luci-base: dispatcher: introduce firstnode() dispatching target

The firstnode target will dispatch the request to the first eligible menu
subtree node that is not a redirect to another node, a special action or
post security enabled page.

That action is specifically useful for global category toplevel nodes like
"admin" which are supposed to simply direct access to the first installed
page node without having to hardcode specific choices.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>

modules/luci-base/luasrc/dispatcher.lua

@@ -724,33 +724,82 @@ end
 
 -- Subdispatchers --
 
+function _find_eligible_node(root, prefix, deep, types, descend)
+	local _, cur_name, cur_node
+	local childs = { }
+
+	for cur_name, cur_node in pairs(root.nodes) do
+		childs[#childs+1] = {
+			node = cur_node,
+			name = cur_name,
+			order = cur_node.order or 100
+		}
+	end
+
+	table.sort(childs, function(a, b)
+		if a.order == b.order then
+			return a.name < b.name
+		else
+			return a.order < b.order
+		end
+	end)
+
+	if not root.leaf and deep ~= nil then
+		local sub_path = { unpack(prefix) }
+
+		if deep == false then
+			deep = nil
+		end
+
+		for _, cur_node in ipairs(childs) do
+			sub_path[#prefix+1] = cur_node.name
+
+			local res_path = _find_eligible_node(cur_node.node, sub_path,
+			                                     deep, types, true)
+
+			if res_path then
+				return res_path
+			end
+		end
+	end
+
+	if descend and
+	   (not types or
+	    (type(root.target) == "table" and
+	     util.contains(types, root.target.type)))
+	then
+		return prefix
+	end
+end
+
+function _find_node(recurse, types)
+	local path = { unpack(context.path) }
+	local name = table.concat(path, ".")
+	local node = context.treecache[name]
+
+	path = _find_eligible_node(node, path, recurse, types)
+
+	if path then
+		dispatch(path)
+	else
+		require "luci.template".render("empty_node_placeholder")
+	end
+end
+
 function _firstchild()
-   local path = { unpack(context.path) }
-   local name = table.concat(path, ".")
-   local node = context.treecache[name]
-
-   local lowest
-   if node and node.nodes and next(node.nodes) then
-	  local k, v
-	  for k, v in pairs(node.nodes) do
-		 if not lowest or
-			(v.order or 100) < (node.nodes[lowest].order or 100)
-		 then
-			lowest = k
-		 end
-	  end
-   end
-
-   if lowest == nil then
-	require "luci.template".render("empty_node_placeholder")
-   else
-	path[#path+1] = lowest
-	dispatch(path)
-   end
+	return _find_node(false, nil)
 end
 
 function firstchild()
-   return { type = "firstchild", target = _firstchild }
+	return { type = "firstchild", target = _firstchild }
+end
+
+function _firstnode()
+	return _find_node(true, { "cbi", "form", "template", "arcombine" })
+end
+
+function firstnode()
+	return { type = "firstnode", target = _firstnode }
 end
 
 function alias(...)