Export the accounts block's user entry, so a kept user is written in ComposeAccounts' own lines (issue 774)

mesh-grants' refused.go keptEntry wrote a user of the accounts block from grants it keeps by copying
ComposeAccounts' lines; a change to one would leave the other writing a list the bus reads differently.
buscompose.AccountsEntry(user, passwordHash, perms) is now the one writer, and ComposeAccounts uses it. No change
to what is written.

go/buscompose/buscompose_test.go

@@ -573,6 +573,13 @@ func TestTheCopiedComposerComposesAsTheControllers(t *testing.T) {
 		if reads[VaultSeat] != ControllerLease+","+GrantsAccounts || reads["mesh-config"] != "" || reads["restic"] != "" || reads["messenger"] != "" {
 			t.Errorf("the reads granted for their readers: %v", reads)
 		}
+		// AccountsEntry is the very lines ComposeAccounts writes for a user, so a writer of kept users repeats none.
+		filled, _ := WithPasswords(judged, map[string]string{judged[0].Username(): "$2a$11$h"})
+		list, err := ComposeAccounts(filled[:1])
+		perms, perr := PermissionsFor(filled[0])
+		if err != nil || perr != nil || !strings.Contains(list, "    users = [\n"+AccountsEntry(filled[0].Username(), "$2a$11$h", perms)+"    ]\n") {
+			t.Errorf("the accounts entry is not ComposeAccounts' own: %v %v\n%s", err, perr, list)
+		}
 		c, _ := ConsumerFor(Principal{Kind: KindModule, Node: "a", Module: "m", Consumes: []string{"x.happened"}})
 		cfg := ConsumerConfig(c)
 		if cfg.Durable != "a_m" || cfg.AckPolicy != jetstream.AckExplicitPolicy || cfg.AckWait != 30*time.Second ||

go/buscompose/nats.go

@@ -1680,17 +1680,7 @@ func ComposeAccounts(principals []Principal) (string, error) {
 		if p.PasswordHash == "" {
 			return "", fmt.Errorf("%s has no password hash: a user without one is a user anybody is", p.Username())
 		}
-		fmt.Fprintf(&b, "      { user: %q, password: %q, permissions: {\n", p.Username(), p.PasswordHash)
-		if len(perms.PublishDeny) > 0 {
-			fmt.Fprintf(&b, "        publish: { allow: [%s], deny: [%s] }\n", quoted(perms.Publish), quoted(perms.PublishDeny))
-		} else {
-			fmt.Fprintf(&b, "        publish: { allow: [%s] }\n", quoted(perms.Publish))
-		}
-		fmt.Fprintf(&b, "        subscribe: { allow: [%s] }\n", quoted(perms.Subscribe))
-		if perms.AllowResponses {
-			fmt.Fprintf(&b, "        allow_responses: { max: 1, ttl: \"%dm\" }\n", int(ResponseTTL/time.Minute))
-		}
-		b.WriteString("      } }\n")
+		b.WriteString(AccountsEntry(p.Username(), p.PasswordHash, perms))
 	}
 	b.WriteString("    ]\n  }\n")
 	b.WriteString(systemAccount)
@@ -1698,6 +1688,25 @@ func ComposeAccounts(principals []Principal) (string, error) {
 	return b.String(), nil
 }
 
+// AccountsEntry is one user of the accounts block, as ComposeAccounts writes each: its name, its password hash and its
+// permissions, in the lines the bus reads. Exported so a writer that keeps a user from grants it holds rather than
+// composes (mesh-grants' refused users) writes the same lines, never a copy of them (issue 774).
+func AccountsEntry(user, passwordHash string, perms Permissions) string {
+	var b strings.Builder
+	fmt.Fprintf(&b, "      { user: %q, password: %q, permissions: {\n", user, passwordHash)
+	if len(perms.PublishDeny) > 0 {
+		fmt.Fprintf(&b, "        publish: { allow: [%s], deny: [%s] }\n", quoted(perms.Publish), quoted(perms.PublishDeny))
+	} else {
+		fmt.Fprintf(&b, "        publish: { allow: [%s] }\n", quoted(perms.Publish))
+	}
+	fmt.Fprintf(&b, "        subscribe: { allow: [%s] }\n", quoted(perms.Subscribe))
+	if perms.AllowResponses {
+		fmt.Fprintf(&b, "        allow_responses: { max: 1, ttl: \"%dm\" }\n", int(ResponseTTL/time.Minute))
+	}
+	b.WriteString("      } }\n")
+	return b.String()
+}
+
 // WatchmanModule is the one module whose principal hears the bus's connect and disconnect events and asks its list of
 // connections (novox/hq ADR 0431 §2, issue 1147): the watchman of the repository mesh-delivery, which writes presence
 // from them. No other principal is granted the subjects below, and nothing is a wildcard over them.