Fix udp multicast test when no exernal interfaces are available

Fixes luvit#539

tests/test-udp.lua

@@ -179,7 +179,7 @@ return require('lib/tap')(function (test)
       assert(uv.udp_bind(server, bind_addr, TEST_PORT))
       local _, err, errname = uv.udp_set_membership(server, multicast_addr, interface_addr, "join")
       if errname == "ENODEV" then
-        print("no ipv6 multicast route, skipping")
+        print("no multicast route, skipping")
         server:close()
         return
       elseif errname == "EADDRNOTAVAIL" and multicast_addr == "ff02::1" then
@@ -240,26 +240,39 @@ return require('lib/tap')(function (test)
     end
   end
 
-  test("udp multicast join ipv4", multicast_join_test("0.0.0.0", "239.255.0.1", nil))
-
-  test("udp multicast join ipv6", function(print, p, expect, uv)
-    local function can_ipv6_external()
-      local addresses = assert(uv.interface_addresses())
-      for _, vals in pairs(addresses) do
-        for _, info in ipairs(vals) do
-          if info.family == "inet6" and not info.internal then
-            return true
-          end
+  -- TODO This might be overkill, but the multicast
+  -- tests seem to rely on external interfaces being
+  -- available on some platforms. So, we use this to skip
+  -- the tests when there are no relevant exernal interfaces
+  -- available. Note: The Libuv multicast join test does use this
+  -- same check for skipping the ipv6 test; we just expanded it to
+  -- the ipv4 test as well.
+  local function has_external_interface(uv, family)
+    local addresses = assert(uv.interface_addresses())
+    for _, vals in pairs(addresses) do
+      for _, info in ipairs(vals) do
+        if (not family or info.family == family) and not info.internal then
+          return true
         end
       end
-      return false
     end
+    return false
+  end
 
-    if not can_ipv6_external() then
-      print("no ipv6 support, skipping")
+  test("udp multicast join ipv4", function(print, p, expect, uv)
+    if not has_external_interface(uv, "inet") then
+      print("no external ipv4 interface, skipping")
       return
     end
+    local testfn = multicast_join_test("0.0.0.0", "239.255.0.1", nil)
+    return testfn(print, p, expect, uv)
+  end)
 
+  test("udp multicast join ipv6", function(print, p, expect, uv)
+    if not has_external_interface(uv, "inet6") then
+      print("no external ipv6 interface, skipping")
+      return
+    end
     local testfn = multicast_join_test("::", "ff02::1", nil)
     return testfn(print, p, expect, uv)
   end)