[Lldb-commits] [PATCH] D86652: [lldb] Fix ProcessEventData::DoOnRemoval to support multiple listeners

[Ilya Nozhkin via Phabricator via lldb-commits]

ilya-nozhkin added a comment.

Yeah, I understand the problem of two listeners trying to change process state simultaneously and I agree that informing other listeners once the primary one finished its job would be a great solution. But the problem is that primary listener just provides an event to some user code (via GetEvent) and there is no way to find out when this code has finished without requiring it to invoke a handshake explicitly.

One of the possible solutions is to send a specifically marked event to a primary listener first. Then, make this event to invoke a handshake in its destructor (which would mean that primary listener doesn't need the event anymore). And handshake can be implemented as just a rebroadcasting a new instance of the event to other listeners (or via unlocking some additional mutex / notifying some conditional variable). However, destructor may be invoked not immediately after event processing is finished. For example, if a primary listener's thread defines an EventSP before the listening cycle and doesn't reset it until GetEvent returns a new one (which may not happen until the next stop, for example).

Anyway, the concept of primary and secondary listeners requires to forbid secondary listeners to do "step" or "continue" at all. So, why can't we do the same without any additional code? I mean, I think that the most common case of multiple listeners is a Python extension adding a listener to read/write some memory/registers on each stop taking the RunLock. So, it does not interfere with a primary listener except for simultaneous DoOnRemoval invocation (which is fixed by this patch). Moreover, as I understand, there is no any way to implement such extension without adding a new listener (because if one would try to use Debugger's default listener then all other users such as lldb-vscode, for example, couldn't handle process events anymore).


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https://reviews.llvm.org/D86652