Fix panic in speculative execution #1086
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Oh, it looks like passing only completed futures to select! without complete branch causes panic instead of deadlock - in that case it may be possible to write a test. |
This commit fixes a panic in `scylla::transport::speculative_execution::execute`. It is possible for all speculative fibers to finish without returning from the function. In that situation `select!` macro will panic because all futures passed to it are completed. Consider the scenario where all executions return BrokenConnection: we will only assign to last_error and never return. There are more ways that this bug can happen, but this is the simplest one to explain and reproduce. This commit fixes the bug by simplifying the logic inside the function. The new logic always checks if the resolved task is the last one, and if so it returns, so it's not possible for executions to be exhausted when calling `select!`.
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Good that they designed |
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I renamed the test, because the bug is a panic not a deadlock. PR is ready for review. |
muzarski
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wprzytula
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Addressed review comments |
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This PR fixes linked bug by simplifying the logic inside the function.
The new logic always checks if the resolved task is the last one, and if so it returns, so it's not possible for the tasks to be exhausted without finishing the
select!.This PR also adds regression test for the issue. I verified that the test actually panics without the fix.
Fixes: #1085
Pre-review checklist
I have provided docstrings for the public items that I want to introduce.I have adjusted the documentation in./docs/source/.I added appropriateFixes:annotations to PR description.