gRPC: RST_STREAM(NO_ERROR) handling after "trailer only" responses. Similarly to the problem fixed in 2096b21fcd10 (ticket #1792), when a "trailer only" gRPC response (that is, a response with the END_STREAM flag in the HEADERS frame) was immediately followed by RST_STREAM(NO_ERROR) in the data preread along with the response header, RST_STREAM wasn't properly skipped and caused "upstream rejected request with error 0" errors. Observed with "unknown service" gRPC errors returned by grpc-go. Fix is to set ctx->done if we are going to parse additional data, so the RST_STREAM(NO_ERROR) is properly skipped. Additionally, now ngx_http_grpc_filter() will complain about frames sent for closed stream if there are any.

--- a/src/http/modules/ngx_http_grpc_module.c
+++ b/src/http/modules/ngx_http_grpc_module.c
@@ -1969,6 +1969,7 @@ ngx_http_grpc_filter_init(void *data)
         }
 
         u->length = 0;
+        ctx->done = 1;
 
     } else {
         u->length = 1;