Add systemd/dtrace probes for DBus APIs

When debugging libvirt it is helpful to set probes around RPC
calls. We already have probes for libvirt's native RPC layer,
so it makes sense to add them for the DBus RPC layer too.

src/libvirt_probes.d

@@ -15,6 +15,11 @@ provider libvirt {
 
 	probe event_poll_run(int nfds, int timeout);
 
+	# file: src/util/virdbus.c
+	# prefix: dbus
+	probe dbus_method_call(const char *interface, const char *member, const char *object, const char *destination);
+	probe dbus_method_error(const char *interface, const char *member, const char *object, const char *destination, const char *name, const char *message);
+	probe dbus_method_reply(const char *interface, const char *member, const char *object, const char *destination);
 
         # file: src/util/virobject.c
         # prefix: object

src/util/virdbus.c

@@ -27,6 +27,7 @@
 #include "virlog.h"
 #include "virthread.h"
 #include "virstring.h"
+#include "virprobe.h"
 
 #define VIR_FROM_THIS VIR_FROM_DBUS
 
@@ -1521,20 +1522,35 @@ static int
 virDBusCall(DBusConnection *conn,
             DBusMessage *call,
             DBusMessage **replyout,
-            DBusError *error,
-            const char *member)
+            DBusError *error)
+
 {
     DBusMessage *reply = NULL;
     DBusError localerror;
     int ret = -1;
+    const char *iface, *member, *path, *dest;
 
     if (!error)
         dbus_error_init(&localerror);
 
+    iface = dbus_message_get_interface(call);
+    member = dbus_message_get_member(call);
+    path = dbus_message_get_path(call);
+    dest = dbus_message_get_destination(call);
+
+    PROBE(DBUS_METHOD_CALL,
+          "'%s.%s' on '%s' at '%s'",
+          iface, member, path, dest);
+
     if (!(reply = dbus_connection_send_with_reply_and_block(conn,
                                                             call,
                                                             VIR_DBUS_METHOD_CALL_TIMEOUT_MILLIS,
                                                             error ? error : &localerror))) {
+        PROBE(DBUS_METHOD_ERROR,
+              "'%s.%s' on '%s' at '%s' error %s: %s",
+              iface, member, path, dest,
+              error ? error->name : localerror.name,
+              error ? error->message : localerror.message);
         if (error) {
             ret = 0;
         } else {
@@ -1544,6 +1560,10 @@ virDBusCall(DBusConnection *conn,
         goto cleanup;
     }
 
+    PROBE(DBUS_METHOD_REPLY,
+          "'%s.%s' on '%s' at '%s'",
+          iface, member, path, dest);
+
     ret = 0;
 
  cleanup:
@@ -1616,7 +1636,7 @@ int virDBusCallMethod(DBusConnection *conn,
 
     ret = -1;
 
-    ret = virDBusCall(conn, call, replyout, error, member);
+    ret = virDBusCall(conn, call, replyout, error);
 
  cleanup:
     if (call)