pacman keeps one mirror list for the mesh, ranked in the write that opens each round and written by every node (ADR 0585, issue 1876)

Each node's reflector ranked its own list on its own timer, and on 2026-10-31 one node's
lagging first mirror left it a Node.js version behind the rest. The list is now the
module's state mirrors, key list: the node that opens a week's round ranks it with the
module's reflector options, keeps only mirrors whose last sync is no older than the
newest found, and the round's key names the list's revision. Every keeper writes
/etc/pacman.d/mirrorlist whole from the key through the root helper's new verb
mirrorlist, when the key changes and when the file differs, and never writes an empty
or unreadable list. reflector.timer is declared stopped and disabled, and reflector.conf
saves nothing. pacman_mirrors answers the shared list, its ranker and age and whether
this node's file matches; with refresh it asks this node's keeper to rank again,
refused while a round runs, naming it. The round's integration test proves the opening
write and both nodes' files.

Claude-Session: https://claude.ai/code/session_01NcbgfJ9Gum47zkFKWRaovL

modules/pacman/cmd/pacman-tools/root.go

@@ -51,6 +51,7 @@ const (
 	VerbRollback    = "rollback"
 	VerbMaps        = "maps"
 	VerbJournal     = "journal"
+	VerbMirrorlist  = "mirrorlist"
 )
 
 // The helper's environment, and the keeper's word for where the helper listens.
@@ -78,6 +79,8 @@ type RootRequest struct {
 	Plan   []RollbackItem `json:"plan,omitempty"`
 	Unit   string         `json:"unit,omitempty"`
 	Lines  int            `json:"lines,omitempty"`
+	// Servers are the mirror list's servers, in order, for the verb mirrorlist.
+	Servers []string `json:"servers,omitempty"`
 }
 
 // RootAnswer is the helper's answer: an error, or what the verb read.
@@ -95,7 +98,9 @@ type RootHelper struct {
 	ProcRoot string
 	PlanDir  string
 	Self     string
-	Peer     int
+	// MirrorlistPath is the mirror list file the verb mirrorlist writes: Mirrorlist, a test's own elsewhere.
+	MirrorlistPath string
+	Peer           int
 	// PeerOf is the account at the other end of a connection; nil reads it from the socket (SO_PEERCRED).
 	PeerOf func(net.Conn) (int, error)
 	Logf   func(format string, a ...any)
@@ -221,8 +226,17 @@ func (h *RootHelper) Handle(req RootRequest) RootAnswer {
 			return RootAnswer{Error: failure("journalctl", "journalctl", r).Error()}
 		}
 		return RootAnswer{Lines: lines(r.Stdout)}
+	case VerbMirrorlist:
+		if err := checkServers(req.Servers); err != nil {
+			return refused("%v", err)
+		}
+		if err := writeWhole(h.MirrorlistPath, RenderMirrorlist(req.Servers), 0o644); err != nil {
+			return RootAnswer{Error: "writing the mirror list: " + err.Error()}
+		}
+		return RootAnswer{}
 	}
-	return refused("%q is not a verb of the root helper (%s, %s, %s, %s)", req.Verb, VerbTransaction, VerbRollback, VerbMaps, VerbJournal)
+	return refused("%q is not a verb of the root helper (%s, %s, %s, %s, %s)", req.Verb, VerbTransaction, VerbRollback, VerbMaps,
+		VerbJournal, VerbMirrorlist)
 }
 
 // startUnit starts command as a transient unit of the service manager and answers once it started.
@@ -341,6 +355,28 @@ func (c rootClient) Rollback(run string, plan []RollbackItem) error {
 	return err
 }
 
+func (c rootClient) Mirrorlist(servers []string) error {
+	_, err := c.ask(RootRequest{Verb: VerbMirrorlist, Servers: servers})
+	return err
+}
+
+// writeWhole writes a file whole: into a file beside it, then renamed over it, so a reader never meets half of it.
+func writeWhole(path string, content []byte, mode os.FileMode) error {
+	tmp := filepath.Join(filepath.Dir(path), "."+filepath.Base(path)+".mesh")
+	if err := os.WriteFile(tmp, content, mode); err != nil {
+		return err
+	}
+	if err := os.Chmod(tmp, mode); err != nil {
+		_ = os.Remove(tmp)
+		return err
+	}
+	if err := os.Rename(tmp, path); err != nil {
+		_ = os.Remove(tmp)
+		return err
+	}
+	return nil
+}
+
 func (c rootClient) Journal(unit string, n int) ([]string, error) {
 	a, err := c.ask(RootRequest{Verb: VerbJournal, Unit: unit, Lines: n})
 	return a.Lines, err
@@ -398,7 +434,8 @@ func rootHelper() int {
 	}
 	ctx, stop := signal.NotifyContext(context.Background(), os.Interrupt, syscall.SIGTERM)
 	defer stop()
-	h := &RootHelper{Run: ExecRunner, Proc: osProc{}, ProcRoot: "/proc", PlanDir: filepath.Join(dir, "plans"), Self: self, Peer: uid, Logf: logf}
+	h := &RootHelper{Run: ExecRunner, Proc: osProc{}, ProcRoot: "/proc", PlanDir: filepath.Join(dir, "plans"), Self: self,
+		MirrorlistPath: Mirrorlist, Peer: uid, Logf: logf}
 	logf("[pacman] root helper listening on %s for account %s (%d)", sock, peerName, uid)
 	if err := h.Serve(ctx, l); err != nil {
 		logf("[pacman] root helper stopped: %v", err)