mesh-host/internal/apply/apply.go

package apply

import "fmt"

// Result is what one apply did: which resources it brought to the declared state, and which it
// removed because they were applied before and are no longer declared.
type Result struct {
	Applied []Record
	Removed []Record
}

// Apply makes this machine match decl, records what it did, and removes what it once applied and
// decl no longer names.
//
// The three properties that govern it are each a recorded decision, not a preference:
//
//   - A failed step fails the apply (ADR 0008). This function stops at the first resource it
//     cannot bring to state and returns the error. It does not log and continue: a partial apply
//     that reports success is the mesh's most expensive shape.
//   - What was applied is recorded after it works, never before (ADR 0035). Each success is
//     appended to what the store will hold; a failure leaves the machine in whatever state it
//     reached, and the store is saved reflecting exactly that — never more.
//   - The host is authoritative over its own footprint and inert everywhere else (ADR 0043).
//     Removal touches only resources the store recorded as created by the host.
//
// identity is this node's own name. A declaration addressed to another node is refused; a host
// with no identity yet — the first node — applies whatever it is handed, because it has nothing
// to check against (ADR 0043).
func Apply(decl Declaration, identity string, store *Store, appliers map[string]Applier) (Result, error) {
	if decl.For != "" && identity != "" && decl.For != identity {
		return Result{}, fmt.Errorf(
			"declaration is for %q and this node is %q — refused, a node applies only what is "+
				"addressed to it", decl.For, identity)
	}

	prior := store.Records()
	declared := make(map[string]bool, len(decl.Resources))
	for _, r := range decl.Resources {
		declared[r.ID] = true
	}

	// Who created what, carried across applies. "created" must be sticky: once the host brought a
	// resource into being, it stays the creator through every re-apply, or a second apply would
	// see the resource already present, record created=false, and then decline to remove
	// something it in fact created. That flip would leak a host-created resource on the next
	// declaration that drops it — reported handled, actually orphaned.
	priorCreated := make(map[string]bool, len(prior))
	for _, rec := range prior {
		priorCreated[rec.ID] = rec.Created
	}

	// Apply in the stated order, recording each success as it lands.
	applied := make([]Record, 0, len(decl.Resources))
	for _, r := range decl.Resources {
		a, ok := appliers[r.Type]
		if !ok {
			// Parse already refused unknown types, so this is a host wired inconsistently with
			// its own shape table — a bug, surfaced rather than skipped.
			err := fmt.Errorf("resource %q is a %q with no applier — refusing", r.ID, r.Type)
			saveMerged(store, prior, applied)
			return Result{}, err
		}
		created, err := a.Apply(r)
		if err != nil {
			// The resource is not at the declared state. Record what did land (this one did not,
			// so it is not appended), persist that, and fail.
			saveMerged(store, prior, applied)
			return Result{}, fmt.Errorf("applying %s %q: %w", r.Type, r.ID, err)
		}
		applied = append(applied, Record{
			ID: r.ID, Type: r.Type, Path: r.Path(),
			Created: created || priorCreated[r.ID],
		})
	}

	// Remove what was applied before and is no longer declared, in reverse application order so
	// a file goes before the directory that held it. Only host-created resources are touched.
	removedIDs := map[string]bool{}
	var removed []Record
	for i := len(prior) - 1; i >= 0; i-- {
		rec := prior[i]
		if declared[rec.ID] {
			continue
		}
		if rec.Created {
			if a, ok := appliers[rec.Type]; ok {
				if err := a.Remove(rec); err != nil {
					// A removal that failed leaves the resource present. That is a failed step,
					// so the apply fails — and the store must reflect reality: the declared set
					// that landed, plus every undeclared prior record not yet removed (this one
					// included), in application order.
					store.replace(survivorsAfterFailedRemoval(applied, prior, declared, removedIDs))
					_ = store.Save()
					return Result{}, fmt.Errorf("removing %s %q: %w", rec.Type, rec.ID, err)
				}
			}
		}
		removedIDs[rec.ID] = true
		removed = append(removed, rec)
	}

	// Success: the store now holds exactly the declared set, freshly recorded.
	store.replace(applied)
	if err := store.Save(); err != nil {
		return Result{}, fmt.Errorf("apply succeeded but its record could not be saved: %w", err)
	}
	return Result{Applied: applied, Removed: removed}, nil
}

// survivorsAfterFailedRemoval is what the machine still holds when a removal fails: the declared
// set that was just applied, plus every prior undeclared record not successfully removed —
// including the one whose removal failed — kept in application order.
func survivorsAfterFailedRemoval(applied, prior []Record, declared, removedIDs map[string]bool) []Record {
	survivors := append([]Record{}, applied...)
	for _, rec := range prior {
		if declared[rec.ID] || removedIDs[rec.ID] {
			continue
		}
		survivors = append(survivors, rec)
	}
	return survivors
}

// saveMerged persists prior records overlaid with what was just applied, for the failure path:
// the machine holds both the untouched prior resources and the ones that landed before the
// failure, so the store must record both.
func saveMerged(store *Store, prior, applied []Record) {
	store.replace(mergeByID(prior, applied))
	_ = store.Save()
}

// mergeByID returns base with overlay applied on top, overlay winning on a shared id, preserving
// base order and appending overlay-only records.
func mergeByID(base, overlay []Record) []Record {
	byID := make(map[string]Record, len(overlay))
	for _, r := range overlay {
		byID[r.ID] = r
	}
	out := make([]Record, 0, len(base)+len(overlay))
	seen := map[string]bool{}
	for _, r := range base {
		if o, ok := byID[r.ID]; ok {
			out = append(out, o)
			seen[r.ID] = true
			continue
		}
		out = append(out, r)
	}
	for _, r := range overlay {
		if !seen[r.ID] {
			out = append(out, r)
		}
	}
	return out
}