[all-commits] [llvm/llvm-project] 98b419: [lldb] Don't exit the main loop when in runs out o...

[Pavel Labath via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 98b419ca7688aa2823df1e87f58051aaa8d9c37f
      https://github.com/llvm/llvm-project/commit/98b419ca7688aa2823df1e87f58051aaa8d9c37f
  Author: Pavel Labath <pavel at labath.sk>
  Date:   2024-10-17 (Thu, 17 Oct 2024)

  Changed paths:
    M lldb/source/Host/posix/MainLoopPosix.cpp
    M lldb/source/Host/windows/MainLoopWindows.cpp
    M lldb/tools/lldb-server/lldb-platform.cpp
    M lldb/unittests/Host/MainLoopTest.cpp

  Log Message:
  -----------
  [lldb] Don't exit the main loop when in runs out of things to listen on (#112565)

This behavior made sense in the beginning as the class was completely
single threaded, so if the source count ever reached zero, there was no
way to add new ones. In https://reviews.llvm.org/D131160, the class
gained the ability to add events (callbacks) from other threads, which
means that is no longer the case (and indeed, one possible use case for
this class -- acting as a sort of arbiter for multiple threads wanting
to run code while making sure it runs serially -- has this class sit in
an empty Run call most of the time). I'm not aware of us having a use
for such a thing right now, but one of my tests in another patch turned
into something similar by accident.

Another problem with the current approach is that, in a
distributed/dynamic setup (multiple things using the main loop without a
clear coordinator), one can never be sure whether unregistering a
specific event will terminate the loop (it depends on whether there are
other listeners). We had this problem in lldb-platform.cpp, where we had
to add an additional layer of synchronization to avoid premature
termination. We can remove this if we can rely on the loop terminating
only when we tell it to.



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