SCIP solver termination condition is "other" when gap limit reached

[adbuerger]

Summary

I solved the same sample integer program with different solvers (CBC, SCIP, HiGHS) until a defined gap limit was reached (but not to full optimality). I noticed that CBC and HiGHS solver stated "optimal" as termination condition, while SCIP stated "other". This appears inconsistent to me and I assume that (since I think a termination condition "gap limit reached" does not exist in Pyomo?) SCIP should also return "optimal" in that case.

Thank you for your consideration.

Steps to reproduce the issue

You can see the behavior when running the following example:

import pyomo.environ as pyo
import random

random.seed(100)
MIP_GAP = 1.0
TEE = False

MIP_GAP_KEYWORD = {
    "cbc": "ratioGap",
    "appsi_highs": "mip_rel_gap",
    "scip": "limits/gap",
}

for solver_name in ["cbc", "appsi_highs", "scip"]:
    m = pyo.ConcreteModel()
    m.x = pyo.Var(range(101), domain=pyo.Binary)
    m.o = pyo.Objective(expr=sum(m.x[j] for j in m.x), sense=pyo.minimize)
    m.c = pyo.ConstraintList()
    for _ in range(200):
        m.c.add(sum(random.choice([0, 1]) * m.x[j] for j in m.x) >= 1)
    m.c.add(sum(m.x[j] for j in m.x) <= 30)

    solver = pyo.SolverFactory(solver_name)
    solver.options[MIP_GAP_KEYWORD[solver_name]] = MIP_GAP

    results = solver.solve(m, tee=TEE)

    print(solver_name, ":", results.solver.termination_condition)

The above code returns:

cbc : optimal
appsi_highs : optimal
scip : other

Information on your system

Pyomo version: 6.9.1
Python version: 3.11.2
Operating system: Debian 12
How Pyomo was installed (PyPI, conda, source): PyPI
Solver (if applicable): CBC, HiGHS, SCIP

Additional information

I think the termination condition is set here:

pyomo/pyomo/solvers/plugins/solvers/SCIPAMPL.py

Line 342 in 457f436

elif "gap limit reached" in results.solver.message:

[odow]

Coincidentally, this came up in SCIP.jl last week: scipopt/SCIP.jl#304 (comment)

We map SCIP_STATUS_GAPLIMIT to MOI.OPTIMAL (code).

The SCIP log for this status is

SCIP Status        : solving was interrupted [gap limit reached]

which will probably confuse users regardless of which convention you choose.

[michaelbynum]

This should be resolved in #3744.