nbd/client-connection: add option for non-blocking connection attempt

We'll need a possibility of non-blocking nbd_co_establish_connection(),
so that it returns immediately, and it returns success only if a
connections was previously established in background.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20210610100802.5888-30-vsementsov@virtuozzo.com>
Signed-off-by: Eric Blake <eblake@redhat.com>

block/nbd.c

@@ -364,7 +364,7 @@ static int coroutine_fn nbd_co_do_establish_connection(BlockDriverState *bs,
 
     assert(!s->ioc);
 
-    s->ioc = nbd_co_establish_connection(s->conn, &s->info, errp);
+    s->ioc = nbd_co_establish_connection(s->conn, &s->info, true, errp);
     if (!s->ioc) {
         return -ECONNREFUSED;
     }

include/block/nbd.h

@@ -420,7 +420,7 @@ void nbd_client_connection_release(NBDClientConnection *conn);
 
 QIOChannel *coroutine_fn
 nbd_co_establish_connection(NBDClientConnection *conn, NBDExportInfo *info,
-                            Error **errp);
+                            bool blocking, Error **errp);
 
 void coroutine_fn nbd_co_establish_connection_cancel(NBDClientConnection *conn);
 

nbd/client-connection.c

@@ -266,6 +266,8 @@ void nbd_client_connection_release(NBDClientConnection *conn)
  *   otherwise the thread is not running, so start a thread and wait for
  *     completion
  *
+ * If @blocking is false, don't wait for the thread, return immediately.
+ *
  * If @info is not NULL, also do nbd-negotiation after successful connection.
  * In this case info is used only as out parameter, and is fully initialized by
  * nbd_co_establish_connection(). "IN" fields of info as well as related only to
@@ -274,7 +276,7 @@ void nbd_client_connection_release(NBDClientConnection *conn)
  */
 QIOChannel *coroutine_fn
 nbd_co_establish_connection(NBDClientConnection *conn, NBDExportInfo *info,
-                            Error **errp)
+                            bool blocking, Error **errp)
 {
     QemuThread thread;
 
@@ -315,6 +317,10 @@ nbd_co_establish_connection(NBDClientConnection *conn, NBDExportInfo *info,
                                connect_thread_func, conn, QEMU_THREAD_DETACHED);
         }
 
+        if (!blocking) {
+            return NULL;
+        }
+
         conn->wait_co = qemu_coroutine_self();
     }