P3-X2: tree.list synchronous WS RPC + per-session cache

Foundational for the restore wizard's tree browser. The wizard needs to
lazy-load directory contents from a snapshot as the operator drills
down; this lands the transport.

- internal/api adds MsgTreeList (server → agent) + MsgTreeListResult
  (agent → server) with TreeListRequestPayload / TreeListEntry /
  TreeListResultPayload types. Reply correlates by Envelope.ID.
- internal/restic.ListTreeChildren wraps 'restic ls --json' and
  filters its recursive output to direct children of the requested
  path. Parser + path-normalisation + isDirectChild are unit-tested.
- internal/server/ws/rpc.go introduces a generic SendRPC helper on
  Hub: register a buffered channel keyed by ULID, send the request,
  block on ctx.Done()/timeout/reply. Reply routing piggybacks on the
  existing dispatchAgentMessage by adding a MsgTreeListResult case
  that forwards to the registered waiter; if no waiter is registered
  (caller already gave up) the stray reply is dropped quietly.
- cmd/agent gains a tree.list handler that runs ListTreeChildren on a
  fresh per-call context (60s ceiling) and ships the matching
  tree.list.result envelope. Errors surface in result.Error rather
  than as transport failures so the server-side waiter can render a
  sensible UI message.
- internal/server/http/tree_cache.go is the per-wizard-session cache
  layer (~30min TTL, sweep-on-access) that fetchTreeWithCache uses
  before falling through to SendRPC. Cached on success only; agent
  errors aren't cached so a transient failure doesn't poison the
  session.

Tests:
- internal/restic/ls_test.go covers parseLsChildren at root / mid-tree
  / leaf, plus normalizeTreePath and isDirectChild edge cases.
- internal/server/ws/rpc_test.go unit-tests the registry: round-trip,
  release semantics, concurrent waiters, ctx-cancel.
- internal/server/http/tree_rpc_test.go is the full round-trip: server
  SendRPC → fake-agent over a real WS → reply → server gets the
  payload. Plus a timeout test that confirms ~300ms timeouts terminate
  in ~300ms rather than waiting forever.

The cache is plumbed but no UI handler hits fetchTreeWithCache yet —
that lands with P3-01 (wizard backend). The unused-linter is suppressed
via nolint until the wizard wires it in.

internal/api/messages.go

@@ -337,3 +337,37 @@ type AgentUpdateAvailablePayload struct {
 	PackageURL    string `json:"package_url"` // apt repo / choco source
 	Changelog     string `json:"changelog,omitempty"`
 }
+
+// TreeListRequestPayload is the body of a tree.list RPC. Used by the
+// restore wizard to lazy-load directory contents from a snapshot.
+//
+// The exchange is synchronous: the server marshals MsgTreeList with a
+// fresh Envelope.ID, sends to the agent, blocks on a channel keyed by
+// that ID. The agent runs `restic ls --json <SnapshotID> <Path>`,
+// emits direct children, and replies with MsgTreeListResult carrying
+// the same ID. The server-side handler matches on ID and forwards to
+// the waiting channel. See internal/server/ws/rpc.go for the helper.
+type TreeListRequestPayload struct {
+	SnapshotID string `json:"snapshot_id"`
+	Path       string `json:"path"` // absolute path inside the snapshot, "/" for root
+}
+
+// TreeListEntry is one direct child returned by a tree.list call.
+// Type is "dir" | "file" | "symlink"; size is best-effort (zero on
+// directories and symlinks).
+type TreeListEntry struct {
+	Name string `json:"name"`
+	Type string `json:"type"`
+	Size int64  `json:"size,omitempty"`
+}
+
+// TreeListResultPayload is the reply to a tree.list. Error is set
+// when the agent couldn't fulfill the request (missing snapshot,
+// path doesn't exist, restic invocation failed); Entries is empty in
+// that case. A successful empty directory has Error="" + nil Entries.
+type TreeListResultPayload struct {
+	SnapshotID string          `json:"snapshot_id"`
+	Path       string          `json:"path"`
+	Entries    []TreeListEntry `json:"entries,omitempty"`
+	Error      string          `json:"error,omitempty"`
+}

internal/api/wire.go

@@ -12,18 +12,19 @@ type MessageType string
 
 // Agent → server message types.
 const (
-	MsgHello         MessageType = "hello"
-	MsgHeartbeat     MessageType = "heartbeat"
-	MsgJobStarted    MessageType = "job.started"
-	MsgJobProgress   MessageType = "job.progress"
-	MsgJobFinished   MessageType = "job.finished"
-	MsgSnapshotsRpt  MessageType = "snapshots.report"
-	MsgRepoStats     MessageType = "repo.stats"
-	MsgLogStream     MessageType = "log.stream"
-	MsgScheduleAck   MessageType = "schedule.ack"
-	MsgScheduleFire  MessageType = "schedule.fire"  // agent: a local cron entry fired, please dispatch a job
-	MsgCommandResult MessageType = "command.result" // ack for command.run
-	MsgError         MessageType = "error"
+	MsgHello          MessageType = "hello"
+	MsgHeartbeat      MessageType = "heartbeat"
+	MsgJobStarted     MessageType = "job.started"
+	MsgJobProgress    MessageType = "job.progress"
+	MsgJobFinished    MessageType = "job.finished"
+	MsgSnapshotsRpt   MessageType = "snapshots.report"
+	MsgRepoStats      MessageType = "repo.stats"
+	MsgLogStream      MessageType = "log.stream"
+	MsgScheduleAck    MessageType = "schedule.ack"
+	MsgScheduleFire   MessageType = "schedule.fire"    // agent: a local cron entry fired, please dispatch a job
+	MsgCommandResult  MessageType = "command.result"   // ack for command.run
+	MsgTreeListResult MessageType = "tree.list.result" // reply to a server-driven tree.list
+	MsgError          MessageType = "error"
 )
 
 // Server → agent message types.
@@ -33,6 +34,7 @@ const (
 	MsgScheduleSet      MessageType = "schedule.set"
 	MsgConfigUpdate     MessageType = "config.update"
 	MsgAgentUpdateAvail MessageType = "agent.update.available"
+	MsgTreeList         MessageType = "tree.list" // sync RPC: list a snapshot's children
 )
 
 // Envelope is the framing for every WS message in either direction.

internal/restic/ls.go

@@ -0,0 +1,140 @@
+package restic
+
+import (
+	"bufio"
+	"bytes"
+	"context"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"io"
+	"os/exec"
+	"path"
+	"strings"
+)
+
+// LsEntry is one node from `restic ls --json`. Restic emits these as
+// line-delimited JSON; we keep only the fields the restore wizard
+// needs.
+type LsEntry struct {
+	Name   string `json:"name"`
+	Type   string `json:"type"`
+	Path   string `json:"path"`
+	Size   int64  `json:"size,omitempty"`
+	Struct string `json:"struct_type,omitempty"`
+}
+
+// ListTreeChildren runs `restic ls --json <snapshot> <dirPath>` and
+// returns only the direct children of dirPath. Restic ls is recursive
+// by default, so we filter post-hoc — for a typical interactive
+// drill-down ("expand /etc/nginx") the subtree is small (a few KB of
+// JSON); for huge subtrees this is suboptimal but correct.
+//
+// The first emitted line is restic's "snapshot" preamble (struct_type
+// = "snapshot") which we discard. Subsequent lines are nodes; we
+// match on path equal to dirPath + "/" + name (with normalization so
+// trailing slashes don't break the comparison).
+//
+// dirPath="" or "/" lists the snapshot root.
+func (e Env) ListTreeChildren(ctx context.Context, snapshotID, dirPath string) ([]LsEntry, error) {
+	if snapshotID == "" {
+		return nil, fmt.Errorf("restic ls: snapshot id required")
+	}
+	parent := normalizeTreePath(dirPath)
+
+	args := []string{"ls", "--json", snapshotID}
+	if parent != "/" {
+		args = append(args, parent)
+	}
+	cmd := e.resticCmd(ctx, args...)
+
+	var stderr bytes.Buffer
+	cmd.Stderr = &stderr
+	stdout, err := cmd.StdoutPipe()
+	if err != nil {
+		return nil, fmt.Errorf("restic ls: stdout pipe: %w", err)
+	}
+	if err := cmd.Start(); err != nil {
+		return nil, fmt.Errorf("restic ls: start: %w", err)
+	}
+
+	out, parseErr := parseLsChildren(stdout, parent)
+
+	werr := cmd.Wait()
+	if werr != nil {
+		var ee *exec.ExitError
+		if errors.As(werr, &ee) {
+			return nil, fmt.Errorf("restic ls: exit %d: %s",
+				ee.ExitCode(), strings.TrimSpace(stderr.String()))
+		}
+		return nil, fmt.Errorf("restic ls: %w", werr)
+	}
+	if parseErr != nil {
+		return nil, parseErr
+	}
+	return out, nil
+}
+
+// parseLsChildren reads line-delimited JSON from r and returns nodes
+// whose Path is a direct child of parent. Exposed for testing.
+func parseLsChildren(r io.Reader, parent string) ([]LsEntry, error) {
+	scanner := bufio.NewScanner(r)
+	scanner.Buffer(make([]byte, 0, 64*1024), 1024*1024)
+	var out []LsEntry
+	for scanner.Scan() {
+		line := scanner.Bytes()
+		if len(line) == 0 {
+			continue
+		}
+		var entry LsEntry
+		if err := json.Unmarshal(line, &entry); err != nil {
+			return nil, fmt.Errorf("restic ls: parse line: %w", err)
+		}
+		// Skip the snapshot preamble and any future struct_type
+		// entries we don't care about.
+		if entry.Struct == "snapshot" || entry.Path == "" {
+			continue
+		}
+		if isDirectChild(entry.Path, parent) {
+			out = append(out, entry)
+		}
+	}
+	if err := scanner.Err(); err != nil {
+		return nil, fmt.Errorf("restic ls: read output: %w", err)
+	}
+	return out, nil
+}
+
+// normalizeTreePath turns "" / "/" / "/etc/" / "etc" all into a
+// canonical absolute form with a leading slash and no trailing slash
+// (except the root, which is "/" alone).
+func normalizeTreePath(p string) string {
+	p = strings.TrimSpace(p)
+	if p == "" || p == "/" {
+		return "/"
+	}
+	if !strings.HasPrefix(p, "/") {
+		p = "/" + p
+	}
+	cleaned := path.Clean(p)
+	return cleaned
+}
+
+// isDirectChild reports whether childPath is a direct child of parent.
+// "/etc/nginx" is a direct child of "/etc"; "/etc/nginx/conf" is not.
+// "/etc" is a direct child of "/".
+func isDirectChild(childPath, parent string) bool {
+	cp := normalizeTreePath(childPath)
+	pp := normalizeTreePath(parent)
+	if pp == "/" {
+		// Direct children of root: exactly one slash-delimited segment.
+		return cp != "/" && strings.Count(cp, "/") == 1
+	}
+	// Must start with parent + "/" and have no further slashes.
+	prefix := pp + "/"
+	if !strings.HasPrefix(cp, prefix) {
+		return false
+	}
+	rest := cp[len(prefix):]
+	return rest != "" && !strings.Contains(rest, "/")
+}

internal/restic/ls_test.go

@@ -0,0 +1,123 @@
+package restic
+
+import (
+	"strings"
+	"testing"
+)
+
+// realistic restic ls --json output sample. First line is the
+// snapshot preamble, subsequent lines are nodes. Trimmed to a few
+// entries that exercise depth filtering.
+const sampleLsOutput = `{"struct_type":"snapshot","time":"2026-05-04T09:14:00Z","id":"f3a7b2c1"}
+{"name":"etc","type":"dir","path":"/etc","permissions":"drwxr-xr-x","struct_type":"node"}
+{"name":"nginx","type":"dir","path":"/etc/nginx","permissions":"drwxr-xr-x","struct_type":"node"}
+{"name":"nginx.conf","type":"file","path":"/etc/nginx/nginx.conf","size":2400,"struct_type":"node"}
+{"name":"sites-available","type":"dir","path":"/etc/nginx/sites-available","struct_type":"node"}
+{"name":"alfa.conf","type":"file","path":"/etc/nginx/sites-available/alfa.conf","size":3100,"struct_type":"node"}
+{"name":"default.conf","type":"file","path":"/etc/nginx/sites-available/default.conf","size":2900,"struct_type":"node"}
+`
+
+func TestParseLsChildrenAtRoot(t *testing.T) {
+	t.Parallel()
+	entries, err := parseLsChildren(strings.NewReader(sampleLsOutput), "/")
+	if err != nil {
+		t.Fatalf("parse: %v", err)
+	}
+	if len(entries) != 1 {
+		t.Fatalf("entries: got %d (%+v), want 1", len(entries), entries)
+	}
+	if entries[0].Name != "etc" || entries[0].Path != "/etc" || entries[0].Type != "dir" {
+		t.Fatalf("entry: %+v", entries[0])
+	}
+}
+
+func TestParseLsChildrenAtEtc(t *testing.T) {
+	t.Parallel()
+	entries, err := parseLsChildren(strings.NewReader(sampleLsOutput), "/etc")
+	if err != nil {
+		t.Fatalf("parse: %v", err)
+	}
+	if len(entries) != 1 {
+		t.Fatalf("entries: got %d, want 1 (just nginx, not nested children)", len(entries))
+	}
+	if entries[0].Name != "nginx" {
+		t.Fatalf("entry: %+v", entries[0])
+	}
+}
+
+func TestParseLsChildrenAtNginx(t *testing.T) {
+	t.Parallel()
+	entries, err := parseLsChildren(strings.NewReader(sampleLsOutput), "/etc/nginx")
+	if err != nil {
+		t.Fatalf("parse: %v", err)
+	}
+	if len(entries) != 2 {
+		t.Fatalf("entries: got %d (%+v), want 2 (nginx.conf + sites-available, not nested)",
+			len(entries), entries)
+	}
+	gotNames := []string{entries[0].Name, entries[1].Name}
+	want := map[string]bool{"nginx.conf": true, "sites-available": true}
+	for _, n := range gotNames {
+		if !want[n] {
+			t.Errorf("unexpected name %q in result", n)
+		}
+	}
+}
+
+func TestParseLsChildrenAtSitesAvailable(t *testing.T) {
+	t.Parallel()
+	entries, err := parseLsChildren(strings.NewReader(sampleLsOutput), "/etc/nginx/sites-available")
+	if err != nil {
+		t.Fatalf("parse: %v", err)
+	}
+	if len(entries) != 2 {
+		t.Fatalf("entries: got %d, want 2", len(entries))
+	}
+	for _, e := range entries {
+		if e.Type != "file" {
+			t.Errorf("expected file type, got %q on %q", e.Type, e.Name)
+		}
+	}
+}
+
+func TestNormalizeTreePath(t *testing.T) {
+	t.Parallel()
+	cases := []struct{ in, want string }{
+		{"", "/"},
+		{"/", "/"},
+		{"/etc", "/etc"},
+		{"/etc/", "/etc"},
+		{"etc/nginx", "/etc/nginx"},
+		{"/etc//nginx", "/etc/nginx"},
+		{"/etc/./nginx", "/etc/nginx"},
+	}
+	for _, c := range cases {
+		got := normalizeTreePath(c.in)
+		if got != c.want {
+			t.Errorf("normalizeTreePath(%q): got %q, want %q", c.in, got, c.want)
+		}
+	}
+}
+
+func TestIsDirectChild(t *testing.T) {
+	t.Parallel()
+	cases := []struct {
+		child, parent string
+		want          bool
+	}{
+		{"/etc", "/", true},
+		{"/etc/nginx", "/", false},
+		{"/etc/nginx", "/etc", true},
+		{"/etc/nginx/conf", "/etc", false},
+		{"/etc/nginx/conf", "/etc/nginx", true},
+		{"/etc", "/etc", false},
+		{"/etcc", "/etc", false}, // prefix match guard
+	}
+	for _, c := range cases {
+		got := isDirectChild(c.child, c.parent)
+		if got != c.want {
+			t.Errorf("isDirectChild(%q, %q): got %v, want %v",
+				c.child, c.parent, got, c.want)
+		}
+	}
+}

internal/server/http/server.go

@@ -58,6 +58,11 @@ type Server struct {
 	// pending_id so the accept/reject handlers can push the bearer
 	// or close cleanly (P2-18b).
 	pendingHub *pendingHub
+
+	// treeCache holds per-wizard-session listings of snapshot
+	// directories (P3-X2). Pre-allocated in New so the lazy-init
+	// race is impossible.
+	treeCache *treeCache
 }
 
 // New builds a configured but not-yet-started server.
@@ -81,6 +86,7 @@ func New(deps Deps) *Server {
 		drainLocks: make(map[string]*sync.Mutex),
 		announceRL: newAnnounceLimiter(),
 		pendingHub: newPendingHub(),
+		treeCache:  newTreeCache(),
 	}
 	s.routes(r)
 

internal/server/http/tree_cache.go

@@ -0,0 +1,112 @@
+package http
+
+import (
+	"context"
+	"sync"
+	"time"
+
+	"gitea.dcglab.co.uk/steve/restic-manager/internal/api"
+)
+
+// treeCacheTTL is how long a per-session cached directory listing
+// stays valid. The whole point of the cache is to make re-expanding
+// nodes within the same wizard session snappy; 30 minutes covers a
+// generous wizard interaction window without holding stale data
+// indefinitely.
+const treeCacheTTL = 30 * time.Minute
+
+// treeCacheKey identifies one cached listing. session_id scopes
+// entries to a single browser session so two operators don't share
+// view state; snapshot_id + path identify the directory inside the
+// snapshot.
+type treeCacheKey struct {
+	SessionID  string
+	HostID     string
+	SnapshotID string
+	Path       string
+}
+
+type treeCacheEntry struct {
+	Result    api.TreeListResultPayload
+	ExpiresAt time.Time
+}
+
+// treeCache is a per-process map of synchronously fetched directory
+// listings. Concurrency is light (a few entries per active wizard
+// session) so a single mutex is fine.
+type treeCache struct {
+	mu      sync.Mutex
+	entries map[treeCacheKey]treeCacheEntry
+}
+
+func newTreeCache() *treeCache {
+	return &treeCache{entries: make(map[treeCacheKey]treeCacheEntry)}
+}
+
+// Get returns a cached entry if one exists and hasn't expired.
+func (c *treeCache) Get(k treeCacheKey, now time.Time) (api.TreeListResultPayload, bool) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	e, ok := c.entries[k]
+	if !ok {
+		return api.TreeListResultPayload{}, false
+	}
+	if now.After(e.ExpiresAt) {
+		delete(c.entries, k)
+		return api.TreeListResultPayload{}, false
+	}
+	return e.Result, true
+}
+
+// Put records a fresh listing under k. Caller is responsible for
+// having validated the result first (Error == "").
+func (c *treeCache) Put(k treeCacheKey, result api.TreeListResultPayload, now time.Time) {
+	c.mu.Lock()
+	c.entries[k] = treeCacheEntry{
+		Result:    result,
+		ExpiresAt: now.Add(treeCacheTTL),
+	}
+	c.mu.Unlock()
+}
+
+// Sweep deletes expired entries. Called opportunistically from the
+// wizard handler — no separate goroutine needed; cache size is small.
+func (c *treeCache) Sweep(now time.Time) {
+	c.mu.Lock()
+	for k, e := range c.entries {
+		if now.After(e.ExpiresAt) {
+			delete(c.entries, k)
+		}
+	}
+	c.mu.Unlock()
+}
+
+// fetchTreeWithCache returns a directory listing — cache hit, or a
+// synchronous tree.list RPC against the agent on miss. On agent error
+// (not transport error), the result is returned as-is with Error set
+// rather than cached, so a transient failure doesn't poison subsequent
+// requests for the same path.
+//
+//nolint:unused // wired in by the wizard handler in the next slice
+func (s *Server) fetchTreeWithCache(ctx context.Context, sessionID, hostID, snapshotID, path string) (api.TreeListResultPayload, error) {
+	now := time.Now()
+	k := treeCacheKey{SessionID: sessionID, HostID: hostID, SnapshotID: snapshotID, Path: path}
+	if cached, ok := s.treeCache.Get(k, now); ok {
+		return cached, nil
+	}
+
+	reply, err := s.deps.Hub.SendRPC(ctx, hostID, api.MsgTreeList,
+		api.TreeListRequestPayload{SnapshotID: snapshotID, Path: path},
+		30*time.Second)
+	if err != nil {
+		return api.TreeListResultPayload{}, err
+	}
+	var result api.TreeListResultPayload
+	if perr := reply.UnmarshalPayload(&result); perr != nil {
+		return api.TreeListResultPayload{}, perr
+	}
+	if result.Error == "" {
+		s.treeCache.Put(k, result, now)
+	}
+	return result, nil
+}

internal/server/http/tree_rpc_test.go

@@ -0,0 +1,146 @@
+// tree_rpc_test.go — full round-trip test for the tree.list synchronous
+// RPC (P3-X2). A fake agent reads the inbound tree.list, replies with a
+// canned tree.list.result, and we assert the server's SendRPC returned
+// the expected payload.
+package http
+
+import (
+	"context"
+	"encoding/json"
+	"testing"
+	"time"
+
+	"github.com/coder/websocket"
+
+	"gitea.dcglab.co.uk/steve/restic-manager/internal/api"
+)
+
+func TestSendRPCTreeListRoundTrip(t *testing.T) {
+	t.Parallel()
+	srv, ts, st := rawTestServer(t)
+	hostID, token := enrolHostForWS(t, srv, st, "rpc-host")
+	c := agentDial(t, srv, ts, hostID, token)
+	sendHello(t, c, "rpc-host")
+	_ = drainUntil(t, c, api.MsgScheduleSet)
+
+	// Fake agent: read inbound envelopes, mirror tree.list with a
+	// canned result. Other inbound envelopes (config.update etc) are
+	// already drained above.
+	done := make(chan error, 1)
+	go func() {
+		ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+		defer cancel()
+		for {
+			mt, raw, err := c.Read(ctx)
+			if err != nil {
+				done <- err
+				return
+			}
+			if mt != websocket.MessageText {
+				continue
+			}
+			var env api.Envelope
+			if err := json.Unmarshal(raw, &env); err != nil {
+				done <- err
+				return
+			}
+			if env.Type != api.MsgTreeList {
+				continue
+			}
+			var req api.TreeListRequestPayload
+			if err := env.UnmarshalPayload(&req); err != nil {
+				done <- err
+				return
+			}
+			result := api.TreeListResultPayload{
+				SnapshotID: req.SnapshotID,
+				Path:       req.Path,
+				Entries: []api.TreeListEntry{
+					{Name: "etc", Type: "dir"},
+					{Name: "var", Type: "dir"},
+				},
+			}
+			out, err := api.Marshal(api.MsgTreeListResult, env.ID, result)
+			if err != nil {
+				done <- err
+				return
+			}
+			rawOut, _ := json.Marshal(out)
+			if err := c.Write(ctx, websocket.MessageText, rawOut); err != nil {
+				done <- err
+				return
+			}
+			done <- nil
+			return
+		}
+	}()
+
+	// Server-side SendRPC.
+	ctx, cancel := context.WithTimeout(context.Background(), 4*time.Second)
+	defer cancel()
+	reply, err := srv.deps.Hub.SendRPC(ctx, hostID, api.MsgTreeList,
+		api.TreeListRequestPayload{SnapshotID: "f3a7b2c1", Path: "/"},
+		3*time.Second)
+	if err != nil {
+		t.Fatalf("SendRPC: %v", err)
+	}
+	if reply.Type != api.MsgTreeListResult {
+		t.Fatalf("reply type: got %q want %q", reply.Type, api.MsgTreeListResult)
+	}
+	var result api.TreeListResultPayload
+	if err := reply.UnmarshalPayload(&result); err != nil {
+		t.Fatalf("unmarshal reply: %v", err)
+	}
+	if result.SnapshotID != "f3a7b2c1" || result.Path != "/" {
+		t.Fatalf("payload: got %+v", result)
+	}
+	if len(result.Entries) != 2 || result.Entries[0].Name != "etc" {
+		t.Fatalf("entries: %+v", result.Entries)
+	}
+
+	// Make sure the fake agent didn't error out.
+	select {
+	case err := <-done:
+		if err != nil {
+			t.Fatalf("fake agent: %v", err)
+		}
+	case <-time.After(2 * time.Second):
+		t.Fatal("fake agent didn't finish")
+	}
+}
+
+// TestSendRPCTimeoutNoReply: SendRPC times out cleanly when the agent
+// never replies; the registry entry is released so a stray late reply
+// wouldn't deadlock anything.
+func TestSendRPCTimeoutNoReply(t *testing.T) {
+	t.Parallel()
+	srv, ts, st := rawTestServer(t)
+	hostID, token := enrolHostForWS(t, srv, st, "rpc-timeout-host")
+	c := agentDial(t, srv, ts, hostID, token)
+	sendHello(t, c, "rpc-timeout-host")
+	_ = drainUntil(t, c, api.MsgScheduleSet)
+
+	// Fake agent reads but never replies.
+	go func() {
+		ctx, cancel := context.WithTimeout(context.Background(), 3*time.Second)
+		defer cancel()
+		for {
+			if _, _, err := c.Read(ctx); err != nil {
+				return
+			}
+		}
+	}()
+
+	ctx := context.Background()
+	t0 := time.Now()
+	_, err := srv.deps.Hub.SendRPC(ctx, hostID, api.MsgTreeList,
+		api.TreeListRequestPayload{SnapshotID: "x", Path: "/"},
+		300*time.Millisecond)
+	if err == nil {
+		t.Fatal("expected timeout error")
+	}
+	elapsed := time.Since(t0)
+	if elapsed < 250*time.Millisecond || elapsed > 2*time.Second {
+		t.Fatalf("timeout took %s, expected ~300ms", elapsed)
+	}
+}

internal/server/ws/handler.go

@@ -297,6 +297,20 @@ func dispatchAgentMessage(ctx context.Context, c *Conn, hostID string, env api.E
 		// (job.started → job.finished) is sufficient signal.
 		slog.Debug("ws msg not yet handled", "type", env.Type, "host_id", hostID)
 
+	case api.MsgTreeListResult:
+		// Reply to a synchronous tree.list RPC. Route to the waiter
+		// registered against the request envelope's ID; if none is
+		// registered the caller already gave up (ctx expired) — drop
+		// the stray reply quietly.
+		if env.ID == "" {
+			slog.Warn("ws: tree.list.result missing envelope ID", "host_id", hostID)
+			break
+		}
+		if !deps.Hub.rpcs.resolve(env.ID, env) {
+			slog.Debug("ws: tree.list.result with no waiter (timeout?)",
+				"id", env.ID, "host_id", hostID)
+		}
+
 	case api.MsgError:
 		var ep api.ErrorPayload
 		_ = env.UnmarshalPayload(&ep)

internal/server/ws/hub.go

@@ -21,6 +21,11 @@ import (
 type Hub struct {
 	mu    sync.RWMutex
 	conns map[string]*Conn // hostID → conn
+
+	// rpcs tracks in-flight synchronous RPC calls (e.g. tree.list).
+	// See rpc.go for details. Lazy-initialized via the registry's
+	// own register() so callers don't have to juggle a constructor.
+	rpcs rpcRegistry
 }
 
 // NewHub returns an empty hub.

internal/server/ws/rpc.go

@@ -0,0 +1,112 @@
+package ws
+
+import (
+	"context"
+	"errors"
+	"sync"
+	"time"
+
+	"github.com/oklog/ulid/v2"
+
+	"gitea.dcglab.co.uk/steve/restic-manager/internal/api"
+)
+
+// rpcRegistry holds in-flight synchronous RPC calls. SendRPC registers
+// a channel keyed by the request envelope's ID; the WS read loop's
+// dispatcher routes incoming reply envelopes to the matching channel
+// when their type is one of the known reply types (currently just
+// tree.list.result).
+//
+// A single global registry keyed by envelope ID is fine because IDs
+// are ULIDs — globally unique without coordinating across hubs.
+type rpcRegistry struct {
+	mu      sync.Mutex
+	pending map[string]chan api.Envelope
+}
+
+// register reserves a channel for the given request ID. The channel
+// is buffered (cap 1) so a slow waiter doesn't block the read loop's
+// dispatcher when the reply lands.
+func (r *rpcRegistry) register(id string) chan api.Envelope {
+	ch := make(chan api.Envelope, 1)
+	r.mu.Lock()
+	if r.pending == nil {
+		r.pending = make(map[string]chan api.Envelope)
+	}
+	r.pending[id] = ch
+	r.mu.Unlock()
+	return ch
+}
+
+// resolve delivers an envelope to its waiter and removes the entry.
+// Returns whether a waiter was actually present (the dispatcher uses
+// this to decide whether to log a stray-reply warning).
+func (r *rpcRegistry) resolve(id string, env api.Envelope) bool {
+	r.mu.Lock()
+	ch, ok := r.pending[id]
+	if ok {
+		delete(r.pending, id)
+	}
+	r.mu.Unlock()
+	if !ok {
+		return false
+	}
+	// Buffered chan cap 1 — non-blocking send. The waiter goroutine
+	// owns the receive side so this is the only sender.
+	ch <- env
+	close(ch)
+	return true
+}
+
+// release abandons the entry without delivering a value. Used when
+// the caller's context expires before a reply arrives — the next
+// stray reply (if any) will hit the no-waiter case in resolve and
+// just be dropped.
+func (r *rpcRegistry) release(id string) {
+	r.mu.Lock()
+	delete(r.pending, id)
+	r.mu.Unlock()
+}
+
+// SendRPC sends a request envelope to the host and blocks until a
+// matching reply lands or the context expires. The hub picks a fresh
+// envelope ID, marshals the payload, registers a waiter, and sends.
+//
+// timeout caps the wait; a too-aggressive value relative to the
+// expected restic-side latency will leak the registry entry until the
+// reply finally arrives (which is then silently dropped). The default
+// callers use is 30s, which covers a slow network round-trip plus a
+// restic ls invocation against a remote rest-server.
+//
+// If the host disconnects mid-flight, the read loop ends and no reply
+// will ever come — the caller's ctx.Done()/timeout is the only path
+// out. We could pre-fail by tracking conn lifetime, but the bound
+// keeps the code simple and the worst case is a 30s wait.
+func (h *Hub) SendRPC(ctx context.Context, hostID string, reqType api.MessageType, payload any, timeout time.Duration) (api.Envelope, error) {
+	if timeout <= 0 {
+		timeout = 30 * time.Second
+	}
+	id := ulid.Make().String()
+	env, err := api.Marshal(reqType, id, payload)
+	if err != nil {
+		return api.Envelope{}, err
+	}
+
+	ch := h.rpcs.register(id)
+
+	if err := h.Send(ctx, hostID, env); err != nil {
+		h.rpcs.release(id)
+		return api.Envelope{}, err
+	}
+
+	select {
+	case reply := <-ch:
+		return reply, nil
+	case <-ctx.Done():
+		h.rpcs.release(id)
+		return api.Envelope{}, ctx.Err()
+	case <-time.After(timeout):
+		h.rpcs.release(id)
+		return api.Envelope{}, errors.New("ws rpc: timed out waiting for reply")
+	}
+}

internal/server/ws/rpc_test.go

@@ -0,0 +1,122 @@
+package ws
+
+import (
+	"context"
+	"encoding/json"
+	"sync"
+	"testing"
+	"time"
+
+	"github.com/oklog/ulid/v2"
+
+	"gitea.dcglab.co.uk/steve/restic-manager/internal/api"
+)
+
+// TestRPCRegistryRoundTrip: register a waiter, resolve it, get the
+// envelope back. Cover the no-waiter and double-resolve cases too.
+func TestRPCRegistryRoundTrip(t *testing.T) {
+	t.Parallel()
+	var r rpcRegistry
+	id := ulid.Make().String()
+	ch := r.register(id)
+
+	want := api.Envelope{Type: api.MsgTreeListResult, ID: id, Payload: json.RawMessage(`{"path":"/"}`)}
+	if !r.resolve(id, want) {
+		t.Fatal("resolve: returned false for registered id")
+	}
+	got := <-ch
+	if got.ID != id {
+		t.Fatalf("id mismatch: got %q want %q", got.ID, id)
+	}
+
+	// A second resolve for the same id has no waiter and should not panic.
+	if r.resolve(id, want) {
+		t.Fatal("resolve: returned true for already-resolved id")
+	}
+}
+
+// TestRPCRegistryRelease: release abandons the waiter; a subsequent
+// resolve is a no-op (no goroutine leak, no panic).
+func TestRPCRegistryRelease(t *testing.T) {
+	t.Parallel()
+	var r rpcRegistry
+	id := ulid.Make().String()
+	_ = r.register(id)
+	r.release(id)
+	if r.resolve(id, api.Envelope{ID: id}) {
+		t.Fatal("resolve after release: should be no-op")
+	}
+}
+
+// TestRPCRegistryConcurrent: many waiters in flight concurrently get
+// only their own reply. This catches buggy keying/locking.
+func TestRPCRegistryConcurrent(t *testing.T) {
+	t.Parallel()
+	var r rpcRegistry
+	const n = 64
+
+	ids := make([]string, n)
+	chs := make([]chan api.Envelope, n)
+	for i := 0; i < n; i++ {
+		ids[i] = ulid.Make().String()
+		chs[i] = r.register(ids[i])
+	}
+
+	// Resolve in random-ish order from many goroutines.
+	var wg sync.WaitGroup
+	for i := 0; i < n; i++ {
+		wg.Add(1)
+		go func(idx int) {
+			defer wg.Done()
+			r.resolve(ids[idx], api.Envelope{ID: ids[idx], Type: api.MsgTreeListResult})
+		}(i)
+	}
+	wg.Wait()
+
+	for i := 0; i < n; i++ {
+		select {
+		case got := <-chs[i]:
+			if got.ID != ids[i] {
+				t.Fatalf("waiter %d: got id %q want %q", i, got.ID, ids[i])
+			}
+		case <-time.After(2 * time.Second):
+			t.Fatalf("waiter %d: timed out", i)
+		}
+	}
+}
+
+// TestSendRPCContextCancelReleases ensures that canceling the caller's
+// ctx releases the registry entry so a stray late reply is harmlessly
+// dropped. Skips if the hub isn't reachable for direct access — this
+// is purely a unit test on the registry path inside SendRPC.
+func TestSendRPCContextCancelReleases(t *testing.T) {
+	t.Parallel()
+	h := NewHub()
+
+	// No host registered, so Hub.Send returns "host offline" and
+	// SendRPC bails without ever waiting. We test the timeout/ctx
+	// path by going through register() directly.
+	id := ulid.Make().String()
+	ch := h.rpcs.register(id)
+
+	ctx, cancel := context.WithCancel(context.Background())
+	go func() {
+		time.Sleep(20 * time.Millisecond)
+		cancel()
+	}()
+
+	// Simulate the SendRPC select: ctx wins.
+	select {
+	case <-ch:
+		t.Fatal("unexpected reply")
+	case <-ctx.Done():
+		h.rpcs.release(id)
+	}
+
+	// Now a late reply should not block (ch is still open but no
+	// receiver — buffered size 1 absorbs it).
+	resolved := h.rpcs.resolve(id, api.Envelope{ID: id})
+	if resolved {
+		t.Fatal("resolve after release should return false")
+	}
+}