Louvain context API for v1.4.4

pkg/louvain, apoc/community.Louvain and the apoc.algo.louvain caller now take the caller's context and report a run's interruption as an error. The previous opaque signatures are retired; there is no forwarding adapter and no optional context in configuration.

func louvain.Run(ctx context.Context, graph *Graph, options Options) (Result, error)
func louvain.RunUndirected(ctx context.Context, size, relationshipCount int,
    relationship func(int) (int, int, float64, bool), options Options) (Result, error)
func community.Louvain(ctx context.Context, nodes []*Node, rels []*Relationship,
    config LouvainConfig) ([]CommunityResult, error)

louvain.Final(Result) is unchanged: a stopped run returns the zero Result, so callers must check the error before reading a partition. Standalone callers choose context.Background() themselves; callApocAlgoLouvain passes the statement context, and the registered apoc.community.louvain handler receives it through the existing plugin dispatch.

What the context observes

The engine checks the context before every relationship it adapts, before it reserves the adjacency buffer those relationships size, before every adjacency read, and at each of its own work boundaries: per level, per pass, per colour class, per node of a moving pass, per node of a modularity or aggregation sweep, and per node of the output pass. The passes a level runs over every node also check per node: the degree total, the initial membership and representative map, the representative numbering, the dense renumbering, and the level's membership together with the copy it publishes. A scan whose length one node's arcs or candidate communities control checks once per stride (the checkStride constant, 1024 steps): a node's arcs in the moving pass, its candidate communities, its arcs in a modularity or aggregation sweep, its neighbours while colouring, and the distinct targets an aggregate emits. apoc.algo.louvain additionally checks before each node's adjacency enumeration, so a cancelled call stops reading adjacency instead of finishing the population pass.

A stopped call returns the caller's error unchanged (context.Canceled or context.DeadlineExceeded) and publishes nothing: apoc.algo.louvain returns no rows, community.Louvain returns no communities, and louvain.Run returns the zero Result. The statement's own cancellation gate and final observation are unchanged.

What stays uninterruptible

Each check is a boundary, not a preemption point. Work already in flight completes: the relationship callback currently running (in apoc.algo.louvain, one ReadAdjacentEdges call), at most one stride of the arc, candidate or neighbour scan in flight, the sort.Ints calls that order a node's neighbours and an aggregate's targets, the math.Pow of a pass, and the reservation NewGraph makes once a level's arc counts are complete (the checks before it run first). A cancelled run therefore stops at the next boundary rather than inside one, and the returned error is observed by whichever boundary comes first. louvain.Renumber is a caller-facing helper without a context and is not part of a run, and NewGraph keeps its exported signature, so a reservation it is asked to make is never interruptible once started.

Unchanged surface

Cypher, HTTP and Bolt syntax, apoc.algo.louvain's (nodes, relationshipTypes, config) arguments, community identifiers and their numbering, the weighted, parallel, self-loop, seeded and tie behaviour, Badger keys, persisted values, WAL and the storage version are unchanged; the existing GDS reference partitions still match exactly. No dependency is patched. Go consumers and the bundled plugin must be rebuilt against the new signatures.