announce: rename the host on a probe conflict instead of giving up

announce.c

@@ -37,6 +37,8 @@ enum {
 
 int announce_ttl = 75 * 60;
 
+static void announce_restart(void);
+
 static void
 announce_timer(struct uloop_timeout *timeout)
 {
@@ -58,7 +60,24 @@ announce_timer(struct uloop_timeout *timeout)
 
 		case STATE_PROBE_END:
 			if (cache_host_is_known(mdns_hostname_local)) {
-				fprintf(stderr, "the host %s already exists. stopping announce service\n", mdns_hostname_local);
+				char conflict[sizeof(mdns_hostname_local)];
+
+				snprintf(conflict, sizeof(conflict), "%s", mdns_hostname_local);
+
+				if (!rename_hostname()) {
+					fprintf(stderr, "the host %s already exists. stopping announce service\n",
+						conflict);
+					return;
+				}
+
+				fprintf(stderr, "the host %s already exists, renaming to %s\n",
+					conflict, mdns_hostname_local);
+
+				/*
+				 * The host name is global, so every interface has to probe
+				 * again for the new one rather than announce the old one.
+				 */
+				announce_restart();
 				return;
 			}
 			iface->announce_state++;
@@ -81,6 +100,15 @@ announce_init(struct interface *iface)
 	uloop_timeout_set(&iface->announce_timer, 100);
 }
 
+static void
+announce_restart(void)
+{
+	struct interface *iface;
+
+	vlist_for_each_element(&interfaces, iface, node)
+		announce_init(iface);
+}
+
 void
 announce_free(struct interface *iface)
 {

util.c

@@ -38,6 +38,9 @@ int debug = 0;
 char umdns_host_label[HOSTNAME_LEN];
 char mdns_hostname_local[HOSTNAME_LEN + 6];
 
+static char umdns_host_base[HOSTNAME_LEN];
+static unsigned int umdns_host_suffix;
+
 uint32_t
 rand_time_delta(uint32_t t)
 {
@@ -61,6 +64,21 @@ rand_time_delta(uint32_t t)
 	return val;
 }
 
+/* "-" plus the widest suffix an unsigned int can print, plus the terminator */
+#define HOSTNAME_SUFFIX_LEN	12
+
+static void apply_hostname(void)
+{
+	if (umdns_host_suffix)
+		snprintf(umdns_host_label, sizeof(umdns_host_label), "%.*s-%u",
+			 (int)sizeof(umdns_host_label) - HOSTNAME_SUFFIX_LEN,
+			 umdns_host_base, umdns_host_suffix + 1);
+	else
+		snprintf(umdns_host_label, sizeof(umdns_host_label), "%s", umdns_host_base);
+
+	snprintf(mdns_hostname_local, sizeof(mdns_hostname_local), "%s.local", umdns_host_label);
+}
+
 void get_hostname(void)
 {
 	struct utsname utsname;
@@ -71,8 +89,28 @@ void get_hostname(void)
 	if (uname(&utsname) < 0)
 		return;
 
-	snprintf(umdns_host_label, sizeof(umdns_host_label), "%s", utsname.nodename);
-	snprintf(mdns_hostname_local, sizeof(mdns_hostname_local), "%s.local", utsname.nodename);
+	/*
+	 * Only start over from the unsuffixed name when the system host name
+	 * itself changed, so that a name picked to resolve a conflict survives
+	 * an unrelated reload.
+	 */
+	if (strcmp(umdns_host_base, utsname.nodename)) {
+		snprintf(umdns_host_base, sizeof(umdns_host_base), "%s", utsname.nodename);
+		umdns_host_suffix = 0;
+	}
+
+	apply_hostname();
+}
+
+bool rename_hostname(void)
+{
+	if (umdns_host_suffix >= HOSTNAME_MAX_SUFFIX)
+		return false;
+
+	umdns_host_suffix++;
+	apply_hostname();
+
+	return true;
 }
 
 time_t monotonic_time(void)

util.h

@@ -26,6 +26,9 @@
 #define MDNS_BUF_LEN	(8 * 1024)
 #define HOSTNAME_LEN	256
 
+/* Highest -N suffix appended to the host name to resolve a conflict. */
+#define HOSTNAME_MAX_SUFFIX	9
+
 extern int debug;
 extern uint8_t mdns_buf[MDNS_BUF_LEN];
 
@@ -39,6 +42,14 @@ extern char umdns_host_label[HOSTNAME_LEN];
 extern char mdns_hostname_local[HOSTNAME_LEN + 6];
 
 extern void get_hostname(void);
+
+/**
+ * Append or bump the -N suffix on the host name after a probe conflict
+ *
+ * Returns false once HOSTNAME_MAX_SUFFIX is reached, leaving the name as is.
+ */
+extern bool rename_hostname(void);
+
 extern uint32_t rand_time_delta(uint32_t t);
 extern time_t monotonic_time(void);