Ditch `LLEventTrackable` aka `boost::signals2::trackable`.

Remove documented `LLEventPump` support for `LLEventTrackable`. That claimed
support was always a little bit magical/fragile. IF:

* a class included `LLEventTrackable` as a base class AND
* an instance of that class was managed by `boost::shared_ptr` AND
* you passed one of that class's methods and the `boost::shared_ptr`
  specifically to `boost::bind()` AND
* the resulting `boost::bind()` object was passed into `LLEventPump::listen()`

THEN the promise was that on destruction of that object, that listener would
automatically be disconnected -- instead of leaving a dangling pointer bound
into the `LLEventPump`, causing a crash on the next `LLEventPump::post()` call.

The only existing code in the viewer code base that exercised `LLEventTrackable`
functionality was in test programs. When the viewer calls `LLEventPump::listen()`,
it typically stores the resulting connection object in an `LLTempBoundListener`
variable, which guarantees disconnection on destruction of that variable.

The fact that `LLEventTrackable` support is specific to `boost::bind()`, that it
silently fails to keep its promise with `std::bind()` or a lambda or any other
form of C++ callable, makes it untrustworthy for new code.

Note that the code base still uses `boost::signals2::trackable` for other
`boost::signals2::signal` instances not associated with `LLEventPump`. We are
not changing those at this time.

1 files changed, 9 insertions, 7 deletions

diff --git a/indra/llui/llnotificationslistener.cpp b/indra/llui/llnotificationslistener.cpp
index ace9e37e25..2ad1689a45 100644
--- a/indra/llui/llnotificationslistener.cpp
+++ b/indra/llui/llnotificationslistener.cpp
@@ -204,7 +204,7 @@ void LLNotificationsListener::ignore(const LLSD& params) const
     }
 }
 
-class LLNotificationsListener::Forwarder: public LLEventTrackable
+class LLNotificationsListener::Forwarder: public boost::signals2::trackable
 {
     LOG_CLASS(LLNotificationsListener::Forwarder);
 public:
@@ -213,8 +213,10 @@ public:
         mRespond(false)
     {
         // Connect to the specified channel on construction. Because
-        // LLEventTrackable is a base, we should automatically disconnect when
-        // destroyed.
+        // boost::signals2::trackable is a base, because we use boost::bind()
+        // below, and because connectPassedFilter() directly calls
+        // boost::signals2::signal::connect(), we should automatically
+        // disconnect when destroyed.
         LLNotificationChannelPtr channelptr(llnotifications.getChannel(channel));
         if (channelptr)
         {
@@ -252,10 +254,10 @@ void LLNotificationsListener::forward(const LLSD& params)
     if (! forward)
     {
         // This is a request to stop forwarding notifications on the specified
-        // channel. The rest of the params don't matter.
-        // Because mForwarders contains scoped_ptrs, erasing the map entry
-        // DOES delete the heap Forwarder object. Because Forwarder derives
-        // from LLEventTrackable, destroying it disconnects it from the
+        // channel. The rest of the params don't matter. Because mForwarders
+        // contains scoped_ptrs, erasing the map entry DOES delete the heap
+        // Forwarder object. Because Forwarder derives from
+        // boost::signals2::trackable, destroying it disconnects it from the
         // channel.
         mForwarders.erase(channel);
         return;