Fix const-ness of callback client read RPC requests (#25763)

* Fix const-ness of callback client read RPC requests

* Fix golden file

src/compiler/cpp_generator.cc

@@ -633,12 +633,12 @@ void PrintHeaderClientMethodCallbackInterfaces(
     printer->Print(*vars,
                    "#ifdef GRPC_CALLBACK_API_NONEXPERIMENTAL\n"
                    "virtual void $Method$(::grpc::ClientContext* context, "
-                   "$Request$* request, "
+                   "const $Request$* request, "
                    "::grpc::ClientReadReactor< $Response$>* "
                    "reactor) = 0;\n"
                    "#else\n"
                    "virtual void $Method$(::grpc::ClientContext* context, "
-                   "$Request$* request, "
+                   "const $Request$* request, "
                    "::grpc::experimental::ClientReadReactor< $Response$>* "
                    "reactor) = 0;\n"
                    "#endif\n");
@@ -730,12 +730,12 @@ void PrintHeaderClientMethodCallback(grpc_generator::Printer* printer,
     printer->Print(*vars,
                    "#ifdef GRPC_CALLBACK_API_NONEXPERIMENTAL\n"
                    "void $Method$(::grpc::ClientContext* context, "
-                   "$Request$* request, "
+                   "const $Request$* request, "
                    "::grpc::ClientReadReactor< $Response$>* "
                    "reactor) override;\n"
                    "#else\n"
                    "void $Method$(::grpc::ClientContext* context, "
-                   "$Request$* request, "
+                   "const $Request$* request, "
                    "::grpc::experimental::ClientReadReactor< $Response$>* "
                    "reactor) override;\n"
                    "#endif\n");
@@ -2005,7 +2005,7 @@ void PrintSourceClientMethod(grpc_generator::Printer* printer,
         *vars,
         "void $ns$$Service$::Stub::experimental_async::$Method$(::grpc::"
         "ClientContext* context, "
-        "$Request$* request, "
+        "const $Request$* request, "
         "::grpc::experimental::ClientReadReactor< $Response$>* reactor) {\n");
     printer->Print(*vars,
                    "  ::grpc::internal::ClientCallbackReaderFactory< "

test/cpp/codegen/compiler_test_golden

@@ -130,9 +130,9 @@ class ServiceA final {
       // MethodA2 trailing comment 1
       // Method A3 leading comment 1
       #ifdef GRPC_CALLBACK_API_NONEXPERIMENTAL
-      virtual void MethodA3(::grpc::ClientContext* context, ::grpc::testing::Request* request, ::grpc::ClientReadReactor< ::grpc::testing::Response>* reactor) = 0;
+      virtual void MethodA3(::grpc::ClientContext* context, const ::grpc::testing::Request* request, ::grpc::ClientReadReactor< ::grpc::testing::Response>* reactor) = 0;
       #else
-      virtual void MethodA3(::grpc::ClientContext* context, ::grpc::testing::Request* request, ::grpc::experimental::ClientReadReactor< ::grpc::testing::Response>* reactor) = 0;
+      virtual void MethodA3(::grpc::ClientContext* context, const ::grpc::testing::Request* request, ::grpc::experimental::ClientReadReactor< ::grpc::testing::Response>* reactor) = 0;
       #endif
       // Method A3 trailing comment 1
       // Method A4 leading comment 1
@@ -215,9 +215,9 @@ class ServiceA final {
       void MethodA2(::grpc::ClientContext* context, ::grpc::testing::Response* response, ::grpc::experimental::ClientWriteReactor< ::grpc::testing::Request>* reactor) override;
       #endif
       #ifdef GRPC_CALLBACK_API_NONEXPERIMENTAL
-      void MethodA3(::grpc::ClientContext* context, ::grpc::testing::Request* request, ::grpc::ClientReadReactor< ::grpc::testing::Response>* reactor) override;
+      void MethodA3(::grpc::ClientContext* context, const ::grpc::testing::Request* request, ::grpc::ClientReadReactor< ::grpc::testing::Response>* reactor) override;
       #else
-      void MethodA3(::grpc::ClientContext* context, ::grpc::testing::Request* request, ::grpc::experimental::ClientReadReactor< ::grpc::testing::Response>* reactor) override;
+      void MethodA3(::grpc::ClientContext* context, const ::grpc::testing::Request* request, ::grpc::experimental::ClientReadReactor< ::grpc::testing::Response>* reactor) override;
       #endif
       #ifdef GRPC_CALLBACK_API_NONEXPERIMENTAL
       void MethodA4(::grpc::ClientContext* context, ::grpc::ClientBidiReactor< ::grpc::testing::Request,::grpc::testing::Response>* reactor) override;