Modeling Algorithms - Guard silently-uncapped solid in BRepOffsetAPI_ThruSections

[gsdali]

Pre-Submission Checks

• I checked existing issues, pull requests, discussions, and forum topics for related work.
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Problem / Motivation

BRepOffsetAPI_ThruSections(isSolid: true) silently omits both end-cap faces when a closed
section wire has two or more periods of out-of-plane variation around the loop, a genuinely
non-planar closed curve. Build() still calls Done() and reports IsDone() == true, and
Shape() returns a TopoDS_Solid that is missing two faces and is not actually closed.
BRepCheck_Analyzer catches this later, but with no localized error (errorCount == 0, no
detailed check statuses), so a caller has already trusted IsDone() by the time anything flags
it.

Root cause: MakeSolid() (the static helper both CreateRuled() and CreateSmoothed() call to
close a loft's two open ends) caps each end via PerformPlan(), which tries
BRepBuilderAPI_FindPlane and then a BRepLib_FindSurface-backed MakeFace(wire) fallback.
Neither finds anything but a plane or a surface already attached to the wire's edges. A section
wire with k >= 2 periods of out-of-plane variation around the loop has no such surface, so
PerformPlan() fails for it. MakeSolid() already tracks this in its own local B, threaded
through both PerformPlan() calls — and discards it: it force-marks the shell and solid
Closed(true) unconditionally before returning, and the caller then calls Done()
unconditionally too.

A single period of out-of-plane variation (e.g. z = amp * cos(theta) at constant radius) caps
fine, because that curve is secretly planar: it is exactly the intersection of the cylinder
r = const with a tilted plane, so BRepBuilderAPI_FindPlane finds it. k >= 2 has no such
plane and no other analytic surface either.

Found downstream of a bevel gear ThruSections loft in the OCCTSwift ecosystem, where each
section is a toothed profile whose out-of-plane period equals the tooth count, always >= 2 for
a real gear.

Proposed Solution

MakeSolid()'s local B already distinguishes exactly the two cases that matter: it stays
true when capping either succeeded or wasn't needed (PerformPlan()'s own degenerate-wire
shortcut — every edge of the wire is degenerate, a single-vertex "point" section built via
AddVertex(), e.g. a cone's apex, which needs no cap face and correctly reports success with a
null face), and ends false only when capping was genuinely attempted and failed. After the
capping block, if B is still false, throw StdFail_NotDone, matching the exception this same
function already throws for a null shell two lines above:

if (!B)
{
  throw StdFail_NotDone("BRepOffsetAPI_ThruSections: could not close a non-planar extremity");
}

No signature change and no call-site change: both call sites already run inside Build()'s only
try/catch (catch (Standard_Failure const&) { NotDone(); return; }), so the throw propagates
there and IsDone() correctly reads false.

This does not make BRepOffsetAPI_ThruSections able to cap a genuinely non-planar wire (that
would need a real capping algorithm, e.g. an N-sided patch, out of scope here). It converts a
silent wrong success into a correctly reported failure, matching every other failure path in this
file.

A null-face check (myFirst.IsNull() || myLast.IsNull()) looks like an equally natural fix and
is not: PerformPlan() also leaves the output face null, and returns true (no failure), when
the wire is degenerate, so a null-face check cannot tell "no cap needed" apart from "cap needed
and failed." Checked directly against BOPAlgo_PaveFillerTest.FuseConeLoftWithBox_DegeneratedEdge
(a circle-to-vertex loft, in this same PR's GTest additions) before settling on the B-based fix
— a null-face guard regresses that test, B does not.

Limitation: GetStatus() is not updated by this fix and stays
BRepFill_ThruSectionErrorStatus_Done on this path. Build()'s catch block doesn't touch
myStatus for any exception source, not one this fix introduces — IsDone() is the correct,
fixed signal.

Validation

Checks performed:

• clang-format --dry-run --Werror against this tree's own .clang-format: zero violations
  on both changed files.
• Added GTest cases in BRepOffsetAPI_ThruSections_Test.cxx:
  NonPlanarClosedWireCappingFails (a two-wire k=2 loft, must now fail) and
  DegenerateVertexEndStillSucceeds (a circle-to-vertex loft, must still succeed — the same
  shape as BOPAlgo_PaveFillerTest.FuseConeLoftWithBox_DegeneratedEdge, added as a direct,
  local regression guard for it in this file too).
• CI on this branch (macOS/Linux/Windows builds, all Run GTest/Test/Regression Test
  jobs including the No-PCH debug build) is green.
• Locally, before pushing: override-link tested (compile the changed .cxx standalone, link
  it ahead of a prebuilt libOCCT-macos.a, no full OCCT rebuild) both an unpatched and a
  patched build. Against the unpatched build, NonPlanarClosedWireCappingFails fails exactly
  as described above (IsDone() reads true). Against the patched build, it passes, along
  with DegenerateVertexEndStillSucceeds and all pre-existing tests in the file. Also
  compiled and ran the actual BOPAlgo_PaveFiller_Test.cxx
FuseConeLoftWithBox_DegeneratedEdge test, unmodified, against the patched build: passes.

🤖 Generated with Claude Code

[gsdali]

Pushed a small strengthening found during downstream review (SecondMouseAU/OCCTSwift#909): if
wire1's PerformPlan() succeeds but wire2's fails, face1 (aliased to myFirst, exposed via
FirstShape()) could still hold a real face that was never added to the shell, even though the
overall build correctly reports IsDone() == false. Nullifying face1/face2 in the failure
branch closes that. No change to the core logic (B, the throw, the exception type) — re-verified
with the same override-link technique against both the two new GTests and
FuseConeLoftWithBox_DegeneratedEdge, all still pass.