nbd/client-connection: implement connection retry

Add an option for a thread to retry connecting until it succeeds. We'll
use nbd/client-connection both for reconnect and for initial connection
in nbd_open(), so we need a possibility to use same NBDClientConnection
instance to connect once in nbd_open() and then use retry semantics for
reconnect.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20210610100802.5888-21-vsementsov@virtuozzo.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
[eblake: grammar tweak]
Signed-off-by: Eric Blake <eblake@redhat.com>

include/block/nbd.h

@@ -409,6 +409,8 @@ const char *nbd_err_lookup(int err);
 /* nbd/client-connection.c */
 typedef struct NBDClientConnection NBDClientConnection;
 
+void nbd_client_connection_enable_retry(NBDClientConnection *conn);
+
 NBDClientConnection *nbd_client_connection_new(const SocketAddress *saddr,
                                                bool do_negotiation,
                                                const char *export_name,

nbd/client-connection.c

@@ -35,6 +35,7 @@ struct NBDClientConnection {
     QCryptoTLSCreds *tlscreds;
     NBDExportInfo initial_info;
     bool do_negotiation;
+    bool do_retry;
 
     QemuMutex mutex;
 
@@ -61,6 +62,15 @@ struct NBDClientConnection {
     Coroutine *wait_co;
 };
 
+/*
+ * The function isn't protected by any mutex, only call it when the client
+ * connection attempt has not yet started.
+ */
+void nbd_client_connection_enable_retry(NBDClientConnection *conn)
+{
+    conn->do_retry = true;
+}
+
 NBDClientConnection *nbd_client_connection_new(const SocketAddress *saddr,
                                                bool do_negotiation,
                                                const char *export_name,
@@ -155,7 +165,10 @@ static void *connect_thread_func(void *opaque)
     NBDClientConnection *conn = opaque;
     int ret;
     bool do_free;
+    uint64_t timeout = 1;
+    uint64_t max_timeout = 16;
 
+    while (true) {
         conn->sioc = qio_channel_socket_new();
 
         error_free(conn->err);
@@ -165,13 +178,30 @@ static void *connect_thread_func(void *opaque)
         ret = nbd_connect(conn->sioc, conn->saddr,
                           conn->do_negotiation ? &conn->updated_info : NULL,
                           conn->tlscreds, &conn->ioc, &conn->err);
+
+        /*
+         * conn->updated_info will finally be returned to the user. Clear the
+         * pointers to our internally allocated strings, which are IN parameters
+         * of nbd_receive_negotiate() and therefore nbd_connect(). Caller
+         * shoudn't be interested in these fields.
+         */
+        conn->updated_info.x_dirty_bitmap = NULL;
+        conn->updated_info.name = NULL;
+
         if (ret < 0) {
             object_unref(OBJECT(conn->sioc));
             conn->sioc = NULL;
+            if (conn->do_retry) {
+                sleep(timeout);
+                if (timeout < max_timeout) {
+                    timeout *= 2;
+                }
+                continue;
+            }
         }
 
-    conn->updated_info.x_dirty_bitmap = NULL;
+        break;
-    conn->updated_info.name = NULL;
+    }
 
     qemu_mutex_lock(&conn->mutex);