plan: show what a module would open and why

Every module.json already declares a why for each port under listens,
but plan only ever used it to build the firewall's rule set — nothing
printed it. An operator deciding whether to assign a module had no way
to see what it would open without reading the manifest by hand.

plan <node> now prints each assigned module's listens entries — port,
protocol, source, and its why — right under the module line, so the
same text that feeds the firewall is visible at the point someone is
actually deciding whether to open it.

cmd/mesh-controller/plan.go

@@ -800,6 +800,22 @@ func grantsFor(ctx context.Context, open *stores, node string) ([]catalogue.Gran
 	return out, nil
 }
 
+// listensLines is what a person is told about what this module would open, and why — the same
+// `why` every listens entry already carries for the firewall it also feeds (novox/hq ADR 0007), so
+// deciding whether to assign a module can see what it would open before it opens it, not only
+// after. A module with nothing to listen on prints nothing extra, same as today.
+func listensLines(m catalogue.Manifest) []string {
+	var out []string
+	for _, l := range m.Listens {
+		if l.Why == "" {
+			out = append(out, fmt.Sprintf("    listens %d/%s from %s", l.Port, l.At(), l.From))
+			continue
+		}
+		out = append(out, fmt.Sprintf("    listens %d/%s from %s — %s", l.Port, l.At(), l.From, l.Why))
+	}
+	return out
+}
+
 func planCommand(ctx context.Context, args []string) error {
 	set := flag.NewFlagSet("plan", flag.ContinueOnError)
 	// Because "one resource" does not tell you whether the settings landed. Being able to read
@@ -853,6 +869,9 @@ func planCommand(ctx context.Context, args []string) error {
 	fmt.Printf("%s would run:\n", args[0])
 	for _, m := range plan.Modules {
 		fmt.Printf("  %-20s %s\n", m.Module, plan.Because[m.Module])
+		for _, line := range listensLines(m) {
+			fmt.Println(line)
+		}
 	}
 	// What was assigned here and cannot run here. Said with the rest rather than as a refusal: it is
 	// one module on the wrong machine, the others still run, and the remedy is to move this one.

cmd/mesh-controller/plan_test.go

@@ -0,0 +1,37 @@
+package main
+
+import (
+	"strings"
+	"testing"
+
+	"github.com/novox/mesh-controller/internal/catalogue"
+)
+
+// `plan` tells a person what a module would open and why, from the same `why` every listens
+// entry already carries for the firewall (novox/hq ADR 0007) — so deciding whether to assign a
+// module does not need reading its manifest first.
+func TestListensLinesShowWhatAModuleWouldOpenAndWhy(t *testing.T) {
+	m := catalogue.Manifest{Module: "minio", Listens: []catalogue.Listening{
+		{Port: 9000, From: catalogue.FromMesh, Why: "the S3 endpoint"},
+		{Port: 9001, From: catalogue.FromMesh},
+	}}
+	got := listensLines(m)
+	if len(got) != 2 {
+		t.Fatalf("two listens entries, got %d: %v", len(got), got)
+	}
+	if !strings.Contains(got[0], "9000/tcp") || !strings.Contains(got[0], "the S3 endpoint") {
+		t.Errorf("the port and its why did not both appear: %q", got[0])
+	}
+	if strings.Contains(got[1], "—") {
+		t.Errorf("a listens entry with no why should not print a dash: %q", got[1])
+	}
+	if !strings.Contains(got[1], "9001/tcp") {
+		t.Errorf("the port still appears without a why: %q", got[1])
+	}
+}
+
+func TestListensLinesAreEmptyForAModuleWithNothingToListenOn(t *testing.T) {
+	if got := listensLines(catalogue.Manifest{Module: "board"}); len(got) != 0 {
+		t.Errorf("a module with no listens should print nothing, got %v", got)
+	}
+}