/* $Id: tuplock.c.src 137 2017-10-08 16:28:45Z coelho $
 *
 * tuplock: PostgreSQL extension to lock tuples
 * version: 1.2.2
 * url: http://www.coelho.net/tuplock
 *
 * (c) 2004-2017 Fabien Coelho < fabien at coelho dot net >
 *
 * This is free software, both inexpensive and provided with sources.
 * The GNU General Public Licence v3 applies, for details, see:
 *     http://www.gnu.org/copyleft/gpl.html
 * The summary is: you get as much as you paid for, and I am not
 * responsible for anything. Beware that you may lose your data, your
 * business or your friends because of this software.
 *
 * A conservative trigger for locking tuples. See README for usage.
 */

#include "postgres.h"
#include "catalog/pg_type.h" // for BOOLOID
#include "executor/spi.h"
#include "commands/trigger.h"
#include "funcapi.h" // for RelationNameGetTupleDesc
#include "access/htup_details.h" // for heap_getattr

// check server compatibility
PG_MODULE_MAGIC;

// declare trigger
extern Datum tuplock(PG_FUNCTION_ARGS);
PG_FUNCTION_INFO_V1(tuplock);

/* this trigger expects a BOOLEAN attribute name as an argument.
 * should be called as BEFORE UPDATE OR DELETE
 */
Datum
tuplock(PG_FUNCTION_ARGS)
{
  TriggerData * context;
  Trigger * trig;
  char * attname;
  bool isnull, locked;
  TupleDesc tupdes;
  int i, attnum;

  // sanity checks
  if (!CALLED_AS_TRIGGER(fcinfo))
    elog(ERROR, "lock_tuple_trig: not called by trigger manager!");

  context = (TriggerData *) fcinfo->context;
  trig = context->tg_trigger;

  // this special case is okay and blocked
  if (!TRIGGER_FIRED_FOR_ROW(context->tg_event) &&
      TRIGGER_FIRED_BY_TRUNCATE(context->tg_event))
    elog(ERROR, "trigger \"%s\" on \"%s\": "
         "items may be locked, cannot TRUNCATE",
         trig->tgname, SPI_getrelname(context->tg_relation));

  if (!TRIGGER_FIRED_FOR_ROW(context->tg_event))
    elog(ERROR, "trigger \"%s\" on \"%s\": must be called FOR EACH ROW",
         trig->tgname, SPI_getrelname(context->tg_relation));

  if (!TRIGGER_FIRED_BY_UPDATE(context->tg_event) &&
      !TRIGGER_FIRED_BY_DELETE(context->tg_event))
    elog(ERROR, "trigger \"%s\" on \"%s\":"
         " must be called for UPDATE or DELETE",
         trig->tgname, SPI_getrelname(context->tg_relation));

  // should I check that is is fired as BEFORE? no, not an issue.

  if (trig->tgnargs!=1)
    elog(ERROR, "trigger \"%s\" on \"%s\":"
         " expecting attribute name as only argument",
         trig->tgname, SPI_getrelname(context->tg_relation));

  // look for relation.attname boolean attribute.
  // can't use GetAttributeByName here, not the right kind of HeapTuple.
  // moreover, we want to check that the type is really bool.
  attname = trig->tgargs[0];
  // tupdes = context->tg_relation->rd_att;
  tupdes = RelationNameGetTupleDesc(SPI_getrelname(context->tg_relation));
  attnum = 0; // 0 is not found
  for (i=0; i<tupdes->natts; i++)
  {
    Form_pg_attribute a = tupdes->attrs[i];

    if (strcasecmp(NameStr(a->attname), attname) == 0)
    {
      if (a->atttypid!=BOOLOID)
        elog(ERROR,
             "trigger \"%s\" on \"%s\": attribute \"%s\" is not BOOL",
             trig->tgname, SPI_getrelname(context->tg_relation), attname);

      // found and type is ok
      attnum = a->attnum;
      break;
    }
  }

  if (attnum == 0)
    elog(ERROR, "trigger \"%s\" on \"%s\": attribute \"%s\" not found",
         trig->tgname, SPI_getrelname(context->tg_relation), attname);

  locked = DatumGetBool(heap_getattr(context->tg_trigtuple,
                                     attnum, tupdes, &isnull));

  // raise exception if NULL or locked.
  if (isnull || locked)
    elog(ERROR, "trigger \"%s\" on \"%s\": item locked by attribute \"%s\"",
         trig->tgname, SPI_getrelname(context->tg_relation), attname);

  // else return tuple to executor for update or delete.
  return PointerGetDatum(TRIGGER_FIRED_BY_UPDATE(context->tg_event) ?
                         context->tg_newtuple : context->tg_trigtuple);
}