dbus: say why the system-bus attach failed

The attach is best effort and its diagnostics were tuned for the case
where no broker exists, so a broker that answers but refuses us was
reported as a bare rc=1 at debug level.  Chasing that meant reading
the header builder to find out what the number meant.

Failures now quote the error name the broker sent, and the one for a
name we could not claim says which of the three ways it went wrong.

Repeats stay quiet.  The probe runs on every service and condition
change, and before syslog is up each line is an open, write and close
on /dev/kmsg, so a broker that keeps refusing would otherwise flood
the console during boot.

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

src/dbus.c

@@ -1201,9 +1201,27 @@ void dbus_notify_condition_change(const char *name, const char *state)
  * rather log than silently sit in the queue). */
 #define DBUS_NAME_FLAG_DO_NOT_QUEUE  0x04
 
+/* sysbus_probe() re-runs on every service and condition change, so a
+ * broker that keeps refusing would repeat itself for every event, and
+ * before syslog is up each line is an open/write/close on /dev/kmsg.
+ * Say it once, then trace, until an attach succeeds. */
+static int sysbus_warned;
+
+#define sysbus_level() (sysbus_warned ? LOG_DEBUG : LOG_WARNING)
+
+/* What the broker said, for the log.  A refused call carries an error
+ * name; anything that failed below that has nothing to quote. */
+static const char *sysbus_errstr(link_client_t *c)
+{
+	const link_reply_t *r = link_client_reply(c);
+
+	return (r && r->error_name) ? r->error_name : "transport or parse failure";
+}
+
 static int sysbus_request_name(link_client_t *c)
 {
-	uint32_t result = 0;
+	const char *reason;
+	uint32_t result;
 	int rc;
 
 	rc = link_client_call_v(c, "/org/freedesktop/DBus",
@@ -1211,11 +1229,19 @@ static int sysbus_request_name(link_client_t *c)
 				"su", FINIT_BUS_NAME,
 				(uint32_t)DBUS_NAME_FLAG_DO_NOT_QUEUE);
 	if (rc != LINK_CALL_OK)
-		return -1;
-	if (link_reply_get_u32(link_client_reply(c), &result) < 0)
-		return -1;
-	/* 1 = primary owner; 2/3/4 mean we didn't get the name */
-	return (result == 1) ? 0 : -1;
+		reason = sysbus_errstr(c);
+	else if (link_reply_get_u32(link_client_reply(c), &result) < 0)
+		reason = "malformed RequestName reply";
+	else if (result != 1)	/* 2/3/4 mean we did not get the name */
+		reason = "name already owned";
+	else
+		return 0;
+
+	logit(sysbus_level(), "Failed to claim %s on system bus: %s",
+	      FINIT_BUS_NAME, reason);
+	sysbus_warned = 1;
+
+	return -1;
 }
 
 static int try_attach_system_bus(uev_ctx_t *ctx)
@@ -1232,16 +1258,21 @@ static int try_attach_system_bus(uev_ctx_t *ctx)
 		return -1;
 	}
 
+	/* Unlike the local bus, a broker routes by destination, and it
+	 * drops anything not addressed to the driver before Hello. */
+	link_client_set_destination(c, "org.freedesktop.DBus");
+
 	rc = link_client_call_v(c, "/org/freedesktop/DBus",
 				"org.freedesktop.DBus", "Hello", NULL);
 	if (rc != LINK_CALL_OK) {
-		dbg("System-bus Hello failed (rc=%d); skipping", rc);
+		logit(sysbus_level(), "System-bus Hello failed: %s",
+		      sysbus_errstr(c));
+		sysbus_warned = 1;
 		link_client_close(c);
 		return -1;
 	}
 
 	if (sysbus_request_name(c) < 0) {
-		logit(LOG_WARNING, "Failed to claim %s on system bus", FINIT_BUS_NAME);
 		link_client_close(c);
 		return -1;
 	}
@@ -1260,6 +1291,7 @@ static int try_attach_system_bus(uev_ctx_t *ctx)
 	}
 
 	sysbus_peer = p;
+	sysbus_warned = 0;	/* arm the warning for a later broker restart */
 	logit(LOG_NOTICE, "Registered %s on system bus", FINIT_BUS_NAME);
 	return 0;
 }