Check vhost-user is available to build vhost-user-blk-server to fix:

 $ ../configure \
      --disable-vhost-user --enable-vhost-user-blk-server && \
   make qemu-nbd
 ...
 [505/505] Linking target qemu-nbd
 FAILED: qemu-nbd
 cc  -o qemu-nbd qemu-nbd.p/qemu-nbd.c.o -Wl,--as-needed -Wl,--no-undefined -pie -Wl,--whole-archive libblockdev.fa libblock.fa libcrypto.fa libauthz.fa libqom.fa libio.fa -Wl,--no-whole-archive -Wl,--warn-common -Wl,-z,relro -Wl,-z,now -m64 -fstack-protector-strong -Wl,--start-group libqemuutil.a libblockdev.fa libblock.fa libcrypto.fa libauthz.fa libqom.fa libio.fa @block.syms -lgio-2.0 -lgobject-2.0 -lglib-2.0 -lgio-2.0 -lgobject-2.0 -lglib-2.0 -pthread -lgnutls -lutil -lm -lgthread-2.0 -lglib-2.0 -lbz2 -lgthread-2.0 -lglib-2.0 -lssh -lrbd -lrados -lcurl -lxml2 -lzstd -lacl -lgfapi -lglusterfs -lgfrpc -lgfxdr -luuid -laio /usr/lib64/libz.so -L/usr/lib64/iscsi -liscsi -lnettle -lgnutls -lpam -Wl,--end-group
 /usr/bin/ld: libblockdev.fa(block_export_vhost-user-blk-server.c.o): in function `vu_blk_process_vq':
 block/export/vhost-user-blk-server.c:203: undefined reference to `vu_get_queue'
 /usr/bin/ld: block/export/vhost-user-blk-server.c:208: undefined reference to `vu_queue_pop'
 /usr/bin/ld: libblockdev.fa(block_export_vhost-user-blk-server.c.o): in function `vu_blk_queue_set_started':
 block/export/vhost-user-blk-server.c:228: undefined reference to `vu_get_queue'
 /usr/bin/ld: libblockdev.fa(block_export_vhost-user-blk-server.c.o): in function `vu_blk_req_complete':
 block/export/vhost-user-blk-server.c:55: undefined reference to `vu_queue_push'
 /usr/bin/ld: block/export/vhost-user-blk-server.c:56: undefined reference to `vu_queue_notify'
 /usr/bin/ld: libblockdev.fa(block_export_vhost-user-blk-server.c.o): in function `vu_blk_queue_set_started':
 block/export/vhost-user-blk-server.c:229: undefined reference to `vu_set_queue_handler'
 /usr/bin/ld: libqemuutil.a(util_vhost-user-server.c.o): in function `vu_client_trip':
 util/vhost-user-server.c:176: undefined reference to `vu_dispatch'
 /usr/bin/ld: util/vhost-user-server.c:180: undefined reference to `vu_deinit'
 /usr/bin/ld: libqemuutil.a(util_vhost-user-server.c.o): in function `vu_accept':
 util/vhost-user-server.c:291: undefined reference to `vu_init'
 collect2: error: ld returned 1 exit status
 ninja: build stopped: subcommand failed.
 make: *** [Makefile:171: run-ninja] Error 1

Fixes: bc15e44cb21 ("configure: introduce --enable-vhost-user-blk-server")
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
---
 configure | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/configure b/configure
index 805f7791503..5079ebb416a 100755
--- a/configure
+++ b/configure
@@ -2390,6 +2390,9 @@ fi
 
 # libvhost-user is Linux-only
 test "$vhost_user_blk_server" = "" && vhost_user_blk_server=$linux
+if test "$vhost_user_blk_server" = "yes" && test "$vhost_user" = "no"; then
+  error_exit "--enable-vhost-user-blk-server requires --enable-vhost-user"
+fi
 if test "$vhost_user_blk_server" = "yes" && test "$linux" = "no"; then
   error_exit "--enable-vhost-user-blk-server is only available on Linux"
 fi
-- 
2.26.2

On Fri, Nov 06, 2020 at 08:04:09PM +0100, Philippe Mathieu-Daudé wrote:
> Check vhost-user is available to build vhost-user-blk-server to fix:

I sent a different patch to address this. The vhost-user-blk server does
not depend on CONFIG_VHOST_USER, just CONFIG_LINUX.

Stefan