timedated: roll back state on config write failure

SetTimezone() and SetLocalRTC() updated the Context before writing
/etc/localtime or /etc/adjtime. If the write failed, D-Bus properties
showed the new value and a repeated request with the same value returned
success without retrying the file update.

Keep the write helpers Context-based, but save the old value before
installing the requested state. Restore the old Context value when the
backing file update fails.

Add TEST-45 coverage for failed alternate localtime and adjtime paths.

Follow-up for: f401e48c2d

src/timedate/timedated.c

@@ -672,6 +672,7 @@ static int property_get_ntp(
 
 static int method_set_timezone(sd_bus_message *m, void *userdata, sd_bus_error *error) {
         Context *c = ASSERT_PTR(userdata);
+        _cleanup_free_ char *old_zone = NULL, *zone = NULL;
         int interactive, r;
         const char *z;
 
@@ -701,13 +702,17 @@ static int method_set_timezone(sd_bus_message *m, void *userdata, sd_bus_error *
         if (r == 0)
                 return 1; /* No authorization for now, but the async polkit stuff will call us again when it has it */
 
-        r = free_and_strdup(&c->zone, z);
-        if (r < 0)
-                return r;
+        zone = strdup(z);
+        if (!zone)
+                return -ENOMEM;
+
+        old_zone = TAKE_PTR(c->zone);
+        c->zone = TAKE_PTR(zone);
 
         /* 1. Write new configuration file */
         r = context_write_data_timezone(c);
         if (r < 0) {
+                free_and_replace(c->zone, old_zone);
                 log_error_errno(r, "Failed to set time zone: %m");
                 return sd_bus_error_set_errnof(error, r, "Failed to set time zone: %m");
         }
@@ -778,11 +783,14 @@ static int method_set_local_rtc(sd_bus_message *m, void *userdata, sd_bus_error
                 return 1;
 
         if (lrtc != c->local_rtc) {
+                bool old_local_rtc = c->local_rtc;
+
                 c->local_rtc = lrtc;
 
                 /* 1. Write new configuration file */
                 r = context_write_data_local_rtc(c);
                 if (r < 0) {
+                        c->local_rtc = old_local_rtc;
                         log_error_errno(r, "Failed to set RTC to %s: %m", lrtc ? "local" : "UTC");
                         return sd_bus_error_set_errnof(error, r, "Failed to set RTC to %s: %m", lrtc ? "local" : "UTC");
                 }

test/units/TEST-45-TIMEDATE.sh

@@ -484,6 +484,7 @@ teardown_timedated_alternate_paths() {
     set +eu
 
     rm -rf /run/systemd/system/systemd-timedated.service.d
+    rm -rf /run/alternate-path
     systemctl daemon-reload
     systemctl restart systemd-timedated
 }
@@ -507,11 +508,25 @@ EOF
     assert_eq "$(readlink /run/alternate-path/mylocaltime | sed 's#^.*zoneinfo/##')" "Europe/Kyiv"
     assert_in "Time zone: Europe/Kyiv \(EES*T, \+0[0-9]00\)" "$(timedatectl)"
 
+    rm -f /run/alternate-path/mylocaltime
+    mkdir /run/alternate-path/mylocaltime
+    assert_rc 1 timedatectl --no-pager set-timezone UTC
+    assert_eq "$(timedatectl show -p Timezone --value)" "Europe/Kyiv"
+    assert_rc 1 timedatectl --no-pager set-timezone UTC
+    rmdir /run/alternate-path/mylocaltime
+    assert_rc 0 timedatectl --no-pager set-timezone UTC
+    assert_rc 0 timedatectl --no-pager set-timezone Europe/Kyiv
+
     # Restart to force using get_timezine
     systemctl restart systemd-timedated
     assert_in "Time zone: Europe/Kyiv \(EES*T, \+0[0-9]00\)" "$(timedatectl)"
 
     assert_in "RTC in local TZ: no" "$(timedatectl --no-pager)"
+    mkdir /run/alternate-path/myadjtime
+    assert_rc 1 timedatectl set-local-rtc 1
+    assert_in "RTC in local TZ: no" "$(timedatectl --no-pager)"
+    assert_rc 1 timedatectl set-local-rtc 1
+    rmdir /run/alternate-path/myadjtime
     assert_rc 0 timedatectl set-local-rtc 1
     assert_in "RTC in local TZ: yes" "$(timedatectl --no-pager)"
     assert_eq "$(cat /run/alternate-path/myadjtime)" "0.0 0 0