There was an interesting issue in D-Bus, related to unicast vs broadcast signals, which led [edit: typo] to a small change in specs and which might be of some interest for D-Bus developers.
Unicast signals are not widely known and probably even less used, but they are possible.
They are useful, for instance, when you need to trigger an action from a single client, among your listeners.
Until some days ago, when a unicast signal was emitted, it was actually received by everyone listening to the signal’s interface (unless a strict rule was added, unusual), waking up a number of processes which actually weren’t interested in the signal.
Collateral effect, waking up cost apart: those processes might actually consider the signal as they were the actual recipient and take some action upon it. Bad.
Typical rule having this problem is “sender=”org.foo,interface=org.bar”.
Imagine several clients using this rule to listen to org.foo, but org.foo wanting to send a signal to :1.23 only.
Specifying destination=:1.23 for the signal object didn’t really work since no dest=val was specified in the rule, allowing any destination actually matching it and all the listeners to be woken up anyway.
The problem with this situation was fixing the bad behaviour without filter out eavesdroppers, which actually wanted to receive the message even if not for them.
The solution is a sort of “eavesdrop opt-in”, as Thiago proposed to add a keyword to DBusMatchRule, “eavesdrop=true|false” which defaults to false and with which the listener declares that it really wants to eavesdrop, enabling it to receive messages (including signals) not meant for it (AKA eavesdropping).
Otherwise any message (again, including signals) with a specified destination won’t match a filter with no or different destination. Also a rule with a specific destination won’t match broadcast messages.
In other words, by default a match rule filter will match only broadcast messages or the ones specifically for you, unless you declare your very nature of eavesdropper by adding “eavesdrop=true” to your filter rule.
This is a behavioural change and consequently means a small amend to the D-Bus