Go refactor

Go Refactor

You perform safe refactors by locking existing behavior with exhaustive tests before changing anything. The principle: if you can't prove the system still works identically after the refactor, you haven't finished.

The Three Phases

Phase 1: Document (read-only — zero code changes)

Phase 2: Lock (tests only — zero implementation changes)

Phase 3: Rewrite (implementation only — zero test changes)

Never mix phases. Each phase has a clear gate before proceeding to the next.

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Phase 1: Document All Inbound Surfaces

Before touching any code, produce a complete inventory of every way the system receives input and produces output.

What to document

For each inbound surface, create .refactor/<surface>.md:

HTTP Endpoints

Read the HTTP Surface Documentation pattern in patterns.md when writing this.

gRPC Services

Read the gRPC Surface Documentation pattern in patterns.md when writing this.

Message Queue Consumers

Read the Queue Surface Documentation pattern in patterns.md when writing this.

Scheduled Jobs / Cron

Read the Cron Surface Documentation pattern in patterns.md when writing this.

How to discover surfaces

1. Read main.go — find all route registrations, gRPC server registrations, queue consumers, cron setups
2. Read handler files — document every public method, its request/response types, error codes
3. Read proto files — document every RPC, message type, enum
4. Read queue consumer files — document every subscription, message schema, side effects
5. Grep for HandleFunc, RegisterService, Subscribe, Consume, AddFunc — catch anything main.go missed

Gate: Phase 1 → Phase 2

Create .refactor/SURFACES.md summarizing all surfaces found.

Read the Surfaces Summary Template pattern in patterns.md when writing this.

Ask the user to review .refactor/SURFACES.md before proceeding. Missing a surface means the refactor could silently break an integration.

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Phase 2: Lock Behavior with Tests

Create exhaustive e2e tests that exercise every documented surface. These tests are the safety net — they must all pass before AND after the refactor. If a test fails after the refactor, the refactor broke something.

Test principles

1. Test the contract, not the implementation. Assert on input types, output types, status codes, error codes, side effects — not on internal function calls.
2. Use testcontainers for everything. Real database, real message queue, real cache. No mocks.
3. Seed deterministic data. Fixed UUIDs, fixed timestamps where possible. Tests must be reproducible.
4. Test exact types. Unmarshal responses into the actual response struct types. If a field is renamed, added, or removed, the test fails at compile time or assertion time.

Test structure

Create one test file per surface in tests/e2e-refactor/.

Read the E2E Test Directory Structure pattern in patterns.md when writing this.

HTTP tests

For each endpoint documented in Phase 1:

• Happy path: valid request → expected response (unmarshal into typed struct, assert every field)
• Every error code: trigger each documented error → assert status code + error JSON
• IDOR: cross-scope access → 404
• Edge cases: empty lists return [], boundary values, missing optional params

gRPC tests

For each RPC:

• Happy path: valid request → expected response (assert every field of the protobuf message)
• Every error code: trigger each → assert gRPC status code
• Type compatibility: the request and response proto messages must match the documented schema

Queue tests

For each consumer:

• Publish a test message → assert the expected side effect (e.g., notification created in DB)
• Publish duplicate message → assert idempotency (no duplicate entity)
• Publish malformed message → assert it's dead-lettered or logged, not silently dropped
• Verify the message schema: marshal a struct to JSON, publish it, verify the consumer accepts it

Type compatibility tests

Read the Type Compatibility Tests pattern in patterns.md when writing this.

Gate: Phase 2 → Phase 3

Run all tests: go test ./tests/e2e-refactor/... -count=1 -v

Every test must pass. If any test fails, the existing code has a bug — fix the test or the code BEFORE proceeding to the refactor. The tests must faithfully represent the current behavior, even if that behavior is wrong (document it as a known issue in .refactor/KNOWN_ISSUES.md).

Ask the user to review the test results and confirm the behavior is correctly captured.

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Phase 3: Plan and Execute the Rewrite

Now that behavior is locked, plan the refactor.

Create .refactor/REWRITE_PLAN.md

Read the Rewrite Plan Template pattern in patterns.md when writing this.

Execute the rewrite

Use the go-runner agent to dispatch tasks. Each task:

• Modifies ONLY implementation files (never tests)
• Is followed by a full test run to verify no regression
• Is small enough to revert if tests fail

Type compatibility enforcement

During the rewrite, the type tests from Phase 2 act as compile-time and runtime guards:

• If you rename a JSON field → json.Decode fails or assertion fails
• If you remove a response field → assertion on that field fails
• If you change a status code → status code assertion fails
• If you change an error code string → error code assertion fails
• If you change a protobuf field number → gRPC client gets wrong data
• If you change a queue message schema → consumer test fails to process

The tests are the contract. The contract doesn't change. Only the implementation behind it changes.

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Guidelines

• Read each file at most once.
• Phase 1 is read-only. Do not modify any code. Only create .refactor/ documentation files.
• Phase 2 creates tests only. Do not modify implementation files. Tests must pass against the CURRENT code.
• Phase 3 modifies implementation only. Do not modify test files. Tests must pass against the NEW code.
• If a test fails in Phase 3, the refactor broke something. Fix the implementation, not the test.
• If the current code has a bug discovered in Phase 2, document it in .refactor/KNOWN_ISSUES.md and write the test to match current (buggy) behavior. Fix the bug as a separate task after the refactor.
• Ask the user to review after Phase 1 and Phase 2 before proceeding. Missing a surface or misunderstanding current behavior makes the refactor unsafe.
• The refactor is successful when: all Phase 2 tests pass, all existing tests pass, go build ./... passes, and the user confirms the new structure is correct.