edgeKind=attachesTo deletes producer-first, breaking GCP LB chain teardown

[naxty]

Summary

edgeKind = "attachesTo" orders deletes producer-first, which breaks teardown for resources whose cloud enforces referential integrity (delete the referencer before the referenced). This surfaced via a new conformance test for GCP::Compute::UrlMap (RFC-0043 annotated the whole GCE LB chain with attachesTo).

Opening for discussion on whether the fix belongs in the GCP schema, in core, or both.

Mechanic

internal/metastructure/changeset/changeset.go, buildDeleteDependencies:

• EdgeKindDefault (L344-347): depNode.LinkWith(dependentGroup) → consumer deleted first, producer second.
• EdgeKindAttachesTo (L305-308): dependentGroup.LinkWith(depNode) → consumer waits for producer → producer deleted first, consumer second.
• Comment L229-230 documents this ("the producer is deleted first"), and L363-364 notes attachesTo has no effect on create (it only inverts destroy). So the behavior is intentional/self-consistent — the question is whether the direction is right.

Why it breaks GCP

GCP enforces referential integrity across the LB chain:

ForwardingRule -> TargetProxy -> UrlMap -> BackendService -> HealthCheck

Each referencer must be deleted before what it references. With attachesTo, formae deletes e.g. the BackendService before the UrlMap that still points at it → GCP returns resourceInUseByAnotherResource → Destroy fails and cascades.

Confirmed by conformance: UrlMap with defaultService on attachesTo fails Destroy; switching that field to the default edgeKind makes the full lifecycle pass.

Cross-check: AWS plugin

formae-plugin-aws uses no edgeKind/attachesTo anywhere. The analogous ELBv2 chain (elasticloadbalancingv2/listener.pkl: loadBalancerArn, defaultActions[].targetGroupArn) and deeper chains (API Gateway RestApi → Resource → Method → Deployment/Stage) use plain String|formae.Resolvable references → default (consumer-first) edge → create and destroy cleanly. No producer-first case exists in that plugin.

Observation

attachesTo's only effect is producer-first destroy. For reference fields under referential integrity that is never valid (the referencer must go first), and the name "X attaches to Y" intuitively reads as "detach X first" (consumer-first) — opposite to the implementation. No plugin currently relies on producer-first ordering.

Options

1. Schema (plugin): change the GCP LB reference fields from attachesTo to default. (Done for UrlMap in formae-plugin-gcp PR fix: update expected values of e2e tests #68; remaining chain — target proxies, forwarding rules, regionUrlMap — pending this discussion.)
2. Core: if attachesTo was intended to mean consumer-first (or if producer-first has no real use case), correct the direction / semantics in changeset.go.

What was attachesTo intended for? Is there a resource relationship where producer-first destroy is correct (e.g. host-deletion auto-cleans an attachment)? That determines whether this is a schema mis-annotation or a core direction bug.