qemu-nbd: Pass max connections to blockdev layer

The next patch wants to adjust whether the NBD server code advertises
MULTI_CONN based on whether it is known if the server limits to
exactly one client.  For a server started by QMP, this information is
obtained through nbd_server_start (which can support more than one
export); but for qemu-nbd (which supports exactly one export), it is
controlled only by the command-line option -e/--shared.  Since we
already have a hook function used by qemu-nbd, it's easiest to just
alter its signature to fit our needs.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20220512004924.417153-2-eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

blockdev-nbd.c

@@ -30,18 +30,18 @@ typedef struct NBDServerData {
 } NBDServerData;
 
 static NBDServerData *nbd_server;
-static bool is_qemu_nbd;
+static int qemu_nbd_connections = -1; /* Non-negative if this is qemu-nbd */
 
 static void nbd_update_server_watch(NBDServerData *s);
 
-void nbd_server_is_qemu_nbd(bool value)
+void nbd_server_is_qemu_nbd(int max_connections)
 {
-    is_qemu_nbd = value;
+    qemu_nbd_connections = max_connections;
 }
 
 bool nbd_server_is_running(void)
 {
-    return nbd_server || is_qemu_nbd;
+    return nbd_server || qemu_nbd_connections >= 0;
 }
 
 static void nbd_blockdev_client_closed(NBDClient *client, bool ignored)

include/block/nbd.h

@@ -344,7 +344,7 @@ void nbd_client_new(QIOChannelSocket *sioc,
 void nbd_client_get(NBDClient *client);
 void nbd_client_put(NBDClient *client);
 
-void nbd_server_is_qemu_nbd(bool value);
+void nbd_server_is_qemu_nbd(int max_connections);
 bool nbd_server_is_running(void);
 void nbd_server_start(SocketAddress *addr, const char *tls_creds,
                       const char *tls_authz, uint32_t max_connections,

qemu-nbd.c

@@ -1095,7 +1095,7 @@ int main(int argc, char **argv)
 
     bs->detect_zeroes = detect_zeroes;
 
-    nbd_server_is_qemu_nbd(true);
+    nbd_server_is_qemu_nbd(shared);
 
     export_opts = g_new(BlockExportOptions, 1);
     *export_opts = (BlockExportOptions) {