Harden vGPU release ordering in the instance lifecycle

[yummybomb]

Summary

Layer 2 of the vendor VFIO vGPU stack (#366 ← this ← #363 ← #364 ← #321). Where #366 is a behavior-preserving refactor, this layer changes lifecycle semantics — release ordering and retention — still on mdev-era machinery, before the vendor VFIO backend lands above:

• Delete releases the vGPU before other teardown — a successful release persists the cleared assignment before later teardown. A failed release keeps the pre-refactor contract: it is logged and the delete continues, since the release guards never destroy a device they cannot prove is unowned and a skipped release is recovered by startup reconciliation. Stopped instances retry a previously failed release.
• Stop keeps its no-op contract — a failed retained-release on stop logs and retains the assignment metadata instead of failing the stop; assignments are passed to DestroyVGPU as a struct.
• Forks don't inherit retained assignments — a retained (failed-release) vGPU assignment is stripped from fork metadata so a fork can't release its parent's device.
• Start persists a stale release immediately — releasing a stale assignment before start writes the cleared metadata at once, so a later start failure can't leave metadata pointing at a device that's already gone.

Testing

• go build ./..., go vet clean
• go test -race targeted lib/instances lifecycle suites pass

---

Note

Medium Risk
Changes ordering and persistence of GPU teardown and restart-policy blocking on delete; failures are mostly best-effort but incorrect behavior could leak vGPU slots or leave stale metadata until reconciliation.

Overview
Tightens vGPU assignment handling across instance lifecycle so metadata, forks, and snapshots cannot double-release or point at devices that are already gone.

Delete blocks restart policy before teardown (with a metadata reload so later saves keep that marker), then releases vGPU before network, devices, and volumes; successful release is persisted early, while failures stay best-effort and delete continues.

Stop no longer clears assignment fields on failed release—it retains metadata for retry. Stop on an already-stopped instance retries via releaseRetainedVGPULocked. Start releases any stale retained assignment and persists cleared fields before the rest of start.

Fork (instance and snapshot) strips live vGPU assignment from fork metadata while keeping GPUProfile; restore snapshot keeps the instance’s current assignment instead of resurrecting the snapshot’s embedded copy.

Adds releaseRetainedVGPULocked and targeted lifecycle tests for ordering, persistence, and fork/restore behavior.

^{Reviewed by Cursor Bugbot for commit 82e452f. Bugbot is set up for automated code reviews on this repo. Configure here.}

[yummybomb]

Trimmed over-engineering after an architectural review against the parent task (host kernel 6.8 / Ubuntu 24.04 support):

• 1c1f52e — dropped the HasMdev/IsAllocated() compatibility shim. VirtualFunction is internal and never serialized in an API response or persisted, so a single Allocated field is enough.

Kept as-is:

• the delete/stop behavior change (fail and retain assignment metadata when release fails) — orphan reconciliation only runs at startup, so failing the delete is the correct fail-closed behavior;
• the darwin DestroyVGPU unknown-framework error — it keeps the fail-closed rollback semantics identical on macOS, which the lib/instances tests exercise in CI.

Tests: go test -race ./lib/devices, ./lib/resources, ./lib/hypervisor/qemu, ./lib/hypervisor/cloudhypervisor, and the vGPU-targeted ./lib/instances tests all pass; the remaining lib/instances E2E failures require KVM/images and fail identically on an unmodified checkout.

[cursor]

Cursor Bugbot has reviewed your changes using high effort and found 1 potential issue.

^{❌ Bugbot Autofix is OFF. To automatically fix reported issues with cloud agents, enable autofix in the Cursor dashboard.}

^{Reviewed by Cursor Bugbot for commit 7b1f995. Configure here.}