Artifact [e8c23ddf24]

Artifact e8c23ddf24c98ea7723ae443ca53545d4221d773363b77c9f8ee7a7d08297804:

• File internal/merge/doc.go — part of check-in [a23c7b12a9] at 2026-10-22 20:12:19 on branch feat/horizon-cones — fix(merge): choose merge bases as fossil's pivot_find does

  go-libfossil picked a merge base with a bidirectional breadth-first walk, the common ancestor nearest by edge count. fossil's merge uses pivot.c's pivot_find: a queue ordered by pending DESC, mtime DESC that expands the newest pending check-in first and returns the first one with a child queued from the other side, the most recent common ancestor by event time. The two differ on criss-cross merges and skewed histories, so a merge here could diff against another base than fossil's and write other bytes.

  merge.FindPivot ports pivot_find in memory: (rid, side) rows with REPLACE re-pending, ties broken as a reverse scan of aqueue_idx1 yields them (rid, then side, descending), a missing event mtime counted as 0.0, ignoreMerges following primary links only, and the walk bound of the horizon merge-base query kept as ErrBound. FindCommonAncestor and FindCommonAncestorWithin now ask it.

  Call sites: - Checkout.Merge takes fossil's pid: the merged-in version primary, the checkout's version and every vmerge id=0 merge the secondaries (merge.c 1076-1083), and gains Baseline, fossil's --baseline. The name pivot nid (pivot_find(1), merge.c 1089-1096) feeds only add_renames and the rename fill-in (merge.c 1192-1257); Merge does not follow renames, so it is not ported. Update and stash use the checkout version as their pivot and pick no base. - Repo.Merge (branch merge) makes the source branch tip the primary. - The origin's horizon-mergebase answer makes the merged-in check-in primary and declines to answer from a walk that reached history it does not hold. - Deepen's local deepen point keeps a boundary target among C's ancestors as its own point, since pivot_find seeds from check-ins only.

  Horizon client rule (draft-fossil-horizon-00 §5.5, agreed for the draft): a common ancestor found in part of the graph need not be the one the whole graph gives, so a merge asks the origin whenever the walk reaches a check-in the store does not hold, not only when the base it finds is not held. MergeBaseCrossing takes that from Pivot.Unheld, and MergeContext merges over the origin's base as a baseline, deepening only when the store does not hold it.

  Changed expectations: the horizon merge fixtures (hzStruct's y into feat2, structuralMergeSource, TestBranchMergeRefusesBaseBelowHorizon) fork f below the horizon, merge c2 into f's line and f into c2's: fossil picks c2, not f. On that shape fossil 2.28 answers fossil merge-base y x -> pivot=52d5923b8d (c2) with f=8ac24055 also a common ancestor. The fixtures now add a check-in on f's line newer than the horizon, which fossil does pick, to keep a base below the horizon, and the crossing tests check both the held-base walk that crosses and the base below. (user: [email protected], size: 732) [annotate] [blame] [check-ins using] [more...]

// Package merge provides three-way merge with pluggable strategies.
//
// Register strategies at init time via [Register], then look them up by
// name with [StrategyByName]. The built-in strategies are:
//
//   - ThreeWayText: fossil's line-level merge (merge3.c), byte for byte,
//     with its conflict markers
//   - Binary: treats any difference as a conflict
//   - LastWriterWins: always picks the remote side
//   - ConflictFork: creates a fork instead of merging
//
// [FindPivot] is fossil's pivot_find: it walks the plink DAG, newest
// check-in first, to the merge base fossil's merge chooses, and
// [FindCommonAncestor] asks it for two check-ins. [DetectForks]
// identifies open forks in a repository.
package merge