python: Bump maximum version for Python 3 module generator check.

The proximate problem is that the Python Module generator cmake script
has started failing for people with Python 3.10, which a CMake backtrace
pointing into FindPythonModuleGeneration.cmake with an error of the form
"The max python version in PythonModuleGeneration must be updated."

At least one distro has addressed this by simply patching out modules
that happen to use this CMake module [1].

From what I can tell and the testing I've done, the cause is pretty
simple: The CMake script attempts to find the best Python 3 version by
starting from an impossible version and working backwards until it finds
a version that is installed.

As a sanity check, if the "impossible" version is actually present, it
aborts. But this appears to be just a sanity check, and not any sort of
guard against buggy version handling code later.

While the best fix is probably to start from a known *good* version and
move up until we stop finding better versions, there's problems here
(e.g. a user with 3.6 and 3.8 installed would fail to see 3.7 and so be
left with 3.6 as the "best" match), so I opted just to increase the max
version significantly, and improve the documentation as to what's
happening and whether it is safe to repeat the step again later.

[1]: https://bugs.gentoo.org/746866

1 files changed, 7 insertions, 2 deletions

diff --git a/find-modules/FindPythonModuleGeneration.cmake b/find-modules/FindPythonModuleGeneration.cmake
index 210ba662..6104c1f4 100644
--- a/find-modules/FindPythonModuleGeneration.cmake
+++ b/find-modules/FindPythonModuleGeneration.cmake
@@ -162,14 +162,19 @@ endif()
 
 if (NOT GPB_PYTHON3_LIBRARY)
   set(_PYTHON3_MIN_VERSION 4)
-  set(_PYTHON3_MAX_VERSION 10)
 
-  _find_python(3 ${_PYTHON3_MAX_VERSION}) # Canary check
+  # This value is safe to increment over time, it is used only as a reasonable
+  # upper bound to start searching from
+  set(_PYTHON3_MAX_VERSION 50)
+
+  _find_python(3 ${_PYTHON3_MAX_VERSION})
 
   if (GPB_PYTHON3_LIBRARY)
     message(FATAL_ERROR "The max python version in ${CMAKE_FIND_PACKAGE_NAME} must be updated.")
   endif()
 
+  # Look for the highest supported version of Python 3 by looking for a minor
+  # version that doesn't exist and decrementing until we find a match.
   set(_PYTHON3_FIND_VERSION ${_PYTHON3_MAX_VERSION})
 
   while(NOT GPB_PYTHON3_LIBRARY