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>

1 files changed, 95 insertions, 7 deletions

diff --git a/src/backend/executor/spi.c b/src/backend/executor/spi.c
index 995f67d2662..9fc4431b80c 100644
--- a/src/backend/executor/spi.c
+++ b/src/backend/executor/spi.c
@@ -83,6 +83,12 @@ static bool _SPI_checktuples(void);
 int
 SPI_connect(void)
 {
+	return SPI_connect_ext(0);
+}
+
+int
+SPI_connect_ext(int options)
+{
 	int			newdepth;
 
 	/* Enlarge stack if necessary */
@@ -92,7 +98,7 @@ SPI_connect(void)
 			elog(ERROR, "SPI stack corrupted");
 		newdepth = 16;
 		_SPI_stack = (_SPI_connection *)
-			MemoryContextAlloc(TopTransactionContext,
+			MemoryContextAlloc(TopMemoryContext,
 							   newdepth * sizeof(_SPI_connection));
 		_SPI_stack_depth = newdepth;
 	}
@@ -124,19 +130,25 @@ SPI_connect(void)
 	_SPI_current->execCxt = NULL;
 	_SPI_current->connectSubid = GetCurrentSubTransactionId();
 	_SPI_current->queryEnv = NULL;
+	_SPI_current->atomic = (options & SPI_OPT_NONATOMIC ? false : true);
+	_SPI_current->internal_xact = false;
 
 	/*
 	 * Create memory contexts for this procedure
 	 *
-	 * XXX it would be better to use PortalContext as the parent context, but
-	 * we may not be inside a portal (consider deferred-trigger execution).
-	 * Perhaps CurTransactionContext would do?	For now it doesn't matter
-	 * because we clean up explicitly in AtEOSubXact_SPI().
+	 * In atomic contexts (the normal case), we use TopTransactionContext,
+	 * otherwise PortalContext, so that it lives across transaction
+	 * boundaries.
+	 *
+	 * 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().
 	 */
-	_SPI_current->procCxt = AllocSetContextCreate(TopTransactionContext,
+	_SPI_current->procCxt = AllocSetContextCreate(_SPI_current->atomic ? TopTransactionContext : PortalContext,
 												  "SPI Proc",
 												  ALLOCSET_DEFAULT_SIZES);
-	_SPI_current->execCxt = AllocSetContextCreate(TopTransactionContext,
+	_SPI_current->execCxt = AllocSetContextCreate(_SPI_current->atomic ? TopTransactionContext : _SPI_current->procCxt,
 												  "SPI Exec",
 												  ALLOCSET_DEFAULT_SIZES);
 	/* ... and switch to procedure's context */
@@ -181,6 +193,73 @@ SPI_finish(void)
 	return SPI_OK_FINISH;
 }
 
+void
+SPI_start_transaction(void)
+{
+	MemoryContext oldcontext = CurrentMemoryContext;
+
+	StartTransactionCommand();
+	MemoryContextSwitchTo(oldcontext);
+}
+
+void
+SPI_commit(void)
+{
+	MemoryContext oldcontext = CurrentMemoryContext;
+
+	if (_SPI_current->atomic)
+		ereport(ERROR,
+				(errcode(ERRCODE_INVALID_TRANSACTION_TERMINATION),
+				 errmsg("invalid transaction termination")));
+
+	/*
+	 * This restriction is required by PLs implemented on top of SPI.  They
+	 * use subtransactions to establish exception blocks that are supposed to
+	 * be rolled back together if there is an error.  Terminating the
+	 * 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.
+	 */
+	if (IsSubTransaction())
+		ereport(ERROR,
+				(errcode(ERRCODE_INVALID_TRANSACTION_TERMINATION),
+				 errmsg("cannot commit while a subtransaction is active")));
+
+	_SPI_current->internal_xact = true;
+
+	if (ActiveSnapshotSet())
+		PopActiveSnapshot();
+	CommitTransactionCommand();
+	MemoryContextSwitchTo(oldcontext);
+
+	_SPI_current->internal_xact = false;
+}
+
+void
+SPI_rollback(void)
+{
+	MemoryContext oldcontext = CurrentMemoryContext;
+
+	if (_SPI_current->atomic)
+		ereport(ERROR,
+				(errcode(ERRCODE_INVALID_TRANSACTION_TERMINATION),
+				 errmsg("invalid transaction termination")));
+
+	/* see under SPI_commit() */
+	if (IsSubTransaction())
+		ereport(ERROR,
+				(errcode(ERRCODE_INVALID_TRANSACTION_TERMINATION),
+				 errmsg("cannot roll back while a subtransaction is active")));
+
+	_SPI_current->internal_xact = true;
+
+	AbortCurrentTransaction();
+	MemoryContextSwitchTo(oldcontext);
+
+	_SPI_current->internal_xact = false;
+}
+
 /*
  * Clean up SPI state at transaction commit or abort.
  */
@@ -188,6 +267,12 @@ void
 AtEOXact_SPI(bool isCommit)
 {
 	/*
+	 * Do nothing if the transaction end was initiated by SPI.
+	 */
+	if (_SPI_current && _SPI_current->internal_xact)
+		return;
+
+	/*
 	 * Note that memory contexts belonging to SPI stack entries will be freed
 	 * automatically, so we can ignore them here.  We just need to restore our
 	 * static variables to initial state.
@@ -224,6 +309,9 @@ AtEOSubXact_SPI(bool isCommit, SubTransactionId mySubid)
 		if (connection->connectSubid != mySubid)
 			break;				/* couldn't be any underneath it either */
 
+		if (connection->internal_xact)
+			break;
+
 		found = true;
 
 		/*