Fix SPI's handling of errors during transaction commit.

SPI_commit previously left it up to the caller to recover from any error
occurring during commit.  Since that's complicated and requires use of
low-level xact.c facilities, it's not too surprising that no caller got
it right.  Let's move the responsibility for cleanup into spi.c.  Doing
that requires redefining SPI_commit as starting a new transaction, so
that it becomes equivalent to SPI_commit_and_chain except that you get
default transaction characteristics instead of preserving the prior
transaction's characteristics.  We can make this pretty transparent
API-wise by redefining SPI_start_transaction() as a no-op.  Callers
that expect to do something in between might be surprised, but
available evidence is that no callers do so.

Having made that API redefinition, we can fix this mess by having
SPI_commit[_and_chain] trap errors and start a new, clean transaction
before re-throwing the error.  Likewise for SPI_rollback[_and_chain].
Some cleanup is also needed in AtEOXact_SPI, which was nowhere near
smart enough to deal with SPI contexts nested inside a committing
context.

While plperl and pltcl need no changes beyond removing their now-useless
SPI_start_transaction() calls, plpython needs some more work because it
hadn't gotten the memo about catching commit/rollback errors in the
first place.  Such an error resulted in longjmp'ing out of the Python
interpreter, which leaks Python stack entries at present and is reported
to crash Python 3.11 altogether.  Add the missing logic to catch such
errors and convert them into Python exceptions.

This is a back-patch of commit 2e517818f.  That's now aged long enough
to reduce the concerns about whether it will break something, and we
do need to ensure that supported branches will work with Python 3.11.

Peter Eisentraut and Tom Lane

Discussion: https://postgr.es/m/3375ffd8-d71c-2565-e348-a597d6e739e3@enterprisedb.com
Discussion: https://postgr.es/m/17416-ed8fe5d7213d6c25@postgresql.org

doc/src/sgml/spi.sgml

@@ -99,10 +99,9 @@ int SPI_connect_ext(int <parameter>options</parameter>)
      <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.
+       means that transaction control calls (<function>SPI_commit</function>,
+       <function>SPI_rollback</function>) are allowed.  Otherwise,
+       calling those functions will result in an immediate error.
       </para>
      </listitem>
     </varlistentry>
@@ -4405,15 +4404,17 @@ void SPI_commit_and_chain(void)
   <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.
+   command <command>COMMIT</command>.  After the transaction is committed, a
+   new transaction is automatically started using default transaction
+   characteristics, so that the caller can continue using SPI facilities.
+   If there is a failure during commit, the current transaction is instead
+   rolled back and a new transaction is started, after which the error is
+   thrown in the usual way.
   </para>
 
   <para>
-   <function>SPI_commit_and_chain</function> is the same, but a new
-   transaction is immediately started with the same transaction
+   <function>SPI_commit_and_chain</function> is the same, but the new
+   transaction is started with the same transaction
    characteristics as the just finished one, like with the SQL command
    <command>COMMIT AND CHAIN</command>.
   </para>
@@ -4458,14 +4459,13 @@ void SPI_rollback_and_chain(void)
   <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.
+   command <command>ROLLBACK</command>.  After the transaction is rolled back,
+   a new transaction is automatically started using default transaction
+   characteristics, so that the caller can continue using SPI facilities.
   </para>
   <para>
-   <function>SPI_rollback_and_chain</function> is the same, but a new
-   transaction is immediately started with the same transaction
+   <function>SPI_rollback_and_chain</function> is the same, but the new
+   transaction is started with the same transaction
    characteristics as the just finished one, like with the SQL command
    <command>ROLLBACK AND CHAIN</command>.
   </para>
@@ -4489,7 +4489,7 @@ void SPI_rollback_and_chain(void)
 
  <refnamediv>
   <refname>SPI_start_transaction</refname>
-  <refpurpose>start a new transaction</refpurpose>
+  <refpurpose>obsolete function</refpurpose>
  </refnamediv>
 
  <refsynopsisdiv>
@@ -4502,17 +4502,12 @@ void SPI_start_transaction(void)
   <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-using 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>.
+   <function>SPI_start_transaction</function> does nothing, and exists
+   only for code compatibility with
+   earlier <productname>PostgreSQL</productname> releases.  It used to
+   be required after calling <function>SPI_commit</function>
+   or <function>SPI_rollback</function>, but now those functions start
+   a new transaction automatically.
   </para>
  </refsect1>
 </refentry>