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>,
+       means that transaction control calls (<function>SPI_commit</function>,
-       <function>SPI_rollback</function>, and
+       <function>SPI_rollback</function>) are allowed.  Otherwise,
-       <function>SPI_start_transaction</function> are allowed.  Otherwise,
+       calling those functions will result in an immediate error.
-       calling these functions will result in an immediate error.
       </para>
      </listitem>
     </varlistentry>
@@ -4414,15 +4413,17 @@ 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
+   command <command>COMMIT</command>.  After the transaction is committed, a
-   transaction has to be started
+   new transaction is automatically started using default transaction
-   using <function>SPI_start_transaction</function> before further database
+   characteristics, so that the caller can continue using SPI facilities.
-   actions can be executed.
+   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
+   <function>SPI_commit_and_chain</function> is the same, but the new
-   transaction is immediately started with the same transaction
+   transaction is started with the same transaction
    characteristics as the just finished one, like with the SQL command
    <command>COMMIT AND CHAIN</command>.
   </para>
@@ -4467,14 +4468,13 @@ 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
+   command <command>ROLLBACK</command>.  After the transaction is rolled back,
-   new transaction has to be started
+   a new transaction is automatically started using default transaction
-   using <function>SPI_start_transaction</function> before further database
+   characteristics, so that the caller can continue using SPI facilities.
-   actions can be executed.
   </para>
   <para>
-   <function>SPI_rollback_and_chain</function> is the same, but a new
+   <function>SPI_rollback_and_chain</function> is the same, but the new
-   transaction is immediately started with the same transaction
+   transaction is started with the same transaction
    characteristics as the just finished one, like with the SQL command
    <command>ROLLBACK AND CHAIN</command>.
   </para>
@@ -4498,7 +4498,7 @@ void SPI_rollback_and_chain(void)
 
  <refnamediv>
   <refname>SPI_start_transaction</refname>
-  <refpurpose>start a new transaction</refpurpose>
+  <refpurpose>obsolete function</refpurpose>
  </refnamediv>
 
  <refsynopsisdiv>
@@ -4511,17 +4511,12 @@ void SPI_start_transaction(void)
   <title>Description</title>
 
   <para>
-   <function>SPI_start_transaction</function> starts a new transaction.  It
+   <function>SPI_start_transaction</function> does nothing, and exists
-   can only be called after <function>SPI_commit</function>
+   only for code compatibility with
-   or <function>SPI_rollback</function>, as there is no transaction active at
+   earlier <productname>PostgreSQL</productname> releases.  It used to
-   that point.  Normally, when an SPI-using procedure is called, there is already a
+   be required after calling <function>SPI_commit</function>
-   transaction active, so attempting to start another one before closing out
+   or <function>SPI_rollback</function>, but now those functions start
-   the current one will result in an error.
+   a new transaction automatically.
-  </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>

src/backend/executor/spi.c

@@ -150,7 +150,8 @@ SPI_connect_ext(int options)
 	 * XXX It could be better to use PortalContext as the parent context in
 	 * all cases, but we may not be inside a portal (consider deferred-trigger
 	 * execution).  Perhaps CurTransactionContext could be an option?  For now
-	 * it doesn't matter because we clean up explicitly in AtEOSubXact_SPI().
+	 * it doesn't matter because we clean up explicitly in AtEOSubXact_SPI();
+	 * but see also AtEOXact_SPI().
 	 */
 	_SPI_current->procCxt = AllocSetContextCreate(_SPI_current->atomic ? TopTransactionContext : PortalContext,
 												  "SPI Proc",
@@ -208,13 +209,13 @@ SPI_finish(void)
 	return SPI_OK_FINISH;
 }
 
+/*
+ * SPI_start_transaction is a no-op, kept for backwards compatibility.
+ * SPI callers are *always* inside a transaction.
+ */
 void
 SPI_start_transaction(void)
 {
-	MemoryContext oldcontext = CurrentMemoryContext;
-
-	StartTransactionCommand();
-	MemoryContextSwitchTo(oldcontext);
 }
 
 static void
@@ -222,6 +223,12 @@ _SPI_commit(bool chain)
 {
 	MemoryContext oldcontext = CurrentMemoryContext;
 
+	/*
+	 * Complain if we are in a context that doesn't permit transaction
+	 * termination.  (Note: here and _SPI_rollback should be the only places
+	 * that throw ERRCODE_INVALID_TRANSACTION_TERMINATION, so that callers can
+	 * test for that with security that they know what happened.)
+	 */
 	if (_SPI_current->atomic)
 		ereport(ERROR,
 				(errcode(ERRCODE_INVALID_TRANSACTION_TERMINATION),
@@ -234,40 +241,74 @@ _SPI_commit(bool chain)
 	 * top-level transaction in such a block violates that idea.  A future PL
 	 * implementation might have different ideas about this, in which case
 	 * this restriction would have to be refined or the check possibly be
-	 * moved out of SPI into the PLs.
+	 * moved out of SPI into the PLs.  Note however that the code below relies
+	 * on not being within a subtransaction.
 	 */
 	if (IsSubTransaction())
 		ereport(ERROR,
 				(errcode(ERRCODE_INVALID_TRANSACTION_TERMINATION),
 				 errmsg("cannot commit while a subtransaction is active")));
 
-	/*
+	/* XXX this ain't re-entrant enough for my taste */
-	 * Hold any pinned portals that any PLs might be using.  We have to do
-	 * this before changing transaction state, since this will run
-	 * user-defined code that might throw an error.
-	 */
-	HoldPinnedPortals();
-
-	/* Start the actual commit */
-	_SPI_current->internal_xact = true;
-
-	/* Release snapshots associated with portals */
-	ForgetPortalSnapshots();
-
 	if (chain)
 		SaveTransactionCharacteristics();
 
-	CommitTransactionCommand();
+	/* Catch any error occurring during the COMMIT */
-
+	PG_TRY();
-	if (chain)
 	{
+		/* Protect current SPI stack entry against deletion */
+		_SPI_current->internal_xact = true;
+
+		/*
+		 * Hold any pinned portals that any PLs might be using.  We have to do
+		 * this before changing transaction state, since this will run
+		 * user-defined code that might throw an error.
+		 */
+		HoldPinnedPortals();
+
+		/* Release snapshots associated with portals */
+		ForgetPortalSnapshots();
+
+		/* Do the deed */
+		CommitTransactionCommand();
+
+		/* Immediately start a new transaction */
 		StartTransactionCommand();
-		RestoreTransactionCharacteristics();
+		if (chain)
+			RestoreTransactionCharacteristics();
+
+		MemoryContextSwitchTo(oldcontext);
+
+		_SPI_current->internal_xact = false;
 	}
+	PG_CATCH();
+	{
+		ErrorData  *edata;
 
-	MemoryContextSwitchTo(oldcontext);
+		/* Save error info in caller's context */
+		MemoryContextSwitchTo(oldcontext);
+		edata = CopyErrorData();
+		FlushErrorState();
 
-	_SPI_current->internal_xact = false;
+		/*
+		 * Abort the failed transaction.  If this fails too, we'll just
+		 * propagate the error out ... there's not that much we can do.
+		 */
+		AbortCurrentTransaction();
+
+		/* ... and start a new one */
+		StartTransactionCommand();
+		if (chain)
+			RestoreTransactionCharacteristics();
+
+		MemoryContextSwitchTo(oldcontext);
+
+		_SPI_current->internal_xact = false;
+
+		/* Now that we've cleaned up the transaction, re-throw the error */
+		ReThrowError(edata);
+	}
+	PG_END_TRY();
 }
 
 void
@@ -287,6 +328,7 @@ _SPI_rollback(bool chain)
 {
 	MemoryContext oldcontext = CurrentMemoryContext;
 
+	/* see under SPI_commit() */
 	if (_SPI_current->atomic)
 		ereport(ERROR,
 				(errcode(ERRCODE_INVALID_TRANSACTION_TERMINATION),
@@ -298,34 +340,68 @@ _SPI_rollback(bool chain)
 				(errcode(ERRCODE_INVALID_TRANSACTION_TERMINATION),
 				 errmsg("cannot roll back while a subtransaction is active")));
 
-	/*
+	/* XXX this ain't re-entrant enough for my taste */
-	 * Hold any pinned portals that any PLs might be using.  We have to do
-	 * this before changing transaction state, since this will run
-	 * user-defined code that might throw an error, and in any case couldn't
-	 * be run in an already-aborted transaction.
-	 */
-	HoldPinnedPortals();
-
-	/* Start the actual rollback */
-	_SPI_current->internal_xact = true;
-
-	/* Release snapshots associated with portals */
-	ForgetPortalSnapshots();
-
 	if (chain)
 		SaveTransactionCharacteristics();
 
-	AbortCurrentTransaction();
+	/* Catch any error occurring during the ROLLBACK */
-
+	PG_TRY();
-	if (chain)
 	{
+		/* Protect current SPI stack entry against deletion */
+		_SPI_current->internal_xact = true;
+
+		/*
+		 * Hold any pinned portals that any PLs might be using.  We have to do
+		 * this before changing transaction state, since this will run
+		 * user-defined code that might throw an error, and in any case
+		 * couldn't be run in an already-aborted transaction.
+		 */
+		HoldPinnedPortals();
+
+		/* Release snapshots associated with portals */
+		ForgetPortalSnapshots();
+
+		/* Do the deed */
+		AbortCurrentTransaction();
+
+		/* Immediately start a new transaction */
 		StartTransactionCommand();
-		RestoreTransactionCharacteristics();
+		if (chain)
+			RestoreTransactionCharacteristics();
+
+		MemoryContextSwitchTo(oldcontext);
+
+		_SPI_current->internal_xact = false;
 	}
+	PG_CATCH();
+	{
+		ErrorData  *edata;
 
-	MemoryContextSwitchTo(oldcontext);
+		/* Save error info in caller's context */
+		MemoryContextSwitchTo(oldcontext);
+		edata = CopyErrorData();
+		FlushErrorState();
 
-	_SPI_current->internal_xact = false;
+		/*
+		 * Try again to abort the failed transaction.  If this fails too,
+		 * we'll just propagate the error out ... there's not that much we can
+		 * do.
+		 */
+		AbortCurrentTransaction();
+
+		/* ... and start a new one */
+		StartTransactionCommand();
+		if (chain)
+			RestoreTransactionCharacteristics();
+
+		MemoryContextSwitchTo(oldcontext);
+
+		_SPI_current->internal_xact = false;
+
+		/* Now that we've cleaned up the transaction, re-throw the error */
+		ReThrowError(edata);
+	}
+	PG_END_TRY();
 }
 
 void
@@ -340,38 +416,55 @@ SPI_rollback_and_chain(void)
 	_SPI_rollback(true);
 }
 
-/*
- * Clean up SPI state.  Called on transaction end (of non-SPI-internal
- * transactions) and when returning to the main loop on error.
- */
-void
-SPICleanup(void)
-{
-	_SPI_current = NULL;
-	_SPI_connected = -1;
-	/* Reset API global variables, too */
-	SPI_processed = 0;
-	SPI_tuptable = NULL;
-	SPI_result = 0;
-}
-
 /*
  * Clean up SPI state at transaction commit or abort.
  */
 void
 AtEOXact_SPI(bool isCommit)
 {
-	/* Do nothing if the transaction end was initiated by SPI. */
+	bool		found = false;
-	if (_SPI_current && _SPI_current->internal_xact)
-		return;
 
-	if (isCommit && _SPI_connected != -1)
+	/*
+	 * Pop stack entries, stopping if we find one marked internal_xact (that
+	 * one belongs to the caller of SPI_commit or SPI_abort).
+	 */
+	while (_SPI_connected >= 0)
+	{
+		_SPI_connection *connection = &(_SPI_stack[_SPI_connected]);
+
+		if (connection->internal_xact)
+			break;
+
+		found = true;
+
+		/*
+		 * We need not release the procedure's memory contexts explicitly, as
+		 * they'll go away automatically when their parent context does; see
+		 * notes in SPI_connect_ext.
+		 */
+
+		/*
+		 * Restore outer global variables and pop the stack entry.  Unlike
+		 * SPI_finish(), we don't risk switching to memory contexts that might
+		 * be already gone.
+		 */
+		SPI_processed = connection->outer_processed;
+		SPI_tuptable = connection->outer_tuptable;
+		SPI_result = connection->outer_result;
+
+		_SPI_connected--;
+		if (_SPI_connected < 0)
+			_SPI_current = NULL;
+		else
+			_SPI_current = &(_SPI_stack[_SPI_connected]);
+	}
+
+	/* We should only find entries to pop during an ABORT. */
+	if (found && isCommit)
 		ereport(WARNING,
 				(errcode(ERRCODE_WARNING),
 				 errmsg("transaction left non-empty SPI stack"),
 				 errhint("Check for missing \"SPI_finish\" calls.")));
-
-	SPICleanup();
 }
 
 /*

src/backend/tcop/postgres.c

@@ -42,7 +42,6 @@
 #include "catalog/pg_type.h"
 #include "commands/async.h"
 #include "commands/prepare.h"
-#include "executor/spi.h"
 #include "jit/jit.h"
 #include "libpq/libpq.h"
 #include "libpq/pqformat.h"
@@ -4125,7 +4124,6 @@ PostgresMain(int argc, char *argv[],
 			WalSndErrorCleanup();
 
 		PortalErrorCleanup();
-		SPICleanup();
 
 		/*
 		 * We can't release replication slots inside AbortTransaction() as we

src/backend/utils/mmgr/portalmem.c

@@ -1275,7 +1275,7 @@ HoldPinnedPortals(void)
 			 */
 			if (portal->strategy != PORTAL_ONE_SELECT)
 				ereport(ERROR,
-						(errcode(ERRCODE_INVALID_TRANSACTION_TERMINATION),
+						(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
 						 errmsg("cannot perform transaction commands inside a cursor loop that is not read-only")));
 
 			/* Verify it's in a suitable state to be held */

src/include/executor/spi.h

@@ -167,7 +167,6 @@ extern void SPI_commit_and_chain(void);
 extern void SPI_rollback(void);
 extern void SPI_rollback_and_chain(void);
 
-extern void SPICleanup(void);
 extern void AtEOXact_SPI(bool isCommit);
 extern void AtEOSubXact_SPI(bool isCommit, SubTransactionId mySubid);
 extern bool SPI_inside_nonatomic_context(void);

src/pl/plperl/expected/plperl_transaction.out

@@ -192,5 +192,53 @@ SELECT * FROM pg_cursors;
 ------+-----------+-------------+-----------+---------------+---------------
 (0 rows)
 
+-- check handling of an error during COMMIT
+CREATE TABLE testpk (id int PRIMARY KEY);
+CREATE TABLE testfk(f1 int REFERENCES testpk DEFERRABLE INITIALLY DEFERRED);
+DO LANGUAGE plperl $$
+# this insert will fail during commit:
+spi_exec_query("INSERT INTO testfk VALUES (0)");
+spi_commit();
+elog(WARNING, 'should not get here');
+$$;
+ERROR:  insert or update on table "testfk" violates foreign key constraint "testfk_f1_fkey" at line 4.
+CONTEXT:  PL/Perl anonymous code block
+SELECT * FROM testpk;
+ id 
+----
+(0 rows)
+
+SELECT * FROM testfk;
+ f1 
+----
+(0 rows)
+
+DO LANGUAGE plperl $$
+# this insert will fail during commit:
+spi_exec_query("INSERT INTO testfk VALUES (0)");
+eval {
+    spi_commit();
+};
+if ($@) {
+    elog(INFO, $@);
+}
+# these inserts should work:
+spi_exec_query("INSERT INTO testpk VALUES (1)");
+spi_exec_query("INSERT INTO testfk VALUES (1)");
+$$;
+INFO:  insert or update on table "testfk" violates foreign key constraint "testfk_f1_fkey" at line 5.
+
+SELECT * FROM testpk;
+ id 
+----
+  1
+(1 row)
+
+SELECT * FROM testfk;
+ f1 
+----
+  1
+(1 row)
+
 DROP TABLE test1;
 DROP TABLE test2;

src/pl/plperl/plperl.c

@@ -3988,7 +3988,6 @@ plperl_spi_commit(void)
 	PG_TRY();
 	{
 		SPI_commit();
-		SPI_start_transaction();
 	}
 	PG_CATCH();
 	{
@@ -4015,7 +4014,6 @@ plperl_spi_rollback(void)
 	PG_TRY();
 	{
 		SPI_rollback();
-		SPI_start_transaction();
 	}
 	PG_CATCH();
 	{

src/pl/plperl/sql/plperl_transaction.sql

@@ -159,5 +159,37 @@ SELECT * FROM test1;
 SELECT * FROM pg_cursors;
 
 
+-- check handling of an error during COMMIT
+CREATE TABLE testpk (id int PRIMARY KEY);
+CREATE TABLE testfk(f1 int REFERENCES testpk DEFERRABLE INITIALLY DEFERRED);
+
+DO LANGUAGE plperl $$
+# this insert will fail during commit:
+spi_exec_query("INSERT INTO testfk VALUES (0)");
+spi_commit();
+elog(WARNING, 'should not get here');
+$$;
+
+SELECT * FROM testpk;
+SELECT * FROM testfk;
+
+DO LANGUAGE plperl $$
+# this insert will fail during commit:
+spi_exec_query("INSERT INTO testfk VALUES (0)");
+eval {
+    spi_commit();
+};
+if ($@) {
+    elog(INFO, $@);
+}
+# these inserts should work:
+spi_exec_query("INSERT INTO testpk VALUES (1)");
+spi_exec_query("INSERT INTO testfk VALUES (1)");
+$$;
+
+SELECT * FROM testpk;
+SELECT * FROM testfk;
+
+
 DROP TABLE test1;
 DROP TABLE test2;

src/pl/plpgsql/src/pl_exec.c

@@ -4894,10 +4894,7 @@ exec_stmt_commit(PLpgSQL_execstate *estate, PLpgSQL_stmt_commit *stmt)
 	if (stmt->chain)
 		SPI_commit_and_chain();
 	else
-	{
 		SPI_commit();
-		SPI_start_transaction();
-	}
 
 	/*
 	 * We need to build new simple-expression infrastructure, since the old
@@ -4921,10 +4918,7 @@ exec_stmt_rollback(PLpgSQL_execstate *estate, PLpgSQL_stmt_rollback *stmt)
 	if (stmt->chain)
 		SPI_rollback_and_chain();
 	else
-	{
 		SPI_rollback();
-		SPI_start_transaction();
-	}
 
 	/*
 	 * We need to build new simple-expression infrastructure, since the old

src/pl/plpython/expected/plpython_transaction.out

@@ -55,8 +55,11 @@ for i in range(0, 10):
 return 1
 $$;
 SELECT transaction_test2();
-ERROR:  invalid transaction termination
+ERROR:  spiexceptions.InvalidTransactionTermination: invalid transaction termination
-CONTEXT:  PL/Python function "transaction_test2"
+CONTEXT:  Traceback (most recent call last):
+  PL/Python function "transaction_test2", line 5, in <module>
+    plpy.commit()
+PL/Python function "transaction_test2"
 SELECT * FROM test1;
  a | b 
 ---+---
@@ -70,7 +73,7 @@ plpy.execute("CALL transaction_test1()")
 return 1
 $$;
 SELECT transaction_test3();
-ERROR:  spiexceptions.InvalidTransactionTermination: invalid transaction termination
+ERROR:  spiexceptions.InvalidTransactionTermination: spiexceptions.InvalidTransactionTermination: invalid transaction termination
 CONTEXT:  Traceback (most recent call last):
   PL/Python function "transaction_test3", line 2, in <module>
     plpy.execute("CALL transaction_test1()")
@@ -88,7 +91,7 @@ plpy.execute("DO LANGUAGE plpythonu $x$ plpy.commit() $x$")
 return 1
 $$;
 SELECT transaction_test4();
-ERROR:  spiexceptions.InvalidTransactionTermination: invalid transaction termination
+ERROR:  spiexceptions.InvalidTransactionTermination: spiexceptions.InvalidTransactionTermination: invalid transaction termination
 CONTEXT:  Traceback (most recent call last):
   PL/Python function "transaction_test4", line 2, in <module>
     plpy.execute("DO LANGUAGE plpythonu $x$ plpy.commit() $x$")
@@ -100,8 +103,11 @@ s.enter()
 plpy.commit()
 $$;
 WARNING:  forcibly aborting a subtransaction that has not been exited
-ERROR:  cannot commit while a subtransaction is active
+ERROR:  spiexceptions.InvalidTransactionTermination: cannot commit while a subtransaction is active
-CONTEXT:  PL/Python anonymous code block
+CONTEXT:  Traceback (most recent call last):
+  PL/Python anonymous code block, line 4, in <module>
+    plpy.commit()
+PL/Python anonymous code block
 -- commit inside cursor loop
 CREATE TABLE test2 (x int);
 INSERT INTO test2 VALUES (0), (1), (2), (3), (4);
@@ -191,5 +197,54 @@ SELECT * FROM pg_cursors;
 ------+-----------+-------------+-----------+---------------+---------------
 (0 rows)
 
+-- check handling of an error during COMMIT
+CREATE TABLE testpk (id int PRIMARY KEY);
+CREATE TABLE testfk(f1 int REFERENCES testpk DEFERRABLE INITIALLY DEFERRED);
+DO LANGUAGE plpythonu $$
+# this insert will fail during commit:
+plpy.execute("INSERT INTO testfk VALUES (0)")
+plpy.commit()
+plpy.warning('should not get here')
+$$;
+ERROR:  spiexceptions.ForeignKeyViolation: insert or update on table "testfk" violates foreign key constraint "testfk_f1_fkey"
+DETAIL:  Key (f1)=(0) is not present in table "testpk".
+CONTEXT:  Traceback (most recent call last):
+  PL/Python anonymous code block, line 4, in <module>
+    plpy.commit()
+PL/Python anonymous code block
+SELECT * FROM testpk;
+ id 
+----
+(0 rows)
+
+SELECT * FROM testfk;
+ f1 
+----
+(0 rows)
+
+DO LANGUAGE plpythonu $$
+# this insert will fail during commit:
+plpy.execute("INSERT INTO testfk VALUES (0)")
+try:
+    plpy.commit()
+except Exception as e:
+    plpy.info('sqlstate: %s' % (e.sqlstate))
+# these inserts should work:
+plpy.execute("INSERT INTO testpk VALUES (1)")
+plpy.execute("INSERT INTO testfk VALUES (1)")
+$$;
+INFO:  sqlstate: 23503
+SELECT * FROM testpk;
+ id 
+----
+  1
+(1 row)
+
+SELECT * FROM testfk;
+ f1 
+----
+  1
+(1 row)
+
 DROP TABLE test1;
 DROP TABLE test2;

src/pl/plpython/plpy_plpymodule.c

@@ -40,8 +40,6 @@ static PyObject *PLy_fatal(PyObject *self, PyObject *args, PyObject *kw);
 static PyObject *PLy_quote_literal(PyObject *self, PyObject *args);
 static PyObject *PLy_quote_nullable(PyObject *self, PyObject *args);
 static PyObject *PLy_quote_ident(PyObject *self, PyObject *args);
-static PyObject *PLy_commit(PyObject *self, PyObject *args);
-static PyObject *PLy_rollback(PyObject *self, PyObject *args);
 
 
 /* A list of all known exceptions, generated from backend/utils/errcodes.txt */
@@ -578,31 +576,3 @@ PLy_output(volatile int level, PyObject *self, PyObject *args, PyObject *kw)
 	 */
 	Py_RETURN_NONE;
 }
-
-static PyObject *
-PLy_commit(PyObject *self, PyObject *args)
-{
-	PLyExecutionContext *exec_ctx = PLy_current_execution_context();
-
-	SPI_commit();
-	SPI_start_transaction();
-
-	/* was cleared at transaction end, reset pointer */
-	exec_ctx->scratch_ctx = NULL;
-
-	Py_RETURN_NONE;
-}
-
-static PyObject *
-PLy_rollback(PyObject *self, PyObject *args)
-{
-	PLyExecutionContext *exec_ctx = PLy_current_execution_context();
-
-	SPI_rollback();
-	SPI_start_transaction();
-
-	/* was cleared at transaction end, reset pointer */
-	exec_ctx->scratch_ctx = NULL;
-
-	Py_RETURN_NONE;
-}

src/pl/plpython/plpy_spi.c

@@ -456,6 +456,100 @@ PLy_spi_execute_fetch_result(SPITupleTable *tuptable, uint64 rows, int status)
 	return (PyObject *) result;
 }
 
+PyObject *
+PLy_commit(PyObject *self, PyObject *args)
+{
+	MemoryContext oldcontext = CurrentMemoryContext;
+	PLyExecutionContext *exec_ctx = PLy_current_execution_context();
+
+	PG_TRY();
+	{
+		SPI_commit();
+
+		/* was cleared at transaction end, reset pointer */
+		exec_ctx->scratch_ctx = NULL;
+	}
+	PG_CATCH();
+	{
+		ErrorData  *edata;
+		PLyExceptionEntry *entry;
+		PyObject   *exc;
+
+		/* Save error info */
+		MemoryContextSwitchTo(oldcontext);
+		edata = CopyErrorData();
+		FlushErrorState();
+
+		/* was cleared at transaction end, reset pointer */
+		exec_ctx->scratch_ctx = NULL;
+
+		/* Look up the correct exception */
+		entry = hash_search(PLy_spi_exceptions, &(edata->sqlerrcode),
+							HASH_FIND, NULL);
+
+		/*
+		 * This could be a custom error code, if that's the case fallback to
+		 * SPIError
+		 */
+		exc = entry ? entry->exc : PLy_exc_spi_error;
+		/* Make Python raise the exception */
+		PLy_spi_exception_set(exc, edata);
+		FreeErrorData(edata);
+
+		return NULL;
+	}
+	PG_END_TRY();
+
+	Py_RETURN_NONE;
+}
+
+PyObject *
+PLy_rollback(PyObject *self, PyObject *args)
+{
+	MemoryContext oldcontext = CurrentMemoryContext;
+	PLyExecutionContext *exec_ctx = PLy_current_execution_context();
+
+	PG_TRY();
+	{
+		SPI_rollback();
+
+		/* was cleared at transaction end, reset pointer */
+		exec_ctx->scratch_ctx = NULL;
+	}
+	PG_CATCH();
+	{
+		ErrorData  *edata;
+		PLyExceptionEntry *entry;
+		PyObject   *exc;
+
+		/* Save error info */
+		MemoryContextSwitchTo(oldcontext);
+		edata = CopyErrorData();
+		FlushErrorState();
+
+		/* was cleared at transaction end, reset pointer */
+		exec_ctx->scratch_ctx = NULL;
+
+		/* Look up the correct exception */
+		entry = hash_search(PLy_spi_exceptions, &(edata->sqlerrcode),
+							HASH_FIND, NULL);
+
+		/*
+		 * This could be a custom error code, if that's the case fallback to
+		 * SPIError
+		 */
+		exc = entry ? entry->exc : PLy_exc_spi_error;
+		/* Make Python raise the exception */
+		PLy_spi_exception_set(exc, edata);
+		FreeErrorData(edata);
+
+		return NULL;
+	}
+	PG_END_TRY();
+
+	Py_RETURN_NONE;
+}
+
 /*
  * Utilities for running SPI functions in subtransactions.
  *

src/pl/plpython/plpy_spi.h

@@ -12,6 +12,9 @@ extern PyObject *PLy_spi_prepare(PyObject *self, PyObject *args);
 extern PyObject *PLy_spi_execute(PyObject *self, PyObject *args);
 extern PyObject *PLy_spi_execute_plan(PyObject *ob, PyObject *list, long limit);
 
+extern PyObject *PLy_commit(PyObject *self, PyObject *args);
+extern PyObject *PLy_rollback(PyObject *self, PyObject *args);
+
 typedef struct PLyExceptionEntry
 {
 	int			sqlstate;		/* hash key, must be first */

src/pl/plpython/sql/plpython_transaction.sql

@@ -148,5 +148,35 @@ SELECT * FROM test1;
 SELECT * FROM pg_cursors;
 
 
+-- check handling of an error during COMMIT
+CREATE TABLE testpk (id int PRIMARY KEY);
+CREATE TABLE testfk(f1 int REFERENCES testpk DEFERRABLE INITIALLY DEFERRED);
+
+DO LANGUAGE plpythonu $$
+# this insert will fail during commit:
+plpy.execute("INSERT INTO testfk VALUES (0)")
+plpy.commit()
+plpy.warning('should not get here')
+$$;
+
+SELECT * FROM testpk;
+SELECT * FROM testfk;
+
+DO LANGUAGE plpythonu $$
+# this insert will fail during commit:
+plpy.execute("INSERT INTO testfk VALUES (0)")
+try:
+    plpy.commit()
+except Exception as e:
+    plpy.info('sqlstate: %s' % (e.sqlstate))
+# these inserts should work:
+plpy.execute("INSERT INTO testpk VALUES (1)")
+plpy.execute("INSERT INTO testfk VALUES (1)")
+$$;
+
+SELECT * FROM testpk;
+SELECT * FROM testfk;
+
+
 DROP TABLE test1;
 DROP TABLE test2;

src/pl/tcl/expected/pltcl_transaction.out

@@ -96,5 +96,54 @@ SELECT * FROM test1;
 ---+---
 (0 rows)
 
+-- check handling of an error during COMMIT
+CREATE TABLE testpk (id int PRIMARY KEY);
+CREATE TABLE testfk(f1 int REFERENCES testpk DEFERRABLE INITIALLY DEFERRED);
+CREATE PROCEDURE transaction_testfk()
+LANGUAGE pltcl
+AS $$
+# this insert will fail during commit:
+spi_exec "INSERT INTO testfk VALUES (0)"
+commit
+elog WARNING "should not get here"
+$$;
+CALL transaction_testfk();
+ERROR:  insert or update on table "testfk" violates foreign key constraint "testfk_f1_fkey"
+SELECT * FROM testpk;
+ id 
+----
+(0 rows)
+
+SELECT * FROM testfk;
+ f1 
+----
+(0 rows)
+
+CREATE OR REPLACE PROCEDURE transaction_testfk()
+LANGUAGE pltcl
+AS $$
+# this insert will fail during commit:
+spi_exec "INSERT INTO testfk VALUES (0)"
+if [catch {commit} msg] {
+    elog INFO $msg
+}
+# these inserts should work:
+spi_exec "INSERT INTO testpk VALUES (1)"
+spi_exec "INSERT INTO testfk VALUES (1)"
+$$;
+CALL transaction_testfk();
+INFO:  insert or update on table "testfk" violates foreign key constraint "testfk_f1_fkey"
+SELECT * FROM testpk;
+ id 
+----
+  1
+(1 row)
+
+SELECT * FROM testfk;
+ f1 
+----
+  1
+(1 row)
+
 DROP TABLE test1;
 DROP TABLE test2;

src/pl/tcl/pltcl.c

@@ -2930,7 +2930,6 @@ pltcl_commit(ClientData cdata, Tcl_Interp *interp,
 	PG_TRY();
 	{
 		SPI_commit();
-		SPI_start_transaction();
 	}
 	PG_CATCH();
 	{
@@ -2970,7 +2969,6 @@ pltcl_rollback(ClientData cdata, Tcl_Interp *interp,
 	PG_TRY();
 	{
 		SPI_rollback();
-		SPI_start_transaction();
 	}
 	PG_CATCH();
 	{

src/pl/tcl/sql/pltcl_transaction.sql

@@ -94,5 +94,42 @@ CALL transaction_test4b();
 SELECT * FROM test1;
 
 
+-- check handling of an error during COMMIT
+CREATE TABLE testpk (id int PRIMARY KEY);
+CREATE TABLE testfk(f1 int REFERENCES testpk DEFERRABLE INITIALLY DEFERRED);
+
+CREATE PROCEDURE transaction_testfk()
+LANGUAGE pltcl
+AS $$
+# this insert will fail during commit:
+spi_exec "INSERT INTO testfk VALUES (0)"
+commit
+elog WARNING "should not get here"
+$$;
+
+CALL transaction_testfk();
+
+SELECT * FROM testpk;
+SELECT * FROM testfk;
+
+CREATE OR REPLACE PROCEDURE transaction_testfk()
+LANGUAGE pltcl
+AS $$
+# this insert will fail during commit:
+spi_exec "INSERT INTO testfk VALUES (0)"
+if [catch {commit} msg] {
+    elog INFO $msg
+}
+# these inserts should work:
+spi_exec "INSERT INTO testpk VALUES (1)"
+spi_exec "INSERT INTO testfk VALUES (1)"
+$$;
+
+CALL transaction_testfk();
+
+SELECT * FROM testpk;
+SELECT * FROM testfk;
+
+
 DROP TABLE test1;
 DROP TABLE test2;