fix(grpc): empty message cannot be unframed (#10836)

We found an issue when testing the fix of #10801. When a service responds with a message with all default value fields, the JSON response would be totally empty.

The reason for this issue is that we assume a message to be at least of length 1 when unframing.

Removing this restriction (a message can be of length 0) fixes this issue.

Fix FTI-5054
Fix #10802

CHANGELOG.md

@@ -33,6 +33,9 @@
 
 #### Plugins
 
+- **grpc-gateway**: Fixed an issue that empty (all default value) messages cannot be unframed correctly.
+  [#10836](https://github.com/Kong/kong/pull/10836)
+
 #### PDK
 
 ### Fixes

kong/tools/grpc.lua

@@ -141,6 +141,9 @@ end
 
 --- wraps a binary payload into a grpc stream frame.
 function _M.frame(ftype, msg)
+  -- byte 0: frame type
+  -- byte 1-4: frame size in big endian (could be zero)
+  -- byte 5-: frame content
   return bpack("C>I", ftype, #msg) .. msg
 end
 
@@ -149,7 +152,8 @@ end
 --- If heading frame isn't complete, returns `nil, body`,
 --- try again with more data.
 function _M.unframe(body)
-  if not body or #body <= 5 then
+  -- must be at least 5 bytes(frame header)
+  if not body or #body < 5 then
     return nil, body
   end
 

spec/03-plugins/28-grpc-gateway/01-proxy_spec.lua

@@ -218,7 +218,7 @@ for _, strategy in helpers.each_strategy() do
       end)
 
       -- Bug found when test FTI-5002's fix. It will be fixed in another PR.
-      pending("empty message #10802", function()
+      test("empty message #10802", function()
         local req_body = { array = {}, nullable = "" }
         local res, _ = proxy_client:post("/v1/echo", {
           headers = { ["Content-Type"] = "application/json" },