Don't call stat(2) on nsswitch.conf(5) every time nsdispatch(3)

and dependent functions (eg getpwname(3)) get called. This can
improve performance of binaries that perform a lot of name
lookups, such as gssd(8). It also matches documented behaviour
of Linux and Solaris.

The old code is left in place, should anyone need it, guarded
by #ifdef NS_REREAD_CONF.

Reviewed by:	imp, bcr
MFC after:	2 weeks
Relnotes:	yes
Sponsored by:	DARPA, AFRL
Differential Revision:	https://reviews.freebsd.org/D17934

lib/libc/net/nsdispatch.c

@@ -335,6 +335,7 @@ static int
 nss_configure(void)
 {
 	static time_t	 confmod;
+	static int	 already_initialized = 0;
 	struct stat	 statbuf;
 	int		 result, isthreaded;
 	const char	*path;
@@ -352,6 +353,16 @@ nss_configure(void)
 	if (path == NULL)
 #endif
 		path = _PATH_NS_CONF;
+#ifndef NS_REREAD_CONF
+	/*
+	 * Define NS_REREAD_CONF to have nsswitch notice changes
+	 * to nsswitch.conf(5) during runtime.  This involves calling
+	 * stat(2) every time, which can result in performance hit.
+	 */
+	if (already_initialized)
+		return (0);
+	already_initialized = 1;
+#endif /* NS_REREAD_CONF */
 	if (stat(path, &statbuf) != 0)
 		return (0);
 	if (statbuf.st_mtime <= confmod)

share/man/man5/nsswitch.conf.5

@@ -33,7 +33,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd June 6, 2016
+.Dd November 10, 2018
 .Dt NSSWITCH.CONF 5
 .Os
 .Sh NAME
@@ -340,6 +340,12 @@ sets a policy of "if the user is notfound in nis, do not try files."
 This treats nis as the authoritative source of information, except
 when the server is down.
 .Sh NOTES
+The
+.Nm
+file is parsed by each program only once.
+Subsequent changes will not be applied until the program
+is restarted.
+.Pp
 If system got compiled with
 .Va WITHOUT_NIS
 you have to remove