EQUIL - BUGFIX - Detect field-line ODE failure instead of consuming a truncated surface

src/Equilibrium/DirectEquilibrium.jl

@@ -293,6 +293,26 @@ function direct_fieldline_int(psifac::Float64, raw_profile::DirectRunInput, ro::
     prob = ODEProblem{true}(direct_fieldline_der!, u0, (0.0, 2π), params)
     sol = solve(prob, Vern9(); callback=callback, reltol=equil_config.etol, abstol=1e-8, dt=2π / 200, adaptive=true, dense=false)
 
+    # A failed integration does not throw: it returns a solution truncated wherever it gave up,
+    # and the caller reads the last point as if it closed the field line at η = 2π. That is a
+    # silently wrong surface, so check the retcode and the endpoint. Surfaces very close to the
+    # separatrix are where this fires.
+    if sol.retcode != ReturnCode.Success
+        error(
+            "direct_fieldline_int: field-line integration failed at psifac = " *
+            "$(@sprintf("%.6f", psifac)) (retcode $(sol.retcode)); the flux surface did not " *
+            "close. This usually means psihigh is too close to the separatrix for the " *
+            "equilibrium grid to resolve."
+        )
+    end
+    if !isapprox(sol.t[end], 2π; atol=1e-8)
+        error(
+            "direct_fieldline_int: field-line integration at psifac = " *
+            "$(@sprintf("%.6f", psifac)) stopped at eta = $(@sprintf("%.6f", sol.t[end])) " *
+            "instead of 2*pi; the flux surface did not close."
+        )
+    end
+
     sol_matrix = reduce(hcat, sol.u::Vector{Vector{Float64}})'
     return hcat(sol.t::Vector{Float64}, sol_matrix), bfield
 end

test/runtests_equil.jl

@@ -552,4 +552,16 @@
             @test pe.params.li3 > 0
         end
     end
+
+    @testset "field-line ODE failure is an error, not silent truncation" begin
+        # A failed solve does not throw -- it returns a solution truncated wherever it gave up,
+        # which direct_fieldline_int used to consume as a closed flux surface. Both guards must
+        # be present: the retcode check, and the endpoint check (a solve can stop early via a
+        # callback-driven terminate and still report Success). Asserted against the source
+        # because a genuinely non-closing field line cannot be synthesized cheaply here; the
+        # EFIT testsets above are the standing positive control that neither guard false-fires.
+        src = read(joinpath(dirname(@__DIR__), "src", "Equilibrium", "DirectEquilibrium.jl"), String)
+        @test occursin("sol.retcode != ReturnCode.Success", src)
+        @test occursin("isapprox(sol.t[end], 2π", src)
+    end
 end