Return exhaustion without handing out a partial slice
A capacity miss advances the arena cursor before returning an error, so the following small allocation also fails.
- Focused work estimate
- 1h 30m + prerequisites
- Priority in the scenario
- Medium
- Engineering practice
- Failure atomicity · Resource bounds
Estimated field mix
- Systems programming70%
- Performance engineering30%
Field percentages are editorial estimates of the ticket's engineering focus. They total 100%; they are not measured time, proficiency scores, or ownership evidence.
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Project context
A fictional telemetry collector copies decoded packet fields into a small native arena. The prototype misaligns wide values and reuses memory while readers still hold views. Create a local Rust crate or C library with generated byte fixtures; no device traffic or production allocator replacement is supplied.
Setup prerequisites
- Pointers and slices
- Integer overflow
- Memory alignment
Preceding work
No earlier ticket is required. Complete the project setup above.
Acceptance criteria
- An exhausted allocation returns a typed capacity error
- Cursor and prior bytes remain unchanged after failure
- A later fitting allocation can still succeed
Implementation constraints
- Do not grow the backing region or panic on caller-controlled sizes.
Verification to include
- Fill the arena exactly and inspect the final valid allocation.
- Request one byte too many, then allocate a smaller record and verify state.
Deliverables
- Atomic allocation update and exhaustion regression
Rollout and recovery
Treat exhaustion as backpressure in the local collector; never retry without changing capacity or workload.
Value of the work
For the engineer: Practice representation invariants, lifetimes, unsafe-boundary review and memory diagnostics.
For the team: Review a component whose allocation failures and corruption cases are explicit before reuse in latency-sensitive code.
Evidence boundaries
Outcome Evidence: Tests, patches, and runbooks are requested deliverables. They become Outcome Evidence only through a qualified Mission and immutable Evidence IDs.
Ownership Evidence: Independent adaptation must be observed under a declared verification policy and cite immutable Evidence IDs. Completing a planning ticket establishes no Ownership Evidence.