/*
moddatetime.c

What is this?
It is a function to be called from a trigger for the purpose of updating
a modification datetime stamp in a record when that record is UPDATEd.

Credits
This is 95%+ based on autoinc.c, which I used as a starting point as I do
not really know what I am doing.  I also had help from
Jan Wieck <jwieck@debis.com> who told me about the timestamp_in("now") function.
OH, me, I'm Terry Mackintosh <terry@terrym.com>
*/

#include "executor/spi.h"		/* this is what you need to work with SPI */
#include "commands/trigger.h"	/* -"- and triggers */

extern Datum moddatetime(PG_FUNCTION_ARGS);

PG_FUNCTION_INFO_V1(moddatetime);

Datum
moddatetime(PG_FUNCTION_ARGS)
{
	TriggerData *trigdata = (TriggerData *) fcinfo->context;
	Trigger    *trigger;		/* to get trigger name */
	int			nargs;			/* # of arguments */
	int			attnum;			/* positional number of field to change */
	Datum		newdt;			/* The current datetime. */
	char	  **args;			/* arguments */
	char	   *relname;		/* triggered relation name */
	Relation	rel;			/* triggered relation */
	HeapTuple	rettuple = NULL;
	TupleDesc	tupdesc;		/* tuple description */

	if (!CALLED_AS_TRIGGER(fcinfo))
		/* internal error */
		elog(ERROR, "moddatetime: not fired by trigger manager");

	if (TRIGGER_FIRED_FOR_STATEMENT(trigdata->tg_event))
		/* internal error */
		elog(ERROR, "moddatetime: can't process STATEMENT events");

	if (TRIGGER_FIRED_AFTER(trigdata->tg_event))
		/* internal error */
		elog(ERROR, "moddatetime: must be fired before event");

	if (TRIGGER_FIRED_BY_INSERT(trigdata->tg_event))
		/* internal error */
		elog(ERROR, "moddatetime: must be fired before event");
	else if (TRIGGER_FIRED_BY_UPDATE(trigdata->tg_event))
		rettuple = trigdata->tg_newtuple;
	else
		/* internal error */
		elog(ERROR, "moddatetime: can't process DELETE events");

	rel = trigdata->tg_relation;
	relname = SPI_getrelname(rel);

	trigger = trigdata->tg_trigger;

	nargs = trigger->tgnargs;

	if (nargs != 1)
		/* internal error */
		elog(ERROR, "moddatetime (%s): A single argument was expected", relname);

	args = trigger->tgargs;
	/* must be the field layout? */
	tupdesc = rel->rd_att;

	/* Get the current datetime. */
	newdt = DirectFunctionCall3(timestamp_in,
								CStringGetDatum("now"),
								ObjectIdGetDatum(InvalidOid),
								Int32GetDatum(-1));

	/*
	 * This gets the position in the tuple of the field we want. args[0] being
	 * the name of the field to update, as passed in from the trigger.
	 */
	attnum = SPI_fnumber(tupdesc, args[0]);

	/*
	 * This is were we check to see if the field we are supposed to update
	 * even exits.	The above function must return -1 if name not found?
	 */
	if (attnum < 0)
		ereport(ERROR,
				(errcode(ERRCODE_TRIGGERED_ACTION_EXCEPTION),
				 errmsg("\"%s\" has no attribute \"%s\"",
						relname, args[0])));

	/*
	 * OK, this is where we make sure the timestamp field that we are
	 * modifying is really a timestamp field. Hay, error checking, what a
	 * novel idea !-)
	 */
	if (SPI_gettypeid(tupdesc, attnum) != TIMESTAMPOID)
		ereport(ERROR,
				(errcode(ERRCODE_TRIGGERED_ACTION_EXCEPTION),
				 errmsg("attribute \"%s\" of \"%s\" must be type TIMESTAMP",
						args[0], relname)));

/* 1 is the number of items in the arrays attnum and newdt.
	attnum is the positional number of the field to be updated.
	newdt is the new datetime stamp.
	NOTE that attnum and newdt are not arrays, but then a 1 ellement array
	is not an array any more then they are.  Thus, they can be considered a
	one element array.
*/
	rettuple = SPI_modifytuple(rel, rettuple, 1, &attnum, &newdt, NULL);

	if (rettuple == NULL)
		/* internal error */
		elog(ERROR, "moddatetime (%s): %d returned by SPI_modifytuple",
			 relname, SPI_result);

/* Clean up */
	pfree(relname);

	return PointerGetDatum(rettuple);
}
