Update CheckQuorum and DropIgnoredMessage in ccfraft.tla #7374
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achamayou
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Oct 20, 2025
achamayou
reviewed
Oct 20, 2025
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@cjen1-msft when Model Checking, we pass --disable-check-quorum, which goes through here, here and ultimately here. The simulation is doing some CheckQuorum though. |
achamayou
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Oct 20, 2025
eddyashton
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Oct 20, 2025
Co-authored-by: Eddy Ashton <ashton.eddy@gmail.com>
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Pull Request Overview
This PR fixes two instances where the TLA+ specification over-approximated CCF's behavior, causing false liveness counterexamples. The changes align the specification more closely with the actual implementation by restricting when CheckQuorum can trigger and when messages can be dropped.
- Adds a constraint to prevent CheckQuorum from triggering in single-node configurations
- Prevents DropIgnoredMessage from dropping RequestVoteRequest messages from unknown nodes
achamayou
approved these changes
Oct 22, 2025
github-merge-queue
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Adding backported label - this was backported in #7436. |
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I found two places where our spec overapproximates the behaviours of CCF.
First CheckQuorum can always trigger on a leader, so in the case of a single nodeconfiguration it can trigger in the spec but never in practise.
Next the DropIgnoredMessage was allowed to drop messages if they were ignored. We specifically don't ignore RequestVote messages from unknown replicas, while this is allowed by the spec.
Both of these were causing false liveness counterexamples in testing of the prevote changes.