sd-dhcp-client: report transient DHCP failure to the caller

So far we only reported major state transitions like failure to acquire
the message. Let's report the initial failure after a few timeouts in
a new event type.

The number of timeouts is hardcoded as 3, since Windows seems to be using
that. I don't think we need to make this configurable out of the box. A
reasonable default may be enough.

src/libsystemd-network/sd-dhcp-client.c

@@ -38,6 +38,9 @@
 #define RESTART_AFTER_NAK_MIN_USEC (1 * USEC_PER_SEC)
 #define RESTART_AFTER_NAK_MAX_USEC (30 * USEC_PER_MINUTE)
 
+#define TRANSIENT_FAILURE_ATTEMPTS 3 /* Arbitrary limit: how many attempts are considered enough to report
+                                      * transient failure. */
+
 typedef struct sd_dhcp_client_id {
         uint8_t type;
         union {
@@ -1299,6 +1302,9 @@ static int client_timeout_resend(
                 goto error;
         }
 
+        if (client->attempt >= TRANSIENT_FAILURE_ATTEMPTS)
+                client_notify(client, SD_DHCP_CLIENT_EVENT_TRANSIENT_FAILURE);
+
         return 0;
 
 error:

src/systemd/sd-dhcp-client.h

@@ -40,6 +40,8 @@ enum {
         SD_DHCP_CLIENT_EVENT_EXPIRED            = 3,
         SD_DHCP_CLIENT_EVENT_RENEW              = 4,
         SD_DHCP_CLIENT_EVENT_SELECTING          = 5,
+        SD_DHCP_CLIENT_EVENT_TRANSIENT_FAILURE  = 6, /* Sent when we have not received a reply after the first few attempts.
+                                                      * The client may want to start acquiring link-local addresses. */
 };
 
 enum {