Torch Bisect

Phase 2: Setup

1. Check GitHub connectivity: run curl -s -o /dev/null --connect-timeout 20 -w "%{http_code}" https://github.com. If it fails or returns non-200, tell the user GitHub is not reachable and suggest checking internet connectivity or proxy settings. On Meta internal servers, suggest setting https_proxy=http://fwdproxy:8080. Do not proceed until connectivity is confirmed.
2. cd <pytorch> && git fetch origin (timeout 300000ms). On failure, ask user to fix (remind about proxy/fwdproxy if it looks like a network error).
3. Check git bisect log 2>&1 — if a bisect is active, ask user to reset or abort.
4. Resolve the good commit:
   • Hash: verify with git log -1 <hash>
   • Date: git log origin/main --before="<DATE>T23:59:59" --format="%H %s" -1 — show result and confirm with user.
5. Bad commit is origin/main. Show it to user.
6. Show git rev-list --count <good>..origin/main and estimated steps (~log2).
7. Run each bisect command individually so failures can be reported cleanly:
   • git bisect start
   • git bisect bad origin/main
   • git bisect good <good> If any step fails, show the error and run git bisect reset before asking the user how to proceed.

Phase 3: Bisect Loop

Repeat until bisect finds the first bad commit:

A. Build — run build command in PyTorch dir (timeout 600000ms). On failure: show last 50 lines, ask user to Skip / Retry / Abort.

B. Test — run test command in TorchTitan dir (timeout 600000ms). Exit 0 = good, non-zero = bad. On timeout, follow user's policy from Phase 1. Never analyze logs — use only exit code.

C. Record — git bisect good or git bisect bad in PyTorch dir.

• Output contains "is the first bad commit" → done, go to Phase 4.
• Output contains "Bisecting:" → continue loop.
• Cannot narrow further (too many skips) → tell user, suggest abort.

D. Progress — print step number, commit hash, good/bad, remaining count.

Phase 4: Report and Cleanup

1. Capture git bisect log and git show --stat <bad_commit>.
2. Extract PR number from commit subject ((#NNNNN) pattern). If found, run gh pr view <N> --repo pytorch/pytorch --json title,body,url to get PR details. Fallback URL: https://github.com/pytorch/pytorch/pull/<N>.
3. git bisect reset.
4. Present summary:

## Bisect Complete
**First bad commit:** <hash>
**Commit message:** <subject>
**Author:** <author> | **Date:** <date>

### Associated Pull Request
**PR:** #<N> — <title>
**Link:** https://github.com/pytorch/pytorch/pull/<N>
**Summary:** <first ~20 lines of PR body>

### Changed files
<git show --stat output>

### Full bisect log
<git bisect log output>

Omit the PR section if no PR number was found. Do NOT diagnose root cause.

Rules

• Always use absolute paths.
• Judge test results by exit code only — never analyze output.
• On unexpected errors, show the error and ask the user before proceeding.
• Keep the user informed after every bisect step.