Remove support for Python older than 2.6

Supporting very old Python versions is a maintenance burden,
especially with the several variant test files to maintain for Python
<2.6.

Since we have dropped support for older OpenSSL versions in
7b283d0, RHEL 5 is now effectively
desupported, and that was also the only mainstream operating system
still using Python versions before 2.6, so it's a good time to drop
those as well.

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://www.postgresql.org/message-id/flat/98b69261-298c-13d2-f34d-836fd9c29b21%402ndquadrant.com

Author: petere

config/python.m4

@@ -37,8 +37,8 @@ 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 4; then
-  AC_MSG_ERROR([Python version $python_version is too old (version 2.4 or later is required)])
+if test "$python_majorversion" -lt 3 -a "$python_minorversion" -lt 6; then
+  AC_MSG_ERROR([Python version $python_version is too old (version 2.6 or later is required)])
 fi
 
 AC_MSG_CHECKING([for Python distutils module])

configure

@@ -9616,8 +9616,8 @@ 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 4; then
-  as_fn_error $? "Python version $python_version is too old (version 2.4 or later is required)" "$LINENO" 5
+if test "$python_majorversion" -lt 3 -a "$python_minorversion" -lt 6; then
+  as_fn_error $? "Python version $python_version is too old (version 2.6 or later is required)" "$LINENO" 5
 fi
 
 { $as_echo "$as_me:${as_lineno-$LINENO}: checking for Python distutils module" >&5

doc/src/sgml/installation.sgml

@@ -196,7 +196,7 @@ su - postgres
       language, you need a <productname>Python</productname>
       installation with the header files and
       the <application>distutils</application> module.  The minimum
-      required version is <productname>Python</productname> 2.4.
+      required version is <productname>Python</productname> 2.6.
       <productname>Python 3</productname> is supported if it's
       version 3.1 or later; but see
       <xref linkend="plpython-python23"/>

doc/src/sgml/plpython.sgml

@@ -1335,9 +1335,8 @@ $$ LANGUAGE plpythonu;
 
    <para>
     Context managers syntax using the <literal>with</literal> keyword
-    is available by default in Python 2.6.  If using PL/Python with an
-    older Python version, it is still possible to use explicit
-    subtransactions, although not as transparently.  You can call the
+    is available by default in Python 2.6.  For compatibility with
+    older Python versions, you can call the
     subtransaction manager's <literal>__enter__</literal> and
     <literal>__exit__</literal> functions using the
     <literal>enter</literal> and <literal>exit</literal> convenience
@@ -1367,17 +1366,6 @@ plpy.execute(plan, [result])
 $$ LANGUAGE plpythonu;
 </programlisting>
    </para>
-
-   <note>
-    <para>
-     Although context managers were implemented in Python 2.5, to use
-     the <literal>with</literal> syntax in that version you need to
-     use a <ulink
-     url="https://docs.python.org/release/2.5/ref/future.html">future
-     statement</ulink>.  Because of implementation details, however,
-     you cannot use future statements in PL/Python functions.
-    </para>
-   </note>
   </sect2>
  </sect1>
 

src/pl/plpython/expected/README

@@ -1,12 +1,8 @@
 Guide to alternative expected files:
 
-plpython_error_0.out			Python 2.4 and older
 plpython_error_5.out			Python 3.5 and newer
 
 plpython_unicode.out			server encoding != SQL_ASCII
 plpython_unicode_3.out			server encoding == SQL_ASCII
 
-plpython_subtransaction_0.out	Python 2.4 and older (without with statement)
-plpython_subtransaction_5.out	Python 2.5 (without with statement)
-
 plpython_types_3.out			Python 3.x