gdb/python: remove Python 2 support

New in this version:

 - Add a PY_MAJOR_VERSION check in configure.ac / AC_TRY_LIBPYTHON.  If
   the user passes --with-python=python2, this will cause a configure
   failure saying that GDB only supports Python 3.

Support for Python 2 is a maintenance burden for any patches touching
Python support.  Among others, the differences between Python 2 and 3
string and integer types are subtle.  It requires a lot of effort and
thinking to get something that behaves correctly on both.  And that's if
the author and reviewer of the patch even remember to test with Python
2.

See this thread for an example:

  https://sourceware.org/pipermail/gdb-patches/2021-December/184260.html

So, remove Python 2 support.  Update the documentation to state that GDB
can be built against Python 3 (as opposed to Python 2 or 3).

Update all the spots that use:

 - sys.version_info
 - IS_PY3K
 - PY_MAJOR_VERSION
 - gdb_py_is_py3k

... to only keep the Python 3 portions and drop the use of some
now-removed compatibility macros.

I did not update the configure script more than just removing the
explicit references to Python 2.  We could maybe do more there, like
check the Python version and reject it if that version is not
supported.  Otherwise (with this patch), things will only fail at
compile time, so it won't really be clear to the user that they are
trying to use an unsupported Python version.  But I'm a bit lost in the
configure code that checks for Python, so I kept that for later.

Change-Id: I75b0f79c148afbe3c07ac664cfa9cade052c0c62

1 files changed, 6 insertions, 12 deletions

diff --git a/gdb/configure.ac b/gdb/configure.ac
index bc8925d..310c765 100644
--- a/gdb/configure.ac
+++ b/gdb/configure.ac
@@ -717,7 +717,12 @@ AC_DEFUN([AC_TRY_LIBPYTHON],
   LIBS="$new_LIBS $LIBS"
   found_usable_python=no
   AC_LINK_IFELSE([AC_LANG_PROGRAM([[#include "Python.h"]],
-                                 [[Py_Initialize ();]])],
+                                 [[
+				    #if PY_MAJOR_VERSION != 3
+				    # error "We only support Python 3"
+				    #endif
+				    Py_Initialize ();
+				 ]])],
                  [have_libpython_var=yes
                   found_usable_python=yes
                   PYTHON_CPPFLAGS=$new_CPPFLAGS
@@ -733,7 +738,6 @@ dnl no -   Don't include python support.
 dnl yes -  Include python support, error if it's missing.
 dnl        If we find python in $PATH, use it to fetch configure options,
 dnl        otherwise assume the compiler can find it with no help from us.
-dnl        Python 2.7 and 2.6 are tried in turn.
 dnl auto - Same as "yes", but if python is missing from the system,
 dnl        fall back to "no".
 dnl /path/to/python/exec-prefix -
@@ -742,7 +746,6 @@ dnl        If /path/to/python/exec-prefix/bin/python exists, use it to find
 dnl        the compilation parameters.  Otherwise use
 dnl        -I/path/to/python/exec-prefix/include,
 dnl        -L/path/to/python/exec-prefix/lib.
-dnl        Python 2.7 and 2.6 are tried in turn.
 dnl        NOTE: This case is historical.  It is what was done for 7.0/7.1
 dnl        but is deprecated.
 dnl /path/to/python/executable -
@@ -877,15 +880,6 @@ else
   if test "${have_python_config}" = yes; then
     AC_TRY_LIBPYTHON(have_libpython,
                      ${python_includes}, ${python_libs})
-  elif test "${have_python_config}" != failed; then
-    if test "${have_libpython}" = no; then
-      AC_TRY_LIBPYTHON(have_libpython,
-                       ${python_includes}, "-lpython2.7 ${python_libs}")
-    fi
-    if test "${have_libpython}" = no; then
-      AC_TRY_LIBPYTHON(have_libpython,
-                       ${python_includes}, "-lpython2.6 ${python_libs}")
-    fi
   fi
 
   if test "${have_libpython}" = no; then