portalmem.c

/*-------------------------------------------------------------------------
 *
 * portalmem.c
 *	  backend portal memory management
 *
 * Portals are objects representing the execution state of a query.
 * This module provides memory management services for portals, but it
 * doesn't actually run the executor for them.
 *
 *
 * Portions Copyright (c) 1996-2007, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * IDENTIFICATION
 *	  $PostgreSQL: pgsql/src/backend/utils/mmgr/portalmem.c,v 1.100 2007/03/13 00:33:42 tgl Exp $
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include "access/heapam.h"
#include "access/xact.h"
#include "catalog/pg_type.h"
#include "commands/portalcmds.h"
#include "funcapi.h"
#include "miscadmin.h"
#include "utils/builtins.h"
#include "utils/memutils.h"

/*
 * Estimate of the maximum number of open portals a user would have,
 * used in initially sizing the PortalHashTable in EnablePortalManager().
 * Since the hash table can expand, there's no need to make this overly
 * generous, and keeping it small avoids unnecessary overhead in the
 * hash_seq_search() calls executed during transaction end.
 */
#define PORTALS_PER_USER	   16


/* ----------------
 *		Global state
 * ----------------
 */

#define MAX_PORTALNAME_LEN		NAMEDATALEN

typedef struct portalhashent
{
	char		portalname[MAX_PORTALNAME_LEN];
	Portal		portal;
} PortalHashEnt;

static HTAB *PortalHashTable = NULL;

#define PortalHashTableLookup(NAME, PORTAL) \
do { \
	PortalHashEnt *hentry; \
	\
	hentry = (PortalHashEnt *) hash_search(PortalHashTable, \
										   (NAME), HASH_FIND, NULL); \
	if (hentry) \
		PORTAL = hentry->portal; \
	else \
		PORTAL = NULL; \
} while(0)

#define PortalHashTableInsert(PORTAL, NAME) \
do { \
	PortalHashEnt *hentry; bool found; \
	\
	hentry = (PortalHashEnt *) hash_search(PortalHashTable, \
										   (NAME), HASH_ENTER, &found); \
	if (found) \
		elog(ERROR, "duplicate portal name"); \
	hentry->portal = PORTAL; \
	/* To avoid duplicate storage, make PORTAL->name point to htab entry */ \
	PORTAL->name = hentry->portalname; \
} while(0)

#define PortalHashTableDelete(PORTAL) \
do { \
	PortalHashEnt *hentry; \
	\
	hentry = (PortalHashEnt *) hash_search(PortalHashTable, \
										   PORTAL->name, HASH_REMOVE, NULL); \
	if (hentry == NULL) \
		elog(WARNING, "trying to delete portal name that does not exist"); \
} while(0)

static MemoryContext PortalMemory = NULL;


/* ----------------------------------------------------------------
 *				   public portal interface functions
 * ----------------------------------------------------------------
 */

/*
 * EnablePortalManager
 *		Enables the portal management module at backend startup.
 */
void
EnablePortalManager(void)
{
	HASHCTL		ctl;

	Assert(PortalMemory == NULL);

	PortalMemory = AllocSetContextCreate(TopMemoryContext,
										 "PortalMemory",
										 ALLOCSET_DEFAULT_MINSIZE,
										 ALLOCSET_DEFAULT_INITSIZE,
										 ALLOCSET_DEFAULT_MAXSIZE);

	ctl.keysize = MAX_PORTALNAME_LEN;
	ctl.entrysize = sizeof(PortalHashEnt);

	/*
	 * use PORTALS_PER_USER as a guess of how many hash table entries to
	 * create, initially
	 */
	PortalHashTable = hash_create("Portal hash", PORTALS_PER_USER,
								  &ctl, HASH_ELEM);
}

/*
 * GetPortalByName
 *		Returns a portal given a portal name, or NULL if name not found.
 */
Portal
GetPortalByName(const char *name)
{
	Portal		portal;

	if (PointerIsValid(name))
		PortalHashTableLookup(name, portal);
	else
		portal = NULL;

	return portal;
}

/*
 * PortalListGetPrimaryStmt
 *		Get the "primary" stmt within a portal, ie, the one marked canSetTag.
 *
 * Returns NULL if no such stmt.  If multiple PlannedStmt structs within the
 * portal are marked canSetTag, returns the first one.	Neither of these
 * cases should occur in present usages of this function.
 *
 * Copes if given a list of Querys --- can't happen in a portal, but this
 * code also supports plancache.c, which needs both cases.
 *
 * Note: the reason this is just handed a List is so that plancache.c
 * can share the code.	For use with a portal, use PortalGetPrimaryStmt
 * rather than calling this directly.
 */
Node *
PortalListGetPrimaryStmt(List *stmts)
{
	ListCell   *lc;

	foreach(lc, stmts)
	{
		Node   *stmt = (Node *) lfirst(lc);

		if (IsA(stmt, PlannedStmt))
		{
			if (((PlannedStmt *) stmt)->canSetTag)
				return stmt;
		}
		else if (IsA(stmt, Query))
		{
			if (((Query *) stmt)->canSetTag)
				return stmt;
		}
		else
		{
			/* Utility stmts are assumed canSetTag if they're the only stmt */
			if (list_length(stmts) == 1)
				return stmt;
		}
	}
	return NULL;
}

/*
 * CreatePortal
 *		Returns a new portal given a name.
 *
 * allowDup: if true, automatically drop any pre-existing portal of the
 * same name (if false, an error is raised).
 *
 * dupSilent: if true, don't even emit a WARNING.
 */
Portal
CreatePortal(const char *name, bool allowDup, bool dupSilent)
{
	Portal		portal;

	AssertArg(PointerIsValid(name));

	portal = GetPortalByName(name);
	if (PortalIsValid(portal))
	{
		if (!allowDup)
			ereport(ERROR,
					(errcode(ERRCODE_DUPLICATE_CURSOR),
					 errmsg("cursor \"%s\" already exists", name)));
		if (!dupSilent)
			ereport(WARNING,
					(errcode(ERRCODE_DUPLICATE_CURSOR),
					 errmsg("closing existing cursor \"%s\"",
							name)));
		PortalDrop(portal, false);
	}

	/* make new portal structure */
	portal = (Portal) MemoryContextAllocZero(PortalMemory, sizeof *portal);

	/* initialize portal heap context; typically it won't store much */
	portal->heap = AllocSetContextCreate(PortalMemory,
										 "PortalHeapMemory",
										 ALLOCSET_SMALL_MINSIZE,
										 ALLOCSET_SMALL_INITSIZE,
										 ALLOCSET_SMALL_MAXSIZE);

	/* create a resource owner for the portal */
	portal->resowner = ResourceOwnerCreate(CurTransactionResourceOwner,
										   "Portal");

	/* initialize portal fields that don't start off zero */
	portal->status = PORTAL_NEW;
	portal->cleanup = PortalCleanup;
	portal->createSubid = GetCurrentSubTransactionId();
	portal->strategy = PORTAL_MULTI_QUERY;
	portal->cursorOptions = CURSOR_OPT_NO_SCROLL;
	portal->atStart = true;
	portal->atEnd = true;		/* disallow fetches until query is set */
	portal->visible = true;
	portal->creation_time = GetCurrentStatementStartTimestamp();

	/* put portal in table (sets portal->name) */
	PortalHashTableInsert(portal, name);

	return portal;
}

/*
 * CreateNewPortal
 *		Create a new portal, assigning it a random nonconflicting name.
 */
Portal
CreateNewPortal(void)
{
	static unsigned int unnamed_portal_count = 0;

	char		portalname[MAX_PORTALNAME_LEN];

	/* Select a nonconflicting name */
	for (;;)
	{
		unnamed_portal_count++;
		sprintf(portalname, "<unnamed portal %u>", unnamed_portal_count);
		if (GetPortalByName(portalname) == NULL)
			break;
	}

	return CreatePortal(portalname, false, false);
}

/*
 * PortalDefineQuery
 *		A simple subroutine to establish a portal's query.
 *
 * Notes: commandTag shall be NULL if and only if the original query string
 * (before rewriting) was an empty string.	Also, the passed commandTag must
 * be a pointer to a constant string, since it is not copied.  However,
 * prepStmtName and sourceText, if provided, are copied into the portal's
 * heap context for safekeeping.
 *
 * If cplan is provided, then it is a cached plan containing the stmts,
 * and the caller must have done RevalidateCachedPlan(), causing a refcount
 * increment.  The refcount will be released when the portal is destroyed.
 *
 * If cplan is NULL, then it is the caller's responsibility to ensure that
 * the passed plan trees have adequate lifetime.  Typically this is done by
 * copying them into the portal's heap context.
 */
void
PortalDefineQuery(Portal portal,
				  const char *prepStmtName,
				  const char *sourceText,
				  const char *commandTag,
				  List *stmts,
				  CachedPlan *cplan)
{
	AssertArg(PortalIsValid(portal));
	AssertState(portal->status == PORTAL_NEW);

	Assert(commandTag != NULL || stmts == NIL);

	portal->prepStmtName = prepStmtName ? 
		MemoryContextStrdup(PortalGetHeapMemory(portal), prepStmtName) : NULL;
	portal->sourceText = sourceText ? 
		MemoryContextStrdup(PortalGetHeapMemory(portal), sourceText) : NULL;
	portal->commandTag = commandTag;
	portal->stmts = stmts;
	portal->cplan = cplan;
	portal->status = PORTAL_DEFINED;
}

/*
 * PortalReleaseCachedPlan
 *		Release a portal's reference to its cached plan, if any.
 */
static void
PortalReleaseCachedPlan(Portal portal)
{
	if (portal->cplan)
	{
		ReleaseCachedPlan(portal->cplan, false);
		portal->cplan = NULL;
	}
}

/*
 * PortalCreateHoldStore
 *		Create the tuplestore for a portal.
 */
void
PortalCreateHoldStore(Portal portal)
{
	MemoryContext oldcxt;

	Assert(portal->holdContext == NULL);
	Assert(portal->holdStore == NULL);

	/*
	 * Create the memory context that is used for storage of the tuple set.
	 * Note this is NOT a child of the portal's heap memory.
	 */
	portal->holdContext =
		AllocSetContextCreate(PortalMemory,
							  "PortalHoldContext",
							  ALLOCSET_DEFAULT_MINSIZE,
							  ALLOCSET_DEFAULT_INITSIZE,
							  ALLOCSET_DEFAULT_MAXSIZE);

	/* Create the tuple store, selecting cross-transaction temp files. */
	oldcxt = MemoryContextSwitchTo(portal->holdContext);

	/* XXX: Should maintenance_work_mem be used for the portal size? */
	portal->holdStore = tuplestore_begin_heap(true, true, work_mem);

	MemoryContextSwitchTo(oldcxt);
}

/*
 * PortalDrop
 *		Destroy the portal.
 */
void
PortalDrop(Portal portal, bool isTopCommit)
{
	AssertArg(PortalIsValid(portal));

	/* Not sure if this case can validly happen or not... */
	if (portal->status == PORTAL_ACTIVE)
		elog(ERROR, "cannot drop active portal");

	/*
	 * Remove portal from hash table.  Because we do this first, we will not
	 * come back to try to remove the portal again if there's any error in the
	 * subsequent steps.  Better to leak a little memory than to get into an
	 * infinite error-recovery loop.
	 */
	PortalHashTableDelete(portal);

	/* let portalcmds.c clean up the state it knows about */
	if (PointerIsValid(portal->cleanup))
		(*portal->cleanup) (portal);

	/* drop cached plan reference, if any */
	if (portal->cplan)
		PortalReleaseCachedPlan(portal);

	/*
	 * Release any resources still attached to the portal.	There are several
	 * cases being covered here:
	 *
	 * Top transaction commit (indicated by isTopCommit): normally we should
	 * do nothing here and let the regular end-of-transaction resource
	 * releasing mechanism handle these resources too.	However, if we have a
	 * FAILED portal (eg, a cursor that got an error), we'd better clean up
	 * its resources to avoid resource-leakage warning messages.
	 *
	 * Sub transaction commit: never comes here at all, since we don't kill
	 * any portals in AtSubCommit_Portals().
	 *
	 * Main or sub transaction abort: we will do nothing here because
	 * portal->resowner was already set NULL; the resources were already
	 * cleaned up in transaction abort.
	 *
	 * Ordinary portal drop: must release resources.  However, if the portal
	 * is not FAILED then we do not release its locks.	The locks become the
	 * responsibility of the transaction's ResourceOwner (since it is the
	 * parent of the portal's owner) and will be released when the transaction
	 * eventually ends.
	 */
	if (portal->resowner &&
		(!isTopCommit || portal->status == PORTAL_FAILED))
	{
		bool		isCommit = (portal->status != PORTAL_FAILED);

		ResourceOwnerRelease(portal->resowner,
							 RESOURCE_RELEASE_BEFORE_LOCKS,
							 isCommit, false);
		ResourceOwnerRelease(portal->resowner,
							 RESOURCE_RELEASE_LOCKS,
							 isCommit, false);
		ResourceOwnerRelease(portal->resowner,
							 RESOURCE_RELEASE_AFTER_LOCKS,
							 isCommit, false);
		ResourceOwnerDelete(portal->resowner);
	}
	portal->resowner = NULL;

	/*
	 * Delete tuplestore if present.  We should do this even under error
	 * conditions; since the tuplestore would have been using cross-
	 * transaction storage, its temp files need to be explicitly deleted.
	 */
	if (portal->holdStore)
	{
		MemoryContext oldcontext;

		oldcontext = MemoryContextSwitchTo(portal->holdContext);
		tuplestore_end(portal->holdStore);
		MemoryContextSwitchTo(oldcontext);
		portal->holdStore = NULL;
	}

	/* delete tuplestore storage, if any */
	if (portal->holdContext)
		MemoryContextDelete(portal->holdContext);

	/* release subsidiary storage */
	MemoryContextDelete(PortalGetHeapMemory(portal));

	/* release portal struct (it's in PortalMemory) */
	pfree(portal);
}


/*
 * Pre-commit processing for portals.
 *
 * Any holdable cursors created in this transaction need to be converted to
 * materialized form, since we are going to close down the executor and
 * release locks.  Other portals are not touched yet.
 *
 * Returns TRUE if any holdable cursors were processed, FALSE if not.
 */
bool
CommitHoldablePortals(void)
{
	bool		result = false;
	HASH_SEQ_STATUS status;
	PortalHashEnt *hentry;

	hash_seq_init(&status, PortalHashTable);

	while ((hentry = (PortalHashEnt *) hash_seq_search(&status)) != NULL)
	{
		Portal		portal = hentry->portal;

		/* Is it a holdable portal created in the current xact? */
		if ((portal->cursorOptions & CURSOR_OPT_HOLD) &&
			portal->createSubid != InvalidSubTransactionId &&
			portal->status == PORTAL_READY)
		{
			/*
			 * We are exiting the transaction that created a holdable cursor.
			 * Instead of dropping the portal, prepare it for access by later
			 * transactions.
			 *
			 * Note that PersistHoldablePortal() must release all resources
			 * used by the portal that are local to the creating transaction.
			 */
			PortalCreateHoldStore(portal);
			PersistHoldablePortal(portal);

			/* drop cached plan reference, if any */
			if (portal->cplan)
				PortalReleaseCachedPlan(portal);

			/*
			 * Any resources belonging to the portal will be released in the
			 * upcoming transaction-wide cleanup; the portal will no longer
			 * have its own resources.
			 */
			portal->resowner = NULL;

			/*
			 * Having successfully exported the holdable cursor, mark it as
			 * not belonging to this transaction.
			 */
			portal->createSubid = InvalidSubTransactionId;

			result = true;
		}
	}

	return result;
}

/*
 * Pre-prepare processing for portals.
 *
 * Currently we refuse PREPARE if the transaction created any holdable
 * cursors, since it's quite unclear what to do with one.  However, this
 * has the same API as CommitHoldablePortals and is invoked in the same
 * way by xact.c, so that we can easily do something reasonable if anyone
 * comes up with something reasonable to do.
 *
 * Returns TRUE if any holdable cursors were processed, FALSE if not.
 */
bool
PrepareHoldablePortals(void)
{
	bool		result = false;
	HASH_SEQ_STATUS status;
	PortalHashEnt *hentry;

	hash_seq_init(&status, PortalHashTable);

	while ((hentry = (PortalHashEnt *) hash_seq_search(&status)) != NULL)
	{
		Portal		portal = hentry->portal;

		/* Is it a holdable portal created in the current xact? */
		if ((portal->cursorOptions & CURSOR_OPT_HOLD) &&
			portal->createSubid != InvalidSubTransactionId &&
			portal->status == PORTAL_READY)
		{
			/*
			 * We are exiting the transaction that created a holdable cursor.
			 * Can't do PREPARE.
			 */
			ereport(ERROR,
					(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
					 errmsg("cannot PREPARE a transaction that has created a cursor WITH HOLD")));
		}
	}

	return result;
}

/*
 * Pre-commit processing for portals.
 *
 * Remove all non-holdable portals created in this transaction.
 * Portals remaining from prior transactions should be left untouched.
 */
void
AtCommit_Portals(void)
{
	HASH_SEQ_STATUS status;
	PortalHashEnt *hentry;

	hash_seq_init(&status, PortalHashTable);

	while ((hentry = (PortalHashEnt *) hash_seq_search(&status)) != NULL)
	{
		Portal		portal = hentry->portal;

		/*
		 * Do not touch active portals --- this can only happen in the case of
		 * a multi-transaction utility command, such as VACUUM.
		 *
		 * Note however that any resource owner attached to such a portal is
		 * still going to go away, so don't leave a dangling pointer.
		 */
		if (portal->status == PORTAL_ACTIVE)
		{
			portal->resowner = NULL;
			continue;
		}

		/*
		 * Do nothing to cursors held over from a previous transaction
		 * (including holdable ones just frozen by CommitHoldablePortals).
		 */
		if (portal->createSubid == InvalidSubTransactionId)
			continue;

		/* Zap all non-holdable portals */
		PortalDrop(portal, true);

		/* Restart the iteration */
		hash_seq_init(&status, PortalHashTable);
	}
}

/*
 * Abort processing for portals.
 *
 * At this point we reset "active" status and run the cleanup hook if
 * present, but we can't release the portal's memory until the cleanup call.
 *
 * The reason we need to reset active is so that we can replace the unnamed
 * portal, else we'll fail to execute ROLLBACK when it arrives.
 */
void
AtAbort_Portals(void)
{
	HASH_SEQ_STATUS status;
	PortalHashEnt *hentry;

	hash_seq_init(&status, PortalHashTable);

	while ((hentry = (PortalHashEnt *) hash_seq_search(&status)) != NULL)
	{
		Portal		portal = hentry->portal;

		if (portal->status == PORTAL_ACTIVE)
			portal->status = PORTAL_FAILED;

		/*
		 * Do nothing else to cursors held over from a previous transaction.
		 */
		if (portal->createSubid == InvalidSubTransactionId)
			continue;

		/* let portalcmds.c clean up the state it knows about */
		if (PointerIsValid(portal->cleanup))
		{
			(*portal->cleanup) (portal);
			portal->cleanup = NULL;
		}

		/* drop cached plan reference, if any */
		if (portal->cplan)
			PortalReleaseCachedPlan(portal);

		/*
		 * Any resources belonging to the portal will be released in the
		 * upcoming transaction-wide cleanup; they will be gone before we run
		 * PortalDrop.
		 */
		portal->resowner = NULL;

		/*
		 * Although we can't delete the portal data structure proper, we can
		 * release any memory in subsidiary contexts, such as executor state.
		 * The cleanup hook was the last thing that might have needed data
		 * there.
		 */
		MemoryContextDeleteChildren(PortalGetHeapMemory(portal));
	}
}

/*
 * Post-abort cleanup for portals.
 *
 * Delete all portals not held over from prior transactions.  */
void
AtCleanup_Portals(void)
{
	HASH_SEQ_STATUS status;
	PortalHashEnt *hentry;

	hash_seq_init(&status, PortalHashTable);

	while ((hentry = (PortalHashEnt *) hash_seq_search(&status)) != NULL)
	{
		Portal		portal = hentry->portal;

		/* Do nothing to cursors held over from a previous transaction */
		if (portal->createSubid == InvalidSubTransactionId)
		{
			Assert(portal->status != PORTAL_ACTIVE);
			Assert(portal->resowner == NULL);
			continue;
		}

		/* Else zap it. */
		PortalDrop(portal, false);
	}
}

/*
 * Pre-subcommit processing for portals.
 *
 * Reassign the portals created in the current subtransaction to the parent
 * subtransaction.
 */
void
AtSubCommit_Portals(SubTransactionId mySubid,
					SubTransactionId parentSubid,
					ResourceOwner parentXactOwner)
{
	HASH_SEQ_STATUS status;
	PortalHashEnt *hentry;

	hash_seq_init(&status, PortalHashTable);

	while ((hentry = (PortalHashEnt *) hash_seq_search(&status)) != NULL)
	{
		Portal		portal = hentry->portal;

		if (portal->createSubid == mySubid)
		{
			portal->createSubid = parentSubid;
			if (portal->resowner)
				ResourceOwnerNewParent(portal->resowner, parentXactOwner);
		}
	}
}

/*
 * Subtransaction abort handling for portals.
 *
 * Deactivate portals created during the failed subtransaction.
 * Note that per AtSubCommit_Portals, this will catch portals created
 * in descendants of the subtransaction too.
 *
 * We don't destroy any portals here; that's done in AtSubCleanup_Portals.
 */
void
AtSubAbort_Portals(SubTransactionId mySubid,
				   SubTransactionId parentSubid,
				   ResourceOwner parentXactOwner)
{
	HASH_SEQ_STATUS status;
	PortalHashEnt *hentry;

	hash_seq_init(&status, PortalHashTable);

	while ((hentry = (PortalHashEnt *) hash_seq_search(&status)) != NULL)
	{
		Portal		portal = hentry->portal;

		if (portal->createSubid != mySubid)
			continue;

		/*
		 * Force any active portals of my own transaction into FAILED state.
		 * This is mostly to ensure that a portal running a FETCH will go
		 * FAILED if the underlying cursor fails.  (Note we do NOT want to do
		 * this to upper-level portals, since they may be able to continue.)
		 *
		 * This is only needed to dodge the sanity check in PortalDrop.
		 */
		if (portal->status == PORTAL_ACTIVE)
			portal->status = PORTAL_FAILED;

		/*
		 * If the portal is READY then allow it to survive into the parent
		 * transaction; otherwise shut it down.
		 *
		 * Currently, we can't actually support that because the portal's
		 * query might refer to objects created or changed in the failed
		 * subtransaction, leading to crashes if execution is resumed. So,
		 * even READY portals are deleted.	It would be nice to detect whether
		 * the query actually depends on any such object, instead.
		 */
#ifdef NOT_USED
		if (portal->status == PORTAL_READY)
		{
			portal->createSubid = parentSubid;
			if (portal->resowner)
				ResourceOwnerNewParent(portal->resowner, parentXactOwner);
		}
		else
#endif
		{
			/* let portalcmds.c clean up the state it knows about */
			if (PointerIsValid(portal->cleanup))
			{
				(*portal->cleanup) (portal);
				portal->cleanup = NULL;
			}

			/* drop cached plan reference, if any */
			if (portal->cplan)
				PortalReleaseCachedPlan(portal);

			/*
			 * Any resources belonging to the portal will be released in the
			 * upcoming transaction-wide cleanup; they will be gone before we
			 * run PortalDrop.
			 */
			portal->resowner = NULL;

			/*
			 * Although we can't delete the portal data structure proper, we
			 * can release any memory in subsidiary contexts, such as executor
			 * state.  The cleanup hook was the last thing that might have
			 * needed data there.
			 */
			MemoryContextDeleteChildren(PortalGetHeapMemory(portal));
		}
	}
}

/*
 * Post-subabort cleanup for portals.
 *
 * Drop all portals created in the failed subtransaction (but note that
 * we will not drop any that were reassigned to the parent above).
 */
void
AtSubCleanup_Portals(SubTransactionId mySubid)
{
	HASH_SEQ_STATUS status;
	PortalHashEnt *hentry;

	hash_seq_init(&status, PortalHashTable);

	while ((hentry = (PortalHashEnt *) hash_seq_search(&status)) != NULL)
	{
		Portal		portal = hentry->portal;

		if (portal->createSubid != mySubid)
			continue;

		/* Zap it. */
		PortalDrop(portal, false);
	}
}

/* Find all available cursors */
Datum
pg_cursor(PG_FUNCTION_ARGS)
{
	FuncCallContext *funcctx;
	HASH_SEQ_STATUS *hash_seq;
	PortalHashEnt *hentry;

	/* stuff done only on the first call of the function */
	if (SRF_IS_FIRSTCALL())
	{
		MemoryContext oldcontext;
		TupleDesc	tupdesc;

		/* create a function context for cross-call persistence */
		funcctx = SRF_FIRSTCALL_INIT();

		/*
		 * switch to memory context appropriate for multiple function calls
		 */
		oldcontext = MemoryContextSwitchTo(funcctx->multi_call_memory_ctx);

		if (PortalHashTable)
		{
			hash_seq = (HASH_SEQ_STATUS *) palloc(sizeof(HASH_SEQ_STATUS));
			hash_seq_init(hash_seq, PortalHashTable);
			funcctx->user_fctx = (void *) hash_seq;
		}
		else
			funcctx->user_fctx = NULL;

		/*
		 * build tupdesc for result tuples. This must match the definition of
		 * the pg_cursors view in system_views.sql
		 */
		tupdesc = CreateTemplateTupleDesc(6, false);
		TupleDescInitEntry(tupdesc, (AttrNumber) 1, "name",
						   TEXTOID, -1, 0);
		TupleDescInitEntry(tupdesc, (AttrNumber) 2, "statement",
						   TEXTOID, -1, 0);
		TupleDescInitEntry(tupdesc, (AttrNumber) 3, "is_holdable",
						   BOOLOID, -1, 0);
		TupleDescInitEntry(tupdesc, (AttrNumber) 4, "is_binary",
						   BOOLOID, -1, 0);
		TupleDescInitEntry(tupdesc, (AttrNumber) 5, "is_scrollable",
						   BOOLOID, -1, 0);
		TupleDescInitEntry(tupdesc, (AttrNumber) 6, "creation_time",
						   TIMESTAMPTZOID, -1, 0);

		funcctx->tuple_desc = BlessTupleDesc(tupdesc);
		MemoryContextSwitchTo(oldcontext);
	}

	/* stuff done on every call of the function */
	funcctx = SRF_PERCALL_SETUP();
	hash_seq = (HASH_SEQ_STATUS *) funcctx->user_fctx;

	/* if the hash table is uninitialized, we're done */
	if (hash_seq == NULL)
		SRF_RETURN_DONE(funcctx);

	/* loop until we find a visible portal or hit the end of the list */
	while ((hentry = hash_seq_search(hash_seq)) != NULL)
	{
		if (hentry->portal->visible)
			break;
	}

	if (hentry)
	{
		Portal		portal;
		Datum		result;
		HeapTuple	tuple;
		Datum		values[6];
		bool		nulls[6];

		portal = hentry->portal;
		MemSet(nulls, 0, sizeof(nulls));

		values[0] = DirectFunctionCall1(textin, CStringGetDatum(portal->name));
		if (!portal->sourceText)
			nulls[1] = true;
		else
			values[1] = DirectFunctionCall1(textin,
										CStringGetDatum(portal->sourceText));
		values[2] = BoolGetDatum(portal->cursorOptions & CURSOR_OPT_HOLD);
		values[3] = BoolGetDatum(portal->cursorOptions & CURSOR_OPT_BINARY);
		values[4] = BoolGetDatum(portal->cursorOptions & CURSOR_OPT_SCROLL);
		values[5] = TimestampTzGetDatum(portal->creation_time);

		tuple = heap_form_tuple(funcctx->tuple_desc, values, nulls);
		result = HeapTupleGetDatum(tuple);
		SRF_RETURN_NEXT(funcctx, result);
	}

	SRF_RETURN_DONE(funcctx);
}