Add pg_file_settings view and function

The function and view added here provide a way to look at all settings
in postgresql.conf, any #include'd files, and postgresql.auto.conf
(which is what backs the ALTER SYSTEM command).

The information returned includes the configuration file name, line
number in that file, sequence number indicating when the parameter is
loaded (useful to see if it is later masked by another definition of the
same parameter), parameter name, and what it is set to at that point.
This information is updated on reload of the server.

This is unfiltered, privileged, information and therefore access is
restricted to superusers through the GRANT system.

Author: Sawada Masahiko, various improvements by me.
Reviewers: David Steele

src/backend/utils/misc/guc.c

@@ -3678,6 +3678,22 @@ static struct config_generic **guc_variables;
 /* Current number of variables contained in the vector */
 static int	num_guc_variables;
 
+/*
+ * Lookup of variables for pg_file_settings view.
+ * guc_file_variables is an array of length num_guc_file_variables.
+ */
+typedef struct ConfigFileVariable
+{
+	char	*name;
+	char	*value;
+	char	*filename;
+	int		sourceline;
+} ConfigFileVariable;
+static struct ConfigFileVariable *guc_file_variables;
+
+/* Number of file variables */
+static int	num_guc_file_variables;
+
 /* Vector capacity */
 static int	size_guc_variables;
 
@@ -8148,6 +8164,110 @@ show_all_settings(PG_FUNCTION_ARGS)
 	}
 }
 
+/*
+ * show_all_file_settings
+ *
+ * returns a table of all parameter settings in all configuration files
+ * which includes the config file path/name, filename, a sequence number
+ * indicating when we loaded it, the parameter name, and the value it is
+ * set to.
+ *
+ * Note: no filtering is done here, instead we depend on the GRANT system
+ * to prevent unprivileged users from accessing this function or the view
+ * built on top of it.
+ */
+Datum
+show_all_file_settings(PG_FUNCTION_ARGS)
+{
+#define NUM_PG_FILE_SETTINGS_ATTS 5
+	FuncCallContext *funcctx;
+	TupleDesc	tupdesc;
+	int			call_cntr;
+	int			max_calls;
+	AttInMetadata *attinmeta;
+	MemoryContext oldcontext;
+
+	if (SRF_IS_FIRSTCALL())
+	{
+		funcctx = SRF_FIRSTCALL_INIT();
+
+		oldcontext = MemoryContextSwitchTo(funcctx->multi_call_memory_ctx);
+
+		/*
+		 * need a tuple descriptor representing NUM_PG_SETTINGS_ATTS columns
+		 * of the appropriate types
+		 */
+
+		tupdesc = CreateTemplateTupleDesc(NUM_PG_FILE_SETTINGS_ATTS, false);
+		TupleDescInitEntry(tupdesc, (AttrNumber) 1, "sourcefile",
+						   TEXTOID, -1, 0);
+		TupleDescInitEntry(tupdesc, (AttrNumber) 2, "sourceline",
+						   INT4OID, -1, 0);
+		TupleDescInitEntry(tupdesc, (AttrNumber) 3, "seqno",
+						   INT4OID, -1, 0);
+		TupleDescInitEntry(tupdesc, (AttrNumber) 4, "name",
+						   TEXTOID, -1, 0);
+		TupleDescInitEntry(tupdesc, (AttrNumber) 5, "setting",
+						   TEXTOID, -1, 0);
+
+		attinmeta = TupleDescGetAttInMetadata(tupdesc);
+		funcctx->attinmeta = attinmeta;
+		funcctx->max_calls = num_guc_file_variables;
+		MemoryContextSwitchTo(oldcontext);
+	}
+
+	funcctx = SRF_PERCALL_SETUP();
+
+	call_cntr = funcctx->call_cntr;
+	max_calls = funcctx->max_calls;
+	attinmeta = funcctx->attinmeta;
+
+	if (call_cntr < max_calls)
+	{
+		char	   *values[NUM_PG_FILE_SETTINGS_ATTS];
+		HeapTuple	tuple;
+		Datum		result;
+		ConfigFileVariable conf;
+		char		buffer[12]; /* must be at least 12, per pg_ltoa */
+
+		/* Check to avoid going past end of array */
+		if (call_cntr > num_guc_file_variables)
+			SRF_RETURN_DONE(funcctx);
+
+		conf = guc_file_variables[call_cntr];
+
+		/* sourcefile */
+		values[0] = conf.filename;
+
+		/* sourceline */
+		pg_ltoa(conf.sourceline, buffer);
+		values[1] = pstrdup(buffer);
+
+		/* seqno */
+		pg_ltoa(call_cntr + 1, buffer);
+		values[2] = pstrdup(buffer);
+
+		/* name */
+		values[3] = conf.name;
+
+		/* setting */
+		values[4] = conf.value;
+
+		/* build a tuple */
+		tuple = BuildTupleFromCStrings(attinmeta, values);
+
+		/* make the tuple into a datum */
+		result = HeapTupleGetDatum(tuple);
+
+		SRF_RETURN_NEXT(funcctx, result);
+	}
+	else
+	{
+		SRF_RETURN_DONE(funcctx);
+	}
+
+}
+
 static char *
 _ShowOption(struct config_generic * record, bool use_units)
 {