fix: improve epoxy propose errors, add gcp logging fmt

[MasterPtato]

Description

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Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Please describe the tests that you ran to verify your changes.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[railway-app]

🚅 Deployed to the rivet-pr-4698 environment in rivet-frontend

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[MasterPtato]

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• fix: improve epoxy propose errors, add gcp logging fmt #4698 👈 (View in Graphite)
• fix: autogenerate bare protocol versions #4696
• main

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[claude]

PR #4698 Review: fix: improve epoxy propose errors, add gcp logging fmt

Summary: This PR does two things: (1) restructures ProposalResult in the epoxy consensus subsystem to replace the opaque ConsensusFailed/CommandError variants with a unified ConsensusFailed type that carries structured failure context; and (2) adds a GCP Cloud Logging (Stackdriver) output format selectable via RUST_LOG_FORMAT=gcp.

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Must Fix

Test compilation failures. Three test files still import CommandError which no longer exists in epoxy::ops::propose:

• engine/packages/epoxy/tests/kv.rs:3
• engine/packages/epoxy/tests/migration.rs:10
• engine/packages/epoxy/tests/backfill_snapshot.rs:8

All three files also match against ProposalResult::CommandError(CommandError::ExpectedValueDoesNotMatch) (e.g., kv.rs lines 42-44, 70-72, 107-109; backfill_snapshot.rs lines 100, 226; migration.rs line 36). These will not compile. They need to be updated to match the new ConsensusFailedReason::ExpectedValueDoesNotMatch pattern.

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Should Address

Gasoline activity output serialization break. ProposalResult is used as the return type of the propose activity in engine/packages/pegboard/src/workflows/actor/keys.rs and actor2/keys.rs. Gasoline serializes activity outputs to JSON for workflow replay. The serialized JSON shapes have changed with this refactor. Any in-flight workflows with a persisted Propose activity output using the old JSON shape will fail deserialization on replay after deploy. Consider serde(alias) attributes to bridge the gap if forward-compatibility is needed.

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Should Fix

Em dash in doc comment (engine/packages/runtime/src/traces.rs:17): CLAUDE.md explicitly prohibits em dashes. Change the doc comment to use a period instead.

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Minor

Redundant branch in result_for_committed_value (engine/packages/epoxy/src/ops/propose.rs lines 137-153): The mutable and non-mutable branches now produce identical code after the unification. The branch can be collapsed to a simple conditional on current_value == self.value.

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Good Changes

• ConsensusFailedReason enum: Clear improvement. The new PreparePhaseConsensusFailed, AcceptPhaseConsensusFailed, StaleBallot, and ExpectedValueDoesNotMatch variants are much more expressive than the old opaque ConsensusFailed unit variant and the misleading CommandError name.

• ProposalResult::resolve() helper: Clean ergonomic addition. The await?.resolve()? pattern is a nice improvement over the old ensure!(matches!(...)) boilerplate.

• Catchall error arm in keys.rs: Replaces the old bail!("consensus failed") which gave no context. Unhandled failure reasons now surface with full debug output.

• GCP logging format: The LogFormat enum and RUST_LOG_FORMAT=gcp env-var approach is clean. The tracing-stackdriver layer is correctly placed and default-features = false is appropriate. The RUST_LOG_LOCATION env var consistently controls source location for both backends.

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Summary

Priority	Issue
Must fix	Update 3 test files that import the removed CommandError type - these will not compile
Should address	Serialization break for in-flight Gasoline propose activities
Should fix	Em dash in traces.rs doc comment (CLAUDE.md style violation)
Minor	Redundant mutable branch in result_for_committed_value