plpython: Reject Python 2 during build configuration.

Python 2.7 went EOL 2020-01-01 and the support for Python 2 requires a fair
bit of infrastructure. Therefore we are removing Python 2 support in plpython.

This patch just rejects Python 2 during configure / mkvcbuild.pl. Future
commits will remove the code and infrastructure for Python 2 support and
adjust more of the documentation. This way we can see the buildfarm state
after the removal sooner and we can be sure that failures are due to
desupporting Python 2, rather than caused by infrastructure cleanup.

Reviewed-By: Peter Eisentraut <peter@eisentraut.org>
Discussion: https://postgr.es/m/20211031184548.g4sxfe47n2kyi55r@alap3.anarazel.de

config/python.m4

@@ -9,6 +9,9 @@
 # Look for Python and set the output variable 'PYTHON' if found,
 # fail otherwise.
 #
+# Since we are supporting only Python 3.x, prefer python3 to plain python.  If
+# the latter exists at all, it very possibly points to python2.
+
 # As the Python 3 transition happens and PEP 394 isn't updated, we
 # need to cater to systems that don't have unversioned "python" by
 # default.  Some systems ship with "python3" by default and perhaps
@@ -16,7 +19,7 @@
 # "python2" and "python3", in which case it's reasonable to prefer the
 # newer version.
 AC_DEFUN([PGAC_PATH_PYTHON],
-[PGAC_PATH_PROGS(PYTHON, [python python3 python2])
+[PGAC_PATH_PROGS(PYTHON, [python3 python])
 AC_ARG_VAR(PYTHON, [Python program])dnl
 if test x"$PYTHON" = x""; then
   AC_MSG_ERROR([Python not found])
@@ -37,8 +40,8 @@ python_majorversion=`echo "$python_fullversion" | sed '[s/^\([0-9]*\).*/\1/]'`
 python_minorversion=`echo "$python_fullversion" | sed '[s/^[0-9]*\.\([0-9]*\).*/\1/]'`
 python_version=`echo "$python_fullversion" | sed '[s/^\([0-9]*\.[0-9]*\).*/\1/]'`
 # Reject unsupported Python versions as soon as practical.
-if test "$python_majorversion" -lt 3 -a "$python_minorversion" -lt 7; then
-  AC_MSG_ERROR([Python version $python_version is too old (version 2.7 or later is required)])
+if test "$python_majorversion" -lt 3; then
+  AC_MSG_ERROR([Python version $python_version is too old (version 3 or later is required)])
 fi
 
 AC_MSG_CHECKING([for Python sysconfig module])

configure

@@ -10280,7 +10280,7 @@ fi
 
 if test "$with_python" = yes; then
   if test -z "$PYTHON"; then
-  for ac_prog in python python3 python2
+  for ac_prog in python3 python
 do
   # Extract the first word of "$ac_prog", so it can be a program name with args.
 set dummy $ac_prog; ac_word=$2
@@ -10346,8 +10346,8 @@ python_majorversion=`echo "$python_fullversion" | sed 's/^\([0-9]*\).*/\1/'`
 python_minorversion=`echo "$python_fullversion" | sed 's/^[0-9]*\.\([0-9]*\).*/\1/'`
 python_version=`echo "$python_fullversion" | sed 's/^\([0-9]*\.[0-9]*\).*/\1/'`
 # Reject unsupported Python versions as soon as practical.
-if test "$python_majorversion" -lt 3 -a "$python_minorversion" -lt 7; then
-  as_fn_error $? "Python version $python_version is too old (version 2.7 or later is required)" "$LINENO" 5
+if test "$python_majorversion" -lt 3; then
+  as_fn_error $? "Python version $python_version is too old (version 3 or later is required)" "$LINENO" 5
 fi
 
 { $as_echo "$as_me:${as_lineno-$LINENO}: checking for Python sysconfig module" >&5

doc/src/sgml/install-windows.sgml

@@ -136,7 +136,7 @@
   to specify the location of your <productname>Python</productname> installation,
   put the following in <filename>config.pl</filename>:
 <programlisting>
-$config->{python} = 'c:\python26';
+$config->{python} = 'c:\python310';
 </programlisting>
   You only need to specify those parameters that are different from what's in
   <filename>config_default.pl</filename>.

doc/src/sgml/installation.sgml

@@ -196,11 +196,7 @@ su - postgres
       language, you need a <productname>Python</productname>
       installation with the header files and
       the <application>sysconfig</application> module.  The minimum
-      required version is <productname>Python</productname> 2.7.
-      <productname>Python 3</productname> is supported if it's
-      version 3.2 or later; but see
-      <xref linkend="plpython-python23"/>
-      when using Python 3.
+      required version is <productname>Python</productname> 3.2.
      </para>
 
      <para>
@@ -1868,14 +1864,10 @@ build-postgresql:
        <term><envar>PYTHON</envar></term>
        <listitem>
         <para>
-         Python interpreter program.  This will be used to
-         determine the dependencies for building PL/Python.  Also,
-         whether Python 2 or 3 is specified here (or otherwise
-         implicitly chosen) determines which variant of the PL/Python
-         language becomes available.  See
-         <xref linkend="plpython-python23"/>
-         for more information.  If this is not set, the following are probed
-         in this order: <literal>python python3 python2</literal>.
+         Python interpreter program.  This will be used to determine the
+         dependencies for building PL/Python.  If this is not set, the
+         following are probed in this order:
+         <literal>python3 python</literal>.
         </para>
        </listitem>
       </varlistentry>

src/tools/msvc/Mkvcbuild.pm

@@ -490,6 +490,10 @@ sub mkvcbuild
 		  if (!(defined($pyprefix) && defined($pyver)));
 
 		my $pymajorver = substr($pyver, 0, 1);
+
+		die "Python version $pyver is too old (version 3 or later is required)"
+		  if int($pymajorver) < 3;
+
 		my $plpython = $solution->AddProject('plpython' . $pymajorver,
 			'dll', 'PLs', 'src/pl/plpython');
 		$plpython->AddIncludeDir($pyprefix . '/include');