Continue the main idea: avoid dependency on @tap in net_tap_setup().
So, move QAPI parsing to net_tap_new().
Move setting sndbuf to net_tap_set_fd(), as it's more appropriate place
(other initial fd settings are here).

Note that net_tap_new() and net_tap_set_fd() are shared with
net_init_bridge(), which didn't set sndbuf. Handle this case by sndbuf=0
(we never pass zero to tap_set_sndbuf(), so let this specific value mean
that we don't want touch sndbuf).

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
---
 net/tap.c | 38 ++++++++++++++++++++++++++------------
 1 file changed, 26 insertions(+), 12 deletions(-)

diff --git a/net/tap.c b/net/tap.c
index e6431d0e08..5a74cb325f 100644
--- a/net/tap.c
+++ b/net/tap.c
@@ -80,6 +80,9 @@ typedef struct TAPState {
     VHostNetState *vhost_net;
     unsigned host_vnet_hdr_len;
     Notifier exit;
+
+    bool sndbuf_required;
+    int sndbuf;
 } TAPState;
 
 static void launch_script(const char *setup_script, const char *ifname,
@@ -402,17 +405,25 @@ static NetClientInfo net_tap_info = {
 };
 
 static TAPState *net_tap_new(NetClientState *peer, const char *model,
-                             const char *name)
+                             const char *name, const NetdevTapOptions *tap)
 {
     NetClientState *nc = qemu_new_net_client(&net_tap_info, peer, model, name);
     TAPState *s = DO_UPCAST(TAPState, nc, nc);
 
     s->fd = -1;
 
+    if (!tap) {
+        return s;
+    }
+
+    s->sndbuf_required = tap->has_sndbuf;
+    s->sndbuf =
+        (tap->has_sndbuf && tap->sndbuf) ? MIN(tap->sndbuf, INT_MAX) : INT_MAX;
+
     return s;
 }
 
-static void net_tap_set_fd(TAPState *s, int fd, int vnet_hdr)
+static bool net_tap_set_fd(TAPState *s, int fd, int vnet_hdr, Error **errp)
 {
     s->fd = fd;
     s->host_vnet_hdr_len = vnet_hdr ? sizeof(struct virtio_net_hdr) : 0;
@@ -430,6 +441,15 @@ static void net_tap_set_fd(TAPState *s, int fd, int vnet_hdr)
     }
     tap_read_poll(s, true);
     s->vhost_net = NULL;
+
+    if (s->sndbuf) {
+        Error **e = s->sndbuf_required ? errp : NULL;
+        if (!tap_set_sndbuf(s->fd, s->sndbuf, e) && s->sndbuf_required) {
+            return false;
+        }
+    }
+
+    return true;
 }
 
 static void close_all_fds_after_fork(int excluded_fd)
@@ -647,8 +667,8 @@ int net_init_bridge(const Netdev *netdev, const char *name,
         return -1;
     }
 
-    s = net_tap_new(peer, "bridge", name);
-    net_tap_set_fd(s, fd, vnet_hdr);
+    s = net_tap_new(peer, "bridge", name, NULL);
+    net_tap_set_fd(s, fd, vnet_hdr, &error_abort);
 
     qemu_set_info_str(&s->nc, "helper=%s,br=%s", helper, br);
 
@@ -688,16 +708,10 @@ static void net_init_tap_one(const NetdevTapOptions *tap, NetClientState *peer,
                              const char *downscript, const char *vhostfdname,
                              int vnet_hdr, int fd, Error **errp)
 {
-    TAPState *s = net_tap_new(peer, model, name);
+    TAPState *s = net_tap_new(peer, model, name, tap);
     int vhostfd;
-    bool sndbuf_required = tap->has_sndbuf;
-    int sndbuf =
-        (tap->has_sndbuf && tap->sndbuf) ? MIN(tap->sndbuf, INT_MAX) : INT_MAX;
-
-    net_tap_set_fd(s, fd, vnet_hdr);
 
-    if (!tap_set_sndbuf(fd, sndbuf, sndbuf_required ? errp : NULL) &&
-        sndbuf_required) {
+    if (!net_tap_set_fd(s, fd, vnet_hdr, errp)) {
         goto failed;
     }
 
-- 
2.48.1