undo task template merge with a task linked twice

Recipe 022 writes every old template_task before the old task_template. When the snapshot has two Tasks on the same template task (the site allows it), the apply that follows re-syncs and wires only one, the server's pick, and 11 of 14 times it was the Task that held no edge (probe 112). Relink one Task per template task before the entity write, the one that held the edges, and the rest after it, in the same batch. A template_task write on its own changes nothing else (probe 096).

Call

# snapshot() from recipe 019 before the merge (its KEEP holds content, which the apply renames on the
# Task it re-syncs), plus the Task ids its edges touch, read with fields ["task", "dependent_task"]:
#   snap["edged"] = {i for d in deps for i in (link(d, "task")["id"], link(d, "dependent_task")["id"])}
# Recipe 022's undo_merge builds `reqs`; replace its step 1 and step 2 with:


def claims_around_entity_write(entity, snap, now):
    """Step 1 and 2 of recipe 022, with a second link to one template task deferred past step 2."""
    before, first, late, seen = snap["tasks"], [], [], set()
    # the Task that held edges goes first, then the lowest id
    order = sorted(now, key=lambda t: (t["id"] not in snap["edged"], t["id"]))
    for t in order:
        old = before.get(t["id"])
        if not old:
            continue
        target = (old["template_task"] or {}).get("id")
        dup = target is not None and target in seen
        seen.add(target)
        if (link(t, "template_task") or {}).get("id") != target:
            (late if dup else first).append(upd("Task", t["id"], {"template_task": old["template_task"]}))
    return first + [upd(entity["type"], entity["id"], {"task_template": snap["task_template"]})] + late

Response

A: w, x, y; x on w, y on x.  xa (edges) and xb (none) both on A.x.  Merged into B: xa B.x, xb null
batch [xa A.x, Shot A, xb A.x, DELETE z] -> 200, 3 of 3
  xa re-synced, xa on w, y on xa; xb unchanged, linked to A.x, no edge; 4 Tasks, none made
the same with xb before Shot A: xb wired in 11 of 14, xa left with B's fields and no edge
then the snapshot's fields -> Tasks, links and fields equal the pre-merge read

Notes

• The end state equals the one-link undo, which is not the pre-merge state to the id: an edge the merge erased, a claimed Task on a Task outside the new template (probe 109), comes back new.
• Only the case of one Task that held A's edges and one that held none was measured. Two Tasks that both held edges: not measured.
• The deferred link leaves the second Task pointing at the template task, so a later apply of A meets the same pick (probe 106). Leave it null instead if nothing reads the link.