net/filter-redirector: add support for dynamic status on/off switching

Currently, filter-redirector does not implement the status_changed
callback, which means the 'status' property cannot be used to
dynamically enable/disable the filter at runtime. When status is
set to 'off' via QMP/HMP, the filter still receives data from the
indev chardev because the chardev handlers remain registered.

This patch adds proper support for the 'status' property:

1. Implement filter_redirector_status_changed() callback:
   - When status changes to 'off': remove chardev read handlers
   - When status changes to 'on': re-register chardev handlers
     (only if chardev is already open)

2. Update filter_redirector_setup() to respect initial status:
   - If filter is created with status=off, do not register handlers
   - This allows creating disabled filters via command line or QMP

3. Handle chardev OPENED/CLOSED events to re-arm handlers on reconnect:
   - Keep the chr_event callback installed on CLOSE so a later OPENED
     can re-register the read handlers when nf->on
   - Use qemu_chr_fe_set_handlers_full(..., set_open=false, sync_state=false)
     instead of qemu_chr_fe_set_handlers() because the latter forces
     sync_state=true and may emit CHR_EVENT_OPENED for an already-open
     backend. Doing that from inside the chr_event callback would cause
     recursive/re-entrant OPENED handling.

Signed-off-by: Jason Wang <jasowang@redhat.com>

net/filter-mirror.c

@@ -179,9 +179,16 @@ static void redirector_chr_event(void *opaque, QEMUChrEvent event)
     MirrorState *s = FILTER_REDIRECTOR(nf);
 
     switch (event) {
+    case CHR_EVENT_OPENED:
+        if (nf->on) {
+            qemu_chr_fe_set_handlers_full(&s->chr_in, redirector_chr_can_read,
+                                          redirector_chr_read, redirector_chr_event,
+                                          NULL, nf, NULL, false, false);
+        }
+        break;
     case CHR_EVENT_CLOSED:
-        qemu_chr_fe_set_handlers(&s->chr_in, NULL, NULL, NULL,
-                                 NULL, NULL, NULL, true);
+        qemu_chr_fe_set_handlers_full(&s->chr_in, NULL, NULL, redirector_chr_event,
+                                      NULL, nf, NULL, false, false);
         break;
     default:
         break;
@@ -306,9 +313,11 @@ static void filter_redirector_setup(NetFilterState *nf, Error **errp)
             return;
         }
 
-        qemu_chr_fe_set_handlers(&s->chr_in, redirector_chr_can_read,
-                                 redirector_chr_read, redirector_chr_event,
-                                 NULL, nf, NULL, true);
+        if (nf->on) {
+            qemu_chr_fe_set_handlers(&s->chr_in, redirector_chr_can_read,
+                                     redirector_chr_read, redirector_chr_event,
+                                     NULL, nf, NULL, true);
+        }
     }
 
     if (s->outdev) {
@@ -324,6 +333,24 @@ static void filter_redirector_setup(NetFilterState *nf, Error **errp)
     }
 }
 
+static void filter_redirector_status_changed(NetFilterState *nf, Error **errp)
+{
+    MirrorState *s = FILTER_REDIRECTOR(nf);
+
+    if (!s->indev) {
+        return;
+    }
+
+    if (nf->on) {
+        qemu_chr_fe_set_handlers(&s->chr_in, redirector_chr_can_read,
+                                 redirector_chr_read, redirector_chr_event,
+                                 NULL, nf, NULL, true);
+    } else {
+        qemu_chr_fe_set_handlers(&s->chr_in, NULL, NULL, NULL,
+                                 NULL, NULL, NULL, true);
+    }
+}
+
 static char *filter_redirector_get_indev(Object *obj, Error **errp)
 {
     MirrorState *s = FILTER_REDIRECTOR(obj);
@@ -440,6 +467,7 @@ static void filter_redirector_class_init(ObjectClass *oc, const void *data)
     nfc->setup = filter_redirector_setup;
     nfc->cleanup = filter_redirector_cleanup;
     nfc->receive_iov = filter_redirector_receive_iov;
+    nfc->status_changed = filter_redirector_status_changed;
 }
 
 static void filter_mirror_init(Object *obj)