indexer_placement reads where modules run as the indexer wrote it, by node or by module (issue 1126, ADR 0549 §2)

modules/mesh-delivery-view/internal/view/read.go

@@ -20,6 +20,8 @@ import (
 const (
 	ModuleReadTool  = "indexer_module"
 	ModulesReadTool = "indexer_modules"
+	// PlacementReadTool reads the indexer's placement (novox/hq ADR 0549 §2, issue 1126).
+	PlacementReadTool = "indexer_placement"
 )
 
 // StateRead reads the indexer's modules state, as the view's reads grant it: its keys and a key's stored value (nil for a key with none).
@@ -39,6 +41,7 @@ type Reader struct {
 	WantsState       StateRead // the assigner's wants and holds
 	DifferencesState StateRead // the assigner's differences
 	HavesState       StateRead // the node-engine's haves
+	PlacementState   StateRead // the indexer's placement
 }
 
 // Module is the stored value of modules.<module>, exactly as the indexer wrote it.
@@ -613,3 +616,55 @@ func (r Reader) Differences(node, module string) (KeptAnswer, error) {
 func (r Reader) Haves(node, module string) (KeptAnswer, error) {
 	return r.kept(r.HavesState, "the haves", node, module)
 }
+
+// Placement is the indexer's placement as stored (ADR 0549 §2): one node's key with node, every node placing a module
+// with module (each node's key whole, so its pins, ports and bindings are beside the module), and every node's key with
+// neither. A key that does not read as a placement is listed as unreadable, never dropped.
+func (r Reader) Placement(node, module string) (map[string]any, error) {
+	node, module = strings.TrimSpace(node), strings.TrimSpace(module)
+	if r.PlacementState.Keys == nil || r.PlacementState.Get == nil {
+		return nil, fmt.Errorf("the placement state cannot be read here")
+	}
+	if node != "" {
+		key, err := delivery.PlacementKey(node)
+		if err != nil {
+			return nil, err
+		}
+		raw, err := r.PlacementState.Get(key)
+		if err != nil {
+			return nil, err
+		}
+		if len(raw) == 0 {
+			return nil, fmt.Errorf("no placement of %s: the catalogue holds no placement file for it, or the indexer has read none yet", node)
+		}
+		return map[string]any{"placement": map[string]json.RawMessage{node: raw}}, nil
+	}
+	keys, err := r.PlacementState.Keys()
+	if err != nil {
+		return nil, err
+	}
+	sort.Strings(keys)
+	values, err := r.PlacementState.values(keys)
+	if err != nil {
+		return nil, err
+	}
+	out := map[string]json.RawMessage{}
+	var unreadable []string
+	for i, key := range keys {
+		n, err := delivery.ParsePlacementKey(key)
+		if err != nil {
+			unreadable = append(unreadable, key)
+			continue
+		}
+		p, err := delivery.DecodePlacement(values[i])
+		if err != nil {
+			unreadable = append(unreadable, key)
+			continue
+		}
+		if module != "" && !slices.Contains(p.Assigned(), module) {
+			continue
+		}
+		out[n] = values[i]
+	}
+	return map[string]any{"placement": out, "unreadable": unreadable}, nil
+}