A build machine serves the seat its credential claims (hq ADR 0190, the handover)

After the build role moved to node-build-agent, nothing would hold it until build-agent is
registered — and registering build-agent needs a build outcome that only the running builder
could produce, bound as it was to the old seat by name. One binary, two roles: the seat a machine
serves is the first its credential claims, as the mesh writes the claims beside the credential it
issues (ADR 0159); the old builder keeps draining mesh-build-machine, a build-agent takes
node-build-agent, and what each says about a build goes out as that seat's events, so an outcome
is heard where the asker of that seat listens. A credential naming no claim serves the current role.

internal/link/build_seat_test.go

@@ -0,0 +1,32 @@
+package link
+
+import "testing"
+
+// A build machine serves the seat its credential claims (novox/hq ADR 0190 handover): the old
+// `builder` keeps the old role, a `build-agent` takes the new, from one binary and no flag.
+func TestABuildMachineServesTheSeatItsCredentialClaims(t *testing.T) {
+	if got := BuildSeatClaimed([]string{"mesh-build-machine"}); got != "mesh-build-machine" {
+		t.Errorf("a credential claiming the old role serves %q", got)
+	}
+	if got := BuildSeatClaimed([]string{"node-build-agent"}); got != TheBuildMachine {
+		t.Errorf("a credential claiming the new role serves %q", got)
+	}
+	if got := BuildSeatClaimed(nil); got != TheBuildMachine {
+		t.Errorf("a credential claiming nothing serves %q, want the current role", got)
+	}
+	if got := BuildSeatClaimed([]string{"", "node-build-agent"}); got != TheBuildMachine {
+		t.Errorf("an empty claim is skipped; got %q", got)
+	}
+}
+
+// What a machine says about a build is the event of the seat it took the build from, so an outcome
+// is heard where the asker of that seat listens.
+func TestABuildsEventsAreItsSeats(t *testing.T) {
+	if BuildOutcomeOf(TheBuildMachineBefore) != "mesh.seat.mesh-build-machine.event.built" {
+		t.Error(BuildOutcomeOf(TheBuildMachineBefore))
+	}
+	if BuildWorkOf(TheBuildMachine) != BuildWork() || BuildOutcomeOf(TheBuildMachine) != BuildOutcome() ||
+		BuildStartedOf(TheBuildMachine) != BuildStarted() || BuildLogOf(TheBuildMachine, "b1") != BuildLog("b1") {
+		t.Error("the no-argument forms must name the current role")
+	}
+}

internal/link/builds.go

@@ -31,10 +31,36 @@ import (
 // build-agent to the machines that build; unassign builder and forget it and its seat's stream.
 const TheBuildMachine = "node-build-agent"
 
+// TheBuildMachineBefore is the role a build was submitted to until ADR 0190: the mesh's one build
+// machine, mesh-scoped. Kept named while the handover runs — a machine whose credential claims it
+// still serves it, and the controller still hears its outcomes — and dropped with the retired seat
+// row once nothing claims it.
+const TheBuildMachineBefore = "mesh-build-machine"
+
+// BuildSeatClaimed is the build role a machine serves: the first seat its credential claims, or the
+// current role when the credential names none (a credential from before claims travelled in it, or
+// one written by hand). **The credential decides, not the binary** (ADR 0190 handover): one build
+// machine binary runs as the old `builder` on the old seat and as a `build-agent` on the new one,
+// each taking the work the mesh issued it a credential for, so neither drains the other's queue
+// and the switch needs no flag day.
+func BuildSeatClaimed(claimed []string) string {
+	for _, seat := range claimed {
+		if seat != "" {
+			return seat
+		}
+	}
+	return TheBuildMachine
+}
+
 // BuildWork is where a build request lands, and BuildOutcome is where its result does. Derived from
-// the seat, so both sides name the role and neither names the other.
-func BuildWork() string    { return "mesh.seat." + TheBuildMachine + ".accept.build" }
-func BuildOutcome() string { return "mesh.seat." + TheBuildMachine + ".event.built" }
+// the seat, so both sides name the role and neither names the other. The no-argument forms name the
+// current role; the `Of` forms take the seat, for the handover during which two roles exist.
+func BuildWork() string              { return BuildWorkOf(TheBuildMachine) }
+func BuildOutcome() string           { return BuildOutcomeOf(TheBuildMachine) }
+func BuildWorkOf(seat string) string { return "mesh.seat." + seat + ".accept.build" }
+func BuildOutcomeOf(seat string) string {
+	return "mesh.seat." + seat + ".event.built"
+}
 
 // BuildStarted is where a build machine says it has taken a build, and BuildLog is where it says
 // what it is doing, one line per message, under the build's own id (novox/hq ADR 0157).
@@ -44,8 +70,10 @@ func BuildOutcome() string { return "mesh.seat." + TheBuildMachine + ".event.bui
 // lived in one container's stderr on one machine. Every line is now an event of the role, retained
 // with the rest of the mesh's events, so a reader follows a build live by subscribing its subject,
 // or reads it back afterwards from the stream, and a viewer is a subscriber and nothing more.
-func BuildStarted() string      { return "mesh.seat." + TheBuildMachine + ".event.started" }
-func BuildLog(id string) string { return "mesh.seat." + TheBuildMachine + ".event.log." + id }
+func BuildStarted() string              { return BuildStartedOf(TheBuildMachine) }
+func BuildLog(id string) string         { return BuildLogOf(TheBuildMachine, id) }
+func BuildStartedOf(seat string) string { return "mesh.seat." + seat + ".event.started" }
+func BuildLogOf(seat, id string) string { return "mesh.seat." + seat + ".event.log." + id }
 
 // BuildStart is what a build machine says the moment it takes a build.
 type BuildStart struct {

internal/link/builds_nats.go

@@ -115,9 +115,16 @@ type natsMachine struct {
 	sub  *nats.Subscription
 }
 
-// MachineOverNATS takes build work from the role this machine holds.
+// MachineOverNATS takes build work from the current build role.
 func MachineOverNATS(js *broker.JetStream, on string) BuildMachine {
-	return &natsMachine{js: js, on: on, seat: TheBuildMachine}
+	return MachineOverNATSOn(js, on, TheBuildMachine)
+}
+
+// MachineOverNATSOn takes build work from the role named — the one this machine's credential claims
+// (ADR 0190 handover): its asks come from that seat's worker, and what it says about a build goes
+// out as that seat's events, so an outcome is heard where the asker listens.
+func MachineOverNATSOn(js *broker.JetStream, on, seat string) BuildMachine {
+	return &natsMachine{js: js, on: on, seat: seat}
 }
 
 func (m *natsMachine) Close() {
@@ -194,7 +201,7 @@ func (m *natsMachine) Take(ctx context.Context, do func(context.Context, Build))
 			// the ask to a second machine nor counts the wait against its deliveries.
 			working := make(chan struct{})
 			go stillWorking(msg, working)
-			do(ctx, &natsBuild{request: request, msg: msg, on: m.on, js: m.js})
+			do(ctx, &natsBuild{request: request, msg: msg, on: m.on, js: m.js, seat: m.seat})
 			close(working)
 		}
 	}
@@ -205,7 +212,9 @@ type natsBuild struct {
 	msg     *nats.Msg
 	on      string
 	js      *broker.JetStream
-	seq     int
+	// seat is the role this build was taken from; what the machine says about it is that role's.
+	seat string
+	seq  int
 }
 
 func (b *natsBuild) Request() BuildRequest { return b.request }
@@ -232,7 +241,7 @@ func (b *natsBuild) Announce(ctx context.Context, result BuildResult) error {
 	}
 	publish, cancel := context.WithTimeout(ctx, 30*time.Second)
 	defer cancel()
-	if _, err := b.js.Context().Publish(BuildOutcome(), body, nats.Context(publish)); err != nil {
+	if _, err := b.js.Context().Publish(BuildOutcomeOf(b.seat), body, nats.Context(publish)); err != nil {
 		return fmt.Errorf("cannot announce a build's outcome: %w", err)
 	}
 	return nil
@@ -252,7 +261,7 @@ func (b *natsBuild) Began(ctx context.Context) error {
 	}
 	publish, cancel := context.WithTimeout(ctx, 30*time.Second)
 	defer cancel()
-	if _, err := b.js.Context().Publish(BuildStarted(), body, nats.Context(publish)); err != nil {
+	if _, err := b.js.Context().Publish(BuildStartedOf(b.seat), body, nats.Context(publish)); err != nil {
 		return fmt.Errorf("cannot say a build started: %w", err)
 	}
 	return nil
@@ -270,7 +279,7 @@ func (b *natsBuild) Say(step, message string) {
 	if err != nil {
 		return
 	}
-	_ = b.js.Conn().Publish(BuildLog(b.request.ID), body)
+	_ = b.js.Conn().Publish(BuildLogOf(b.seat, b.request.ID), body)
 }
 
 func (b *natsBuild) Hold(after time.Duration) error { return b.msg.NakWithDelay(after) }