Don't clear current stemcell on VM delete

[Ivaylogi98]

What

Removes the stemcellRepo.ClearCurrent() call from vm.Delete() (deployment/vm/vm.go), along with the now-unused stemcellRepo dependency it pulled onto the vm struct, NewVM/NewVMWithMetadata, NewManager, and NewManagerFactory.

Fixes #731.

Why

vm.Delete() unconditionally cleared current_stemcell_id (set it to "") whenever the old VM was torn down. Inside create-env's delete-then-recreate cycle, the intended sequence is:

1. vm.Delete() clears current_stemcell_id
2. new VM boots → cloudStemcell.PromoteAsCurrent() sets it to the new stemcell record
3. stemcellManager.DeleteUnused() reaps every record whose ID ≠ current

On the happy path, step 2 overwrites the clear from step 1, so it has no observable effect. But when the replacement VM never comes up (agent timeout, network issue, or a failure inside vmManager.Create before promote), step 2 is never reached and bosh-state.json is persisted with current_stemcell_id: "" while the stemcell record and its IaaS image remain.

On the next create-env run, FindUnused (stemcell/manager.go) treats every record as unused when the current pointer is empty (found == false), and DeleteUnused deregisters the still-in-use image (e.g. an AWS AMI). Every subsequent create_vm that references it then fails:

CPI 'create_vm' method responded with error:
CmdError{"type":"Bosh::Clouds::CloudError","message":"could not find AMI 'ami-xxxxxxxxxxxxxxxxx'","ok_to_retry":false}

This is more likely to surface on unattended pipelines that auto-retry a failed create-env.

The clear was always redundant

PromoteAsCurrent (stemcell/cloud_stemcell.go) calls repo.UpdateCurrent(id) unconditionally — it never reads the prior value — so the clear in vm.Delete() contributed nothing on the success path. It was introduced in bd573fe8 (Nov 2014) as defensive symmetry (clear on teardown, set on build), but even in that original code PromoteAsCurrent ran before DeleteUnused, so the clear only ever had an effect in the failure window, where it is purely destructive.

With it removed, a failed deploy leaves current_stemcell_id pointing at the stemcell the deployment is configured to use, so DeleteUnused leaves it alone. A genuinely superseded stemcell is still reaped — but only after a successful deploy where PromoteAsCurrent moves the pointer to a newer record.

Scope / not affected

• delete-env (deployment.Delete()) deletes stemcells through an explicit cloudStemcell.Delete() step, independent of vm.Delete() — unchanged.
• bosh delete-vm is a director API command (director.Deployment.DeleteVM) and never touches the local stemcell repo — unchanged.
• stemcell/manager.go and config/stemcell_repo.go (ClearCurrent is still used by cloudStemcell.Delete()) are left as-is. The VM.Delete() interface signature is unchanged.

Testing

• Dropped the two clears current stemcell in the stemcell repo cases in deployment/vm/vm_test.go (they asserted the removed behavior).
• go build ./... and go test ./deployment/vm/... ./stemcell/... ./config/... pass.

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Walkthrough

The VM and manager constructors no longer accept or store stemcellRepo. Manager factory wiring and VM construction call sites were updated. vm.Delete no longer clears the current stemcell repository. Tests were updated to match the new constructors and deletion behavior.

Suggested reviewers: aramprice

Priority: ⬆️ High

Severity of issue fixed: High

Merge Risk: ⚪ Minimal · up to a3f57

The current stemcell remains selected after VM deletion, so failed replacement attempts will not make the active image eligible for cleanup.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 1 functions across 5 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly states the primary change: preventing VM deletion from clearing the current stemcell. It is concise and specific.
Description check	✅ Passed	The description directly explains the bug, the code changes, the failure scenario, scope, and testing. It is fully related to the changeset.
Linked Issues check	✅ Passed	The changes satisfy issue #731. vm.Delete() no longer calls stemcellRepo.ClearCurrent(), so a failed VM recreation retains current_stemcell_id. The change removes the unused dependency from VM a…
Out of Scope Changes check	✅ Passed	The changes stay within issue #731. Constructor updates and call-site updates support removal of the destructive stemcell clear. The regression test verifies the required failure behavior. No unrelate…

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🟡 Changes recommended

Three unchanged test call sites no longer compile, and the failure-and-retry regression lacks coverage.

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Pull request overview

Prevents failed VM recreation from clearing the current stemcell reference and accidentally deleting its cloud image.

Changes:

• Removes stemcell repository access from VM deletion.
• Removes the dependency throughout VM construction.
• Deletes obsolete assertions for stemcell clearing.

File summaries

File	Description
deployment/vm/vm.go	Stops clearing the current stemcell during VM deletion.
deployment/vm/vm_test.go	Removes obsolete dependency and assertions.
deployment/vm/manager.go	Removes stemcell repository plumbing.
deployment/vm/manager_test.go	Updates constructor calls.
deployment/vm/manager_factory.go	Simplifies manager factory dependencies.
cmd/env_factory.go	Updates production factory construction.

Review details

• Files reviewed: 6/6 changed files
• Comments generated: 2
• Review effort level: Balanced

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Actionable comments posted: 1

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Inline comments:
In `@deployment/vm/vm_test.go`:
- Around line 588-605: Replace the vacuous vm.Delete() regression test with a
deployer test that uses real VM and stemcell repositories backed by the same
DeploymentStateService. Persist and select an old stemcell, force replacement VM
creation to fail, call deployer.Deploy, and immediately assert
stemcellRepo.FindCurrent() still returns the old record; do not rely on
DeleteUnused.

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📒 Files selected for processing (4)

• deployment/deployment_test.go
• deployment/manager_test.go
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