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authorDavid Herrmann <dh.herrmann@gmail.com>2014-03-22 18:06:38 +0100
committerDavid Herrmann <dh.herrmann@gmail.com>2014-03-22 18:06:38 +0100
commiteb33a6f858c2cd39dcf9c2f39514c9f83ed040fe (patch)
tree6bf4d80d8f66d252574407de3b10af7011f42378 /src
parent22fdeadcc06e95fe41ac4de872ec245c0887547f (diff)
sd-bus: add note about sd_bus_unref() recursion
In sd_bus_unref() we check for self-reference loops and destruct our queues in case we're the only reference holders. However, we do _not_ modify our own ref-count, thus effectively causing the message-destructions to enter with the same reference count as we did. The only reason this doesn't cause an endless recursion (or trigger assert(m->n_ref > 0) in sd_bus_message_unref()) is the fact that we decrease queue-counters _before_ calling _unref(). That's not obvious at all, so add a big fat note in bus_reset_queues() to everyone touching that code.
Diffstat (limited to 'src')
-rw-r--r--src/libsystemd/sd-bus/sd-bus.c11
1 files changed, 10 insertions, 1 deletions
diff --git a/src/libsystemd/sd-bus/sd-bus.c b/src/libsystemd/sd-bus/sd-bus.c
index 15c767723c..984611fd4d 100644
--- a/src/libsystemd/sd-bus/sd-bus.c
+++ b/src/libsystemd/sd-bus/sd-bus.c
@@ -111,6 +111,12 @@ static void bus_node_destroy(sd_bus *b, struct node *n) {
static void bus_reset_queues(sd_bus *b) {
assert(b);
+ /* NOTE: We _must_ decrement b->Xqueue_size before calling
+ * sd_bus_message_unref() for _each_ message. Otherwise the
+ * self-reference checks in sd_bus_unref() will fire for each message.
+ * We would thus recurse into sd_bus_message_unref() and trigger the
+ * assert(m->n_ref > 0) */
+
while (b->rqueue_size > 0)
sd_bus_message_unref(b->rqueue[--b->rqueue_size]);
@@ -1408,7 +1414,10 @@ _public_ sd_bus *sd_bus_unref(sd_bus *bus) {
/* We are the only holders on the messages, and the
* messages are the only holders on us, so let's drop
* the messages and thus implicitly also kill our own
- * last references */
+ * last references.
+ * bus_reset_queues() decrements the queue-size before
+ * calling into sd_bus_message_unref(). Thus, it
+ * protects us from recursion. */
if (q)
bus_reset_queues(bus);