review-mr-post

name: review-mr-post description: Review a PETSc GitLab merge request and post the findings back as inline DiffNote comments (with apply-able suggestion blocks where possible). Use when the user asks to "post review comments on MR ", "leave inline comments on the MR", or "review and post" — anything that should land on GitLab, not stdout. argument-hint: <MR_IID | diff-file | empty for current branch>

Adhere to @CLAUDE.md while reviewing and drafting comments.

Identify and fetch

Follow @../review-mr/identify.md (Sections 1–3) to resolve <MR_IID>, fetch metadata, and check for drift.

Review

Follow @../review-mr/review-procedure.md (Sections 4–6) to classify and verify findings. Then continue below to filter and post.

7. Filter findings

Only post findings that have a concrete, actionable fix (a code change the author can apply). Do NOT post:

• Informational or observational notes ("just noting...", "no issue, but...")
• Findings that acknowledge correctness but flag theoretical fragility
• Style nits with no specific suggested change
• Comments on code that was not changed in the MR (surrounding context is for understanding, not reviewing)

Each posted comment opens a discussion thread the author must resolve — avoid noise.

8. Post inline comments as DiffNotes

Use the GitLab Discussions API with JSON input to create inline DiffNote comments.

IMPORTANT: Use --input - with -H "Content-Type: application/json" — the -f flag with bracket notation does NOT work for nested position fields.

For each comment, use this Python pattern:

import json, re, subprocess

body = "<comment body with optional suggestion block>"
# Guard against patterns that break GitLab suggestion blocks:
# 1. Trailing backslash breaks suggestion rendering
# 2. Triple backticks inside suggestion body (e.g., in markdown files)
suggestion_match = re.search(r'(```suggestion[^\n]*\n)(.*?)(```)\s*$', body, re.DOTALL)
if suggestion_match:
    opening, inner, _ = suggestion_match.groups()
    if inner.count('```') > 0 or inner.rstrip('\n').endswith('\\'):
        # Fall back to plain code block
        body = body.replace(opening, '```\n', 1)

payload = {
    "body": body,
    "position": {
        "position_type": "text",
        "base_sha": "<BASE_SHA>",
        "head_sha": "<HEAD_SHA>",
        "start_sha": "<START_SHA>",
        "new_path": "<file_path>",
        "old_path": "<file_path>",
        "new_line": <line_number_in_new_file>
    }
}
p = subprocess.run(
    ["glab", "api", "projects/:id/merge_requests/<MR_IID>/discussions",
     "-X", "POST", "--input", "-", "-H", "Content-Type: application/json"],
    input=json.dumps(payload), capture_output=True, text=True, check=True
)
assert '"DiffNote"' in p.stdout, f"Unexpected response: {p.stdout}"

9. Use GitLab suggestion blocks for concrete fixes

When a comment has a specific code fix, include a suggestion block in the body so the author can click "Apply suggestion":

```suggestion:-0+0
corrected line here
```

• -0+0 means replace just the target line (the new_line in the position). Use -N+M to expand the range to N lines before and M lines after the target line.
• CRITICAL: The suggestion body replaces the entire selected range. You MUST reproduce every line in the range, not just the changed ones. For example, suggestion:-2+0 selects 3 lines (2 before + the target); the body must contain all 3 lines (with your edits applied). Omitting unchanged lines will delete them. When in doubt, prefer suggestion:-0+0 targeting a single line.
• To insert a new line after the target, use suggestion:-0+0 and include both the original target line and the new line.
• To delete a line, use an empty suggestion block.
• Only use suggestions for concrete fixes. Use plain comments for design/architectural feedback.
• Only comment on lines that are part of the MR diff. Do not suggest changes to unchanged code that happens to be near the diff.

10. Line number mapping

• For new files: new_line = the line number in the file itself.
• For modified files: new_line = the line number in the new version of the file. Parse the @@ hunk headers to map correctly.

11. Verify

After posting, confirm each response has "DiffNote" to ensure comments appear inline on the Changes tab.