\Software", "HKLM\SOFTWARE", "HKLM\Software\Wow6432Node") do (
+  call :find-versions %%k
+  if not errorlevel 1 goto :found-python
 )
 
 goto :no-python
 
 
-:: Find Python 2 installations in a registry location
+:: Python 2: Find Python 2 installations in a registry location
 :find-versions-v2
 for /f "delims=" %%a in ('reg query "%~1\Python\PythonCore" /f * /k 2^> nul ^| findstr /r ^^HK ^| findstr "\\2\."') do (
   call :read-installpath %%a
   if not errorlevel 1 exit /b 0
 )
 exit /b 1
 
+:: Find Python installations in a registry location
+:find-versions
+for /f "delims=" %%a in ('reg query "%~1\Python\PythonCore" /f * /k 2^> nul ^| findstr /r ^^HK') do (
+  call :read-installpath %%a
+  if not errorlevel 1 exit /b 0
+)
+exit /b 1
+
 :: Read the InstallPath of a given Environment Key to %p%
 :: https://www.python.org/dev/peps/pep-0514/#installpath
 :read-installpath
@@ -44,22 +72,37 @@ for /f "skip=2 tokens=3*" %%a in ('reg query "%1\InstallPath" /ve /t REG_SZ 2^>
 exit /b 1
 
 
-:: Check if %p% holds a path to a real python2 executable
-:validate
-IF NOT EXIST "%p%python.exe" goto :no-python
+:: Python 2: Check if %p% holds a path to a real python2 executable
+:validate-v2
+IF NOT EXIST "%p%\python.exe" EXIT /B 1
 :: Check if %p% is python2
-"%p%python.exe" -V 2>&1 | findstr /R "^Python.2.*" > NUL
-IF ERRORLEVEL 1 goto :no-python2
-:: We can wrap it up
-ENDLOCAL & SET pt=%p%& SET need_path_ext=%need_path%
-SET VCBUILD_PYTHON_LOCATION=%pt%python.exe
-IF %need_path_ext%==1 SET Path=%Path%;%pt%
-SET need_path_ext=
+"%p%\python.exe" -V 2>&1 | findstr /R "^Python.2.*" > NUL
 EXIT /B %ERRORLEVEL%
 
-:no-python2
-echo Python found in %p%, but it is not v2.x.
-exit /B 1
+
+:: Python 2:
+:found-python2
+echo Python 2 found in %p%\python.exe
+set pyver=2
+goto :done
+
+:found-python
+echo Python found in %p%\python.exe
+echo WARNING: Python 3 is not yet fully supported, to avoid issues Python 2 should be installed.
+set pyver=3
+goto :done
+
+:done
+endlocal ^
+  & set "pt=%p%" ^
+  & set "need_path_ext=%need_path%" ^
+  & set "VCBUILD_PYTHON_VERSION=%pyver%"
+set "VCBUILD_PYTHON_LOCATION=%pt%\python.exe"
+if %need_path_ext%==1 set "PATH=%pt%;%PATH%"
+set "pt="
+set "need_path_ext="
+exit /b 0
+
 :no-python
 echo Could not find Python.
-exit /B 1
+exit /b 1

vcbuild.bat

@@ -689,6 +689,9 @@ goto exit
 
 :create-msvs-files-failed
 echo Failed to create vc project files.
+if %VCBUILD_PYTHON_VERSION%==3 (
+  echo Python 3 is not yet fully supported, to avoid issues Python 2 should be installed.
+)
 del .used_configure_flags
 goto exit