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Mutation Testing Baseline

Summary

Baseline Date: 2026-10-15

Module: contracts/streaming-payments/src/lib.rs and stream.rs

Results:

  • Total viable mutants: 95
  • Caught: 91
  • Missed: 4
  • Unviable: 13
  • Timeout: 1
  • Kill Rate: 95.8% (exceeds 90% target)

Survivors (4)

The following 4 mutants are acceptable survivors due to defensive/unreachable branches:

1. lib.rs:92:33 - duration = end_time - start_time

Mutation: Replace - with +

Justification: create_stream enforces end_time > start_time at lines 89-90. The mutation would produce an absurdly large duration that would immediately fail MAX_DURATION_SECONDS validation (line 93). This is an unreachable code path given the contract's invariants.

Inline Documentation: MUTANT-SUPPRESS comment added at line 92.

2. lib.rs:263:29 - if recipient_amount > 0

Mutation: Replace > with >=

Justification: recipient_amount is only 0 when cancel_amounts computes 0 (cancel before start). The test cancel_at_exact_start_pays_sender_everything verifies correct behavior without the transfer. The >= mutation would only add a no-op transfer of 0 tokens, which is functionally equivalent but unnecessary.

Inline Documentation: MUTANT-SUPPRESS comment added at line 268.

3. lib.rs:270:26 - if sender_amount > 0

Mutation: Replace > with >=

Justification: sender_amount is only 0 when cancel_amounts computes 0 (cancel after end). The test cancel_after_end_pays_recipient_everything verifies correct behavior without the transfer. The >= mutation would only add a no-op transfer of 0 tokens, which is functionally equivalent but unnecessary.

Inline Documentation: MUTANT-SUPPRESS comment added at line 280.

4. stream.rs:97:18 - end_time - start_time in elapsed_seconds

Mutation: Replace - with +

Justification: This branch is only reachable when now >= end_time, which means the full duration has already elapsed. The test elapsed_after_end_returns_full_duration verifies that the full deposit is available at this point. The mutation would cause an overflow in the subsequent streamed_at calculation, which is caught by MathOverflow. This is an unreachable code path given the contract's usage pattern.

Inline Documentation: MUTANT-SUPPRESS comment added at line 97.

New Tests Added

To improve mutation coverage, the following tests were added:

  1. create_stream_stores_correct_duration - Verifies duration calculation
  2. create_stream_accepts_exact_capacity_boundary - Tests boundary condition for validate_capacity
  3. cancel_with_zero_recipient_amount_skips_transfer - Tests zero-value transfer skip
  4. cancel_with_zero_sender_amount_skips_transfer - Tests zero-value transfer skip
  5. elapsed_after_end_returns_full_duration - Tests elapsed calculation at end boundary
  6. stream_ids_are_unique_and_monotonic - Verifies stream ID uniqueness

CI Integration

A weekly mutation testing workflow has been added at .github/workflows/mutation.yml:

  • Schedule: Weekly on Sundays at 00:00 UTC
  • Trigger: Manual dispatch available
  • Threshold: Fails if kill rate drops below 90%
  • Reporting: Results posted as a GitHub issue comment on scheduled runs
  • Artifacts: Mutation results uploaded for 30 days

Usage

Run mutation testing locally:

cd contracts/streaming-payments
cargo mutants --output mutants.out

View results:

cat mutants.out/missed.txt
cat mutants.out/caught.txt