Send the bus's user list alone before a walk's gate, so a new grant is on the bus when the first machine is judged (issue 490)

A walk left the machine holding the bus out of a gated send whenever a
build waited there for that same gate, so a module's new health tool was
refused by the bus on its first machine and the gate could not pass. The
grants step sends that machine its declaration with every build kept,
before the first machine's send; plans and the change plan say it.

cmd/mesh-controller/changeplan.go

@@ -2,6 +2,7 @@ package main
 
 import (
 	"fmt"
+	"slices"
 	"sort"
 	"strings"
 
@@ -28,6 +29,14 @@ func changePlanOf(repository, base, head string, r mergeReach, entries []invento
 	for _, e := range entries {
 		byName[e.Manifest.Module] = e
 	}
+	// The machine holding the bus, whose declaration carries every module's grants (novox/hq issue 490).
+	holder := ""
+	for _, e := range entries {
+		if e.Manifest.BusUsers != "" && e.Manifest.ClaimsSeat(catalogue.BrokerSeat) && len(e.On) > 0 {
+			holder = e.On[0]
+		}
+	}
+	var granting []string
 	machines := map[string]*link.MachinePlan{}
 	machine := func(name string) *link.MachinePlan {
 		if machines[name] == nil {
@@ -50,6 +59,10 @@ func changePlanOf(repository, base, head string, r mergeReach, entries []invento
 					machine(on).Receives = append(machine(on).Receives, name)
 				}
 			}
+			if !waits && holder != "" && !(len(e.On) == 1 && e.On[0] == holder) && len(e.On) > 0 &&
+				!slices.Contains(granting, name) {
+				granting = append(granting, name)
+			}
 			switch {
 			case (name == "nats" || catalogue.ProvidesBus(e.Manifest)) && len(e.On) > 0:
 				// Said before the merge (novox/hq issue 336): a new bus build holds every send to the bus's machine
@@ -76,6 +89,13 @@ func changePlanOf(repository, base, head string, r mergeReach, entries []invento
 			}
 		}
 	}
+	if len(granting) > 0 {
+		// Said before the merge (novox/hq issue 490): what the walk does when the change alters the bus's
+		// user list. Whether it does is known only once the builds are registered, so it is said as a rule.
+		p.Steps = append(p.Steps, fmt.Sprintf("%s: if this changes what the bus's user list says (a new tool, "+
+			"subject or user), %s is sent that list alone, every build there kept, before %s is judged on its first "+
+			"machine", grantsStepWord, holder, strings.Join(granting, ", ")))
+	}
 	var names []string
 	for name := range machines {
 		names = append(names, name)
@@ -88,6 +108,10 @@ func changePlanOf(repository, base, head string, r mergeReach, entries []invento
 	return p
 }
 
+// grantsStepWord names the grants step (novox/hq issue 490): the bus's user list sent alone to the machine
+// holding the bus ahead of a walk's gate. Not the bus step, which replaces the bus itself.
+const grantsStepWord = "grants step"
+
 // busUpgradeNeeded is how a change plan says that merging it holds the bus's machine for a person's step.
 const busUpgradeNeeded = "merging this needs a person's bus upgrade"