Drummer

The collective that processes in rhythm. Holistically review pending PRs, then squash-merge them as a cohesive batch.

Invocation

/drummer

Flow

1. Read dive context (if available) for project background:

   cat .wm/dive_context.md 2>/dev/null || echo "No dive context"
   

   This provides architecture decisions, conventions, and session intent.

2. Find all PRs with drummer-merge label:

   gh pr list --label drummer-merge --json number,title,headRefName,baseRefName,mergeable,additions,deletions
   

   Only PRs with the drummer-merge label are eligible for merge. This finds ALL labeled PRs, not just those for claimed tasks.

3. Build dependency graph and identify stacks:

   • Create adjacency list: baseRefName → [PRs targeting it]
   • Find root PRs: those where baseRefName = main (or master)
   • Identify stacks: chains where child PRs target parent PR branches
   • Detect cycles: If a branch eventually targets itself, report error and skip
   • Example graph:

     main ← PR #42 (ba/abc-123) ← PR #43 (ba/abc-456)
     main ← PR #44 (ba/xyz-789)  [separate stack]
     

4. Select stack to process:

   • If multiple independent stacks exist, pick first by lowest root PR number (FIFO by age)
   • If multiple PRs target the same base branch, order by PR number (lowest first)
   • Report other stacks as "queued for next run"
   • Process only one stack per invocation to keep merges atomic
   • Orphaned children: If a child PR targets a branch that doesn't exist (parent merged externally), update its base to main and treat as a root

5. Batch review - evaluate all PRs in the selected stack together:

   • Collect combined diff of all PRs against main
   • Run sg review on the combined changes
   • Evaluate:
     • Do changes conflict logically? (same code modified differently)
     • Is there duplicate work? (two PRs solving same problem)
     • Do changes compose well? (feature A + feature B = coherent whole)
     • Any cross-cutting concerns? (shared dependencies, API changes)
   • If concerns found:
     • Report issues
     • Ask human whether to proceed or address first

6. Merge stack in dependency order (root first, then children):

   For each PR in the stack, starting from the root:

   a. Verify CI is passing:

   gh pr checks <pr-number> --fail-on-error
   

   If CI is failing, stop and report error.

   b. Rebase onto its target branch:

   git fetch origin
   gh pr checkout <pr-number>
   git rebase origin/<base-branch>  # main for root, parent branch for children
   

   c. If .ba/ conflict, resolve mechanically (each task = one line)

   d. Push rebased branch:

   git push --force-with-lease
   

   e. Squash merge:

   gh pr merge <pr-number> --squash
   

   f. For child PRs in the stack (after parent merged):

   • Update base branch to main:

     gh pr edit <child-pr-number> --base main
     

   • Rebase child onto main:

     gh pr checkout <child-pr-number>
     git rebase origin/main
     git push --force-with-lease
     

   • Force-push triggers new CI run; batch review + parent merge provides confidence
   • Now child PR targets main and is rebased, continue to merge it (step a-e)

   g. Verify task closure - After each merge:

   git pull origin main
   # Extract task ID from branch name: ba/abc-123 → abc-123
   # Note: Assumes branch follows ba/<task-id> convention from mouse skill
   task_id="${headRefName#ba/}"
   ba show $task_id  # Check status
   # If not closed, fix it:
   ba finish $task_id
   # If any fixes were needed:
   git add .ba/ && git commit -m "fix: close task ${task_id} after merge" && git push origin main
   

   Squash merge can lose .ba/ changes during conflict resolution.

   h. On merge failure - If any PR in the stack fails to merge:

   • Stop processing the stack
   • Report which PRs were merged successfully and which failed
   • Signal error: already-merged PRs stay merged, failed PR remains open
   • Next drummer run will see the failed PR as a new root (its parent is now in main)

7. Signal completion (MANDATORY) - This is the LAST thing you do:

   # On success:
   curl -sS -X POST "http://localhost:${MIRANDA_PORT}/complete" \
     -H "Content-Type: application/json" \
     -d "{\"session\": \"$TMUX_SESSION\", \"status\": \"success\"}"
   
   # On error:
   curl -sS -X POST "http://localhost:${MIRANDA_PORT}/complete" \
     -H "Content-Type: application/json" \
     -d "{\"session\": \"$TMUX_SESSION\", \"status\": \"error\", \"error\": \"<reason>\"}"
   

   If you don't signal, Miranda won't know you're done and the session becomes orphaned.

Stacked PRs

When PRs target other PR branches (not main), drummer detects the stack and processes it:

Detection:

gh pr list --label drummer-merge --json number,title,headRefName,baseRefName,mergeable

• PRs with baseRefName = main are roots
• PRs with baseRefName = ba/<task-id> are children targeting that parent

Graph building:

adjacency[baseRefName] = [list of PRs targeting it]

Example:
  adjacency["main"] = [PR #42, PR #44]
  adjacency["ba/abc-123"] = [PR #43]

Stack 1: main ← #42 (ba/abc-123) ← #43 (ba/abc-456)
Stack 2: main ← #44 (ba/xyz-789)

Merge sequence (for Stack 1):

1. Merge #42 to main
2. Update #43: gh pr edit 43 --base main
3. Rebase #43 onto main: git rebase origin/main && git push --force-with-lease
4. Merge #43 to main

After processing:

Before:  PR #43 → ba/abc-123 → main
         PR #42 → main
         PR #44 → main (separate stack)

After:   PR #42 merged to main
         PR #43 rebased onto main, merged to main
         PR #44 remains for next drummer run

Batch Review Criteria

The holistic review checks what individual PR reviews can't:

• Logical conflicts: PR A assumes X, PR B assumes not-X
• Duplication: Both PRs add similar functionality
• Integration issues: Combined changes break something neither breaks alone
• Ordering dependencies: PR B depends on PR A being merged first (auto-detected for stacked PRs)
• Scope creep: Batch as a whole does more than originally intended

Conflict Resolution

.ba/issues.jsonl is line-per-task:

• Each line is independent
• Resolution: union of all lines (dedupe by task ID)
• Never lose a task closure

Prerequisites

• PRs must have the drummer-merge label (human approval gate)
• PRs must have CI passing
• No code conflicts (only .ba/ conflicts handled automatically)
• Batch review must pass (or human override)

Note: The drummer-merge label must be created in the repo. This is opt-in per repo.

Exit Conditions

• Success: Selected stack fully merged → signal status: "success"
• Partial success: Some PRs in stack merged, then failure → signal status: "error" with details
• Needs attention: Batch review raised concerns - waiting for human decision (no signal - awaiting input)
• Error: Unrecoverable failure (code conflicts, CI failing, cycle detected) → signal status: "error" with message
• No work: No PRs with drummer-merge label found → signal status: "success" (nothing to do)

Example

Basic (no stacks)

$ /drummer

Finding PRs with drummer-merge label...
Found 2 PRs:
  PR #42 "Fix validation bug" → main (CI ✓)
  PR #44 "Refactor validator" → main (CI ✓)

Building dependency graph...
  Stack 1: main ← #42
  Stack 2: main ← #44
  2 independent stacks, processing Stack 1

Running batch review on Stack 1 (1 PR)...
Batch review complete: ✓ No issues

Processing PR #42 (Fix validation bug)...
  Verifying CI... ✓
  Rebasing onto main... clean
  Squash merging... ✓
  Verifying task closure... ✓

Merge complete.
  Merged: 1 PR (#42)
  Remaining: 1 PR (#44 - queued for next run)

Signaling completion...
Done.

Stacked PRs

$ /drummer

Finding PRs with drummer-merge label...
Found 3 PRs:
  PR #42 "Fix validation bug" → main (CI ✓)
  PR #43 "Add edge case tests" → ba/abc-123 (CI ✓)
  PR #44 "Refactor validator" → main (CI ✓)

Building dependency graph...
  Stack 1: main ← #42 (ba/abc-123) ← #43 (ba/abc-456)
  Stack 2: main ← #44
  2 stacks found, processing Stack 1 (2 PRs)

Running batch review on Stack 1...
Collecting diffs: +547 -103 across 8 files
Batch review complete: ✓ No issues

Processing stack root: PR #42 (Fix validation bug)...
  Verifying CI... ✓
  Rebasing onto main... clean
  Squash merging... ✓
  Verifying task closure... ✓

Processing stack child: PR #43 (Add edge case tests)...
  Verifying CI... ✓
  Updating base branch to main... done
  Rebasing onto main... clean
  Conflict in .ba/issues.jsonl (expected)
  Resolving: keeping all task closures
  Squash merging... ✓
  Verifying task closure... ✓

Stack merged.
  Merged: 2 PRs (#42, #43)
  Remaining: 1 PR (#44 - queued for next run)

Signaling completion...
Done.