reload /etc/resolv.conf when it has changed

if /etc/resolv.conf is updated, then make sure applications
already running get the updated information.

https://bugs.gentoo.org/177416
https://sourceware.org/bugzilla/show_bug.cgi?id=984
https://sourceware.org/ml/libc-alpha/2004-09/msg00130.html
https://sourceware.org/ml/libc-alpha/2016-12/msg00023.html
https://build.opensuse.org/package/view_file/openSUSE:Factory/glibc/glibc-resolv-reload.diff?expand=1

diff --git a/resolv/res_libc.c b/resolv/res_libc.c
index a4b376f15b0b0e9830cc08b8212a71da4bb10684..2d9b0265afa0dd98e6e21c9c28e055d76e732b17 100644 (file)
--- a/resolv/res_libc.c
+++ b/resolv/res_libc.c
@@ -25,6 +25,7 @@
 #include <arpa/nameser.h>
 #include <resolv.h>
 #include <libc-lock.h>
+#include <sys/stat.h>
 
 extern unsigned long long int __res_initstamp attribute_hidden;
 /* We have atomic increment operations on 64-bit platforms.  */
@@ -87,12 +88,34 @@ res_init(void) {
        return (__res_vinit(&_res, 1));
 }
 
+static time_t resconf_mtime;
+__libc_lock_define_initialized (static, resconf_mtime_lock);
+
+/* Check if the modification time of resolv.conf has changed.
+   If so, have all threads re-initialize their resolver states */
+static void
+__res_check_resconf (void)
+{
+       struct stat statbuf;
+       if (stat (_PATH_RESCONF, &statbuf) == 0) {
+               __libc_lock_lock (resconf_mtime_lock);
+               if (statbuf.st_mtime != resconf_mtime) {
+                       resconf_mtime = statbuf.st_mtime;
+                       atomicinclock (lock);
+                       atomicinc (__res_initstamp);
+                       atomicincunlock (lock);
+               }
+               __libc_lock_unlock (resconf_mtime_lock);
+       }
+}
+
 /* Initialize resp if RES_INIT is not yet set or if res_init in some other
    thread requested re-initializing.  */
 int
 __res_maybe_init (res_state resp, int preinit)
 {
        if (resp->options & RES_INIT) {
+               __res_check_resconf ();
                if (__res_initstamp != resp->_u._ext.initstamp) {
                        if (resp->nscount > 0)
                                __res_iclose (resp, true);