network/ipv4ll: do not start sd-ipv4ll on exit

When assert_return() is critical, the following assertion is triggered
on exit:
---
 #0  0x00007f8b1f6b0884 in __pthread_kill_implementation () from target:/lib64/libc.so.6
 #1  0x00007f8b1f65fafe in raise () from target:/lib64/libc.so.6
 #2  0x00007f8b1f64887f in abort () from target:/lib64/libc.so.6
 #3  0x00007f8b208d02d6 in log_assert_failed (text=0x7f8b210009e0 "e->state != SD_EVENT_FINISHED", file=0x7f8b20fff403 "src/libsystemd/sd-event/sd-event.c",
     line=1252, func=0x7f8b21004400 <__func__.154> "sd_event_add_io") at ../src/basic/log.c:948
 #4  0x00007f8b208d0457 in log_assert_failed_return (text=0x7f8b210009e0 "e->state != SD_EVENT_FINISHED",
     file=0x7f8b20fff403 "src/libsystemd/sd-event/sd-event.c", line=1252, func=0x7f8b21004400 <__func__.154> "sd_event_add_io") at ../src/basic/log.c:967
 #5  0x00007f8b20c7d102 in sd_event_add_io (e=0x617000000080, ret=0x60c000000a20, fd=11, events=1, callback=0x7dfd85 <ipv4acd_on_packet>,
     userdata=0x60c000000a00) at ../src/libsystemd/sd-event/sd-event.c:1252
 #6  0x00000000007e3934 in sd_ipv4acd_start (acd=0x60c000000a00, reset_conflicts=true) at ../src/libsystemd-network/sd-ipv4acd.c:597
 #7  0x00000000007e72b9 in ipv4ll_start_internal (ll=0x6080000006a0, reset_generation=true) at ../src/libsystemd-network/sd-ipv4ll.c:278
 #8  0x00000000007e7462 in sd_ipv4ll_start (ll=0x6080000006a0) at ../src/libsystemd-network/sd-ipv4ll.c:298
 #9  0x00000000006047a1 in dhcp4_handler (client=0x617000000400, event=0, userdata=0x61a000000680) at ../src/network/networkd-dhcp4.c:1183
 #10 0x000000000075b1ed in client_notify (client=0x617000000400, event=0) at ../src/libsystemd-network/sd-dhcp-client.c:783
 #11 0x000000000075bf8d in client_stop (client=0x617000000400, error=0) at ../src/libsystemd-network/sd-dhcp-client.c:821
 #12 0x000000000077710f in sd_dhcp_client_stop (client=0x617000000400) at ../src/libsystemd-network/sd-dhcp-client.c:2388
 #13 0x000000000065cdd1 in link_stop_engines (link=0x61a000000680, may_keep_dhcp=true) at ../src/network/networkd-link.c:336
 #14 0x000000000041f214 in manager_free (m=0x618000000080) at ../src/network/networkd-manager.c:613
 #15 0x00000000004124e3 in manager_freep (p=0x7f8b1c800040) at ../src/network/networkd-manager.h:128
 #16 0x00000000004139f6 in run (argc=1, argv=0x7ffffe4522e8) at ../src/network/networkd.c:24
 #17 0x0000000000413b20 in main (argc=1, argv=0x7ffffe4522e8) at ../src/network/networkd.c:119
---
Prompted by systemd#30049 (comment).

src/libsystemd-network/sd-ipv4acd.c

@@ -585,6 +585,12 @@ int sd_ipv4acd_start(sd_ipv4acd *acd, bool reset_conflicts) {
         assert_return(!ether_addr_is_null(&acd->mac_addr), -EINVAL);
         assert_return(acd->state == IPV4ACD_STATE_INIT, -EBUSY);
 
+        r = sd_event_get_state(acd->event);
+        if (r < 0)
+                return r;
+        if (r == SD_EVENT_FINISHED)
+                return -ESTALE;
+
         r = arp_network_bind_raw_socket(acd->ifindex, &acd->address, &acd->mac_addr);
         if (r < 0)
                 return r;

src/network/networkd-dhcp4.c

@@ -1181,7 +1181,7 @@ static int dhcp4_handler(sd_dhcp_client *client, int event, void *userdata) {
                                 }
 
                                 r = sd_ipv4ll_start(link->ipv4ll);
-                                if (r < 0)
+                                if (r < 0 && r != -ESTALE) /* On exit, we cannot and should not start sd-ipv4ll. */
                                         return log_link_warning_errno(link, r, "Could not acquire IPv4 link-local address: %m");
                         }