[code-review] error_swallow_check delta detection misses new instances when code is prepended — count-based offset assumes new instances always appear after old ones

[topcheer]

File and Lines

internal/agent/error_swallow_check.go lines 69-83 (checkErrorSwallowing)

Problem

The delta detection logic uses a count-based offset (newInstances[oldCount+i]) that assumes new error-swallowing instances always appear AFTER old instances in the position-ordered list. When new code is added at the BEGINNING of a file, the new instance appears at index 0, and the logic reports an OLD instance as "newly introduced" while missing the actual new instance.

Trigger Scenario

Before edit (oldContent) — 2 existing swallows:

func oldFunc1() error {
    err := doSomething()
    if err != nil {} // OLD instance 1
    return nil
}
func oldFunc2() error {
    err := doSomething()
    if err != nil {} // OLD instance 2
    return nil
}

oldCount = 2

After edit (newContent) — new function added at top:

func newFunc() error {
    err := doSomething()
    if err != nil {} // NEW instance at index 0
    return nil
}
func oldFunc1() error { ... if err != nil {} ... } // OLD at index 1
func oldFunc2() error { ... if err != nil {} ... } // OLD at index 2

Delta logic execution:

1. oldCount = 2, len(newInstances) = 3
2. newCount = 3 - 2 = 1
3. Loop: i=0, accesses newInstances[0+2] = newInstances[2] = OLD instance at line 22
4. Warning issued for OLD instance — NEW instance at index 0 is MISSED

Expected vs Actual Behavior

• Expected: New error-swallowing at the beginning of a file should be detected and flagged.
• Actual: New instance is missed (false negative), old instance is misattributed as "newly introduced".

Root Cause

findErrorSwallows() returns instances ordered by source position (top to bottom). The delta logic assumes indices 0..oldCount-1 = OLD, oldCount..end = NEW. This only works when code is appended to the end. When code is prepended, old instances shift to higher indices and the offset breaks.

Severity

MEDIUM — Only affects delta detection (new files and appends work correctly). Causes both false negatives (missed new bugs) and misattribution (flagging old code as new).

Fix Suggestion

Since error swallowing detection is fast (<1ms), the simplest robust fix is to drop delta optimization entirely and report all instances each time. Alternatively, use position-based matching (store token.Pos for old instances and match against new ones).

[topcheer]

已修复 (commit 7264faa)。

问题：delta 检测使用基于计数的偏移量 (newInstances[oldCount+i])，假设新增实例总是出现在列表末尾。当新代码被预置到文件开头时，旧实例的索引偏移导致：旧实例被误报为新实例，真正的新实例被漏检。

修复：用内容指纹匹配替代计数偏移。每个实例生成稳定指纹 funcName:errName:patternType，通过指纹集合匹配来判断是否为新增。指纹不依赖行号或位置，因此无论代码插入在文件开头还是末尾都能正确工作。

（注：最初尝试了 token.Pos 匹配，但两次独立 parse 的 Pos 值不同，同样无法处理预置场景。）

测试：新增 TestCheckErrorSwallowing_DeltaPrependedNewInstance 验证：当新函数被预置到文件开头时，只报告 1 个新实例（而非 3 个）。

--- PASS: TestCheckErrorSwallowing_DeltaPrependedNewInstance (0.00s)

Closes #98