fix(api) fix output of records in /plugins/schema

* Ensure that plugin schemas return sub-fields of nested records in the same
  format as the toplevel `fields` entry (i.e., as an array of single-fields
  objects, so that the field order can be retrieved).
* Ensure `fields` in `/plugins/schema` always returns a JSON array,
  even when it is empty (as in the prometheus plugin).

This produces an incompatible output from that of 1.0.0, but the one in
1.0.0 is arguably broken, so this produces the intended result.

Fixes #4137.

kong/api/routes/plugins.lua

@@ -70,14 +70,52 @@ end
 -- cjson can encode the schema.
 local schema_to_jsonable
 do
-  local function fdata_to_jsonable(fdata)
+  local insert = table.insert
+  local ipairs = ipairs
+  local next = next
+
+  local fdata_to_jsonable
+
+
+  local function fields_to_jsonable(fields)
+    local out = {}
+    for _, field in ipairs(fields) do
+      local fname = next(field)
+      local fdata = field[fname]
+      insert(out, { [fname] = fdata_to_jsonable(fdata, "no") })
+    end
+    setmetatable(out, cjson.array_mt)
+    return out
+  end
+
+
+  -- Convert field data from schemas into something that can be
+  -- passed to a JSON encoder.
+  -- @tparam table fdata A Lua table with field data
+  -- @tparam string is_array A three-state enum: "yes", "no" or "maybe"
+  -- @treturn table A JSON-convertible Lua table
+  fdata_to_jsonable = function(fdata, is_array)
     local out = {}
-    for k, v in pairs(fdata) do
+    local iter = is_array == "yes" and ipairs or pairs
+
+    for k, v in iter(fdata) do
+      if is_array == "maybe" and type(k) ~= "number" then
+        is_array = "no"
+      end
+
       if k == "schema" then
         out[k] = schema_to_jsonable(v)
 
       elseif type(v) == "table" then
-        out[k] = fdata_to_jsonable(v)
+        if k == "fields" and fdata.type == "record" then
+          out[k] = fields_to_jsonable(v)
+
+        elseif k == "default" and fdata.type == "array" then
+          out[k] = fdata_to_jsonable(v, "yes")
+
+        else
+          out[k] = fdata_to_jsonable(v, "maybe")
+        end
 
       elseif type(v) == "number" then
         if v ~= v then
@@ -94,20 +132,15 @@ do
         out[k] = v
       end
     end
+    if is_array == "yes" or is_array == "maybe" then
+      setmetatable(out, cjson.array_mt)
+    end
     return out
   end
 
-  local insert = table.insert
-  local ipairs = ipairs
-  local next = next
 
   schema_to_jsonable = function(schema)
-    local fields = {}
-    for _, field in ipairs(schema.fields) do
-      local fname = next(field)
-      local fdata = field[fname]
-      insert(fields, { [fname] = fdata_to_jsonable(fdata) })
-    end
+    local fields = fields_to_jsonable(schema.fields)
     return { fields = fields }
   end
 end
@@ -172,7 +205,7 @@ return {
 
   ["/plugins/enabled"] = {
     GET = function(_, _, helpers)
-      local enabled_plugins = setmetatable({}, cjson.empty_array_mt)
+      local enabled_plugins = setmetatable({}, cjson.array_mt)
       for k in pairs(singletons.configuration.loaded_plugins) do
         enabled_plugins[#enabled_plugins+1] = k
       end

spec/02-integration/04-admin_api/04-plugins_routes_spec.lua

@@ -241,6 +241,17 @@ for _, strategy in helpers.each_strategy() do
             assert.is_table(json.fields)
           end
         end)
+        it("returns nested records and empty array defaults as arrays", function()
+          local res = assert(client:send {
+            method = "GET",
+            path = "/plugins/schema/request-transformer",
+          })
+          local body = assert.res_status(200, res)
+          assert.match('{"fields":[{', body, 1, true)
+          assert.not_match('"fields":{', body, 1, true)
+          assert.match('"default":[]', body, 1, true)
+          assert.not_match('"default":{}', body, 1, true)
+        end)
         it("returns 404 on invalid plugin", function()
           local res = assert(client:send {
             method = "GET",