fix(json): preserve empty arrays across decode + re-encode

lib/resty/session/utils.lua

@@ -246,6 +246,10 @@ local encode_json, decode_json do
   encode_json = function(value)
     if not cjson then
       cjson = require("cjson.safe").new()
+      -- decode arrays with the array metatable so an empty array round-trips
+      -- as `[]` instead of being re-encoded as `{}` (encode and decode share
+      -- this instance, so the option is set wherever it is created first).
+      cjson.decode_array_with_array_mt(true)
     end
     encode_json = cjson.encode
     return encode_json(value)
@@ -263,6 +267,10 @@ local encode_json, decode_json do
   decode_json = function(value)
     if not cjson then
       cjson = require("cjson.safe").new()
+      -- decode arrays with the array metatable so an empty array round-trips
+      -- as `[]` instead of being re-encoded as `{}` (encode and decode share
+      -- this instance, so the option is set wherever it is created first).
+      cjson.decode_array_with_array_mt(true)
     end
     decode_json = cjson.decode
     return decode_json(value)

spec/01-utils_spec.lua

@@ -52,6 +52,16 @@ describe("Testing utils", function()
 
       assert.same(decoded, input)
     end)
+
+    it("preserves empty arrays across decode then re-encode", function()
+      -- An empty JSON array must survive a decode + re-encode round-trip as
+      -- `[]`, not turn into `{}`. Without decode_array_with_array_mt the decoded
+      -- empty array loses its array metatable and is re-encoded as an object.
+      local decoded = utils.decode_json('{"items":[]}')
+      local reencoded = utils.encode_json(decoded)
+
+      assert.equals('{"items":[]}', reencoded)
+    end)
   end)
 
   describe("encode/decode base64url", function()