net: Fix handling of id in netdev_add and netdev_del

CLI -netdev accumulates in option group "netdev".

Before commit 08712fcb85 "net: Track netdevs in NetClientState rather
than QemuOpt", netdev_add added to the option group, and netdev_del
removed from it, both HMP and QMP.  Thus, every netdev had a
corresponding QemuOpts in this option group.

Commit 08712fcb85 dropped this for QMP netdev_add and both netdev_del.
Now a netdev has a corresponding QemuOpts only when it was created
with CLI or HMP.  Two issues:

* QMP and HMP netdev_del can leave QemuOpts behind, breaking HMP
  netdev_add.  Reproducer:

    $ qemu-system-x86_64 -S -display none -nodefaults -monitor stdio
    QEMU 5.1.92 monitor - type 'help' for more information
    (qemu) netdev_add user,id=net0
    (qemu) info network
    net0: index=0,type=user,net=10.0.2.0,restrict=off
    (qemu) netdev_del net0
    (qemu) info network
    (qemu) netdev_add user,id=net0
    upstream-qemu: Duplicate ID 'net0' for netdev
    Try "help netdev_add" for more information

  Fix by restoring the QemuOpts deletion in qmp_netdev_del(), but with
  a guard, because the QemuOpts need not exist.

* QMP netdev_add loses its "no duplicate ID" check.  Reproducer:

    $ qemu-system-x86_64 -S -display none -qmp stdio
    {"QMP": {"version": {"qemu": {"micro": 92, "minor": 1, "major": 5}, "package": "v5.2.0-rc2-1-g02c1f0142c"}, "capabilities": ["oob"]}}
    {"execute": "qmp_capabilities"}
    {"return": {}}
    {"execute": "netdev_add", "arguments": {"type": "user", "id":"net0"}}
    {"return": {}}
    {"execute": "netdev_add", "arguments": {"type": "user", "id":"net0"}}
    {"return": {}}

  Fix by adding a duplicate ID check to net_client_init1() to replace
  the lost one.  The check is redundant for callers where QemuOpts
  still checks, i.e. for CLI and HMP.

Reported-by: Andrew Melnichenko <andrew@daynix.com>
Fixes: 08712fcb85
Cc: qemu-stable@nongnu.org
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>

net/net.c

@@ -983,6 +983,7 @@ static int (* const net_client_init_fun[NET_CLIENT_DRIVER__MAX])(
 static int net_client_init1(const Netdev *netdev, bool is_netdev, Error **errp)
 {
     NetClientState *peer = NULL;
+    NetClientState *nc;
 
     if (is_netdev) {
         if (netdev->type == NET_CLIENT_DRIVER_NIC ||
@@ -1010,6 +1011,12 @@ static int net_client_init1(const Netdev *netdev, bool is_netdev, Error **errp)
         }
     }
 
+    nc = qemu_find_netdev(netdev->id);
+    if (nc) {
+        error_setg(errp, "Duplicate ID '%s'", netdev->id);
+        return -1;
+    }
+
     if (net_client_init_fun[netdev->type](netdev, netdev->id, peer, errp) < 0) {
         /* FIXME drop when all init functions store an Error */
         if (errp && !*errp) {
@@ -1020,8 +1027,6 @@ static int net_client_init1(const Netdev *netdev, bool is_netdev, Error **errp)
     }
 
     if (is_netdev) {
-        NetClientState *nc;
-
         nc = qemu_find_netdev(netdev->id);
         assert(nc);
         nc->is_netdev = true;
@@ -1135,6 +1140,7 @@ void qmp_netdev_add(Netdev *netdev, Error **errp)
 void qmp_netdev_del(const char *id, Error **errp)
 {
     NetClientState *nc;
+    QemuOpts *opts;
 
     nc = qemu_find_netdev(id);
     if (!nc) {
@@ -1149,6 +1155,16 @@ void qmp_netdev_del(const char *id, Error **errp)
     }
 
     qemu_del_net_client(nc);
+
+    /*
+     * Wart: we need to delete the QemuOpts associated with netdevs
+     * created via CLI or HMP, to avoid bogus "Duplicate ID" errors in
+     * HMP netdev_add.
+     */
+    opts = qemu_opts_find(qemu_find_opts("netdev"), id);
+    if (opts) {
+        qemu_opts_del(opts);
+    }
 }
 
 static void netfilter_print_info(Monitor *mon, NetFilterState *nf)