Bug Regression Preventer

Systematic checklist to ensure bug fixes don't introduce new problems.

Pre-Fix Checklist

1. Reproduction Verification

• Bug can be consistently reproduced
• Reproduction steps documented
• Environment conditions noted
• Expected vs actual behavior clear

2. Test Coverage Check

• Existing tests identified
• Missing test cases noted
• Flaky tests identified
• Performance benchmarks available (if relevant)

3. Impact Assessment

• Callers identified
• Dependents mapped
• Events/messages reviewed
• API contracts reviewed

Fix Validation Checklist

API Compatibility

Method Signature

// BEFORE
public function process(Order $order): OrderResult

// AFTER - Must maintain compatibility
public function process(Order $order): OrderResult  // ✓ Same signature
public function process(Order $order): Result       // ✗ Return type changed
public function process(Order $order, bool $flag): OrderResult // ✗ New required param
public function process(Order $order, bool $flag = false): OrderResult // ✓ Optional param OK

Checklist:

• Return type unchanged
• Parameter types unchanged
• Parameter order unchanged
• No new required parameters
• No removed parameters

Exception Contract

// BEFORE - throws ValidationException
public function validate(Data $data): void

// AFTER - Must throw same or subtype
public function validate(Data $data): void  // throws ValidationException ✓
public function validate(Data $data): void  // throws DataValidationException extends ValidationException ✓
public function validate(Data $data): void  // throws RuntimeException ✗ Different hierarchy

Checklist:

• Same exception types thrown
• New exceptions are subtypes of existing
• Exception messages format preserved (if parsed downstream)
• Exception codes unchanged (if used)

Behavior Preservation

Return Value Semantics

// BEFORE
public function findUser(UserId $id): ?User
// Returns null for non-existent user

// AFTER - Preserve null semantics
public function findUser(UserId $id): ?User
// Must still return null for non-existent user
// NOT throw NotFoundException (behavior change)

Checklist:

• Null return preserved (if applicable)
• Empty collection return preserved (if applicable)
• Default values unchanged
• Error conditions unchanged

Side Effects

// BEFORE - Has side effects
public function completeOrder(Order $order): void
{
    $order->complete();                    // 1. State change
    $this->repository->save($order);       // 2. Database write
    $this->events->publish($event);        // 3. Event published
}

// AFTER - Must preserve all side effects
// Unless the bug IS one of these side effects

Checklist:

• State changes preserved
• Database writes preserved
• Events still published
• Messages still sent
• Logs still written
• Metrics still recorded

Data Integrity

Database Schema

• No schema changes required
• Existing data remains valid
• Indexes still effective
• Constraints not violated

Data Format

// BEFORE - stored as "2024-01-15"
// AFTER - must stay "2024-01-15"
// NOT "2024-01-15T00:00:00Z" (format change)

Checklist:

• Serialization format unchanged
• JSON structure unchanged
• Date/time formats unchanged
• Numeric precision unchanged

Performance

Query Patterns

• No new N+1 queries
• No removed indexes usage
• No added full table scans
• Transaction scope unchanged

Resource Usage

• Memory usage not increased significantly
• CPU usage not increased significantly
• Network calls not increased
• File I/O not increased

Test Requirements

Mandatory Tests

1. Reproduction Test

/**
 * This test reproduces the original bug.
 * It MUST fail before the fix and pass after.
 */
#[Test]
public function testBugReproduction(): void
{
    // Arrange: Set up conditions that trigger the bug
    $order = OrderBuilder::create()
        ->withItem(null) // The bug trigger
        ->build();

    // Act & Assert: Verify bug is fixed
    $result = $this->service->calculateTotal($order);

    // Before fix: throws NullPointerException
    // After fix: returns Money::zero()
    $this->assertEquals(Money::zero('USD'), $result);
}

2. Edge Case Tests

#[Test]
public function testEdgeCases(): void
{
    // Test boundary conditions around the fix
    $this->assertEquals($expected, $this->service->process($emptyInput));
    $this->assertEquals($expected, $this->service->process($maxInput));
    $this->assertEquals($expected, $this->service->process($nullableInput));
}

3. Regression Tests

#[Test]
public function testExistingBehaviorPreserved(): void
{
    // Test that normal cases still work
    $normalOrder = OrderBuilder::create()
        ->withItem($validItem)
        ->build();

    $result = $this->service->calculateTotal($normalOrder);

    $this->assertEquals($expectedTotal, $result);
}

Test Coverage Matrix

Scenario	Before Fix	After Fix	Test Required
Bug trigger case	Fails/crashes	Works correctly	✓ Reproduction
Normal case	Works	Must still work	✓ Regression
Edge cases	May vary	Defined behavior	✓ Edge case
Related features	Work	Must still work	✓ Integration

Common Regression Patterns

1. Over-Fixing

// Bug: Null pointer when item is null
// WRONG: Remove null items entirely (behavior change)
$items = array_filter($items, fn($i) => $i !== null);

// CORRECT: Handle null gracefully
foreach ($items as $item) {
    if ($item === null) {
        continue; // Skip null, preserve others
    }
    // ... process item
}

2. Breaking Callers

// Bug: Method should return early for invalid state
// WRONG: Change return type
public function process(): void // Was: ?Result

// CORRECT: Return null for invalid state (preserve contract)
public function process(): ?Result
{
    if (!$this->isValid()) {
        return null;
    }
    // ...
}

3. Hiding Errors

// Bug: Exception crashes application
// WRONG: Swallow exception
try {
    $this->service->process($data);
} catch (Exception $e) {
    // Silent failure - bug hidden
}

// CORRECT: Handle specific exception appropriately
try {
    $this->service->process($data);
} catch (ValidationException $e) {
    return Result::invalid($e->getErrors());
}

4. Performance Regression

// Bug: Missing validation
// WRONG: Add validation that queries database in loop
foreach ($items as $item) {
    if (!$this->repository->exists($item->getId())) { // N+1 query!
        throw new NotFoundException();
    }
}

// CORRECT: Batch validation
$ids = array_map(fn($i) => $i->getId(), $items);
$existing = $this->repository->findByIds($ids);
if (count($existing) !== count($items)) {
    throw new NotFoundException();
}

Post-Fix Verification

Manual Testing

• Bug no longer reproducible manually
• Normal workflows still work
• Edge cases handled correctly
• Error messages appropriate

Automated Testing

• All unit tests pass
• All integration tests pass
• All E2E tests pass
• No new test failures

Code Review Points

• Fix is minimal (only affected code changed)
• No unrelated changes
• No commented-out code
• No debug statements left
• Proper exception handling
• No new code smells

Documentation

• PHPDoc updated if public API affected
• CHANGELOG entry added
• Issue linked in commit
• Breaking changes documented (if any)

Rollback Plan

Before Deployment

1. Tag current version: git tag pre-fix-{issue-id}
2. Document rollback command: git revert {commit-hash}
3. Identify monitoring dashboards
4. Set alert thresholds

Monitoring After Deployment

• Error rates normal
• Latency normal
• Resource usage normal
• No new exception types
• Business metrics stable

Rollback Triggers

• Error rate increase > 5%
• Latency increase > 20%
• New critical errors appearing
• Business metric anomaly

Quick Checklist Summary

## Pre-Fix
- [ ] Bug reproduced
- [ ] Impact assessed
- [ ] Existing tests identified

## Fix Applied
- [ ] API compatible
- [ ] Behavior preserved
- [ ] Side effects intact
- [ ] Data integrity maintained

## Tests Added
- [ ] Reproduction test
- [ ] Edge case tests
- [ ] Regression tests

## Post-Fix
- [ ] All tests pass
- [ ] Code reviewed
- [ ] Documentation updated
- [ ] Monitoring ready