From: merch-redmine@...
Date: 2021-02-22T19:18:25+00:00
Subject: [ruby-core:102569] [Ruby master Bug#17650] TracePoint doesn't receive :fiber_switch events when raising exceptions

Issue #17650 has been updated by jeremyevans0 (Jeremy Evans).

Status changed from Open to Closed

PR merged at commit:3ee4fa9491d0b2b5fb40deea8e93e797924de789.

----------------------------------------
Bug #17650: TracePoint doesn't receive :fiber_switch events when raising exceptions
https://bugs.ruby-lang.org/issues/17650#change-90551

* Author: nevans (Nicholas Evans)
* Status: Closed
* Priority: Normal
* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN, 3.0: UNKNOWN
----------------------------------------
It seems to me the `:fiber_switch` event should be raised for *every* fiber switch, unless there's some very important reason not to, e.g. maybe it makes sense to skip the event when a fiber terminates with a fatal error.  In addition to sending the event, there's some other fiber cleanup code in `fiber_switch` that is skipped after raising an exception: checking thread interrupts and incrementing `th->blocking`.  Maybe this is intended behavior?  But it all seems like a bug to me.

I've captured the expected behavior in this test (which fails on `master`):

```ruby
    evs = []
    f = nil
    TracePoint.new(:raise, :fiber_switch){|tp|
      next unless target_thread?
      evs << [tp.event, Fiber.current]
    }.enable{
      f = Fiber.new{
        Fiber.yield # will raise
        Fiber.yield # unreachable
      }
      begin
        f.resume
        f.raise StopIteration
      rescue StopIteration
        evs << :rescued
      end
    }
    assert_equal [:fiber_switch, f],             evs[0], "initial resume"
    assert_equal [:fiber_switch, Fiber.current], evs[1], "Fiber.yield"
    # fiber switch event comes before the exception is raised:
    assert_equal [:fiber_switch, f],             evs[2], "fiber.raise"
    assert_equal [:raise, f],                    evs[3], "fiber.raise"
    # fiber switch event comes before the exception is raised:
    assert_equal [:fiber_switch, Fiber.current], evs[4], "terminated with raise"
    assert_equal [:raise, Fiber.current],        evs[5], "terminated with raise"
    assert_equal :rescued,                       evs[6]
    assert_equal 7, evs.size
```

I've attempted to fix this in a PR here: https://github.com/ruby/ruby/pull/4207

That PR moves the `rb_exc_raise` after all of the other `fiber_switch`cleanup code. It also changes fiber termination with an unhandled exception to pass the exception to its `return_fiber` in the same way as `Fiber#raise` (via `fiber->cont.value` and `fiber->cont.argc == -1`) instead of via a thread interrupt, which can lead to unexpected and difficult to debug behavior.  Fatal errors are still passed via `rb_threadptr_pending_interrupt_enque`.



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