Bug Root Cause Tracing

Bug Root Cause Tracing

Overview

Bugs often manifest deep in the call stack — wrong directory, bad parameter, corrupted data — far from where the invalid value originated. Fixing where the error appears treats the symptom. Tracing backward to find the original trigger fixes the cause.

Core principle: Never fix where the error surfaces. Trace back until you find what actually caused it.

For coding-related work, load engineering-principles before this skill. Root cause tracing applies its fail-fast, explicit-state, and boundary principles to the path that introduced bad state.

engineering-principles
  -> systematic-debugging
  -> bug-root-cause-tracing  (you are here, when root cause is unclear/deep)
       -> trace backward to original trigger
       -> fix at source
       -> test-driven-development / manual-testing  (prove original symptom)
       -> bug-protection-multi-layered              (if class can recur)
       -> verification-before-completion

When to Use

• Error appears deep in execution (not at the entry point)
• Stack trace shows a long call chain
• Unclear where invalid data originated
• You need to find which code path triggers the problem
• Bad state, wrong files, unexpected side effects, or test pollution appear far from their origin
• systematic-debugging Phase 2 investigation didn't immediately reveal the root cause

The Tracing Process

Step 1: Observe the Symptom

Identify what exactly fails and where:

Error: git init failed in /home/user/project/packages/core

Step 2: Find the Immediate Cause

What code directly produces this failure?

subprocess.run(["git", "init"], cwd=project_dir, cwd=project_dir)

Step 3: Trace Up One Level

Ask: what called this code, and with what values?

WorktreeManager.create_session_worktree(project_dir)
  → called by Session.initialize_workspace()
  → called by Session.create()
  → called by test at Project.create()

Step 4: Keep Tracing

At each level, identify the parameter values and whether they're valid:

project_dir = ""   ← empty string!
  → Empty cwd resolves to process.cwd()
  → That's the source code directory!

Step 5: Find the Original Trigger

Where did the bad value originate?

context = setup_core_test()   # Returns { temp_dir: "" }
Project.create("name", context.temp_dir)   # Accessed before setUp!

Root cause: The test accessed a variable before it was initialized. Fix at this level, not at the git init call.

Stack Trace Instrumentation

When you can't trace the call chain manually, add instrumentation:

# Before the problematic operation
import traceback

def git_init(directory: str) -> None:
    print(f"DEBUG git init:", file=sys.stderr)
    print(f"  directory: {directory}", file=sys.stderr)
    print(f"  cwd: {os.getcwd()}", file=sys.stderr)
    traceback.print_stack(file=sys.stderr)

    subprocess.run(["git", "init"], cwd=directory)

Critical: Write diagnostics to stderr (not a logger that may be suppressed in tests).

Run and capture:

pytest -v 2>&1 | grep "DEBUG git init"

Analyze stack traces:

• Look for test file names and line numbers
• Find the origin of each call
• Identify the pattern — same test? same parameter?

Key Pattern: Backward Tracing

Found immediate cause
    │
    ├── Can trace one level up?
    │   YES → what called this, with what values?
    │   │       │
    │   │       ├── Is this the source of the bad value?
    │   │       │   YES → fix at source
    │   │       │   NO  → trace further up
    │   │       │
    │   │       └── Can't trace up? Instrument with stack logging
    │   │
    │   NO → you're at a dead end
    │       Add instrumentation and re-run
    │
    └── NEVER fix just the symptom

Adding Stack Traces

When the call chain is unclear:

# Create a stack trace at the point of failure
stack = "".join(traceback.format_stack())
logging.debug("About to perform operation:\n%s", stack)

This reveals who called this code, with what arguments, in what context.

Minimizing the Trace

Once you have a stack trace showing the full chain:

1. Find the outermost caller that passes a bad value
2. Verify by tracing forward: if you fix this value, does the symptom disappear?
3. If yes — you've found the root cause. Fix at this level.
4. If no — there's another source upstream. Keep tracing.

Common Mistakes

• Fixing the innermost error — the place where the crash happens is rarely where the bug lives
• Stopping at "this library is broken" — trace further: are you using it correctly?
• Adding null checks downstream — downstream validation hides bugs; upstream fixes eliminate them
• Skipping instrumentation — "I know what this value should be" is not evidence. Log it and confirm.

Real Example

Symptom: .git directory created inside source code directory

Trace:

1. git init ran in process.cwd() ← empty cwd parameter
2. WorkspaceManager called with empty project directory
3. Session.create() passed empty string
4. Test accessed context.temp_dir before setUp
5. setup_core_test() returned { temp_dir: "" } initially

Root cause: Top-level variable initialization accessing an empty value before setup.

Fix: Made temp_dir a getter that raises if accessed before setUp. Also added validation at each level — the same bug is now structurally impossible.

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