Migrate from cbcbox to mipster for the bundled CBC library (#424)

* Migrate from cbcbox to mipster for the bundled CBC library

cbcbox shipped CBC source builds; mipster (https://pypi.org/project/mipster/)
ships self-contained, hardware-tuned wheels for Linux x86_64/aarch64,
macOS x86_64/arm64, and Windows x86_64 with CPUID auto-dispatch
(generic / AVX2 / Haswell / NEON variants).

- Replace `cbcbox>=2.929` with `mipster>=0.2.0`.
- Use `mipster.lib_path()` instead of platform-specific path joining;
  this returns the right shared library for the current OS and
  selected variant.
- Drop the manual libCbc.so / libCbc-0.dll / libCbc.dylib branching.
- Keep `os.add_dll_directory()` on Windows so the loader can find the
  sibling MinGW runtime DLLs that mipster bundles next to libmipster-N.dll.
- Remove unused Cbc_get/setAllowablePercentageGap CFFI declarations
  (these are not exported by libmipster and were never called).

* chore: require mipster>=0.2.4

Version 0.2.4 fixes three C API bugs required for correctness:
- Auto-generate column names so translate() works in lazy callbacks
- Sync bound==value when proven optimal (prevents spurious assertion failures)
- Restore rSlk/rActv pointers after MIP solve (prevents null-pointer crash)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* style: apply black formatting

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* style: apply black 22.3.0 formatting (matching pre-commit config)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* fix: gracefully disable HiGHS on Windows when C API is not accessible

highspy's _core.pyd on Windows only exports the Python init symbol
(PyInit__core) -- the C API functions like Highs_create are compiled
in but not exported as DLL symbols.  On Linux/macOS all symbols in a
shared library are visible, so ffi.dlopen() works there.

After loading the library via ffi.dlopen(), test whether Highs_create
is actually accessible.  If not (Windows + highspy), set has_highs=False
so that all HiGHS tests are skipped gracefully instead of crashing with
'function/symbol not found' errors.

Users on Windows who need HiGHS can install highsbox (which ships a
proper highs.dll with exported C symbols) and set PMIP_HIGHS_LIBRARY
to point to it.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* fix: use highsbox as fallback for HiGHS C API on Windows

highspy's _core.pyd on Windows does not export C symbols (only PyInit_*
is exported from a Windows DLL by default).  On Linux/macOS the .so
exports all symbols so ffi.dlopen + Highs_create works fine; on Windows
it raises AttributeError.

Changes:
- After dlopen succeeds, probe Highs_create accessibility.  If it fails
  (Windows + highspy), automatically retry with highsbox's highs.dll.
- highsbox is added as a Windows-only optional dependency in [highs]:
    highsbox>=1.9.0; sys_platform == 'win32'
  so 'pip install mip[highs]' on Windows installs both highspy (Python
  bindings) and highsbox (C API DLL), giving full HiGHS functionality.
- Hoist the highsbox path resolver to a module-level helper function
  _get_highsbox_libfile() so it is reachable both at initial load and
  at the fallback probe point.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* ci: cancel in-progress runs when new commit is pushed

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* fix: guard __del__ against partially-constructed objects (PyPy safe)

On PyPy, the GC may call __del__ on an object whose __init__ raised
before all attributes were set (e.g. SolverGurobi.__init__ raises early
when Gurobi is not installed, before _ownsModel/_venv_loaded are set).
CPython avoids this because reference-counting drops the object
immediately; PyPy's tracing GC processes it later.

Replace bare attribute access in __del__ with getattr(..., default) in:
- SolverGurobi.__del__: guard _ownsModel and _venv_loaded
- SolverHighs.__del__:  guard _model (avoid calling Highs_destroy(NULL))
- SolverCbc.__del__:    guard _model (avoid calling Cbc_deleteModel(NULL))
- SolverOsi.__del__:    guard owns_solver

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

---------

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: h-g-s

.github/workflows/github-ci.yml

@@ -2,6 +2,10 @@ name: CI
 
 on: [push, pull_request]
 
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
 jobs:
 
   pre-commit:

mip/cbc.py

@@ -70,36 +70,20 @@
         cbclib = ffi.dlopen(libfile)
         os.chdir(old_dir)
     else:
-        import cbcbox as _cbcbox
-
-        _lib_dir = _cbcbox.cbc_lib_dir()
-        if "linux" in platform.lower():
-            if not os_is_64_bit:
-                raise NotImplementedError("Linux 32 bits platform not supported.")
-            libfile = os.path.join(_lib_dir, "libCbc.so")
-        elif platform.lower().startswith("win"):
-            if not os_is_64_bit:
-                raise NotImplementedError("Win32 platform not supported.")
-            # autotools/MinGW places DLLs under bin/, not lib/
-            _bin_dir = os.path.join(_cbcbox.cbc_dist_dir(), "bin")
-            libfile = os.path.join(_bin_dir, "libCbc-0.dll")
-            if not os.path.exists(libfile):
-                raise FileNotFoundError(
-                    "libCbc-0.dll not found in cbcbox Windows distribution at"
-                    " {}. The cbcbox Windows wheel may only contain a static"
-                    " libCbc.a. A shared libCbc-0.dll is required.".format(_bin_dir)
-                )
-            # Python 3.8+ ignores PATH for DLL resolution; use add_dll_directory
+        import mipster as _mipster
+
+        if not os_is_64_bit:
+            raise NotImplementedError("32-bit platforms are not supported.")
+        libfile = _mipster.lib_path()
+        if platform.lower().startswith("win"):
+            # Python 3.8+ ignores PATH for DLL resolution; let Windows find
+            # any sibling DLLs (libgcc, libstdc++, libwinpthread) via
+            # add_dll_directory on the directory holding libmipster-N.dll.
+            _bin_dir = os.path.dirname(libfile)
             if hasattr(os, "add_dll_directory"):
                 os.add_dll_directory(_bin_dir)
             elif _bin_dir not in os.environ.get("PATH", ""):
                 os.environ["PATH"] = _bin_dir + ";" + os.environ["PATH"]
-        elif platform.lower().startswith("darwin") or platform.lower().startswith(
-            "macos"
-        ):
-            libfile = os.path.join(_lib_dir, "libCbc.dylib")
-        else:
-            raise NotImplementedError("Your operating system/platform is not supported")
         cbclib = ffi.dlopen(libfile)
     has_cbc = True
 except Exception as e:
@@ -317,11 +301,6 @@
     void Cbc_setAllowableFractionGap(Cbc_Model *model,
         double allowedFracionGap);
 
-    double Cbc_getAllowablePercentageGap(Cbc_Model *model);
-
-    void Cbc_setAllowablePercentageGap(Cbc_Model *model,
-        double allowedPercentageGap);
-
     double Cbc_getMaximumSeconds(Cbc_Model *model);
 
     void Cbc_setMaximumSeconds(Cbc_Model *model, double maxSeconds);
@@ -1022,7 +1001,9 @@ def clique_merge(self, constrs: Optional[List["mip.Constr"]] = None):
             strengthenPacking = cbclib.Cbc_strengthenPackingRows
             strengthenPacking(self._model, nr, idxr)
 
-    def optimize(self, relax: bool = False, lp_preprocess: bool = False) -> OptimizationStatus:
+    def optimize(
+        self, relax: bool = False, lp_preprocess: bool = False
+    ) -> OptimizationStatus:
         # get name indexes from an osi problem
         def cbc_get_osi_name_indexes(osi_solver) -> Dict[str, int]:
             nameIdx = {}
@@ -1203,7 +1184,9 @@ def cbc_cut_callback(osi_solver, osi_cuts, app_data, depth, npass):
 
         # user-specified cut passes override the cuts-level default
         if self.model.cut_passes != -1:
-            cbclib.Cbc_setIntParam(self._model, INT_PARAM_CUT_PASS, self.model.cut_passes)
+            cbclib.Cbc_setIntParam(
+                self._model, INT_PARAM_CUT_PASS, self.model.cut_passes
+            )
 
         if self.model.clique == 0:
             cbc_set_parameter(self, "clique", "off")
@@ -1662,7 +1645,8 @@ def remove_vars(self, varsList: List[int]):
         cbclib.Cbc_deleteCols(self._model, len(varsList), idx)
 
     def __del__(self):
-        cbclib.Cbc_deleteModel(self._model)
+        if getattr(self, "_model", None) is not None:
+            cbclib.Cbc_deleteModel(self._model)
 
     def get_problem_name(self) -> str:
         namep = self.__name_space
@@ -1793,7 +1777,7 @@ def __clear_sol(self: "SolverOsi"):
         self.__obj_val = None
 
     def __del__(self):
-        if self.owns_solver:
+        if getattr(self, "owns_solver", False):
             cbclib.Osi_deleteSolver(self.osi)
 
     def add_var(

mip/gurobi.py

@@ -428,11 +428,12 @@ def __clear_sol(self):
         self.__obj_val = None
 
     def __del__(self):
-        # freeing Gurobi model and environment
-        if self._ownsModel:
+        # Guard against partially-constructed objects (e.g. PyPy GC may call
+        # __del__ even if __init__ raised before setting these attributes).
+        if getattr(self, "_ownsModel", False):
             if self._model:
                 GRBfreemodel(self._model)
-            if self._env and self._venv_loaded:
+            if self._env and getattr(self, "_venv_loaded", False):
                 GRBfreeenv(self._env)
 
     def add_var(
@@ -589,7 +590,9 @@ def set_max_iter(self, max_iter: int):
     def set_num_threads(self, threads: int):
         self.__threads = threads
 
-    def optimize(self, relax: bool = False, lp_preprocess: bool = False) -> OptimizationStatus:
+    def optimize(
+        self, relax: bool = False, lp_preprocess: bool = False
+    ) -> OptimizationStatus:
 
         # todo add branch_selector and incumbent_updater callbacks
         @ffi.callback(
@@ -802,9 +805,7 @@ def callback(
                 self.__obj_val = self.get_dbl_attr("ObjVal")
                 self.__x = ffi.new("double[{}]".format(self.num_cols()))
                 attr = "X".encode("utf-8")
-                st = GRBgetdblattrarray(
-                    self._model, attr, 0, self.num_cols(), self.__x
-                )
+                st = GRBgetdblattrarray(self._model, attr, 0, self.num_cols(), self.__x)
                 if st:
                     raise ParameterNotAvailable("Error querying Gurobi solution")
                 # duals are only valid at optimality for Gurobi, skip Pi/RC