List every module's preferences in the settings verb (hq ADR 0262)

One verb is the interface to every preference, so no module builds a settings tool of its own: each
key, its default and why, and every assigned machine's value with its source.

cmd/mesh-controller/modules.go

@@ -8,6 +8,7 @@ import (
 	"fmt"
 	"net"
 	"os"
+	"sort"
 	"strings"
 
 	"github.com/novox/mesh-controller/internal/broker"
@@ -396,7 +397,8 @@ func assignCommand(ctx context.Context, verb string, args []string) error {
 func settingsCommand(ctx context.Context, args []string) error {
 	if len(args) == 0 {
 		return errors.New("settings show <module> [--node <node>] [--history], settings set <module> <file> " +
-			"[--node <node>] [--replace], or settings clear <module> [--node <node>]")
+			"[--node <node>] [--replace], settings clear <module> [--node <node>], or settings preferences " +
+			"[<module>] [--node <node>]")
 	}
 	open, err := openStores(ctx)
 	if err != nil {
@@ -536,6 +538,46 @@ func settingsCommand(ctx context.Context, args []string) error {
 		fmt.Print(describeEffective(positionals[0], where, catalogue.Effective(m, layers)))
 		return nil
 
+	case "preferences":
+		// Every module's preferences, and each machine's value with its source (novox/hq ADR 0262).
+		if len(positionals) > 1 {
+			return errors.New("settings preferences [<module>] [--node <node>]")
+		}
+		only := ""
+		if len(positionals) == 1 {
+			only = positionals[0]
+		}
+		entries, err := inv.Catalogued(ctx)
+		if err != nil {
+			return err
+		}
+		var listed []preferencesOf
+		for _, e := range entries {
+			m := e.Manifest
+			if len(m.Settings) == 0 || (only != "" && m.Module != only) {
+				continue
+			}
+			p := preferencesOf{Manifest: m, On: map[string][]catalogue.SettingSource{}}
+			for _, n := range e.On {
+				if *node != "" && n != *node {
+					continue
+				}
+				p.Nodes = append(p.Nodes, n)
+				layers, err := inv.SettingsFor(ctx, n, m.Module)
+				if err != nil {
+					return err
+				}
+				p.On[n] = catalogue.Effective(m, layers)
+			}
+			listed = append(listed, p)
+		}
+		if only != "" && len(listed) == 0 {
+			fmt.Printf("%s declares no preferences\n", only)
+			return nil
+		}
+		fmt.Print(describePreferences(listed))
+		return nil
+
 	case "clear":
 		if len(positionals) != 1 {
 			return errors.New("settings clear <module> [--node <node>]")
@@ -547,7 +589,7 @@ func settingsCommand(ctx context.Context, args []string) error {
 		return nil
 
 	default:
-		return fmt.Errorf("settings has no %q; it has show, set and clear", args[0])
+		return fmt.Errorf("settings has no %q; it has show, set, clear and preferences", args[0])
 	}
 }
 
@@ -568,6 +610,58 @@ func describeEffective(module, where string, values []catalogue.SettingSource) s
 	return b.String()
 }
 
+// preferencesOf is one module's preferences and its value on each machine it is assigned to.
+type preferencesOf struct {
+	Manifest catalogue.Manifest
+	Nodes    []string
+	On       map[string][]catalogue.SettingSource
+}
+
+// describePreferences lists each module's preferences — key, default and why — and, per machine it is
+// assigned to, the value and where it comes from (novox/hq ADR 0262).
+func describePreferences(modules []preferencesOf) string {
+	if len(modules) == 0 {
+		return "no module declares a preference\n"
+	}
+	var b strings.Builder
+	for i, p := range modules {
+		if i > 0 {
+			b.WriteString("\n")
+		}
+		on := "assigned nowhere"
+		if len(p.Nodes) > 0 {
+			on = "on " + strings.Join(p.Nodes, ", ")
+		}
+		fmt.Fprintf(&b, "%s (%s)\n", p.Manifest.Module, on)
+		keys := make([]string, 0, len(p.Manifest.Settings))
+		for k := range p.Manifest.Settings {
+			keys = append(keys, k)
+		}
+		sort.Strings(keys)
+		for _, k := range keys {
+			d := p.Manifest.Settings[k]
+			def, _ := json.Marshal(d.Default)
+			fmt.Fprintf(&b, "  %s, default %s: %s\n", k, def, d.Why)
+			for _, n := range p.Nodes {
+				for _, s := range p.On[n] {
+					if s.Key != k {
+						continue
+					}
+					v, _ := json.Marshal(s.Value)
+					from := s.From
+					if s.FromDefault {
+						from = catalogue.DefaultLayer
+					} else if s.From != catalogue.MeshWideLayer {
+						from = "the node"
+					}
+					fmt.Fprintf(&b, "    %s: %s (%s)\n", n, v, from)
+				}
+			}
+		}
+	}
+	return b.String()
+}
+
 // nodeFlag is ` --node <node>` for a machine's layer, nothing for the whole mesh's.
 func nodeFlag(node string) string {
 	if node == "" {