A bus account kind ledger, named ledger.<node>.<module> and held in the tool runner's ledgers (issue 1966 S6)

ADR 0597 §9 and the lead's Q1 path: one ledger credential per module
assignment, minted by the vault like every kind; the grants part's copy
of buscompose gains the same kind alike (D5b).

go/buscompose/accounts.go

@@ -31,7 +31,8 @@ type ClaimedSeat struct {
 // WantedAccount is one key of the grants part's state `accounts`: an account the mesh needs, without its password.
 type WantedAccount struct {
 	User string `json:"user"`
-	// Kind is KindModule (KindNodeTools for the runtime, named as a module is), KindProcess or KindProvisioner.
+	// Kind is KindModule (KindNodeTools for the runtime, named as a module is), KindProcess, KindProvisioner or
+	// KindLedger.
 	Kind   Kind   `json:"kind"`
 	Node   string `json:"node"`
 	Module string `json:"module"`
@@ -46,8 +47,12 @@ type WantedAccount struct {
 	At time.Time `json:"at"`
 }
 
+// LedgersSecret is the tool runner's own secret holding its machine's ledger accounts, a map of module to credential
+// (ADR 0597 §9).
+const LedgersSecret = "ledgers"
+
 // mintedKinds are the kinds of account the vault mints (ADR 0561 §1).
-var mintedKinds = []Kind{KindModule, KindNodeTools, KindProcess, KindProvisioner}
+var mintedKinds = []Kind{KindModule, KindNodeTools, KindProcess, KindProvisioner, KindLedger}
 
 // principal is the account as the composer names it.
 func (w WantedAccount) principal() Principal {
@@ -93,9 +98,12 @@ func (w WantedAccount) Same(o WantedAccount) bool {
 }
 
 // OwnSecret is the module's own secret the account is held in: `broker`, `broker-<process id>` or `provisioner-broker`
-// (the controller's names, ADR 0436 §2, ADR 0349 §11).
+// (the controller's names, ADR 0436 §2, ADR 0349 §11); a ledger account in the tool runner's `ledgers`, which keeps every
+// ledger account of its machine, by module (ADR 0597 §9).
 func (w WantedAccount) OwnSecret() string {
 	switch w.Kind {
+	case KindLedger:
+		return LedgersSecret
 	case KindProcess:
 		return "broker-" + w.Process
 	case KindProvisioner:

go/buscompose/names_test.go

@@ -18,4 +18,12 @@ func TestTheBusNamesAgree(t *testing.T) {
 	if LogStreamName("mesh-delivery-builder", "build-log") == "" || LogSubject("mesh-delivery-builder", "build-log") == "" {
 		t.Fatal("a log has no stream or subject")
 	}
+	// 1966/AC23: a wanted account of kind ledger for m on a names the user ledger.a.m and the own secret ledgers.
+	ledger := WantedAccount{User: "ledger.a.m", Kind: KindLedger, Node: "a", Module: "m", URL: "nats://10.0.0.1:4222"}
+	if got := (Principal{Kind: KindLedger, Node: "a", Module: "m"}).Username(); got != "ledger.a.m" {
+		t.Fatalf("a ledger account is named %q", got)
+	}
+	if err := ledger.Valid(); err != nil || ledger.OwnSecret() != "ledgers" {
+		t.Fatalf("a wanted ledger account: %v, held in %q", err, ledger.OwnSecret())
+	}
 }

go/buscompose/nats.go

@@ -44,6 +44,10 @@ const (
 	// exactly what its bus says — never root, never the operator account, and never able to answer a tool
 	// call or a seat's verb (§3). One per process and machine.
 	KindProcess Kind = "process"
+	// KindLedger is one module assignment's ledger credential (novox/hq ADR 0597 §9, design 25 § 12.4): held by that
+	// machine's tool runner, never by the module's own code, and the only account that publishes on that module's ledger
+	// histories and snapshots. One per module assignment, whose module asks for a ledger in `requires`.
+	KindLedger Kind = "ledger"
 )
 
 // Seat is a role on the bus as a principal relates to it: the subjects it accepts, and those it
@@ -272,6 +276,10 @@ func (p Principal) Username() string {
 			return p.Login
 		}
 		return "node." + p.Node
+	case KindLedger:
+		// `ledger.<machine>.<module>`: never `<node>.<module>`, which is the module's own account or the tool runner's,
+		// so the module's code never holds it.
+		return "ledger." + p.Node + "." + p.Module
 	case KindProvisioner:
 		// Named for the machine, as a node is: one client per machine, whichever module runs it, and never
 		// `<node>.<module>`, which is the module's own account or the tool runner's.