Do not duplicate demand in BroadcastDispatcher #319
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Since 5d8193e, the BroadcastDispatcher can send demand upstream even if its consumers did not request that many events. Because `waiting` is set to 0 when a new subscriber subscribes, the demand is repeated when the new subscriber asks for events. I am fixing this by introducing a new counter in state - the number of requested but not yet delivered events. The dispatcher now does not send upstream demand if the demand can be satisfied by the requested events (and sends a lower demand if it can be satisfied partially). Fixes elixir-lang#318.
xxdavid
commented
Jul 15, 2025
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| {:ok, 10, disp} = D.ask(10, {pid2, ref2}, disp) | ||
| assert disp == {[{0, pid2, ref2, nil}], 10, expected_subscribers} | ||
| {:ok, 0, disp} = D.ask(10, {pid2, ref2}, disp) |
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Note that here (and also at line 155) I am reverting the IMO incorrect change in assertions in 5d8193e.
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Thank you, I believe this is indeed the correct fix and it was really well done. I will play with a couple more things locally and then do a new release |
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Since 5d8193e, the BroadcastDispatcher can send demand upstream even if its consumers did not request that many events. Because
waitingis set to 0 when a new subscriber subscribes, the demand is repeated when the new subscriber asks for events.I am fixing this by introducing a new counter in state - the number of requested but not yet delivered events. The dispatcher now does not send upstream demand if the demand can be satisfied by the requested events (and sends a lower demand if it can be satisfied partially).
Fixes #318.
Feel free to close this pull request if you come up with a better solution.