From 45f9ca77300a3a180aa82dbcd8938e4cd4524c94 Mon Sep 17 00:00:00 2001
From: Oscar Dowson <odow@users.noreply.github.com>
Date: Mon, 21 Feb 2022 10:48:23 +1300
Subject: [PATCH] Fix handling of INVALID_MODEL (#303)

---
 src/MOI_wrapper.jl  | 17 +++++++++++++----
 test/MOI_wrapper.jl | 10 ++++------
 2 files changed, 17 insertions(+), 10 deletions(-)

diff --git a/src/MOI_wrapper.jl b/src/MOI_wrapper.jl
index 4d1971b..3c393d9 100644
--- a/src/MOI_wrapper.jl
+++ b/src/MOI_wrapper.jl
@@ -13,6 +13,7 @@ Create a new Ipopt optimizer.
 """
 mutable struct Optimizer <: MOI.AbstractOptimizer
     inner::Union{Nothing,IpoptProblem}
+    invalid_model::Bool
     variables::MOI.Utilities.VariablesContainer{Float64}
     variable_primal_start::Vector{Union{Nothing,Float64}}
     variable_lower_start::Vector{Union{Nothing,Float64}}
@@ -67,6 +68,7 @@ mutable struct Optimizer <: MOI.AbstractOptimizer
     function Optimizer()
         return new(
             nothing,
+            false,
             MOI.Utilities.VariablesContainer{Float64}(),
             Union{Nothing,Float64}[],
             Union{Nothing,Float64}[],
@@ -123,6 +125,7 @@ MOI.eval_hessian_lagrangian(::_EmptyNLPEvaluator, H, x, σ, μ) = nothing
 
 function MOI.empty!(model::Optimizer)
     model.inner = nothing
+    model.invalid_model = false
     MOI.empty!(model.variables)
     empty!(model.variable_primal_start)
     empty!(model.variable_lower_start)
@@ -1119,6 +1122,11 @@ function MOI.optimize!(model::Optimizer)
         push!(g_U, bound.upper)
     end
     start_time = time()
+    if length(model.variables.lower) == 0
+        # Don't attempt to create a problem because Ipopt will error.
+        model.invalid_model = true
+        return
+    end
     model.inner = CreateIpoptProblem(
         length(model.variables.lower),
         model.variables.lower,
@@ -1254,7 +1262,9 @@ end
 ### MOI.TerminationStatus
 
 function MOI.get(model::Optimizer, ::MOI.TerminationStatus)
-    if model.inner === nothing
+    if model.invalid_model
+        return MOI.INVALID_MODEL
+    elseif model.inner === nothing
         return MOI.OPTIMIZE_NOT_CALLED
     end
     status = _STATUS_CODES[model.inner.status]
@@ -1292,10 +1302,9 @@ function MOI.get(model::Optimizer, ::MOI.TerminationStatus)
         return MOI.OTHER_ERROR
     elseif status == :NonIpopt_Exception_Thrown
         return MOI.OTHER_ERROR
-    elseif status == :Insufficient_Memory
-        return MOI.MEMORY_LIMIT
     else
-        error("Unrecognized Ipopt status $status")
+        @assert status == :Insufficient_Memory
+        return MOI.MEMORY_LIMIT
     end
 end
 
diff --git a/test/MOI_wrapper.jl b/test/MOI_wrapper.jl
index 25a4342..0f8ec4c 100644
--- a/test/MOI_wrapper.jl
+++ b/test/MOI_wrapper.jl
@@ -184,12 +184,10 @@ end
 
 function test_empty_optimize()
     model = Ipopt.Optimizer()
-    err = ErrorException(
-        "IPOPT: Failed to construct problem because there are 0 variables. " *
-        "If you intended to construct an empty problem, one work-around is " *
-        "to add a variable fixed to 0.",
-    )
-    @test_throws err MOI.optimize!(model)
+    MOI.optimize!(model)
+    @test MOI.get(model, MOI.TerminationStatus()) == MOI.INVALID_MODEL
+    @test MOI.get(model, MOI.DualStatus()) == MOI.NO_SOLUTION
+    @test MOI.get(model, MOI.PrimalStatus()) == MOI.NO_SOLUTION
     return
 end