Answer a placement candidate as ADR 0559 has it: added and removed modules, per-module refusals, both digests of a difference, and a node's read of every candidate key (issue 1126)

go/compose/composed.go

@@ -4,6 +4,7 @@ import (
 	"encoding/json"
 	"errors"
 	"fmt"
+	"slices"
 	"sort"
 	"strings"
 
@@ -18,7 +19,8 @@ import (
 // (mesh-hand-compose) gets that one. Each module's Data says whether its manifest declares data. The compositions are not
 // signed and not yet by digest: the writer does both (delivery.SignComposed, Composed.ByDigest).
 func Compositions(in Input, node string, fps map[string]string, mode Mode) (out map[string]delivery.Composed, skipped []string, err error) {
-	return compositions(in, node, fps, mode, false)
+	out, skipped, _, err = compositions(in, node, fps, mode, false)
+	return out, skipped, err
 }
 
 // ErrStoodIn is a composition that holds a stand-in for a value the input does not carry (an own secret, a grant's
@@ -34,10 +36,21 @@ const BusUsersStandIn = "the bus's user list"
 // ErrStoodIn, naming each, so a stand-in is never applied as if it were the value; but for the bus's user list
 // (BusUsersStandIn), which the bus node holds out of every apply of a composition it made itself.
 func CompositionsApplied(in Input, node string, fps map[string]string) (map[string]delivery.Composed, []string, error) {
-	return compositions(in, node, fps, Reading, true)
+	out, skipped, _, err := compositions(in, node, fps, Reading, true)
+	return out, skipped, err
 }
 
-func compositions(in Input, node string, fps map[string]string, mode Mode, applied bool) (out map[string]delivery.Composed, skipped []string, err error) {
+// CompositionsShadow is CompositionsApplied for a composition that is compared and never applied (a node's answer to a
+// placement candidate, novox/hq ADR 0559 §4): composed in the Foreseeing mode, so an own secret never issued (a module
+// the candidate adds) is a named stand-in rather than an error, and what it stands in for is answered (stood, sorted,
+// the bus's user list left out) rather than refused, so the caller judges each stand-in: a value not yet issued for a
+// module the candidate adds, or one missing for a module the node runs. A module whose every value is held composes
+// as CompositionsApplied composes it: the modes differ only in an own secret never issued.
+func CompositionsShadow(in Input, node string, fps map[string]string) (out map[string]delivery.Composed, skipped, stood []string, err error) {
+	return compositions(in, node, fps, Foreseeing, false)
+}
+
+func compositions(in Input, node string, fps map[string]string, mode Mode, applied bool) (out map[string]delivery.Composed, skipped, stood []string, err error) {
 	reading := in
 	reading.Nodes = append([]Node(nil), in.Nodes...)
 	found := false
@@ -48,36 +61,34 @@ func compositions(in Input, node string, fps map[string]string, mode Mode, appli
 		}
 	}
 	if !found {
-		return nil, nil, fmt.Errorf("compose: the mesh has no node %q", node)
+		return nil, nil, nil, fmt.Errorf("compose: the mesh has no node %q", node)
 	}
 	d, problems, err := ComposeIn(reading, node, mode)
 	if err != nil {
-		return nil, nil, err
+		return nil, nil, nil, err
 	}
 	if len(problems) > 0 {
-		return nil, nil, fmt.Errorf("compose: %s's declaration is refused by the node-engine's validator: %v", node, problems)
+		return nil, nil, nil, fmt.Errorf("compose: %s's declaration is refused by the node-engine's validator: %v", node, problems)
 	}
-	if applied {
-		var stood []string
-		for _, x := range d.StoodIn {
-			// The bus's user list is the bus node's own to follow, written alone and never from a composition the node
-			// made itself (ADR 0547 §7, §8): its stand-in is held out of the apply, not applied.
-			if x != BusUsersStandIn {
-				stood = append(stood, x)
-			}
-		}
-		stood = append(stood, d.Foreseen...)
-		for _, p := range d.ForeseenPorts {
-			stood = append(stood, "port "+p)
-		}
-		if len(stood) > 0 {
-			sort.Strings(stood)
-			return nil, nil, fmt.Errorf("%w on %s: %s", ErrStoodIn, node, strings.Join(stood, ", "))
+	for _, x := range d.StoodIn {
+		// The bus's user list is the bus node's own to follow, written alone and never from a composition the node
+		// made itself (ADR 0547 §7, §8): its stand-in is held out of the apply, not applied.
+		if x != BusUsersStandIn {
+			stood = append(stood, x)
 		}
 	}
+	stood = append(stood, d.Foreseen...)
+	for _, p := range d.ForeseenPorts {
+		stood = append(stood, "port "+p)
+	}
+	sort.Strings(stood)
+	stood = slices.Compact(stood) // an own secret is both stood in for and foreseen
+	if applied && len(stood) > 0 {
+		return nil, nil, nil, fmt.Errorf("%w on %s: %s", ErrStoodIn, node, strings.Join(stood, ", "))
+	}
 	body, err := d.Body()
 	if err != nil {
-		return nil, nil, err
+		return nil, nil, nil, err
 	}
 	var envelope struct {
 		Resources   []json.RawMessage     `json:"resources"`
@@ -86,7 +97,7 @@ func compositions(in Input, node string, fps map[string]string, mode Mode, appli
 		Provisioner string                `json:"provisioner_account"`
 	}
 	if err := json.Unmarshal(body, &envelope); err != nil {
-		return nil, nil, err
+		return nil, nil, nil, err
 	}
 	var kept []json.RawMessage
 	keptPaths := map[string]bool{}
@@ -98,7 +109,7 @@ func compositions(in Input, node string, fps map[string]string, mode Mode, appli
 			Path   string `json:"path"`
 		}
 		if err := json.Unmarshal(raw, &head); err != nil {
-			return nil, nil, err
+			return nil, nil, nil, err
 		}
 		if _, ok := fps[head.Module]; head.Module != "" && !ok {
 			left[head.Module] = true
@@ -118,7 +129,7 @@ func compositions(in Input, node string, fps map[string]string, mode Mode, appli
 	out, err = delivery.SplitComposed(node, kept, fps, filled,
 		delivery.NodeOwn{Adoption: envelope.Adoption, Provisioner: envelope.Provisioner})
 	if err != nil {
-		return nil, nil, err
+		return nil, nil, nil, err
 	}
 	for key, c := range out {
 		if m, ok := in.Catalogue[c.Module]; ok && c.Module != "" {
@@ -130,5 +141,5 @@ func compositions(in Input, node string, fps map[string]string, mode Mode, appli
 		skipped = append(skipped, m)
 	}
 	sort.Strings(skipped)
-	return out, skipped, nil
+	return out, skipped, stood, nil
 }