Re: Resolving the python 2 -> python 3 mess

Here's an updated pair of patches that attempt to fix the MSVC
scripts (pretty blindly) and provide a very simple regression test.
I'm not too sure whether the regression test will really prove
workable or not: for starters, it'll fail if "2to3" isn't available
in the PATH.  Perhaps there's reason to object to even trying to
test that, on security grounds.

I set up the MSVC scripts to default to building the stub extension.
I don't know if we really want to commit it that way, but the idea
for the moment is to try to get the cfbot to test it on Windows.

            regards, tom lane

diff --git a/doc/src/sgml/plpython.sgml b/doc/src/sgml/plpython.sgml
index 1921915..ac989a3 100644
--- a/doc/src/sgml/plpython.sgml
+++ b/doc/src/sgml/plpython.sgml
@@ -164,13 +164,6 @@
   </para>

   <para>
-   See also the
-   document <ulink url="https://docs.python.org/3/whatsnew/3.0.html">What's
-   New In Python 3.0</ulink> for more information about porting to
-   Python 3.
-  </para>
-
-  <para>
    It is not allowed to use PL/Python based on Python 2 and PL/Python
    based on Python 3 in the same session, because the symbols in the
    dynamic modules would clash, which could result in crashes of the
@@ -179,6 +172,90 @@
    a mismatch is detected.  It is possible, however, to use both
    PL/Python variants in the same database, from separate sessions.
   </para>
+
+  <sect2 id="plpython-python3-conversion">
+   <title>Converting from Python 2 to Python 3</title>
+
+   <para>
+    See the
+    document <ulink url="https://docs.python.org/3/whatsnew/3.0.html">What's
+    New In Python 3.0</ulink> for the Python community's information and
+    recommendations about porting to Python 3.
+   </para>
+
+   <para>
+    <productname>PostgreSQL</productname> provides some support for helping
+    you to convert existing Python 2 routines to Python 3.  In an
+    installation built with Python 3, there is an
+    extension <filename>convert_python3</filename> that changes functions
+    and procedures from the <literal>plpythonu</literal>
+    and <literal>plpython2u</literal> languages to
+    the <literal>plpython3u</literal> language.  While doing so, it applies
+    the <filename>2to3</filename> tool described in the above document to
+    the body of each such routine.
+   </para>
+
+   <para>
+    Using <filename>convert_python3</filename> can be as simple as:
+<programlisting>
+CREATE EXTENSION convert_python3;
+CALL convert_python3_all();
+</programlisting>
+    This must be done as database superuser.  If you wish, you can drop the
+    extension once you're done converting everything.
+   </para>
+
+   <para>
+    Since <filename>convert_python3</filename> is Python 3 code, be careful
+    not to install or run it in a session that has previously executed any
+    Python 2 code.  As explained above, that won't work.
+   </para>
+
+   <para>
+    <function>convert_python3_all</function> has two optional arguments: the
+    name of the conversion tool to use (by default <literal>2to3</literal>,
+    but you might for instance need to provide a full path name) and any
+    special command-line options to provide to it.  You might for example
+    want to adjust the set of <quote>fixer</quote> rules
+    that <literal>2to3</literal> applies:
+<programlisting>
+CALL convert_python3_all(options => '-f idioms -x apply');
+</programlisting>
+    See <literal>2to3</literal>'s
+    <ulink url="https://docs.python.org/3/library/2to3.html">documentation</ulink>
+    for more information.
+   </para>
+
+   <para>
+    The <filename>convert_python3</filename> extension also provides a
+    procedure that converts just one Python 2 function at a time:
+<programlisting>
+CALL convert_python3_one('myfunc(int)');
+</programlisting>
+    The argument is the target function's OID, which can be written as
+    a <type>regprocedure</type> constant (see
+    <xref linkend="datatype-oid"/>).  The main reason to use this would be
+    if you need to use different options for different functions.  It has
+    the same optional arguments as <function>convert_python3_all</function>:
+<programlisting>
+CALL convert_python3_one('otherfunc(text)', tool => '/usr/bin/2to3',
+                         options => '-f idioms');
+</programlisting>
+   </para>
+
+   <para>
+    If you have needs that go beyond this, consult the source code for
+    the <filename>convert_python3</filename> extension (it's just a
+    couple of <literal>plpython3u</literal> procedures) and adapt those
+    procedures as necessary.
+   </para>
+
+   <para>
+    Keep in mind that if you've constructed any <command>DO</command> blocks
+    that use Python 2 code, those will have to be fixed up manually,
+    wherever the source code for them exists.
+   </para>
+  </sect2>
  </sect1>

  <sect1 id="plpython-funcs">
diff --git a/src/pl/plpython/Makefile b/src/pl/plpython/Makefile
index 9e95285..03f858b 100644
--- a/src/pl/plpython/Makefile
+++ b/src/pl/plpython/Makefile
@@ -38,6 +38,9 @@ DATA = $(NAME)u.control $(NAME)u--1.0.sql
 ifeq ($(python_majorversion),2)
 DATA += plpythonu.control plpythonu--1.0.sql
 endif
+ifeq ($(python_majorversion),3)
+DATA += convert_python3.control convert_python3--1.0.sql
+endif

 # header files to install - it's not clear which of these might be needed
 # so install them all.
diff --git a/src/pl/plpython/convert_python3--1.0.sql b/src/pl/plpython/convert_python3--1.0.sql
new file mode 100644
index 0000000..3444ac0
--- /dev/null
+++ b/src/pl/plpython/convert_python3--1.0.sql
@@ -0,0 +1,149 @@
+/* convert_python3--1.0.sql */
+
+-- complain if script is sourced in psql, rather than via CREATE EXTENSION
+\echo Use "CREATE EXTENSION convert_python3" to load this file. \quit
+
+-- This module provides two procedures, one to convert all python2
+-- functions and one to do just one.  They're nearly identical, and
+-- in principle convert_python3_all() could be written as a loop
+-- around convert_python3_one().  It's not done that way since
+-- creating a temp directory for each function in a bulk conversion
+-- could get expensive.
+
+-- For some benighted reason, lib2to3 has exactly no documented API,
+-- so we must use the command-line API "2to3" instead.  User may pass
+-- in the name of that program (in case it's not in the server's PATH)
+-- as well as any desired options for it (perhaps some -f switches).
+
+create procedure convert_python3_all(tool text default '2to3',
+                                     options text default '')
+language plpython3u as $$
+import re, subprocess, tempfile
+
+# pattern to extract just the function header from pg_get_functiondef result
+aspat = re.compile("^(.*?\nAS )", re.DOTALL)
+# pattern for replacing LANGUAGE portion
+langpat = re.compile("\n LANGUAGE plpython2?u\n")
+
+# collect info about functions to update
+rv = plpy.execute("""
+select p.oid::pg_catalog.regprocedure as funcid,
+       pg_catalog.pg_get_functiondef(p.oid) as fd,
+       prosrc as body
+from pg_catalog.pg_proc p join pg_catalog.pg_language l on p.prolang = l.oid
+where lanname in ('plpythonu', 'plpython2u')
+order by proname
+""")
+
+# Make a temp directory to hold the file for 2to3 to work on.
+with tempfile.TemporaryDirectory() as tmpdirname:
+
+    # process each function
+    for r in rv:
+        # emit notices so user can tell which function failed, if one does
+        plpy.notice("converting function " + r["funcid"])
+
+        # extract everything but the body from pg_get_functiondef result
+        m = aspat.match(r["fd"])
+        if not m:
+            raise ValueError('unexpected match failure')
+        fheader = m.group(1)
+
+        # replace the language clause
+        fheader = langpat.sub("\n LANGUAGE plpython3u\n", fheader, 1)
+
+        # put body in a temp file so we can apply 2to3
+        f = open(tmpdirname + "/temp.py", mode = 'w')
+        f.write(r["body"])
+        f.close()
+
+        # apply 2to3 to body
+        subprocess.check_call(tool + " " + options + " --no-diffs -w " + tmpdirname + "/temp.py", shell=True)
+        f = open(tmpdirname + "/temp.py", mode = 'r')
+        fbody = f.read()
+        f.close()
+
+        # ensure check_function_bodies is enabled
+        plpy.execute("set local check_function_bodies = true")
+
+        # construct and execute SQL command to replace the function
+        newstmt = fheader + plpy.quote_literal(fbody)
+        # uncomment this for debugging purposes:
+        # plpy.info(newstmt)
+        plpy.execute(newstmt)
+
+        # commit after each successful replacement, in case a later one fails
+        plpy.commit()
+$$;
+
+-- The above procedure has to be superuser-only since it trivially allows
+-- executing random programs.  But you'd have to be superuser anyway
+-- to replace the definitions of plpython functions.
+
+revoke all on procedure convert_python3_all(text, text) from public;
+
+
+-- Here's the one-function version.
+
+create procedure convert_python3_one(funcid regprocedure,
+                                     tool text default '2to3',
+                                     options text default '')
+language plpython3u as $$
+import re, subprocess, tempfile
+
+# pattern to extract just the function header from pg_get_functiondef result
+aspat = re.compile("^(.*?\nAS )", re.DOTALL)
+# pattern for replacing LANGUAGE portion
+langpat = re.compile("\n LANGUAGE plpython2?u\n")
+
+# collect info about function to update, making sure it's the right language
+plan = plpy.prepare("""
+select p.oid::pg_catalog.regprocedure as funcid,
+       pg_catalog.pg_get_functiondef(p.oid) as fd,
+       prosrc as body
+from pg_catalog.pg_proc p join pg_catalog.pg_language l on p.prolang = l.oid
+where p.oid = $1 and lanname in ('plpythonu', 'plpython2u')
+""", ["pg_catalog.regprocedure"])
+
+rv = plpy.execute(plan, [funcid])
+
+# Make a temp directory to hold the file for 2to3 to work on.
+with tempfile.TemporaryDirectory() as tmpdirname:
+
+    # process each function (we only expect one, but it's easy to loop)
+    for r in rv:
+        # extract everything but the body from pg_get_functiondef result
+        m = aspat.match(r["fd"])
+        if not m:
+            raise ValueError('unexpected match failure')
+        fheader = m.group(1)
+
+        # replace the language clause
+        fheader = langpat.sub("\n LANGUAGE plpython3u\n", fheader, 1)
+
+        # put body in a temp file so we can apply 2to3
+        f = open(tmpdirname + "/temp.py", mode = 'w')
+        f.write(r["body"])
+        f.close()
+
+        # apply 2to3 to body
+        subprocess.check_call(tool + " " + options + " --no-diffs -w " + tmpdirname + "/temp.py", shell=True)
+        f = open(tmpdirname + "/temp.py", mode = 'r')
+        fbody = f.read()
+        f.close()
+
+        # ensure check_function_bodies is enabled
+        plpy.execute("set local check_function_bodies = true")
+
+        # construct and execute SQL command to replace the function
+        newstmt = fheader + plpy.quote_literal(fbody)
+        # uncomment this for debugging purposes:
+        # plpy.info(newstmt)
+        plpy.execute(newstmt)
+$$;
+
+-- The above procedure has to be superuser-only since it trivially allows
+-- executing random programs.  But you'd have to be superuser anyway
+-- to replace the definitions of plpython functions.
+
+revoke all on procedure convert_python3_one(regprocedure, text, text) from public;
diff --git a/src/pl/plpython/convert_python3.control b/src/pl/plpython/convert_python3.control
new file mode 100644
index 0000000..8debb3b
--- /dev/null
+++ b/src/pl/plpython/convert_python3.control
@@ -0,0 +1,5 @@
+# convert_python3 extension
+comment = 'convert plpython[2]u functions to plpython3u'
+default_version = '1.0'
+relocatable = true
+requires = 'plpython3u'
diff --git a/configure b/configure
index d2c74e5..3feebf6 100755
--- a/configure
+++ b/configure
@@ -714,6 +714,7 @@ with_ldap
 with_krb_srvnam
 krb_srvtab
 with_gssapi
+with_python2_stub
 with_python
 with_perl
 with_tcl
@@ -847,6 +848,7 @@ with_tcl
 with_tclconfig
 with_perl
 with_python
+with_python2_stub
 with_gssapi
 with_krb_srvnam
 with_pam
@@ -1546,6 +1548,7 @@ Optional Packages:
   --with-tclconfig=DIR    tclConfig.sh is in DIR
   --with-perl             build Perl modules (PL/Perl)
   --with-python           build Python modules (PL/Python)
+  --with-python2-stub     build Python 2 compatibility stub
   --with-gssapi           build with GSSAPI support
   --with-krb-srvnam=NAME  default service principal name in Kerberos (GSSAPI)
                           [postgres]
@@ -7644,6 +7647,32 @@ fi
 $as_echo "$with_python" >&6; }


+
+
+
+# Check whether --with-python2-stub was given.
+if test "${with_python2_stub+set}" = set; then :
+  withval=$with_python2_stub;
+  case $withval in
+    yes)
+      :
+      ;;
+    no)
+      :
+      ;;
+    *)
+      as_fn_error $? "no argument expected for --with-python2-stub option" "$LINENO" 5
+      ;;
+  esac
+
+else
+  with_python2_stub=no
+
+fi
+
+
+
+
 #
 # GSSAPI
 #
@@ -9745,6 +9774,12 @@ $as_echo "${python_libspec} ${python_additional_libs}" >&6; }



+  # Disable building Python 2 stub if primary version isn't Python 3
+  if test "$python_majorversion" -lt 3; then
+    with_python2_stub=no
+  fi
+else
+  with_python2_stub=no
 fi

 if test "$cross_compiling" = yes && test -z "$with_system_tzdata"; then
diff --git a/configure.in b/configure.in
index 0b0a871..ffa49c9 100644
--- a/configure.in
+++ b/configure.in
@@ -766,6 +766,9 @@ PGAC_ARG_BOOL(with, python, no, [build Python modules (PL/Python)])
 AC_MSG_RESULT([$with_python])
 AC_SUBST(with_python)

+PGAC_ARG_BOOL(with, python2-stub, no, [build Python 2 compatibility stub])
+AC_SUBST(with_python2_stub)
+
 #
 # GSSAPI
 #
@@ -1042,6 +1045,12 @@ fi
 if test "$with_python" = yes; then
   PGAC_PATH_PYTHON
   PGAC_CHECK_PYTHON_EMBED_SETUP
+  # Disable building Python 2 stub if primary version isn't Python 3
+  if test "$python_majorversion" -lt 3; then
+    with_python2_stub=no
+  fi
+else
+  with_python2_stub=no
 fi

 if test "$cross_compiling" = yes && test -z "$with_system_tzdata"; then
diff --git a/doc/src/sgml/installation.sgml b/doc/src/sgml/installation.sgml
index cc242dc..ff81ea8 100644
--- a/doc/src/sgml/installation.sgml
+++ b/doc/src/sgml/installation.sgml
@@ -877,6 +877,23 @@ build-postgresql:
       </varlistentry>

       <varlistentry>
+       <term><option>--with-python2-stub</option></term>
+       <listitem>
+        <para>
+         Build a stub version of the Python
+         2 <application>PL/Python</application> language, to aid in
+         transitioning old <application>PL/Python</application> code.
+         This option is recommended if you are
+         building <application>PL/Python</application> for Python 3
+         and do not intend to also build a version for Python 2.
+         This option is ignored if you do not also
+         specify <option>--with-python</option>, or if the selected Python
+         implementation is Python 2.
+        </para>
+       </listitem>
+      </varlistentry>
+
+      <varlistentry>
        <term><option>--with-tcl</option></term>
        <listitem>
         <para>
diff --git a/doc/src/sgml/plpython.sgml b/doc/src/sgml/plpython.sgml
index ac989a3..9014750 100644
--- a/doc/src/sgml/plpython.sgml
+++ b/doc/src/sgml/plpython.sgml
@@ -77,13 +77,7 @@
      <para>
       The language named <literal>plpythonu</literal> implements
       PL/Python based on the default Python language variant, which is
-      currently Python 2.  (This default is independent of what any
-      local Python installations might consider to be
-      their <quote>default</quote>, for example,
-      what <filename>/usr/bin/python</filename> might be.)  The
-      default will probably be changed to Python 3 in a distant future
-      release of PostgreSQL, depending on the progress of the
-      migration to Python 3 in the Python community.
+      determined by the person who builds and installs PostgreSQL.
      </para>
     </listitem>
    </itemizedlist>
@@ -103,9 +97,22 @@
     The built variant depends on which Python version was found during
     the installation or which version was explicitly set using
     the <envar>PYTHON</envar> environment variable;
-    see <xref linkend="install-procedure"/>.  To make both variants of
-    PL/Python available in one installation, the source tree has to be
-    configured and built twice.
+    see <xref linkend="install-procedure"/>.  To make working versions of
+    both variants of PL/Python available in one installation, the source
+    tree has to be configured and built twice.
+   </para>
+
+   <para>
+    However, if the builder specifies both <option>--with-python</option>
+    and <option>--with-python2-stub</option> while building with Python 3,
+    then in addition to the working Python-3-based PL/Python,
+    a <quote>stub</quote> version of <literal>plpython2u</literal> is
+    produced.  This stub is non-functional and will simply throw errors if
+    any <literal>plpython2u</literal> function is executed, but its
+    presence allows such functions to be loaded into the database in
+    preparation for conversion to <literal>plpython3u</literal>.  This
+    configuration also causes <literal>plpythonu</literal> to become
+    Python 3 rather than Python 2.
    </para>
   </tip>

@@ -115,49 +122,38 @@
    <itemizedlist>
     <listitem>
      <para>
-      Existing users and users who are currently not interested in
-      Python 3 use the language name <literal>plpythonu</literal> and
-      don't have to change anything for the foreseeable future.  It is
-      recommended to gradually <quote>future-proof</quote> the code
-      via migration to Python 2.6/2.7 to simplify the eventual
-      migration to Python 3.
+      Use the language name <literal>plpythonu</literal> if you have
+      simple Python code that works for either Python 2 or Python 3;
+      or if you are prepared to migrate your code whenever your packager
+      decides to change the default Python version.
      </para>

      <para>
       In practice, many PL/Python functions will migrate to Python 3
-      with few or no changes.
+      with few or no changes.  However, there are some cases that
+      will require more work.
      </para>
     </listitem>

     <listitem>
      <para>
-      Users who know that they have heavily Python 2 dependent code
-      and don't plan to ever change it can make use of
+      If you know that you have heavily Python 2 dependent code
+      and don't plan to ever change it, you can make use of
       the <literal>plpython2u</literal> language name.  This will
-      continue to work into the very distant future, until Python 2
-      support might be completely dropped by PostgreSQL.
-     </para>
-    </listitem>
-
-    <listitem>
-     <para>
-      Users who want to dive into Python 3 can use
-      the <literal>plpython3u</literal> language name, which will keep
-      working forever by today's standards.  In the distant future,
-      when Python 3 might become the default, they might like to
-      remove the <quote>3</quote> for aesthetic reasons.
+      continue to work for as long as your operating system platform
+      continues to provide Python 2.  PostgreSQL itself may eventually
+      drop support for this option, but that is unlikely to happen
+      as long as any platform support remains in the wild.
      </para>
     </listitem>

     <listitem>
      <para>
-      Daredevils, who want to build a Python-3-only operating system
-      environment, can change the contents of
-      <literal>plpythonu</literal>'s extension control and script files
-      to make <literal>plpythonu</literal> be equivalent
-      to <literal>plpython3u</literal>, keeping in mind that this
-      would make their installation incompatible with most of the rest
-      of the world.
+      If you know your code requires Python 3, use
+      the <literal>plpython3u</literal> language name.  You can also
+      use this language name to tag functions that have been successfully
+      converted from Python 2, so as to keep track of which functions have
+      been converted and which have not.
      </para>
     </listitem>
    </itemizedlist>
@@ -171,6 +167,8 @@
    Python major versions in a session, which will abort the session if
    a mismatch is detected.  It is possible, however, to use both
    PL/Python variants in the same database, from separate sessions.
+   (Also, the <quote>stub</quote> version of <literal>plpython2u</literal>
+   does not present any conflicts.)
   </para>

   <sect2 id="plpython-python3-conversion">
diff --git a/src/Makefile.global.in b/src/Makefile.global.in
index e4db3e8..1f880b6 100644
--- a/src/Makefile.global.in
+++ b/src/Makefile.global.in
@@ -180,6 +180,7 @@ bitcodedir = $(pkglibdir)/bitcode
 with_icu    = @with_icu@
 with_perl    = @with_perl@
 with_python    = @with_python@
+with_python2_stub = @with_python2_stub@
 with_tcl    = @with_tcl@
 with_openssl    = @with_openssl@
 with_readline    = @with_readline@
diff --git a/src/pl/Makefile b/src/pl/Makefile
index c4a0d1c..fe9cd0f 100644
--- a/src/pl/Makefile
+++ b/src/pl/Makefile
@@ -26,6 +26,12 @@ else
 ALWAYS_SUBDIRS += plpython
 endif

+ifeq ($(with_python2_stub), yes)
+SUBDIRS += stub_plpython2
+else
+ALWAYS_SUBDIRS += stub_plpython2
+endif
+
 ifeq ($(with_tcl), yes)
 SUBDIRS += tcl
 else
diff --git a/src/pl/plpython/Makefile b/src/pl/plpython/Makefile
index 03f858b..ac262fa 100644
--- a/src/pl/plpython/Makefile
+++ b/src/pl/plpython/Makefile
@@ -37,6 +37,9 @@ OBJS = \
 DATA = $(NAME)u.control $(NAME)u--1.0.sql
 ifeq ($(python_majorversion),2)
 DATA += plpythonu.control plpythonu--1.0.sql
+else ifeq ($(with_python2_stub), yes)
+# install extension files for the stub module (see ../stub_plpython2)
+DATA += plpythonu.control plpythonu--1.0.sql plpython2u.control plpython2u--1.0.sql
 endif
 ifeq ($(python_majorversion),3)
 DATA += convert_python3.control convert_python3--1.0.sql
@@ -109,6 +112,7 @@ REGRESS = \
     plpython_composite \
     plpython_subtransaction \
     plpython_transaction \
+    plpython_stub \
     plpython_drop

 REGRESS_PLPYTHON3_MANGLE := $(REGRESS)
diff --git a/src/pl/plpython/expected/plpython_stub.out b/src/pl/plpython/expected/plpython_stub.out
new file mode 100644
index 0000000..2a565b4
--- /dev/null
+++ b/src/pl/plpython/expected/plpython_stub.out
@@ -0,0 +1,71 @@
+--
+-- Test stub plpython2 and convert_python3 extension
+--
+-- skip test unless we have python3 installed and the others are available
+SELECT (SELECT count(*) FROM pg_extension WHERE extname = 'plpython3u') = 0 OR
+       (SELECT count(*) FROM pg_available_extensions WHERE name = 'plpython2u') = 0 OR
+       (SELECT count(*) FROM pg_available_extensions WHERE name = 'plpythonu') = 0 OR
+       (SELECT count(*) FROM pg_available_extensions WHERE name = 'convert_python3') = 0
+       AS skip_test \gset
+\if :skip_test
+\quit
+\endif
+-- Funny formatting of some commands below is to prevent
+-- regress-python3-mangle.mk from being "helpful".
+-- install the additional extensions
+CREATE EXTENSION
+plpython2u;
+CREATE EXTENSION
+plpythonu;
+CREATE EXTENSION convert_python3;
+-- create some functions that need conversion
+create function convert1() returns int
+as 'return 123l'
+language
+plpython2u
+immutable;
+-- disable syntax checking since if plpythonu is python 3, this'll fail
+set check_function_bodies = false;
+create function convert2() returns int
+as 'return 123l'
+language
+plpythonu
+immutable;
+-- can't use \sf here because mangling would affect the LANGUAGE clauses
+select proname, lanname, prosrc
+from pg_proc p join pg_language l on prolang = l.oid
+where proname = 'convert1';
+ proname  |  lanname   |   prosrc
+----------+------------+-------------
+ convert1 | plpython2u | return 123l
+(1 row)
+
+select proname, lanname, prosrc
+from pg_proc p join pg_language l on prolang = l.oid
+where proname = 'convert2';
+ proname  |  lanname  |   prosrc
+----------+-----------+-------------
+ convert2 | plpythonu | return 123l
+(1 row)
+
+call convert_python3_all();
+NOTICE:  converting function convert1()
+NOTICE:  converting function convert2()
+\sf convert1()
+CREATE OR REPLACE FUNCTION public.convert1()
+ RETURNS integer
+ LANGUAGE plpython3u
+ IMMUTABLE
+AS $function$return 123$function$
+\sf convert2()
+CREATE OR REPLACE FUNCTION public.convert2()
+ RETURNS integer
+ LANGUAGE plpython3u
+ IMMUTABLE
+AS $function$return 123$function$
+-- clean up
+DROP EXTENSION
+plpython2u;
+DROP EXTENSION
+plpythonu;
+DROP EXTENSION convert_python3;
diff --git a/src/pl/plpython/expected/plpython_stub_1.out b/src/pl/plpython/expected/plpython_stub_1.out
new file mode 100644
index 0000000..bb625b9
--- /dev/null
+++ b/src/pl/plpython/expected/plpython_stub_1.out
@@ -0,0 +1,11 @@
+--
+-- Test stub plpython2 and convert_python3 extension
+--
+-- skip test unless we have python3 installed and the others are available
+SELECT (SELECT count(*) FROM pg_extension WHERE extname = 'plpython3u') = 0 OR
+       (SELECT count(*) FROM pg_available_extensions WHERE name = 'plpython2u') = 0 OR
+       (SELECT count(*) FROM pg_available_extensions WHERE name = 'plpythonu') = 0 OR
+       (SELECT count(*) FROM pg_available_extensions WHERE name = 'convert_python3') = 0
+       AS skip_test \gset
+\if :skip_test
+\quit
diff --git a/src/pl/plpython/sql/plpython_stub.sql b/src/pl/plpython/sql/plpython_stub.sql
new file mode 100644
index 0000000..8c587fd
--- /dev/null
+++ b/src/pl/plpython/sql/plpython_stub.sql
@@ -0,0 +1,60 @@
+--
+-- Test stub plpython2 and convert_python3 extension
+--
+
+-- skip test unless we have python3 installed and the others are available
+SELECT (SELECT count(*) FROM pg_extension WHERE extname = 'plpython3u') = 0 OR
+       (SELECT count(*) FROM pg_available_extensions WHERE name = 'plpython2u') = 0 OR
+       (SELECT count(*) FROM pg_available_extensions WHERE name = 'plpythonu') = 0 OR
+       (SELECT count(*) FROM pg_available_extensions WHERE name = 'convert_python3') = 0
+       AS skip_test \gset
+\if :skip_test
+\quit
+\endif
+
+-- Funny formatting of some commands below is to prevent
+-- regress-python3-mangle.mk from being "helpful".
+
+-- install the additional extensions
+CREATE EXTENSION
+plpython2u;
+CREATE EXTENSION
+plpythonu;
+CREATE EXTENSION convert_python3;
+
+-- create some functions that need conversion
+create function convert1() returns int
+as 'return 123l'
+language
+plpython2u
+immutable;
+
+-- disable syntax checking since if plpythonu is python 3, this'll fail
+set check_function_bodies = false;
+
+create function convert2() returns int
+as 'return 123l'
+language
+plpythonu
+immutable;
+
+-- can't use \sf here because mangling would affect the LANGUAGE clauses
+select proname, lanname, prosrc
+from pg_proc p join pg_language l on prolang = l.oid
+where proname = 'convert1';
+
+select proname, lanname, prosrc
+from pg_proc p join pg_language l on prolang = l.oid
+where proname = 'convert2';
+
+call convert_python3_all();
+
+\sf convert1()
+\sf convert2()
+
+-- clean up
+DROP EXTENSION
+plpython2u;
+DROP EXTENSION
+plpythonu;
+DROP EXTENSION convert_python3;
diff --git a/src/pl/stub_plpython2/Makefile b/src/pl/stub_plpython2/Makefile
new file mode 100644
index 0000000..67e7559
--- /dev/null
+++ b/src/pl/stub_plpython2/Makefile
@@ -0,0 +1,38 @@
+# src/pl/stub_plpython2/Makefile
+
+# Note that this Makefile only builds and installs a quasi-dummy
+# version of plpython2.so.  The control and script files for the
+# plpythonu and plpython2u extensions are installed by ../plpython;
+# they are the same whether we're using real or stub plpython2.
+# (We'd probably not have this separate subdirectory at all, except
+# that Makefile.shlib can only build one shlib per directory.)
+
+subdir = src/pl/stub_plpython2
+top_builddir = ../../..
+include $(top_builddir)/src/Makefile.global
+
+PGFILEDESC = "PL/Python - procedural language stub for Python 2"
+
+NAME = plpython2
+
+OBJS = \
+    $(WIN32RES) \
+    stub_plpython2.o
+
+include $(top_srcdir)/src/Makefile.shlib
+
+all: all-lib
+
+# Ensure parallel safety if a build is started in this directory
+$(OBJS): | submake-generated-headers
+
+install: all install-lib
+
+installdirs: installdirs-lib
+
+uninstall: uninstall-lib
+
+clean distclean: clean-lib
+    rm -f $(OBJS)
+
+maintainer-clean: distclean
diff --git a/src/pl/stub_plpython2/stub_plpython2.c b/src/pl/stub_plpython2/stub_plpython2.c
new file mode 100644
index 0000000..4ade88b
--- /dev/null
+++ b/src/pl/stub_plpython2/stub_plpython2.c
@@ -0,0 +1,105 @@
+/*
+ * PL/Python stub for Python 2, in an environment that has only Python 3
+ *
+ * Our strategy is to pass through "plpythonu" functions to Python 3,
+ * but throw a not-implemented error for "plpython2u".
+ *
+ * Pass-through is implemented by using dfmgr.c to look up the appropriate
+ * function in plpython3.so, rather than trying to resolve the reference
+ * directly.  This greatly simplifies building this as an independent
+ * shared library, and it ensures that we can't somehow pull in a different
+ * version of plpython3 (and thence libpython) than would get loaded for
+ * a plpython3u function.
+ *
+ * src/pl/stub_plpython2/stub_plpython2.c
+ */
+
+#include "postgres.h"
+
+#include "fmgr.h"
+
+#define PLPYTHON_LIBNAME "$libdir/plpython3"
+
+/*
+ * exported functions
+ */
+
+PG_MODULE_MAGIC;
+
+PG_FUNCTION_INFO_V1(plpython_validator);
+PG_FUNCTION_INFO_V1(plpython_call_handler);
+PG_FUNCTION_INFO_V1(plpython_inline_handler);
+
+PG_FUNCTION_INFO_V1(plpython2_validator);
+PG_FUNCTION_INFO_V1(plpython2_call_handler);
+PG_FUNCTION_INFO_V1(plpython2_inline_handler);
+
+
+Datum
+plpython_validator(PG_FUNCTION_ARGS)
+{
+    static PGFunction plpython3_validator = NULL;
+
+    if (plpython3_validator == NULL)
+        plpython3_validator =
+            load_external_function(PLPYTHON_LIBNAME,
+                                   "plpython3_validator",
+                                   true, NULL);
+
+    return (*plpython3_validator) (fcinfo);
+}
+
+Datum
+plpython_call_handler(PG_FUNCTION_ARGS)
+{
+    static PGFunction plpython3_call_handler = NULL;
+
+    if (plpython3_call_handler == NULL)
+        plpython3_call_handler =
+            load_external_function(PLPYTHON_LIBNAME,
+                                   "plpython3_call_handler",
+                                   true, NULL);
+
+    return (*plpython3_call_handler) (fcinfo);
+}
+
+Datum
+plpython_inline_handler(PG_FUNCTION_ARGS)
+{
+    static PGFunction plpython3_inline_handler = NULL;
+
+    if (plpython3_inline_handler == NULL)
+        plpython3_inline_handler =
+            load_external_function(PLPYTHON_LIBNAME,
+                                   "plpython3_inline_handler",
+                                   true, NULL);
+
+    return (*plpython3_inline_handler) (fcinfo);
+}
+
+Datum
+plpython2_validator(PG_FUNCTION_ARGS)
+{
+    /* It seems more convenient to do nothing here than throw an error. */
+    PG_RETURN_VOID();
+}
+
+Datum
+plpython2_call_handler(PG_FUNCTION_ARGS)
+{
+    ereport(ERROR,
+            (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+             errmsg("Python 2 is no longer supported"),
+             errhint("Convert the function to use plpython3u.")));
+    PG_RETURN_NULL();            /* keep compiler quiet */
+}
+
+Datum
+plpython2_inline_handler(PG_FUNCTION_ARGS)
+{
+    ereport(ERROR,
+            (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+             errmsg("Python 2 is no longer supported"),
+             errhint("Convert the DO block to use plpython3u.")));
+    PG_RETURN_NULL();            /* keep compiler quiet */
+}
diff --git a/src/tools/msvc/Install.pm b/src/tools/msvc/Install.pm
index 1a92ed2..cf8b1e2 100644
--- a/src/tools/msvc/Install.pm
+++ b/src/tools/msvc/Install.pm
@@ -154,6 +154,7 @@ sub Install
         my @pldirs             = ('src/pl/plpgsql/src');
         push @pldirs, "src/pl/plperl"   if $config->{perl};
         push @pldirs, "src/pl/plpython" if $config->{python};
+        push @pldirs, "src/pl/stub_plpython2" if $config->{python2_stub};
         push @pldirs, "src/pl/tcl"      if $config->{tcl};
         File::Find::find(
             {
diff --git a/src/tools/msvc/Mkvcbuild.pm b/src/tools/msvc/Mkvcbuild.pm
index 727a8fd..1d28607 100644
--- a/src/tools/msvc/Mkvcbuild.pm
+++ b/src/tools/msvc/Mkvcbuild.pm
@@ -531,7 +531,12 @@ sub mkvcbuild
             'ltree',                        'contrib');
         $ltree_plpython->AddDefine(
             'PLPYTHON_LIBNAME="plpython' . $pymajorver . '"');
+
+        # Ignore --with-python2-stub if building real plpython2
+        $solution->{options}->{python2_stub} = 0 if $pymajorver < 3;
     }
+    else
+        $solution->{options}->{python2_stub} = 0;

     if ($solution->{options}->{perl})
     {
diff --git a/src/tools/msvc/Solution.pm b/src/tools/msvc/Solution.pm
index 6b4a6ee..848e118 100644
--- a/src/tools/msvc/Solution.pm
+++ b/src/tools/msvc/Solution.pm
@@ -1145,6 +1145,7 @@ sub GetFakeConfigure
     $cfg .= ' --with-tcl'           if ($self->{options}->{tcl});
     $cfg .= ' --with-perl'          if ($self->{options}->{perl});
     $cfg .= ' --with-python'        if ($self->{options}->{python});
+    $cfg .= ' --with-python2-stub'  if ($self->{options}->{python2_stub});

     return $cfg;
 }
diff --git a/src/tools/msvc/clean.bat b/src/tools/msvc/clean.bat
index 672bb2d..4ad5f07 100755
--- a/src/tools/msvc/clean.bat
+++ b/src/tools/msvc/clean.bat
@@ -26,6 +26,7 @@ if exist src\interfaces\ecpg\test\win32ver.rc del /q src\interfaces\ecpg\test\wi
 if exist src\pl\plperl\win32ver.rc del /q src\pl\plperl\win32ver.rc
 if exist src\pl\plpgsql\src\win32ver.rc del /q src\pl\plpgsql\src\win32ver.rc
 if exist src\pl\plpython\win32ver.rc del /q src\pl\plpython\win32ver.rc
+if exist src\pl\stub_plpython2\win32ver.rc del /q src\pl\stub_plpython2\win32ver.rc
 if exist src\pl\tcl\win32ver.rc del /q src\pl\tcl\win32ver.rc
 if exist src\test\isolation\win32ver.rc del /q src\test\isolation\win32ver.rc
 if exist src\test\regress\win32ver.rc del /q src\test\regress\win32ver.rc
diff --git a/src/tools/msvc/config_default.pl b/src/tools/msvc/config_default.pl
index 2ef2cfc..d7f9b60 100644
--- a/src/tools/msvc/config_default.pl
+++ b/src/tools/msvc/config_default.pl
@@ -16,6 +16,7 @@ our $config = {
     tcl       => undef,    # --with-tcl=<path>
     perl      => undef,    # --with-perl=<path>
     python    => undef,    # --with-python=<path>
+    python2_stub => 1,     # --with-python2-stub (ignored unless Python is v3)
     openssl   => undef,    # --with-openssl=<path>
     uuid      => undef,    # --with-uuid=<path>
     xml       => undef,    # --with-libxml=<path>