make objective_scaling_factor optional for gRPC (#1600)

This coupled with a presence check adjusts for the proto3 default 0 on omission of non-optional fields, which differs from the cuopt default of 1.

Previously, omitted objective_scaling_factor fields would cause a remote solve to use objective_scaling_factor = 0, instead of the cuopt default "1".  (If the field was not omitted, it would only be 0 if explicitly set. Zero is not a valid value in any case).

Found during a routine review of gRPC fields versus local solve.

Authors:
  - Trevor McKay (https://github.com/tmckayus)

Approvers:
  - Miles Lubin (https://github.com/mlubin)
  - Ramakrishna Prabhu (https://github.com/ramakrishnap-nv)

URL: #1600

Author: tmckayus

cpp/src/grpc/codegen/FIELD_REGISTRY_REFERENCE.md

@@ -242,9 +242,10 @@ section:
   (the setter / assignment is skipped). This is the case the flag is designed
   for: without `optional:`, an external client that omits the field silently
   overwrites the C++ default with the proto3 zero.
-- **Problem-chunked**: currently *not honored* — the chunked path reads from
-  the hand-written `ChunkedProblemHeader` message which does not declare
-  `optional` on its fields. Tracked for unification (see §6).
+- **Problem-chunked**: the C++ field's in-class default is preserved, provided
+  the matching field in the hand-written `ChunkedProblemHeader` message is
+  also declared `optional`. The generator emits a `has_X()` guard for registry
+  fields marked `optional`.
 - **Solution**: cosmetic. Solutions are constructor-built each call; there's no
   pre-existing default to preserve. The `optional` keyword adds `has_X()` to
   the proto for client-side presence detection, but the from-proto path still

cpp/src/grpc/codegen/field_registry.yaml

@@ -819,6 +819,10 @@ optimization_problem:
       getter: get_sense()
   - objective_scaling_factor:
       field_num: 4
+      # C++ defaults to 1 while an absent proto3 scalar reads as 0. Track
+      # presence so external clients may omit the field without changing the
+      # objective to an unsupported zero scaling factor.
+      optional: true
   - objective_offset:
       field_num: 5
 

cpp/src/grpc/codegen/generate_conversions.py

@@ -2420,22 +2420,22 @@ def _gen_chunked_header_to_problem(registry, indent="  "):
     ind = indent
     lines = []
 
-    # `ChunkedProblemHeader` is hand-written (see cuopt_remote_service.proto);
-    # it does not declare `optional` for any field, so we pass has_check=None
-    # to suppress the has_X() guard.  `sentinel:` still applies — the wire
-    # mapping must stay consistent with the unary path.
+    # `ChunkedProblemHeader` is hand-written (see cuopt_remote_service.proto).
+    # Fields marked optional in the registry must also be declared optional in
+    # that message so the chunked path can preserve non-zero C++ defaults.
     for entry in obj.get("scalars", []):
         f = parse_field(entry)
         pname = _proto_cpp_name(f["name"])
         setter = _default_problem_setter(f)
+        has_check = f"header.has_{pname}()" if f.get("optional") else None
         lines.extend(
             emit_scalar_from_proto_assign(
                 lambda v, s=setter: f"cpu_problem.{s}({v});",
                 f"header.{pname}()",
                 f,
                 registry,
                 ind,
-                has_check=None,
+                has_check=has_check,
             )
         )
 

cpp/src/grpc/codegen/generated/cuopt_remote_data.proto

@@ -137,7 +137,7 @@ message OptimizationProblem {
   string problem_name = 1;
   string objective_name = 2;
   bool maximize = 3;
-  double objective_scaling_factor = 4;
+  optional double objective_scaling_factor = 4;
   double objective_offset = 5;
   repeated string variable_names = 7;
   repeated string row_names = 8;

cpp/src/grpc/codegen/generated/generated_chunked_header_to_problem.inc

@@ -5,7 +5,9 @@
   cpu_problem.set_problem_name(header.problem_name());
   cpu_problem.set_objective_name(header.objective_name());
   cpu_problem.set_maximize(header.maximize());
-  cpu_problem.set_objective_scaling_factor(header.objective_scaling_factor());
+  if (header.has_objective_scaling_factor()) {
+    cpu_problem.set_objective_scaling_factor(header.objective_scaling_factor());
+  }
   cpu_problem.set_objective_offset(header.objective_offset());
 
   if (header.variable_names_size() > 0) {

cpp/src/grpc/codegen/generated/generated_proto_to_problem.inc

@@ -5,7 +5,9 @@
   cpu_problem.set_problem_name(pb_problem.problem_name());
   cpu_problem.set_objective_name(pb_problem.objective_name());
   cpu_problem.set_maximize(pb_problem.maximize());
-  cpu_problem.set_objective_scaling_factor(pb_problem.objective_scaling_factor());
+  if (pb_problem.has_objective_scaling_factor()) {
+    cpu_problem.set_objective_scaling_factor(pb_problem.objective_scaling_factor());
+  }
   cpu_problem.set_objective_offset(pb_problem.objective_offset());
 
   if (pb_problem.a_offsets_size() > 0) {

cpp/src/grpc/cuopt_remote_service.proto

@@ -118,7 +118,7 @@ message ChunkedProblemHeader {
 
   // Problem scalars
   bool maximize = 2;
-  double objective_scaling_factor = 3;
+  optional double objective_scaling_factor = 3;
   double objective_offset = 4;
   string problem_name = 5;
   string objective_name = 6;

cpp/tests/linear_programming/grpc/grpc_client_test.cpp

@@ -1857,6 +1857,53 @@ TEST(MapperRoundtrip, ProblemWithVariableTypes)
   EXPECT_DOUBLE_EQ(restored_obj[2], 3.0);
 }
 
+TEST(MapperRoundtrip, UnaryProblemObjectiveScalingPresence)
+{
+  cuopt::remote::OptimizationProblem omitted;
+  omitted.add_c(0.0);
+  omitted.add_variable_lower_bounds(0.0);
+  omitted.add_variable_upper_bounds(1.0);
+  ASSERT_FALSE(omitted.has_objective_scaling_factor());
+
+  cpu_optimization_problem_t<int32_t, double> restored_default;
+  map_proto_to_problem(omitted, restored_default);
+  EXPECT_DOUBLE_EQ(restored_default.get_objective_scaling_factor(), 1.0);
+
+  cpu_optimization_problem_t<int32_t, double> orig;
+  const std::vector<double> objective{0.0};
+  const std::vector<double> lower_bound{0.0};
+  const std::vector<double> upper_bound{1.0};
+  orig.set_objective_coefficients(objective.data(), objective.size());
+  orig.set_variable_lower_bounds(lower_bound.data(), lower_bound.size());
+  orig.set_variable_upper_bounds(upper_bound.data(), upper_bound.size());
+  orig.set_objective_scaling_factor(2.5);
+  cuopt::remote::OptimizationProblem present;
+  map_problem_to_proto(orig, &present);
+  ASSERT_TRUE(present.has_objective_scaling_factor());
+  EXPECT_DOUBLE_EQ(present.objective_scaling_factor(), 2.5);
+
+  cpu_optimization_problem_t<int32_t, double> restored_present;
+  map_proto_to_problem(present, restored_present);
+  EXPECT_DOUBLE_EQ(restored_present.get_objective_scaling_factor(), 2.5);
+}
+
+TEST(MapperRoundtrip, ChunkedProblemObjectiveScalingPresence)
+{
+  cuopt::remote::ChunkedProblemHeader omitted;
+  ASSERT_FALSE(omitted.has_objective_scaling_factor());
+
+  cpu_optimization_problem_t<int32_t, double> restored_default;
+  map_chunked_header_to_problem(omitted, restored_default);
+  EXPECT_DOUBLE_EQ(restored_default.get_objective_scaling_factor(), 1.0);
+
+  omitted.set_objective_scaling_factor(2.5);
+  ASSERT_TRUE(omitted.has_objective_scaling_factor());
+
+  cpu_optimization_problem_t<int32_t, double> restored_present;
+  map_chunked_header_to_problem(omitted, restored_present);
+  EXPECT_DOUBLE_EQ(restored_present.get_objective_scaling_factor(), 2.5);
+}
+
 TEST(MapperRoundtrip, MIPSolutionAllFields)
 {
   std::vector<double> sol_vec = {1.0, 0.0, 1.0, 0.0, 1.0};