migration/multifd: Add an explicit MultiFDSendData destructor

This way if there are fields there that needs explicit disposal (like, for
example, some attached buffers) they will be handled appropriately.

Add a related assert to multifd_set_payload_type() in order to make sure
that this function is only used to fill a previously empty MultiFDSendData
with some payload, not the other way around.

Reviewed-by: Fabiano Rosas <farosas@suse.de>
Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>
Link: https://lore.kernel.org/qemu-devel/6755205f2b95abbed251f87061feee1c0e410836.1741124640.git.maciej.szmigiero@oracle.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>

migration/multifd-nocomp.c

@@ -42,8 +42,7 @@ void multifd_ram_save_setup(void)
 
 void multifd_ram_save_cleanup(void)
 {
-    g_free(multifd_ram_send);
+    g_clear_pointer(&multifd_ram_send, multifd_send_data_free);
-    multifd_ram_send = NULL;
 }
 
 static void multifd_set_file_bitmap(MultiFDSendParams *p)

migration/multifd.c

@@ -123,6 +123,32 @@ MultiFDSendData *multifd_send_data_alloc(void)
     return g_malloc0(size_minus_payload + max_payload_size);
 }
 
+void multifd_send_data_clear(MultiFDSendData *data)
+{
+    if (multifd_payload_empty(data)) {
+        return;
+    }
+
+    switch (data->type) {
+    default:
+        /* Nothing to do */
+        break;
+    }
+
+    data->type = MULTIFD_PAYLOAD_NONE;
+}
+
+void multifd_send_data_free(MultiFDSendData *data)
+{
+    if (!data) {
+        return;
+    }
+
+    multifd_send_data_clear(data);
+
+    g_free(data);
+}
+
 static bool multifd_use_packets(void)
 {
     return !migrate_mapped_ram();
@@ -496,8 +522,7 @@ static bool multifd_send_cleanup_channel(MultiFDSendParams *p, Error **errp)
     qemu_sem_destroy(&p->sem_sync);
     g_free(p->name);
     p->name = NULL;
-    g_free(p->data);
+    g_clear_pointer(&p->data, multifd_send_data_free);
-    p->data = NULL;
     p->packet_len = 0;
     g_free(p->packet);
     p->packet = NULL;
@@ -695,7 +720,7 @@ static void *multifd_send_thread(void *opaque)
                        (uint64_t)p->next_packet_size + p->packet_len);
 
             p->next_packet_size = 0;
-            multifd_set_payload_type(p->data, MULTIFD_PAYLOAD_NONE);
+            multifd_send_data_clear(p->data);
 
             /*
              * Making sure p->data is published before saying "we're

migration/multifd.h

@@ -149,6 +149,9 @@ static inline bool multifd_payload_empty(MultiFDSendData *data)
 static inline void multifd_set_payload_type(MultiFDSendData *data,
                                             MultiFDPayloadType type)
 {
+    assert(multifd_payload_empty(data));
+    assert(type != MULTIFD_PAYLOAD_NONE);
+
     data->type = type;
 }
 
@@ -365,6 +368,8 @@ static inline void multifd_send_prepare_header(MultiFDSendParams *p)
 void multifd_channel_connect(MultiFDSendParams *p, QIOChannel *ioc);
 bool multifd_send(MultiFDSendData **send_data);
 MultiFDSendData *multifd_send_data_alloc(void);
+void multifd_send_data_clear(MultiFDSendData *data);
+void multifd_send_data_free(MultiFDSendData *data);
 
 static inline uint32_t multifd_ram_page_size(void)
 {