client: fallback to CLOCK_MONOTONIC for timerfd

RHEL7 supports clock_gettime(CLOCK_BOOTIME), but it does not support
timerfd_create(CLOCK_BOOTIME). Creating a timerfd will fail with EINVAL.
Fallback to CLOCK_MONOTONIC.

Compare this to n-acd which also has compatibility code to fallback to
CLOCK_MONOTONIC. However when n-acd falls back to CLOCK_MONOTONIC, it uses
monotonic clock also for clock_gettime().

For n-dhcp4, the timestamps are also exposed in the public API
(n_dhcp4_client_lease_get_lifetime()). Hence, for timestamps n-dhcp4
still uses and requires clock_gettime(CLOCK_BOOTIME). Only the internal
timeout handling with the timerfd falls back to CLOCK_MONOTONIC.

https://github.com/nettools/n-dhcp4/pull/13
https://gitlab.freedesktop.org/NetworkManager/NetworkManager/merge_requests/362
(cherry picked from commit a1771c738d)

shared/n-dhcp4/src/n-dhcp4-client.c

@@ -388,8 +388,14 @@ _c_public_ int n_dhcp4_client_new(NDhcp4Client **clientp, NDhcp4ClientConfig *co
                 return -errno;
 
         client->fd_timer = timerfd_create(CLOCK_BOOTTIME, TFD_CLOEXEC | TFD_NONBLOCK);
+        if (client->fd_timer < 0) {
+                if (errno != EINVAL)
+                        return -errno;
+                client->fd_timer = timerfd_create(CLOCK_MONOTONIC, TFD_CLOEXEC | TFD_NONBLOCK);
                 if (client->fd_timer < 0)
                         return -errno;
+                client->timerfd_is_monotonic = true;
+        }
 
         ev.data.u32 = N_DHCP4_CLIENT_EPOLL_TIMER;
         r = epoll_ctl(client->fd_epoll, EPOLL_CTL_ADD, client->fd_timer, &ev);
@@ -499,8 +505,24 @@ void n_dhcp4_client_arm_timer(NDhcp4Client *client) {
                 n_dhcp4_client_probe_get_timeout(client->current_probe, &timeout);
 
         if (timeout != client->scheduled_timeout) {
+                uint64_t scheduled_timeout = timeout;
+                int flags = TFD_TIMER_ABSTIME;
+
+                if (   timeout != 0
+                    && client->timerfd_is_monotonic) {
+                        uint64_t now;
+
+                        /* the timerfd ticks with CLOCK_MONOTONIC. Calculate and set the relative
+                         * timeout. */
+                        now = n_dhcp4_gettime(CLOCK_BOOTTIME);
+                        if (timeout <= now)
+                                timeout = 1;
+                        else
+                                timeout = timeout - now;
+                        flags = 0;
+                }
                 r = timerfd_settime(client->fd_timer,
-                                    TFD_TIMER_ABSTIME,
+                                    flags,
                                     &(struct itimerspec){
                                         .it_value = {
                                                 .tv_sec = timeout / UINT64_C(1000000000),
@@ -510,7 +532,7 @@ void n_dhcp4_client_arm_timer(NDhcp4Client *client) {
                                     NULL);
                 c_assert(r >= 0);
 
-                client->scheduled_timeout = timeout;
+                client->scheduled_timeout = scheduled_timeout;
         }
 }
 

shared/n-dhcp4/src/n-dhcp4-private.h

@@ -332,6 +332,7 @@ struct NDhcp4Client {
         uint64_t scheduled_timeout;
 
         bool preempted : 1;
+        bool timerfd_is_monotonic : 1;
 };
 
 #define N_DHCP4_CLIENT_NULL(_x) {                                               \