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feat(FN-4071): add overlap guard to prevent concurrent merge conflicts

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Implements an overlap guard for the merger that prevents concurrent merges of tasks with conflicting file changes, wired through a new overlap-aware fallback strategy and exposed via a dashboard setting. The feature includes a 406-line test suite for the overlap guard, a 203-line lifecycle integrati

Fusion-Task-Id: FN-4071
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Fusion
2026-05-12 09:02:55 -07:00
committed by gsxdsm
parent 24cecb76b9
commit 0d0796ab7b
14 changed files with 1043 additions and 74 deletions
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docs/settings-reference.md
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@@ -180,6 +180,8 @@ Defaults from `DEFAULT_PROJECT_SETTINGS`; key scope from `PROJECT_SETTINGS_KEYS`
| `overlapIgnorePaths` | `string[]` | `[]` | Optional project-relative file or directory paths to exclude from overlap blocking (for example `docs` or `generated/openapi.json`). Entries are trimmed, deduplicated, and must not be absolute or contain `..` traversal. |
| `autoMerge` | `boolean` | `true` | Auto-finalize tasks from `in-review`. |
| `mergeStrategy` | `"direct" \| "pull-request"` | `"direct"` | Completion mode (local direct merge vs PR-first). |
| `mergeConflictStrategy` | `"smart-prefer-main" \| "smart-prefer-branch" \| "ai-only" \| "abort"` | `"smart-prefer-main"` | Controls the merger's conflict-resolution cascade. `smart-prefer-main` fast-forwards local main from `origin` when possible, then tries AI resolution, then auto-resolve heuristics, then a final `-X ours` fallback that prefers main unless the overlap guard below says otherwise. `smart-prefer-branch` uses the same cascade but ends with `-X theirs` so the task branch wins. `ai-only` never silently picks a side, and `abort` stops after the first AI attempt. Legacy `smart` / `prefer-main` values are normalized automatically. |
| `mergeStrategyOverlapBehavior` | `"flip-to-prefer-branch" \| "warn-only" \| "ignore"` | `"flip-to-prefer-branch"` | Safety control for `mergeConflictStrategy="smart-prefer-main"`. Before the Attempt 3 `-X ours` fallback, Fusion checks whether the task branch and recent `main` history overlap on the same files (30-commit lookback, matching the squash audit heuristics). `flip-to-prefer-branch` makes overlapping files prefer the task branch so hardening is not silently discarded (the FN-3936 class of regression). `warn-only` logs the overlap but keeps the legacy main-wins fallback. `ignore` disables the overlap guard and preserves legacy behavior exactly. |
| `pushAfterMerge` | `boolean` | `false` | Auto-push to remote after successful direct merge. Includes pulling latest and AI conflict resolution. |
| `pushRemote` | `string` | `"origin"` | Git remote (and optional branch) to push to after merge. |
| `worktreeInitCommand` | `string` | `undefined` | Shell command run after worktree creation. For pnpm repos, prefer `pnpm install --frozen-lockfile` for deterministic bootstrap. |