awful.rules: Handle non-existing tags

If a tag is specified by name, but no such tags exist, awful.rules would
cause an error (attempt to index a nil value). Fix this and add a test
for this case.

Fixes: https://github.com/awesomeWM/awesome/issues/2087
Signed-off-by: Uli Schlachter <psychon@znc.in>

lib/awful/rules.lua

@@ -292,7 +292,13 @@ local force_ignore = {
 function rules.high_priority_properties.tag(c, value, props)
     if value then
         if type(value) == "string" then
+            local name = value
             value = atag.find_by_name(c.screen, value)
+            if not value then
+                require("gears.debug").print_error("awful.rules-rule specified "
+                    .. "tag = '" .. name .. "', but no such tag exists")
+                return
+            end
         end
 
         -- In case the tag has been forced to another screen, move the client

tests/test-awful-rules.lua

@@ -5,6 +5,7 @@ local test_client = require("_client")
 local unpack = unpack or table.unpack -- luacheck: globals unpack (compatibility with Lua 5.1)
 
 local callback_called = false
+local message_printed = false
 
 -- Magic table to store tests
 local tests = {}
@@ -46,9 +47,16 @@ local function get_client_by_class(class)
     end
 end
 
+local orig_error = gears.debug.print_error
+function gears.debug.print_error(msg)
+    assert(not message_printed, msg)
+    assert(msg:find("specified tag = 'does_not_exist', but no such tag exists"), msg)
+    message_printed = true
+end
+
 -- Test callback and floating
 test_rule {
-    properties = { floating = true },
+    properties = { floating = true, tag = "does_not_exist" },
     callback   = function(c)
         assert(type(c) == "client")
         callback_called = true
@@ -57,6 +65,10 @@ test_rule {
         -- Test if callbacks works
         assert(callback_called)
 
+        -- Test that the "does_not_exist"-tag caused an error
+        assert(message_printed)
+        gears.debug.print_error = orig_error
+
         -- Make sure "smart" dynamic properties are applied
         assert(get_client_by_class(class).floating)