Routing logs missing after solving with cp-sat

[igorbanfi]

Hello

What version of OR-Tools and what language are you using?

Version: 9.15.6755
Language: Python

Which solver are you using (e.g. CP-SAT, Routing Solver, GLOP, BOP, Gurobi)

Solver: Routing solver and CP-SAT

What did you do?

After solving with CP-SAT solver the logs from routing solver are missing, even if enabled with search_parameters.log_search.

Example to reproduce the issue:
main.py

What did you expect to see
Logs to be configured independetly

What did you see instead?
Logs from the second run of routing solver are missing.

[jmarca]

You don't actually need the first call to the routing solver, it seems. Calling CP-SAT and then routing is enough to trigger the bug:

$ python main.py

Simple CP-SAT example:
x = 1
y = 0
z = 0

TSP example 2:
Objective: 7293 miles
Route for vehicle 0:
 0 -> 7 -> 2 -> 3 -> 4 -> 12 -> 6 -> 8 -> 1 -> 11 -> 10 -> 5 -> 9 -> 0
Route distance: 7293miles

[jmarca]

Something that has always bothered me is this message in the routing solver:

WARNING: All log messages before absl::InitializeLog() is called are written to STDERR

And because work is slow I decided to try to track this down a bit.

Anyway, this message does not show up in the cpsat solver, so I assume the absl log is initialized properly there.

Searching why that might be, I grepped for intialize log:

james at pineapple in ~/repos/or-tools/ortools on stable
$ grep "absl::InitializeLog()" * -R
base/logging.cc:    absl::InitializeLog();
base/init_google.h:  absl::InitializeLog();
flatzinc/fz.cc:  absl::InitializeLog();
sat/python/gen_proto_builder_pybind11.cc:  absl::InitializeLog();
sat/sat_runner.cc:  absl::InitializeLog();

Following that up, I see that base/logging.cc: absl::InitializeLog(); is contained in a function

void FixFlagsAndEnvironmentForSwig() {
  std::call_once(init_done, [] {
    absl::InitializeLog();
    absl::SetProgramUsageMessage("swig_helper");
  });
  absl::EnableLogPrefix(false);
}

That function is called from cpsat but not constraint_solver:

james at pineapple in ~/repos/or-tools/ortools on stable
$ grep "FixFlags" * -R
base/logging.cc:void FixFlagsAndEnvironmentForSwig() {
base/logging.h:void FixFlagsAndEnvironmentForSwig();
sat/swig_helper.cc:  FixFlagsAndEnvironmentForSwig();

So my guess is that somehow the correct usage in the CP-SAT library is clobbering the incorrect usage in the routing library. The routing library, because the absl::logging is not initialized, defaults to stderr. But when it is properly initialized in CP-SAT, it is then silent and/or logging somewhere other than stderr.