Fix the kind: the seat set a host gives (catalogue.UseSeats) is held atomically, so a reload never races a lookup

Builds: design 47 G §419

go/catalogue/seats.go

@@ -3,6 +3,7 @@ package catalogue
 import (
 	"fmt"
 	"strings"
+	"sync/atomic"
 )
 
 // Seat is one role the mesh defines.
@@ -87,7 +88,15 @@ type Receivable struct {
 // seat list and imports no contracts module (novox/hq ADR 0635 §1). A host reads the list from the bus
 // (mesh-sdk go/seatlist, which holds the foundation's bootstrapped definitions until the indexer is heard) and gives
 // its seats here at start and at each change.
-var seats []Seat
+var seats atomic.Pointer[[]Seat]
+
+// working is the set in force, none before a host gives one.
+func working() []Seat {
+	if p := seats.Load(); p != nil {
+		return *p
+	}
+	return nil
+}
 
 // UseSeats replaces the working set with the seat list a host read (seatlist.Reader.Seats, via SeatsFromDefinitions).
 //
@@ -98,7 +107,8 @@ func UseSeats(s []Seat) {
 	if len(s) == 0 {
 		return
 	}
-	seats = append([]Seat(nil), s...)
+	set := append([]Seat(nil), s...)
+	seats.Store(&set)
 }
 
 // aliases maps a seat's former names to its current canonical name (novox/hq ADR 0122). Loaded from
@@ -108,20 +118,20 @@ var aliases = map[string]string{}
 
 // Seats is every seat the mesh defines, in reading order.
 func Seats() []Seat {
-	return append([]Seat(nil), seats...)
+	return append([]Seat(nil), working()...)
 }
 
 // SeatNamed is the seat a name refers to, whether that is its current name or one it used to have
 // (novox/hq ADR 0122). A former name resolves to the seat's canonical row, so a rename breaks no
 // reference to the old name.
 func SeatNamed(name string) (Seat, bool) {
-	for _, s := range seats {
+	for _, s := range working() {
 		if s.Name == name {
 			return s, true
 		}
 	}
 	if canonical, aliased := aliases[name]; aliased {
-		for _, s := range seats {
+		for _, s := range working() {
 			if s.Name == canonical {
 				return s, true
 			}
@@ -135,7 +145,7 @@ func SeatDelivering(provision string) (Seat, bool) {
 	if provision == "" {
 		return Seat{}, false
 	}
-	for _, s := range seats {
+	for _, s := range working() {
 		if s.Delivers == provision {
 			return s, true
 		}
@@ -208,7 +218,7 @@ func providesAt(m Manifest, provision, scope string) bool {
 // nothing on the bus and needs no queue.
 func SeatsWithAProtocol() []Seat {
 	var out []Seat
-	for _, s := range seats {
+	for _, s := range working() {
 		if len(s.Accepts) > 0 || len(s.Emits) > 0 || len(s.Serves) > 0 {
 			out = append(out, s)
 		}