Add new system view, pg_config

Move and refactor the underlying code for the pg_config client
application to src/common in support of sharing it with a new
system information SRF called pg_config() which makes the same
information available via SQL. Additionally wrap the SRF with a
new system view, as called pg_config.

Patch by me with extensive input and review by Michael Paquier
and additional review by Alvaro Herrera.

3 files changed, 111 insertions, 2 deletions

diff --git a/src/backend/catalog/system_views.sql b/src/backend/catalog/system_views.sql
index 923fe589d5e..abf9a7007c5 100644
--- a/src/backend/catalog/system_views.sql
+++ b/src/backend/catalog/system_views.sql
@@ -433,6 +433,12 @@ CREATE VIEW pg_timezone_abbrevs AS
 CREATE VIEW pg_timezone_names AS
     SELECT * FROM pg_timezone_names();
 
+CREATE VIEW pg_config AS
+    SELECT * FROM pg_config();
+
+REVOKE ALL on pg_config FROM PUBLIC;
+REVOKE EXECUTE ON FUNCTION pg_config() FROM PUBLIC;
+
 -- Statistics views
 
 CREATE VIEW pg_stat_all_tables AS
diff --git a/src/backend/utils/misc/Makefile b/src/backend/utils/misc/Makefile
index 788910157de..a0c82c17438 100644
--- a/src/backend/utils/misc/Makefile
+++ b/src/backend/utils/misc/Makefile
@@ -14,8 +14,8 @@ include $(top_builddir)/src/Makefile.global
 
 override CPPFLAGS := -I. -I$(srcdir) $(CPPFLAGS)
 
-OBJS = guc.o help_config.o pg_rusage.o ps_status.o rls.o \
-       sampling.o superuser.o timeout.o tzparser.o
+OBJS = guc.o help_config.o pg_config.o pg_rusage.o \
+       ps_status.o rls.o sampling.o superuser.o timeout.o tzparser.o
 
 # This location might depend on the installation directories. Therefore
 # we can't subsitute it into pg_config.h.
diff --git a/src/backend/utils/misc/pg_config.c b/src/backend/utils/misc/pg_config.c
new file mode 100644
index 00000000000..3d6b9f27c08
--- /dev/null
+++ b/src/backend/utils/misc/pg_config.c
@@ -0,0 +1,103 @@
+/*-------------------------------------------------------------------------
+ *
+ * pg_config.c
+ *		Expose same output as pg_config except as an SRF
+ *
+ * Portions Copyright (c) 1996-2016, 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 "funcapi.h"
+#include "miscadmin.h"
+#include "catalog/pg_type.h"
+#include "common/config_info.h"
+#include "utils/builtins.h"
+#include "utils/elog.h"
+#include "port.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 ||
+		tupdesc->attrs[0]->atttypid != TEXTOID ||
+		tupdesc->attrs[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;
+}