TEST: add unit and integration tests for GCG attack

[romanlutz]

Summary

Adds 55 new tests for the GCG (Greedy Coordinate Gradient) attack code, bringing total GCG test coverage from 24 to 79 tests. This covers Phase 1a (unit tests) and Phase 1b (integration tests) of a larger GCG refactoring effort.

New unit test files

File	Tests	Coverage Area
test_gcg_core.py	26	Core algorithm: loss computation (shape, finiteness, ordering), candidate sampling (shape, vocab bounds, single-position mutation, non-ASCII filtering), candidate filtering, embedding helpers, attack routing, init validation
test_data_and_config.py	12	YAML config loading, all 11 shipped config files validated (parse, required keys, individual vs transfer), data loading (seed reproducibility, separate test data, n_train_data limiting), run_trainer input validation
test_lifecycle.py	7	GPU memory parsing (nvidia-smi), MLflow lifecycle (run started before training), worker cleanup (stopped after training), bug characterization: workers leak on failure

New integration test file

File	Tests	Coverage Area
test_gcg_integration.py	10	End-to-end GCG pipeline with real GPT-2 model on CPU: token_gradients (gradient shape, finiteness), GCGAttackPrompt (init slices, grad, loss), sample_control (decodable candidates), embedding helpers with real model, get_nonascii_toks

Integration tests use GPT-2 (~124M params) with the llama-2 conversation template. Marked @run_only_if_all_tests. Runs in ~18s on CPU.

Notable finding

generate_suffix() does not use try/finally for worker cleanup. If attack.run() raises an exception, workers are never stopped — this is a resource leak. Documented with a characterization test (test_workers_not_stopped_on_training_failure) that will be fixed in a later phase.

Context

This is the first PR in a planned GCG refactor effort (tracked in #960) that will:

1. Phase 1: Add comprehensive test coverage (this PR = 1a + 1b, 1c Azure e2e to follow)
2. Phase 2: Clean up the public interface for extensibility (GCG variants like AttnGCG, probe sampling, mellowmax)
3. Phase 3: Internal cleanup (split monolith, replace fastchat with apply_chat_template FEAT replace fastchat in GCG #965 building on PR FEAT: Drop fastchat from GCG (#965) #1049, model adapter registry FEAT support more models for evaluating suffixes in GCG #990, fix OOM BUG GCG runs out of memory even on huge machines #961)
4. Phase 4: PyRIT integration + Azure improvements
5. Phase 5: GCG variant extensions (FEAT Faster Greedy Coordinate Gradient algorithm (GCG) #962)

No source code changes

This PR only adds test files — no modifications to existing source code.

[rlundeen2]

Review of PR #1684: GCG Attack Test Coverage

Overall: Excellent PR — well-structured, comprehensive test coverage with clear documentation. The bug characterization test for the worker leak is particularly good engineering practice. One actionable issue below.

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🐛 Missing @pytest.mark.run_only_if_all_tests on integration tests

File: tests/integration/auxiliary_attacks/test_gcg_integration.py

Per test.instructions.md, integration tests must be marked with @pytest.mark.run_only_if_all_tests (skipped unless RUN_ALL_TESTS=true).

The pytest.importorskip at module level handles missing deps, but serves a different purpose — run_only_if_all_tests ensures these slower tests (~18s + potential model download) only run intentionally, not in every CI pass where torch happens to be installed.

The PR description says "Marked @run_only_if_all_tests" but the code doesn't actually have it. Easiest fix is a module-level marker:

python pytestmark = pytest.mark.run_only_if_all_tests

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Minor observations (non-blocking)

• The object.__new__(GCGPromptManager) pattern in the integration test (to call sample_control without full init) is pragmatic but slightly undermines the "integration" nature of the test. If GCGPromptManager.__init__ ever adds required state, this will silently break. Consider a brief inline comment noting why full construction is avoided (e.g., it requires multiple coordinated workers).

• The CONFIGS_DIR relative path in test_data_and_config.py uses 4 levels of .. — fragile if the test file ever moves. A helper like pathlib.Path(__file__).resolve().parents[4] / "pyrit" / ... would be slightly more readable but not critical.