src/backend/utils/misc/pg_config.c

/*-------------------------------------------------------------------------
 *
 * pg_config.c
 *		Expose same output as pg_config except as an SRF
 *
 * Portions Copyright (c) 1996-2021, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * IDENTIFICATION
 *	  src/backend/utils/misc/pg_config.c
 *
 *-------------------------------------------------------------------------
 */

#include "postgres.h"

#include "catalog/pg_type.h"
#include "common/config_info.h"
#include "funcapi.h"
#include "miscadmin.h"
#include "port.h"
#include "utils/builtins.h"

Datum
pg_config(PG_FUNCTION_ARGS)
{
	ReturnSetInfo *rsinfo = (ReturnSetInfo *) fcinfo->resultinfo;
	Tuplestorestate *tupstore;
	HeapTuple	tuple;
	TupleDesc	tupdesc;
	AttInMetadata *attinmeta;
	MemoryContext per_query_ctx;
	MemoryContext oldcontext;
	ConfigData *configdata;
	size_t		configdata_len;
	char	   *values[2];
	int			i = 0;

	/* check to see if caller supports us returning a tuplestore */
	if (!rsinfo || !(rsinfo->allowedModes & SFRM_Materialize))
		ereport(ERROR,
				(errcode(ERRCODE_SYNTAX_ERROR),
				 errmsg("materialize mode required, but it is not "
						"allowed in this context")));

	per_query_ctx = rsinfo->econtext->ecxt_per_query_memory;
	oldcontext = MemoryContextSwitchTo(per_query_ctx);

	/* get the requested return tuple description */
	tupdesc = CreateTupleDescCopy(rsinfo->expectedDesc);

	/*
	 * Check to make sure we have a reasonable tuple descriptor
	 */
	if (tupdesc->natts != 2 ||
		TupleDescAttr(tupdesc, 0)->atttypid != TEXTOID ||
		TupleDescAttr(tupdesc, 1)->atttypid != TEXTOID)
		ereport(ERROR,
				(errcode(ERRCODE_SYNTAX_ERROR),
				 errmsg("query-specified return tuple and "
						"function return type are not compatible")));

	/* OK to use it */
	attinmeta = TupleDescGetAttInMetadata(tupdesc);

	/* let the caller know we're sending back a tuplestore */
	rsinfo->returnMode = SFRM_Materialize;

	/* initialize our tuplestore */
	tupstore = tuplestore_begin_heap(true, false, work_mem);

	configdata = get_configdata(my_exec_path, &configdata_len);
	for (i = 0; i < configdata_len; i++)
	{
		values[0] = configdata[i].name;
		values[1] = configdata[i].setting;

		tuple = BuildTupleFromCStrings(attinmeta, values);
		tuplestore_puttuple(tupstore, tuple);
	}

	/*
	 * no longer need the tuple descriptor reference created by
	 * TupleDescGetAttInMetadata()
	 */
	ReleaseTupleDesc(tupdesc);

	tuplestore_donestoring(tupstore);
	rsinfo->setResult = tupstore;

	/*
	 * SFRM_Materialize mode expects us to return a NULL Datum. The actual
	 * tuples are in our tuplestore and passed back through rsinfo->setResult.
	 * rsinfo->setDesc is set to the tuple description that we actually used
	 * to build our tuples with, so the caller can verify we did what it was
	 * expecting.
	 */
	rsinfo->setDesc = tupdesc;
	MemoryContextSwitchTo(oldcontext);

	return (Datum) 0;
}