Build/droplet loses its lifecycle_data when a shared row is deleted

Summary

buildpack_lifecycle_data and cnb_lifecycle_data rows can be shared between a build and a droplet: during staging a single row ends up with both build_guid and droplet_guid set, pointing at two live parents. Both parents declare an app-level :destroy dependency on that row.

This document describes the issue, inventories the current delete associations and DB foreign keys, and lays out two distinct ways forward.

The issue

How a row becomes shared

The buildpack/CNB staging path creates one lifecycle_data row and attaches it to both parents:

1. A build is created; its lifecycle_data row is created owned by the build (build_guid set).
2. DropletCreate#find_or_create_buildpack_droplet creates the droplet and attaches the build's same row to the droplet by setting droplet_guid on it.

The result is one row with build_guid and droplet_guid both set, both referencing live parents.

Aside: requested vs. detected buildpacks

The build's lifecycle_data holds the requested buildpacks (what the user asked for at build time; empty = auto-detect). When staging finishes, StagingCompletionHandler#save_staging_result writes the detected buildpacks (what staging actually resolved) via droplet.buildpack_lifecycle_data.buildpacks = …. Because build and droplet currently share one row, that write overwrites the requested value - after staging, the (shared) row holds detected, not requested buildpacks. This is a subtle semantic difference, not treated as a bug today.

Why deletion leaves a dangling reference

Both parents remove the row on their own destruction, unconditionally:

• BuildModel: add_association_dependencies buildpack_lifecycle_data: :destroy, cnb_lifecycle_data: :destroy
• DropletModel: add_association_dependencies buildpack_lifecycle_data: :destroy, cnb_lifecycle_data: :destroy

add_association_dependencies … :destroy is an app-level hook (Sequel), not a DB constraint. When the droplet is destroyed it destroys the shared row - even though the build still references it - so the build is left with a dangling reference (build.buildpack_lifecycle_data → nil). The same happens in reverse: destroying the build destroys the shared row and leaves the still-live droplet with a dangling reference.

All deletion paths for a shared row

A shared row (build_guid and droplet_guid both set, both live) can be removed by any of the following. A live parent is left with a dangling reference whenever the row is destroyed while that parent still exists - the parent survives but its lifecycle_data is gone.

Trigger	Mechanism	Effect on the shared row	Dangling reference on a live parent?
Delete the build (BuildDelete)	DB ON DELETE CASCADE via build_guid FK and app-level :destroy on BuildModel	Row destroyed (DB cascade fires regardless of the Ruby hook)	Yes - the still-live droplet dangles (droplet.buildpack_lifecycle_data → nil)
Delete the droplet (DropletDelete)	App-level :destroy on DropletModel	Row destroyed	Yes - the still-live build dangles (build.buildpack_lifecycle_data → nil)
Delete the app (AppDelete#delete_subresources)	Deletes builds then droplets in order (BuildDelete, then DropletDelete)	Row destroyed when the build is deleted (first)	No lasting dangle - the droplet is deleted immediately after; but the build-delete-dangles-droplet mechanism is exercised here mid-transaction
Delete the app's own lifecycle_data	App owns a separate app_guid-keyed row, not the shared one	Shared row untouched	N/A - app-owned rows are single-parent and not involved in sharing

The critical asymmetry: both the build-delete and the droplet-delete paths leave the other live parent dangling. The droplet side has no DB FK (relies on the Ruby :destroy hook); the build side has an ON DELETE CASCADE FK, so it destroys the row at the database level - which no app-level fix can intercept. Any solution that keeps rows shared must reconcile both directions.

Inventory: delete associations

add_association_dependencies is app-level (fires in Ruby on Model#destroy); it is not enforced by the database and is bypassed by raw dataset deletes (Model.where(...).delete).

Parent model	Association	Target row	Dependency (app-level)
BuildModel	buildpack_lifecycle_data	buildpack_lifecycle_data	:destroy
BuildModel	cnb_lifecycle_data	cnb_lifecycle_data	:destroy
DropletModel	buildpack_lifecycle_data	buildpack_lifecycle_data	:destroy
DropletModel	cnb_lifecycle_data	cnb_lifecycle_data	:destroy
*LifecycleDataModel	buildpack_lifecycle_buildpacks	child buildpack rows	:destroy

Back-navigation (lifecycle_data.build / .droplet / .app) exists as many_to_one readers but is not used by production code to drive deletes.

The lifecycle_data readers on both parents already tolerate a missing row (buildpack_lifecycle_data || BuildpackLifecycleDataModel.new.freeze), with the comment "The lifecycle_data row can be destroyed independently of this" - i.e. the codebase already assumes the row's lifetime is decoupled from any single parent.

Inventory: DB foreign keys on the lifecycle_data tables

Owner columns are app_guid, build_guid, droplet_guid on both tables.

Table	Column	SQL foreign key?	on_delete
buildpack_lifecycle_data	build_guid	Yes - fk_buildpack_lifecycle_build_guid → builds.guid	cascade
buildpack_lifecycle_data	droplet_guid	No - index only	-
buildpack_lifecycle_data	app_guid	No - index only	-
cnb_lifecycle_data	build_guid	Yes - fk_cnb_lifecycle_build_guid → builds.guid	cascade
cnb_lifecycle_data	droplet_guid	No - index only	-
cnb_lifecycle_data	app_guid	No - index only	-
buildpack_lifecycle_buildpacks	buildpack_lifecycle_data_guid	Yes - fk_blbuildpack_bldata_guid → buildpack_lifecycle_data.guid	cascade
buildpack_lifecycle_buildpacks	cnb_lifecycle_data_guid	Yes - fk_blcnb_bldata_guid → cnb_lifecycle_data.guid	cascade

Key asymmetry: build_guid is DB-enforced with cascade on both tables, but droplet_guid is not - only an index. So at the DB level, deleting a build cascades to a build-owned row, but deleting a droplet relies entirely on the app-level :destroy hook.

Two ways forward

Both keep both delete orders safe (the hard requirement). They differ in whether they eliminate row-sharing or merely tolerate it, and in how much migration/backfill they require.

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Option A - Stop sharing: droplet gets its own copy + cascade FK

Each parent owns its own single-parent row. The staging path copies the build's lifecycle_data onto a new droplet-owned row instead of attaching the build's row. A cascade FK on droplet_guid becomes the DB-level backstop.

Changes

1. Forward fix (app/actions/droplet_create.rb) - find_or_create_buildpack_droplet copies stack, buildpacks (rebuilding the child buildpack_lifecycle_buildpacks rows), and - for CNB - the encrypted registry_credentials_json, onto a new droplet-owned row. Uses the create(..., droplet:) association-arg form to avoid a MySQL concurrent-insert deadlock (mirrors DropletCopy).
2. Model validate - reject any row with more than one owner column set ([app_guid, build_guid, droplet_guid].compact.length > 1). Single-parent becomes an invariant the model refuses to violate.
3. Migration - add on_delete: :cascade FK on droplet_guid for both tables, with MySQL collation alignment; delete rows whose droplet_guid points at a now-deleted droplet (NULL-guarded so app/build-owned rows survive).
4. Backfill - for every pre-existing shared row (build_guid and droplet_guid both set on a live pair), split it: create a droplet-owned copy and clear droplet_guid on the build's row.

Order within the migration: backfill (split shared rows) → delete rows with a dangling droplet_guid → MySQL collation alignment → add the droplet_guid cascade FK. The FK must be added last, once every row is single-owned.

Pros

• Eliminates the shared-row design entirely; every row has exactly one owner.
• DB-enforced backstop: a raw dataset delete or future migration cannot silently reintroduce a dangling reference - the FK guarantees droplet-owned rows die with their droplet and nothing else.
• Side-effect: because the droplet now has its own row, the staging-completion write lands on the droplet's copy - build's row keeps requested buildpacks, droplet's row holds detected. No production reader depends on this either way; it's a semantic tidy-up, not a requirement.

Cons

• Largest change. Copy logic + validate + migration + a data backfill of existing shared rows, in a specific order.
• Backfill risk. Copying inside a migration bypasses the model layer; CNB's encrypted registry_credentials_json must be copied as raw ciphertext + salt (not re-encrypted). Backfill cost scales with the number of existing shared rows in production CCDBs.
• Multi-step, order-dependent migration; must be idempotent.

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Option B - Tolerate sharing: set null FKs + destroy only when no parent remains

Keep rows shareable. Instead of unconditionally destroying the row when a parent is deleted, let the database null out the departing parent's guid and only destroy the row once no owner is left. No copy, no backfill.

Changes

1. FKs to set null - change the existing build_guid FKs (both tables) from on_delete: :cascade to on_delete: :set null, and add a droplet_guid FK also with on_delete: :set null. Deleting either parent then clears only its own guid column at the DB level; the other owner and the row survive.
2. Remove the unconditional :destroy dependencies on BuildModel / DropletModel for buildpack_lifecycle_data / cnb_lifecycle_data - the set null FK now handles detaching this parent. No model hooks are added.
3. Reap ownerless rows with a background job. With the :destroy hooks gone, a row whose last parent is deleted is left with all owner columns null and would leak. Add a periodic cleanup job (same pattern as pending_build_cleanup / orphaned_blobs_cleanup, registered in the clock scheduler) that deletes *_lifecycle_data rows where app_guid, build_guid, and droplet_guid are all null.
4. Migration - flip the two build_guid FKs to set null, add the droplet_guid set null FK (with MySQL collation alignment). No shared-row split needed - set null tolerates the existing two-owner rows.

Pros

• No data backfill. Existing shared rows keep working; set null handles them on delete. Migration only alters FK definitions.
• No model hooks / no delete-path changes. Detaching is DB-enforced; the only new code is a standalone cleanup job, independent of the delete flows.
• Symmetric by construction - build-delete and droplet-delete both just clear their own guid at the DB level.
• Both delete orders safe.

Cons

• Ownerless rows exist transiently. Between a parent's deletion and the next cleanup-job run, a fully-detached row sits in the table. Harmless (no live parent references it) but not immediately reclaimed.
• Still a schema change (FK redefinition on three columns), though no data migration.

Side-effect: shared rows persist, so the staging-completion write still lands on the shared row - build.lifecycle_data.buildpacks continues to reflect detected rather than requested (same as today). Unlike Option A, the requested/detected separation is not gained. No production reader depends on it.

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Comparison

Dimension	Option A (copy + cascade FK)	Option B (set null FKs)
Eliminates row sharing	Yes	No (tolerates it)
Schema change / migration	Yes	Yes (FK redefinition only)
Data backfill of existing rows	Required	Not needed
Detach-on-parent-delete enforced by	DB cascade FK	DB set null FK
Ownerless-row cleanup	N/A (single owner)	Background cleanup job
Fixes existing shared rows	Via backfill	Via set null on delete
Change size / steps	Large, multi-step	Moderate
Requested vs. detected buildpacks separated	Yes	No
Model hooks / delete-path changes	Forward-copy in staging	None (job is standalone)

Recommendation

• If DB-enforced correctness and eliminating the shared-row design are the priority, Option A - accept the migration + backfill cost.
• If avoiding the data backfill is the priority and keeping shared rows is acceptable, Option B - the set null FKs make both delete directions safe at the DB level with no delete-path or model-hook changes, at the cost of a periodic background job that reaps ownerless rows.

Start by comparing the two proposed approaches, then inspect app/actions/droplet_create.rb, BuildModel, DropletModel, and the lifecycle-data foreign-key migrations. If Option B is chosen, also read the pending_build_cleanup or orphaned_blobs_cleanup pattern and the clock scheduler. Done means a decided, implemented approach that keeps both build-delete and droplet-delete paths safe.