v0.335.0: no app stop for a tier the agent will refuse because its host is busy (host_busy pre-check, R-921)

controller/internal/agentapi/backup_tiers.go

@@ -132,7 +132,7 @@ func (c *Client) BackupStatusFor(ctx context.Context, target string) (StatusResp
 // tier is the agent's first reason to refuse POST /backup (HTTP 409), so the controller asks this BEFORE it
 // stops any app. It reads only GET /backup/tiers and GET /backup/status?target= (both agent >= v0.97.0); a
 // pre-R-82 agent (no /backup/tiers) answers nil, nil. Any other error is returned — the caller fails toward
-// backing up. It does NOT see the agent's host-wide heavy-operation gate: no endpoint serves it.
+// backing up. It does NOT see the agent's host-wide heavy-operation gate; HostBusy does (newer agents).
 func (c *Client) BackupJobsInFlight(ctx context.Context) ([]string, error) {
 	tiers, err := c.BackupTiers(ctx)
 	if errors.Is(err, ErrTiersUnsupported) {
@@ -154,6 +154,21 @@ func (c *Client) BackupJobsInFlight(ctx context.Context) ([]string, error) {
 	return out, nil
 }
 
+// HostBusy (R-921) reads GET /backup/status's host_busy: whether the agent's host-wide heavy-operation gate is held
+// right now (another guest's backup, the restore test, a night OS step …) — the agent's second reason to refuse
+// POST /backup. known=false: the agent does not serve the field (older than the agent release that adds it) — no
+// information, never „free". An error is returned; the caller fails toward backing up.
+func (c *Client) HostBusy(ctx context.Context) (busy bool, what string, known bool, err error) {
+	st, err := c.BackupStatusFor(ctx, "")
+	if err != nil {
+		return false, "", false, err
+	}
+	if st.HostBusy == nil {
+		return false, "", false, nil
+	}
+	return st.HostBusy.Busy, st.HostBusy.What, true, nil
+}
+
 // SetBackupTargetResponse mirrors POST /backup/target (agent >= v0.113.0).
 type SetBackupTargetResponse struct {
 	Target string `json:"target"`