block/nbd: introduce nbd_client_connection_release()

This is a last step of creating bs-independent nbd connection
interface. With next commit we can finally move it to separate file.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20210610100802.5888-17-vsementsov@virtuozzo.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>

block/nbd.c

@@ -127,7 +127,7 @@ typedef struct BDRVNBDState {
     NBDClientConnection *conn;
 } BDRVNBDState;
 
-static void nbd_free_connect_thread(NBDClientConnection *conn);
+static void nbd_client_connection_release(NBDClientConnection *conn);
 static int nbd_establish_connection(BlockDriverState *bs, SocketAddress *saddr,
                                     Error **errp);
 static coroutine_fn QIOChannelSocket *
@@ -139,22 +139,9 @@ static void nbd_yank(void *opaque);
 static void nbd_clear_bdrvstate(BlockDriverState *bs)
 {
     BDRVNBDState *s = (BDRVNBDState *)bs->opaque;
-    NBDClientConnection *conn = s->conn;
-    bool do_free = false;
-
-    qemu_mutex_lock(&conn->mutex);
-    assert(!conn->detached);
-    if (conn->running) {
-        conn->detached = true;
-    } else {
-        do_free = true;
-    }
-    qemu_mutex_unlock(&conn->mutex);
 
-    /* the runaway thread will clean up itself */
-    if (do_free) {
-        nbd_free_connect_thread(conn);
-    }
+    nbd_client_connection_release(s->conn);
+    s->conn = NULL;
 
     yank_unregister_instance(BLOCKDEV_YANK_INSTANCE(bs->node_name));
 
@@ -372,7 +359,7 @@ nbd_client_connection_new(const SocketAddress *saddr)
     return conn;
 }
 
-static void nbd_free_connect_thread(NBDClientConnection *conn)
+static void nbd_client_connection_do_free(NBDClientConnection *conn)
 {
     if (conn->sioc) {
         qio_channel_close(QIO_CHANNEL(conn->sioc), NULL);
@@ -414,12 +401,34 @@ static void *connect_thread_func(void *opaque)
     qemu_mutex_unlock(&conn->mutex);
 
     if (do_free) {
-        nbd_free_connect_thread(conn);
+        nbd_client_connection_do_free(conn);
     }
 
     return NULL;
 }
 
+static void nbd_client_connection_release(NBDClientConnection *conn)
+{
+    bool do_free = false;
+
+    if (!conn) {
+        return;
+    }
+
+    qemu_mutex_lock(&conn->mutex);
+    assert(!conn->detached);
+    if (conn->running) {
+        conn->detached = true;
+    } else {
+        do_free = true;
+    }
+    qemu_mutex_unlock(&conn->mutex);
+
+    if (do_free) {
+        nbd_client_connection_do_free(conn);
+    }
+}
+
 /*
  * Get a new connection in context of @conn:
  *   if the thread is running, wait for completion