Add nullability to SolveInterrupter in linear solver (#4958)

ortools/linear_solver/BUILD.bazel

@@ -598,5 +598,6 @@ cc_library(
         ":linear_solver_cc_proto",
         "//ortools/util:lazy_mutable_copy",
         "//ortools/util:solve_interrupter",
+        "@abseil-cpp//absl/base:nullability",
     ],
 )

ortools/linear_solver/linear_solver.cc

@@ -516,7 +516,6 @@ absl::string_view ToString(
   }
   LOG(FATAL) << "Unrecognized solver type: "
              << static_cast<int>(optimization_problem_type);
-  return "";
 }
 
 bool AbslParseFlag(const absl::string_view text,

ortools/linear_solver/solve_mp_model.cc

@@ -17,6 +17,7 @@
 #include <string>
 #include <utility>
 
+#include "absl/base/nullability.h"
 #include "ortools/linear_solver/linear_solver.h"
 #include "ortools/linear_solver/linear_solver.pb.h"
 #include "ortools/util/lazy_mutable_copy.h"
@@ -26,8 +27,9 @@ namespace operations_research {
 
 // TODO(b/311704821): this function should not delegate to MPSolver, also true
 // for the functions below.
-MPSolutionResponse SolveMPModel(LazyMutableCopy<MPModelRequest> request,
-                                const SolveInterrupter* interrupter) {
+MPSolutionResponse SolveMPModel(
+    LazyMutableCopy<MPModelRequest> request,
+    const SolveInterrupter* absl_nullable interrupter) {
   MPSolutionResponse response;
   if (interrupter != nullptr) {
     std::atomic<bool> atomic_bool = false;

ortools/linear_solver/solve_mp_model.h

@@ -20,6 +20,7 @@
 
 #include <string>
 
+#include "absl/base/nullability.h"
 #include "ortools/linear_solver/linear_solver.pb.h"
 #include "ortools/util/lazy_mutable_copy.h"
 #include "ortools/util/solve_interrupter.h"
@@ -41,8 +42,9 @@ namespace operations_research {
  * Passing a non-null pointer with any other solver type immediately returns an
  * MPSOLVER_INCOMPATIBLE_OPTIONS error.
  */
-MPSolutionResponse SolveMPModel(LazyMutableCopy<MPModelRequest> request,
-                                const SolveInterrupter* interrupter = nullptr);
+MPSolutionResponse SolveMPModel(
+    LazyMutableCopy<MPModelRequest> request,
+    const SolveInterrupter* absl_nullable interrupter = nullptr);
 
 bool SolverTypeSupportsInterruption(MPModelRequest::SolverType solver);
 

ortools/util/python/py_solve_interrupter_testing.cc

@@ -15,11 +15,13 @@
 
 #include <optional>
 
+#include "absl/base/nullability.h"
 #include "ortools/util/python/py_solve_interrupter.h"
 
 namespace operations_research {
 
-std::optional<bool> IsInterrupted(const PySolveInterrupter* interrupter) {
+std::optional<bool> IsInterrupted(
+    const PySolveInterrupter* absl_nullable interrupter) {
   if (interrupter == nullptr) {
     return std::nullopt;
   }

ortools/util/python/py_solve_interrupter_testing.h

@@ -31,7 +31,8 @@ namespace operations_research {
 //
 // The Clif/pybind11 wrapper will return a `bool | None` value, with None for
 // nullopt.
-std::optional<bool> IsInterrupted(const PySolveInterrupter* interrupter);
+std::optional<bool> IsInterrupted(
+    const PySolveInterrupter* absl_nullable interrupter);
 
 // Class that keeps a reference on a std::shared_ptr<PySolveInterrupter> to
 // test that the C++ object survive the cleanup of the Python reference.