systematic-debugging

Jesse Vincent ｜ 除錯與調查 ｜ 原文 ｜ MIT

遇到 bug 先查 root cause，四階段流程走完才准修

---
name: systematic-debugging
description: 遇到任何 bug、測試失敗或非預期行為時，在提出修正方案之前使用
---

# Systematic Debugging

## 概述

**核心原則：** 嘗試修正之前，一定要先找出 root cause。只修表面症狀就是失敗。

**違反這個流程的字面規定，就是違反除錯的精神。**

## 鐵律

```
NO FIXES WITHOUT ROOT CAUSE INVESTIGATION FIRST
```

階段 1 還沒完成，你就不能提出修正方案。

## 何時使用

任何技術問題都要用：
- 測試失敗
- 正式環境的 bug
- 非預期的行為
- 效能問題
- Build 失敗
- 整合問題

**尤其要在這些時候用：**
- 有時間壓力（緊急狀況特別容易讓人想用猜的）
- 「只要快速修一下」看起來很明顯
- 你已經試過好幾個修正
- 前一個修正沒有用
- 你沒有完全理解這個問題

**這些時候也不要跳過：**
- 問題看起來很簡單（簡單的 bug 也有 root cause）
- 你很趕（趕只保證重工）
- 主管要它「現在」就修好（系統化的做法比亂試更快）

## 四個階段

進入下一個階段之前，你一定要先完成每個階段。

### 階段 1：Root Cause 調查

**嘗試「任何」修正之前：**

1. **仔細讀錯誤訊息**
   - 不要略過錯誤或警告
   - 它們常常就包含確切的解法
   - 完整讀完 stack trace
   - 記下行號、檔案路徑、錯誤碼

2. **穩定地重現**
   - 你能可靠地觸發它嗎？
   - 確切的步驟是什麼？
   - 每次都會發生嗎？
   - 如果無法重現 → 收集更多資料，不要用猜的

3. **檢查最近的變更**
   - 什麼改變可能造成這個問題？
   - Git diff、最近的 commit
   - 新的依賴、設定變更
   - 環境差異

4. **在多元件系統中收集證據**

   **當系統有多個元件時（CI → build → signing、API → service → database）：**

   **提出修正方案之前，先加上診斷用的 instrumentation：**
   ```
   For EACH component boundary:
     - Log what data enters component
     - Log what data exits component
     - Verify environment/config propagation
     - Check state at each layer

   Run once to gather evidence showing WHERE it breaks
   THEN analyze evidence to identify failing component
   THEN investigate that specific component
   ```

   **範例（多層系統）：**
   ```bash
   # Layer 1: Workflow
   echo "=== Secrets available in workflow: ==="
   echo "IDENTITY: $([ -n "${IDENTITY:-}" ] && echo SET || echo UNSET)"

   # Layer 2: Build script
   echo "=== Env vars in build script: ==="
   printenv IDENTITY >/dev/null 2>&1 && echo "IDENTITY in environment" || echo "IDENTITY not in environment"

   # Layer 3: Signing script
   echo "=== Keychain state: ==="
   security list-keychains
   security find-identity -v

   # Layer 4: Actual signing
   codesign --sign "$IDENTITY" --verbose=4 "$APP"
   ```

   **這會揭露：** 是哪一層失敗（secrets → workflow ✓，workflow → build ✗）

5. **追蹤資料流**

   **當錯誤在 call stack 很深的地方：**

   完整的反向追蹤技巧，見這個目錄裡的 `root-cause-tracing.md`。

   **快速版：**
   - 壞掉的值是從哪裡來的？
   - 是誰帶著這個壞值呼叫它的？
   - 持續往上追，直到找到源頭
   - 在源頭修，不要在症狀那裡修

### 階段 2：模式分析

**修正之前先找出模式：**

1. **找出能運作的範例**
   - 在同一個程式碼庫裡找類似、而且能運作的程式碼
   - 有什麼東西跟壞掉的很像，卻是能運作的？

2. **對照參考資料**
   - 如果你在實作某個模式，要「完整」讀完參考實作
   - 不要略讀，每一行都要讀
   - 套用之前，要完全理解這個模式

3. **找出差異**
   - 能運作的和壞掉的之間有什麼不同？
   - 列出每一個差異，不管多小
   - 不要假設「那個不會有影響」

4. **理解依賴**
   - 它還需要哪些其他元件？
   - 需要哪些設定、config、環境？
   - 它做了哪些假設？

### 階段 3：假設與測試

**科學方法：**

1. **形成單一假設**
   - 清楚陳述：「I think X is the root cause because Y」
   - 寫下來
   - 要具體，不要含糊

2. **用最小的方式測試**
   - 做出「最小」的可能改動來測試假設
   - 一次一個變數
   - 不要同時修好幾件事

3. **繼續之前先驗證**
   - 有效嗎？有 → 階段 4
   - 沒效？形成「新的」假設
   - 不要在上面再疊更多修正

4. **當你不知道的時候**
   - 說「I don't understand X」
   - 不要假裝知道
   - 求助
   - 做更多研究

### 階段 4：實作

**修 root cause，不要修症狀：**

1. **建立會失敗的測試案例**
   - 盡可能最簡單的重現
   - 可以的話用自動化測試
   - 沒有框架就用一次性的測試 script
   - 修正之前「一定要有」
   - 用 `superpowers:test-driven-development` skill 來寫出像樣的失敗測試

2. **實作單一修正**
   - 處理你找到的 root cause
   - 一次「一個」改動
   - 不要有「反正我在這裡就順便」的改進
   - 不要夾帶重構

3. **驗證修正**
   - 測試現在通過了嗎？
   - 沒有弄壞其他測試嗎？
   - 問題真的解決了嗎？
   - 宣稱成功之前，用 `superpowers:verification-before-completion` skill

4. **如果修正沒有用**
   - 停
   - 數一下：你已經試過幾個修正了？
   - 如果 < 3：回到階段 1，用新的資訊重新分析
   - **如果 ≥ 3：停下來，質疑架構（下面的步驟 5）**
   - 沒有架構層級的討論，不要嘗試第 4 個修正

5. **如果 3 個以上的修正都失敗了：質疑架構**

   **指出架構問題的模式：**
   - 每個修正都在不同的地方揭露新的共享狀態／耦合／問題
   - 修正需要「大規模重構」才能實作
   - 每個修正都在別處製造新的症狀

   **停下來，質疑根本：**
   - 這個模式在根本上是健全的嗎？
   - 我們是不是「只因為慣性就繼續用它」？
   - 我們應該重構架構，還是繼續修症狀？

   **在嘗試更多修正之前，先跟你的搭檔討論**

   這不是失敗的假設，這是錯誤的架構。

## 警訊：停下來，照流程走

如果你發現自己在想：
- 「先快速修一下，之後再調查」
- 「就試著改 X 看看有沒有用」
- 「一次加好幾個改動，然後跑測試」
- 「跳過測試，我手動驗證」
- 「大概是 X，我來修它」
- 「我沒有完全理解，但這可能有用」
- 「模式說 X，但我要用不同的方式調整它」
- 「主要的問題在這裡：[沒有調查就列出一堆修正]」
- 追蹤資料流之前就提出解決方案
- **「再試一次修正」（當你已經試過 2 次以上）**
- **每個修正都在不同的地方揭露新問題**

**這些全部都表示：停。回到階段 1。**

**如果 3 個以上的修正都失敗了：** 質疑架構（見階段 4.5）

## 你的搭檔的訊號：你做錯了

**留意這些導正：**
- 「Is that not happening?」：你沒有驗證就假設了
- 「Will it show us...?」：你本來應該加上收集證據的步驟
- 「Stop guessing」：你在還沒理解的情況下提出修正
- 「Ultra-think this」：質疑根本，不只是症狀
- 「We're stuck?」（沮喪的語氣）：你的做法行不通

**看到這些時：** 停。回到階段 1。

## 常見的合理化藉口

| 藉口 | 現實 |
|--------|---------|
| 「問題很簡單，不需要流程」 | 簡單的問題也有 root cause。簡單的 bug 用流程也很快。 |
| 「很緊急，沒時間走流程」 | 系統化除錯比猜了再試的亂撞更「快」。 |
| 「先試這個，之後再調查」 | 第一個修正會定下模式。從一開始就做對。 |
| 「確認修正有用之後我再寫測試」 | 沒測試過的修正不會穩。先寫測試才能證明。 |
| 「一次做多個修正可以省時間」 | 無法隔離是哪個有用。還會製造新 bug。 |
| 「參考資料太長，我改編一下模式」 | 只理解一部分，保證出 bug。完整讀完它。 |
| 「我看到問題了，我來修」 | 看到症狀 ≠ 理解 root cause。 |
| 「再試一次修正」（失敗 2 次以上之後） | 3 次以上失敗 = 架構問題。質疑模式，不要再修。 |

## 快速參考

| 階段 | 關鍵活動 | 成功標準 |
|-------|---------------|------------------|
| **1. Root Cause** | 讀錯誤、重現、檢查變更、收集證據 | 理解「是什麼」和「為什麼」 |
| **2. 模式** | 找能運作的範例、比較 | 找出差異 |
| **3. 假設** | 形成理論、最小化測試 | 確認，或形成新的假設 |
| **4. 實作** | 建立測試、修正、驗證 | Bug 解決、測試通過 |

## 當流程揭露「沒有 Root Cause」

如果系統化的調查揭露，這個問題確實是環境性的、取決於時序，或來自外部：

1. 你已經完成這個流程了
2. 記錄你調查過什麼
3. 實作適當的處理（retry、timeout、錯誤訊息）
4. 加上監控／logging，供未來調查

**但是：** 95% 的「沒有 root cause」案例，都是調查不完整。

## 輔助技巧

這些技巧是系統化除錯的一部分，放在這個目錄裡：

- **`root-cause-tracing.md`** - 沿著 call stack 往回追 bug，找出原始的觸發點
- **`defense-in-depth.md`** - 找到 root cause 之後，在多個層級加上驗證
- **`condition-based-waiting.md`** - 把任意的 timeout 換成條件輪詢

---
name: systematic-debugging
description: Use when encountering any bug, test failure, or unexpected behavior, before proposing fixes
---

# Systematic Debugging

## Overview

**Core principle:** ALWAYS find root cause before attempting fixes. Symptom fixes are failure.

**Violating the letter of this process is violating the spirit of debugging.**

## The Iron Law

```
NO FIXES WITHOUT ROOT CAUSE INVESTIGATION FIRST
```

If you haven't completed Phase 1, you cannot propose fixes.

## When to Use

Use for ANY technical issue:
- Test failures
- Bugs in production
- Unexpected behavior
- Performance problems
- Build failures
- Integration issues

**Use this ESPECIALLY when:**
- Under time pressure (emergencies make guessing tempting)
- "Just one quick fix" seems obvious
- You've already tried multiple fixes
- Previous fix didn't work
- You don't fully understand the issue

**Don't skip when:**
- Issue seems simple (simple bugs have root causes too)
- You're in a hurry (rushing guarantees rework)
- Manager wants it fixed NOW (systematic is faster than thrashing)

## The Four Phases

You MUST complete each phase before proceeding to the next.

### Phase 1: Root Cause Investigation

**BEFORE attempting ANY fix:**

1. **Read Error Messages Carefully**
   - Don't skip past errors or warnings
   - They often contain the exact solution
   - Read stack traces completely
   - Note line numbers, file paths, error codes

2. **Reproduce Consistently**
   - Can you trigger it reliably?
   - What are the exact steps?
   - Does it happen every time?
   - If not reproducible → gather more data, don't guess

3. **Check Recent Changes**
   - What changed that could cause this?
   - Git diff, recent commits
   - New dependencies, config changes
   - Environmental differences

4. **Gather Evidence in Multi-Component Systems**

   **WHEN system has multiple components (CI → build → signing, API → service → database):**

   **BEFORE proposing fixes, add diagnostic instrumentation:**
   ```
   For EACH component boundary:
     - Log what data enters component
     - Log what data exits component
     - Verify environment/config propagation
     - Check state at each layer

   Run once to gather evidence showing WHERE it breaks
   THEN analyze evidence to identify failing component
   THEN investigate that specific component
   ```

   **Example (multi-layer system):**
   ```bash
   # Layer 1: Workflow
   echo "=== Secrets available in workflow: ==="
   echo "IDENTITY: $([ -n "${IDENTITY:-}" ] && echo SET || echo UNSET)"

   # Layer 2: Build script
   echo "=== Env vars in build script: ==="
   printenv IDENTITY >/dev/null 2>&1 && echo "IDENTITY in environment" || echo "IDENTITY not in environment"

   # Layer 3: Signing script
   echo "=== Keychain state: ==="
   security list-keychains
   security find-identity -v

   # Layer 4: Actual signing
   codesign --sign "$IDENTITY" --verbose=4 "$APP"
   ```

   **This reveals:** Which layer fails (secrets → workflow ✓, workflow → build ✗)

5. **Trace Data Flow**

   **WHEN error is deep in call stack:**

   See `root-cause-tracing.md` in this directory for the complete backward tracing technique.

   **Quick version:**
   - Where does bad value originate?
   - What called this with bad value?
   - Keep tracing up until you find the source
   - Fix at source, not at symptom

### Phase 2: Pattern Analysis

**Find the pattern before fixing:**

1. **Find Working Examples**
   - Locate similar working code in same codebase
   - What works that's similar to what's broken?

2. **Compare Against References**
   - If implementing pattern, read reference implementation COMPLETELY
   - Don't skim - read every line
   - Understand the pattern fully before applying

3. **Identify Differences**
   - What's different between working and broken?
   - List every difference, however small
   - Don't assume "that can't matter"

4. **Understand Dependencies**
   - What other components does this need?
   - What settings, config, environment?
   - What assumptions does it make?

### Phase 3: Hypothesis and Testing

**Scientific method:**

1. **Form Single Hypothesis**
   - State clearly: "I think X is the root cause because Y"
   - Write it down
   - Be specific, not vague

2. **Test Minimally**
   - Make the SMALLEST possible change to test hypothesis
   - One variable at a time
   - Don't fix multiple things at once

3. **Verify Before Continuing**
   - Did it work? Yes → Phase 4
   - Didn't work? Form NEW hypothesis
   - DON'T add more fixes on top

4. **When You Don't Know**
   - Say "I don't understand X"
   - Don't pretend to know
   - Ask for help
   - Research more

### Phase 4: Implementation

**Fix the root cause, not the symptom:**

1. **Create Failing Test Case**
   - Simplest possible reproduction
   - Automated test if possible
   - One-off test script if no framework
   - MUST have before fixing
   - Use the `superpowers:test-driven-development` skill for writing proper failing tests

2. **Implement Single Fix**
   - Address the root cause identified
   - ONE change at a time
   - No "while I'm here" improvements
   - No bundled refactoring

3. **Verify Fix**
   - Test passes now?
   - No other tests broken?
   - Issue actually resolved?
   - Use the `superpowers:verification-before-completion` skill before claiming success

4. **If Fix Doesn't Work**
   - STOP
   - Count: How many fixes have you tried?
   - If < 3: Return to Phase 1, re-analyze with new information
   - **If ≥ 3: STOP and question the architecture (step 5 below)**
   - DON'T attempt Fix #4 without architectural discussion

5. **If 3+ Fixes Failed: Question Architecture**

   **Pattern indicating architectural problem:**
   - Each fix reveals new shared state/coupling/problem in different place
   - Fixes require "massive refactoring" to implement
   - Each fix creates new symptoms elsewhere

   **STOP and question fundamentals:**
   - Is this pattern fundamentally sound?
   - Are we "sticking with it through sheer inertia"?
   - Should we refactor architecture vs. continue fixing symptoms?

   **Discuss with your human partner before attempting more fixes**

   This is NOT a failed hypothesis - this is a wrong architecture.

## Red Flags - STOP and Follow Process

If you catch yourself thinking:
- "Quick fix for now, investigate later"
- "Just try changing X and see if it works"
- "Add multiple changes, run tests"
- "Skip the test, I'll manually verify"
- "It's probably X, let me fix that"
- "I don't fully understand but this might work"
- "Pattern says X but I'll adapt it differently"
- "Here are the main problems: [lists fixes without investigation]"
- Proposing solutions before tracing data flow
- **"One more fix attempt" (when already tried 2+)**
- **Each fix reveals new problem in different place**

**ALL of these mean: STOP. Return to Phase 1.**

**If 3+ fixes failed:** Question the architecture (see Phase 4.5)

## your human partner's Signals You're Doing It Wrong

**Watch for these redirections:**
- "Is that not happening?" - You assumed without verifying
- "Will it show us...?" - You should have added evidence gathering
- "Stop guessing" - You're proposing fixes without understanding
- "Ultra-think this" - Question fundamentals, not just symptoms
- "We're stuck?" (frustrated) - Your approach isn't working

**When you see these:** STOP. Return to Phase 1.

## Common Rationalizations

| Excuse | Reality |
|--------|---------|
| "Issue is simple, don't need process" | Simple issues have root causes too. Process is fast for simple bugs. |
| "Emergency, no time for process" | Systematic debugging is FASTER than guess-and-check thrashing. |
| "Just try this first, then investigate" | First fix sets the pattern. Do it right from the start. |
| "I'll write test after confirming fix works" | Untested fixes don't stick. Test first proves it. |
| "Multiple fixes at once saves time" | Can't isolate what worked. Causes new bugs. |
| "Reference too long, I'll adapt the pattern" | Partial understanding guarantees bugs. Read it completely. |
| "I see the problem, let me fix it" | Seeing symptoms ≠ understanding root cause. |
| "One more fix attempt" (after 2+ failures) | 3+ failures = architectural problem. Question pattern, don't fix again. |

## Quick Reference

| Phase | Key Activities | Success Criteria |
|-------|---------------|------------------|
| **1. Root Cause** | Read errors, reproduce, check changes, gather evidence | Understand WHAT and WHY |
| **2. Pattern** | Find working examples, compare | Identify differences |
| **3. Hypothesis** | Form theory, test minimally | Confirmed or new hypothesis |
| **4. Implementation** | Create test, fix, verify | Bug resolved, tests pass |

## When Process Reveals "No Root Cause"

If systematic investigation reveals issue is truly environmental, timing-dependent, or external:

1. You've completed the process
2. Document what you investigated
3. Implement appropriate handling (retry, timeout, error message)
4. Add monitoring/logging for future investigation

**But:** 95% of "no root cause" cases are incomplete investigation.

## Supporting Techniques

These techniques are part of systematic debugging and available in this directory:

- **`root-cause-tracing.md`** - Trace bugs backward through call stack to find original trigger
- **`defense-in-depth.md`** - Add validation at multiple layers after finding root cause
- **`condition-based-waiting.md`** - Replace arbitrary timeouts with condition polling