Add TestGatewayErrorOnNetDisconnect

Signed-off-by: Rob Murray <rob.murray@docker.com>

integration/networking/bridge_linux_test.go

@@ -2090,3 +2090,52 @@ func TestSetIPWithNoConfiguredSubnet(t *testing.T) {
 		assert.Check(t, is.Contains(res.Stdout.String(), ip6))
 	}
 }
+
+// Regression test for https://github.com/moby/moby/issues/51578
+func TestGatewayErrorOnNetDisconnect(t *testing.T) {
+	ctx := setupTest(t)
+	d := daemon.New(t)
+	d.StartWithBusybox(ctx, t)
+	defer d.Stop(t)
+	c := d.NewClientT(t)
+
+	network.CreateNoError(ctx, t, c, "n1")
+	defer network.RemoveNoError(ctx, t, c, "n1")
+	network.CreateNoError(ctx, t, c, "n2")
+	defer network.RemoveNoError(ctx, t, c, "n2")
+
+	// Run a container attached to both networks, with n1 providing the default gateway
+	// and n2's interface named "eth2".
+	ctrID := container.Run(ctx, t, c,
+		container.WithEndpointSettings("n1", &networktypes.EndpointSettings{GwPriority: 1}),
+		container.WithEndpointSettings("n2", &networktypes.EndpointSettings{DriverOpts: map[string]string{
+			netlabel.Ifname: "eth2",
+		}}),
+		container.WithCapability("NET_ADMIN"),
+	)
+	defer container.Remove(ctx, t, c, ctrID, client.ContainerRemoveOptions{Force: true})
+
+	// Break n2 so it can't be used as a gateway (there will be no route).
+	execRes := container.ExecT(ctx, t, c, ctrID, []string{"ip", "link", "set", "eth2", "down"})
+	assert.Assert(t, is.Equal(execRes.ExitCode, 0))
+
+	// Disconnect n1, n2 will be selected as the gateway and its config will fail.
+	// The error is only logged and the disconnect proceeds.
+	_, err := c.NetworkDisconnect(ctx, "n1", client.NetworkDisconnectOptions{Container: ctrID})
+	assert.Check(t, err)
+
+	// Check n1 can be reconnected.
+	_, err = c.NetworkConnect(ctx, "n1", client.NetworkConnectOptions{Container: ctrID})
+	assert.Check(t, err)
+
+	// Check the container can be restarted.
+	timeout := 0
+	_, err = c.ContainerRestart(ctx, ctrID, client.ContainerRestartOptions{Timeout: &timeout})
+	assert.Check(t, err)
+
+	// Both networks should be attached.
+	ctrInsp := container.Inspect(ctx, t, c, ctrID)
+	assert.Check(t, is.Len(ctrInsp.NetworkSettings.Networks, 2))
+	assert.Check(t, is.Contains(ctrInsp.NetworkSettings.Networks, "n1"))
+	assert.Check(t, is.Contains(ctrInsp.NetworkSettings.Networks, "n2"))
+}