Transaction control in PL procedures

In each of the supplied procedural languages (PL/pgSQL, PL/Perl,
PL/Python, PL/Tcl), add language-specific commit and rollback
functions/commands to control transactions in procedures in that
language.  Add similar underlying functions to SPI.  Some additional
cleanup so that transaction commit or abort doesn't blow away data
structures still used by the procedure call.  Add execution context
tracking to CALL and DO statements so that transaction control commands
can only be issued in top-level procedure and block calls, not function
calls or other procedure or block calls.

- SPI

Add a new function SPI_connect_ext() that is like SPI_connect() but
allows passing option flags.  The only option flag right now is
SPI_OPT_NONATOMIC.  A nonatomic SPI connection can execute transaction
control commands, otherwise it's not allowed.  This is meant to be
passed down from CALL and DO statements which themselves know in which
context they are called.  A nonatomic SPI connection uses different
memory management.  A normal SPI connection allocates its memory in
TopTransactionContext.  For nonatomic connections we use PortalContext
instead.  As the comment in SPI_connect_ext() (previously SPI_connect())
indicates, one could potentially use PortalContext in all cases, but it
seems safest to leave the existing uses alone, because this stuff is
complicated enough already.

SPI also gets new functions SPI_start_transaction(), SPI_commit(), and
SPI_rollback(), which can be used by PLs to implement their transaction
control logic.

- portalmem.c

Some adjustments were made in the code that cleans up portals at
transaction abort.  The portal code could already handle a command
*committing* a transaction and continuing (e.g., VACUUM), but it was not
quite prepared for a command *aborting* a transaction and continuing.

In AtAbort_Portals(), remove the code that marks an active portal as
failed.  As the comment there already predicted, this doesn't work if
the running command wants to keep running after transaction abort.  And
it's actually not necessary, because pquery.c is careful to run all
portal code in a PG_TRY block and explicitly runs MarkPortalFailed() if
there is an exception.  So the code in AtAbort_Portals() is never used
anyway.

In AtAbort_Portals() and AtCleanup_Portals(), we need to be careful not
to clean up active portals too much.  This mirrors similar code in
PreCommit_Portals().

- PL/Perl

Gets new functions spi_commit() and spi_rollback()

- PL/pgSQL

Gets new commands COMMIT and ROLLBACK.

Update the PL/SQL porting example in the documentation to reflect that
transactions are now possible in procedures.

- PL/Python

Gets new functions plpy.commit and plpy.rollback.

- PL/Tcl

Gets new commands commit and rollback.

Reviewed-by: Andrew Dunstan <andrew.dunstan@2ndquadrant.com>

doc/src/sgml/spi.sgml

@@ -64,6 +64,7 @@
 
  <refentry id="spi-spi-connect">
   <indexterm><primary>SPI_connect</primary></indexterm>
+  <indexterm><primary>SPI_connect_ext</primary></indexterm>
 
   <refmeta>
    <refentrytitle>SPI_connect</refentrytitle>
@@ -72,12 +73,17 @@
 
   <refnamediv>
    <refname>SPI_connect</refname>
+   <refname>SPI_connect_ext</refname>
    <refpurpose>connect a procedure to the SPI manager</refpurpose>
  </refnamediv>
 
  <refsynopsisdiv>
 <synopsis>
 int SPI_connect(void)
+</synopsis>
+
+ <synopsis>
+int SPI_connect_ext(int <parameter>options</parameter>)
 </synopsis>
  </refsynopsisdiv>
 
@@ -90,6 +96,31 @@ int SPI_connect(void)
    function if you want to execute commands through SPI.  Some utility
    SPI functions can be called from unconnected procedures.
   </para>
+
+  <para>
+   <function>SPI_connect_ext</function> does the same but has an argument that
+   allows passing option flags.  Currently, the following option values are
+   available:
+   <variablelist>
+    <varlistentry>
+     <term><symbol>SPI_OPT_NONATOMIC</symbol></term>
+     <listitem>
+      <para>
+       Sets the SPI connection to be <firstterm>nonatomic</firstterm>, which
+       means that transaction control calls <function>SPI_commit</function>,
+       <function>SPI_rollback</function>, and
+       <function>SPI_start_transaction</function> are allowed.  Otherwise,
+       calling these functions will result in an immediate error.
+      </para>
+     </listitem>
+    </varlistentry>
+   </variablelist>
+  </para>
+
+  <para>
+   <literal>SPI_connect()</literal> is equivalent to
+   <literal>SPI_connect_ext(0)</literal>.
+  </para>
  </refsect1>
 
  <refsect1>
@@ -4325,6 +4356,152 @@ int SPI_freeplan(SPIPlanPtr <parameter>plan</parameter>)
 
  </sect1>
 
+ <sect1 id="spi-transaction">
+  <title>Transaction Management</title>
+
+  <para>
+   It is not possible to run transaction control commands such
+   as <command>COMMIT</command> and <command>ROLLBACK</command> through SPI
+   functions such as <function>SPI_execute</function>.  There are, however,
+   separate interface functions that allow transaction control through SPI.
+  </para>
+
+  <para>
+   It is not generally safe and sensible to start and end transactions in
+   arbitrary user-defined SQL-callable functions without taking into account
+   the context in which they are called.  For example, a transaction boundary
+   in the middle of a function that is part of a complex SQL expression that
+   is part of some SQL command will probably result in obscure internal errors
+   or crashes.  The interface functions presented here are primarily intended
+   to be used by procedural language implementations to support transaction
+   management in procedures that are invoked by the <command>CALL</command>
+   command, taking the context of the <command>CALL</command> invocation into
+   account.  SPI procedures implemented in C can implement the same logic, but
+   the details of that are beyond the scope of this documentation.
+  </para>
+
+<!-- *********************************************** -->
+
+<refentry id="spi-spi-commit">
+ <indexterm><primary>SPI_commit</primary></indexterm>
+
+ <refmeta>
+  <refentrytitle>SPI_commit</refentrytitle>
+  <manvolnum>3</manvolnum>
+ </refmeta>
+
+ <refnamediv>
+  <refname>SPI_commit</refname>
+  <refpurpose>commit the current transaction</refpurpose>
+ </refnamediv>
+
+ <refsynopsisdiv>
+<synopsis>
+void SPI_commit(void)
+</synopsis>
+ </refsynopsisdiv>
+
+ <refsect1>
+  <title>Description</title>
+
+  <para>
+   <function>SPI_commit</function> commits the current transaction.  It is
+   approximately equivalent to running the SQL
+   command <command>COMMIT</command>.  After a transaction is committed, a new
+   transaction has to be started
+   using <function>SPI_start_transaction</function> before further database
+   actions can be executed.
+  </para>
+
+  <para>
+   This function can only be executed if the SPI connection has been set as
+   nonatomic in the call to <function>SPI_connect_ext</function>.
+  </para>
+ </refsect1>
+</refentry>
+
+<!-- *********************************************** -->
+
+<refentry id="spi-spi-rollback">
+ <indexterm><primary>SPI_rollback</primary></indexterm>
+
+ <refmeta>
+  <refentrytitle>SPI_rollback</refentrytitle>
+  <manvolnum>3</manvolnum>
+ </refmeta>
+
+ <refnamediv>
+  <refname>SPI_rollback</refname>
+  <refpurpose>abort the current transaction</refpurpose>
+ </refnamediv>
+
+ <refsynopsisdiv>
+<synopsis>
+void SPI_rollback(void)
+</synopsis>
+ </refsynopsisdiv>
+
+ <refsect1>
+  <title>Description</title>
+
+  <para>
+   <function>SPI_rollback</function> rolls back the current transaction.  It
+   is approximately equivalent to running the SQL
+   command <command>ROLLBACK</command>.  After a transaction is rolled back, a
+   new transaction has to be started
+   using <function>SPI_start_transaction</function> before further database
+   actions can be executed.
+  </para>
+
+  <para>
+   This function can only be executed if the SPI connection has been set as
+   nonatomic in the call to <function>SPI_connect_ext</function>.
+  </para>
+ </refsect1>
+</refentry>
+
+<!-- *********************************************** -->
+
+<refentry id="spi-spi-start-transaction">
+ <indexterm><primary>SPI_start_transaction</primary></indexterm>
+
+ <refmeta>
+  <refentrytitle>SPI_start_transaction</refentrytitle>
+  <manvolnum>3</manvolnum>
+ </refmeta>
+
+ <refnamediv>
+  <refname>SPI_start_transaction</refname>
+  <refpurpose>start a new transaction</refpurpose>
+ </refnamediv>
+
+ <refsynopsisdiv>
+<synopsis>
+void SPI_start_transaction(void)
+</synopsis>
+ </refsynopsisdiv>
+
+ <refsect1>
+  <title>Description</title>
+
+  <para>
+   <function>SPI_start_transaction</function> starts a new transaction.  It
+   can only be called after <function>SPI_commit</function>
+   or <function>SPI_rollback</function>, as there is no transaction active at
+   that point.  Normally, when an SPI procedure is called, there is already a
+   transaction active, so attempting to start another one before closing out
+   the current one will result in an error.
+  </para>
+
+  <para>
+   This function can only be executed if the SPI connection has been set as
+   nonatomic in the call to <function>SPI_connect_ext</function>.
+  </para>
+ </refsect1>
+</refentry>
+
+ </sect1>
+
  <sect1 id="spi-visibility">
   <title>Visibility of Data Changes</title>