Replace weakref.proxy with strong references for plugin self.model (#… (#1194)

Joao-Dionisio · web-flow · commit e3b28924ce2f · 2026-04-22T16:29:45.000+01:00
* Replace weakref.proxy with strong references for plugin self.model (#1193) * Remove stale Py_DECREF from plugin Free callbacks and fix IISfinder/dealloc * remove need for dropEvent * Fix stubs and comment for weakref removal * Clarify plugin lifecycle and hide eventhdlr bookkeeping * Swallow SCIPfree retcode in _free_scip_instance
diff --git a/CHANGELOG.md b/CHANGELOG.md
@@ -10,6 +10,7 @@
 - Used `getIndex()` instead of `ptr()` for sorting nonlinear expression terms to avoid nondeterministic behavior
 - Fixed stubtest failures with mypy 1.20 by marking dunder method parameters as positional-only
 - Return `MatrixGenExpr` in `buildGenExprObj` instead of `MatrixExpr`
+- Plugins now hold strong references to their `Model` instead of `weakref.proxy`, fixing `ReferenceError` during cleanup callbacks (#1193)
 ### Changed
 - Speed up `constant * Expr` via C-level API
 - Speed up `Term.__eq__` via the C-level API
diff --git a/src/pyscipopt/benders.pxi b/src/pyscipopt/benders.pxi
@@ -76,7 +76,6 @@ cdef SCIP_RETCODE PyBendersFree (SCIP* scip, SCIP_BENDERS* benders) noexcept wit
     bendersdata = SCIPbendersGetData(benders)
     PyBenders = <Benders>bendersdata
     PyBenders.bendersfree()
-    Py_DECREF(PyBenders)
     return SCIP_OKAY
 
 cdef SCIP_RETCODE PyBendersInit (SCIP* scip, SCIP_BENDERS* benders) noexcept with gil:
diff --git a/src/pyscipopt/benderscut.pxi b/src/pyscipopt/benderscut.pxi
@@ -31,7 +31,6 @@ cdef SCIP_RETCODE PyBenderscutFree (SCIP* scip, SCIP_BENDERSCUT* benderscut) noe
     benderscutdata = SCIPbenderscutGetData(benderscut)
     PyBenderscut = <Benderscut>benderscutdata
     PyBenderscut.benderscutfree()
-    Py_DECREF(PyBenderscut)
     return SCIP_OKAY
 
 cdef SCIP_RETCODE PyBenderscutInit (SCIP* scip, SCIP_BENDERSCUT* benderscut) noexcept with gil:
diff --git a/src/pyscipopt/branchrule.pxi b/src/pyscipopt/branchrule.pxi
@@ -47,7 +47,6 @@ cdef SCIP_RETCODE PyBranchruleFree (SCIP* scip, SCIP_BRANCHRULE* branchrule) noe
     branchruledata = SCIPbranchruleGetData(branchrule)
     PyBranchrule = <Branchrule>branchruledata
     PyBranchrule.branchfree()
-    Py_DECREF(PyBranchrule)
     return SCIP_OKAY
 
 cdef SCIP_RETCODE PyBranchruleInit (SCIP* scip, SCIP_BRANCHRULE* branchrule) noexcept with gil:
diff --git a/src/pyscipopt/conshdlr.pxi b/src/pyscipopt/conshdlr.pxi
@@ -159,7 +159,6 @@ cdef SCIP_RETCODE PyConshdlrCopy (SCIP* scip, SCIP_CONSHDLR* conshdlr, SCIP_Bool
 cdef SCIP_RETCODE PyConsFree (SCIP* scip, SCIP_CONSHDLR* conshdlr) noexcept with gil:
     PyConshdlr = getPyConshdlr(conshdlr)
     PyConshdlr.consfree()
-    Py_DECREF(PyConshdlr)
     return SCIP_OKAY
 
 cdef SCIP_RETCODE PyConsInit (SCIP* scip, SCIP_CONSHDLR* conshdlr, SCIP_CONS** conss, int nconss) noexcept with gil:
diff --git a/src/pyscipopt/cutsel.pxi b/src/pyscipopt/cutsel.pxi
@@ -37,7 +37,6 @@ cdef SCIP_RETCODE PyCutselFree (SCIP* scip, SCIP_CUTSEL* cutsel) noexcept with g
   cutseldata = SCIPcutselGetData(cutsel)
   PyCutsel = <Cutsel>cutseldata
   PyCutsel.cutselfree()
-  Py_DECREF(PyCutsel)
   return SCIP_OKAY
 
 cdef SCIP_RETCODE PyCutselInit (SCIP* scip, SCIP_CUTSEL* cutsel) noexcept with gil:
diff --git a/src/pyscipopt/event.pxi b/src/pyscipopt/event.pxi
@@ -3,6 +3,11 @@
 cdef class Eventhdlr:
     cdef public Model model
     cdef public str name
+    # eventtypes caught via `Model.catchEvent`, auto-dropped on `eventexit`.
+    cdef list _caught_events
+
+    def __cinit__(self):
+        self._caught_events = []
 
     def eventcopy(self):
         '''sets copy callback for all events of this event handler '''
@@ -51,7 +56,6 @@ cdef SCIP_RETCODE PyEventCopy (SCIP* scip, SCIP_EVENTHDLR* eventhdlr) noexcept w
 cdef SCIP_RETCODE PyEventFree (SCIP* scip, SCIP_EVENTHDLR* eventhdlr) noexcept with gil:
     PyEventhdlr = getPyEventhdlr(eventhdlr)
     PyEventhdlr.eventfree()
-    Py_DECREF(PyEventhdlr)
     return SCIP_OKAY
 
 cdef SCIP_RETCODE PyEventInit (SCIP* scip, SCIP_EVENTHDLR* eventhdlr) noexcept with gil:
@@ -62,6 +66,10 @@ cdef SCIP_RETCODE PyEventInit (SCIP* scip, SCIP_EVENTHDLR* eventhdlr) noexcept w
 cdef SCIP_RETCODE PyEventExit (SCIP* scip, SCIP_EVENTHDLR* eventhdlr) noexcept with gil:
     PyEventhdlr = getPyEventhdlr(eventhdlr)
     PyEventhdlr.eventexit()
+    # Auto-drop any events not explicitly dropped by the user
+    for eventtype in PyEventhdlr._caught_events:
+        SCIPdropEvent(scip, eventtype, eventhdlr, NULL, -1)
+    PyEventhdlr._caught_events = []
     return SCIP_OKAY
 
 cdef SCIP_RETCODE PyEventInitsol (SCIP* scip, SCIP_EVENTHDLR* eventhdlr) noexcept with gil:
diff --git a/src/pyscipopt/heuristic.pxi b/src/pyscipopt/heuristic.pxi
@@ -38,7 +38,6 @@ cdef SCIP_RETCODE PyHeurFree (SCIP* scip, SCIP_HEUR* heur) noexcept with gil:
     heurdata = SCIPheurGetData(heur)
     PyHeur = <Heur>heurdata
     PyHeur.heurfree()
-    Py_DECREF(PyHeur)
     return SCIP_OKAY
 
 cdef SCIP_RETCODE PyHeurInit (SCIP* scip, SCIP_HEUR* heur) noexcept with gil:
diff --git a/src/pyscipopt/iisfinder.pxi b/src/pyscipopt/iisfinder.pxi
@@ -1,8 +1,9 @@
 ##@file iisfinder.pxi
 #@brief Base class of the IIS finder Plugin
 cdef class IISfinder:
+    cdef public Model model
     cdef public IIS iis
-    cdef SCIP_IIS* scip_iis 
+    cdef SCIP_IIS* scip_iis
     cdef SCIP_IISFINDER* scip_iisfinder
 
     def iisfinderfree(self):
@@ -22,7 +23,6 @@ cdef SCIP_RETCODE PyiisfinderFree (SCIP* scip, SCIP_IISFINDER* iisfinder) noexce
     iisfinderdata = SCIPiisfinderGetData(iisfinder)
     PyIIS = <IISfinder>iisfinderdata
     PyIIS.iisfinderfree()
-    Py_DECREF(PyIIS)
     return SCIP_OKAY
 
 cdef SCIP_RETCODE PyiisfinderExec (SCIP_IIS* iis, SCIP_IISFINDER* iisfinder, SCIP_RESULT* result) noexcept with gil:
diff --git a/src/pyscipopt/nodesel.pxi b/src/pyscipopt/nodesel.pxi
@@ -50,7 +50,6 @@ cdef SCIP_RETCODE PyNodeselFree (SCIP* scip, SCIP_NODESEL* nodesel) noexcept wit
   nodeseldata = SCIPnodeselGetData(nodesel)
   PyNodesel = <Nodesel>nodeseldata
   PyNodesel.nodefree()
-  Py_DECREF(PyNodesel)
   return SCIP_OKAY
 
 cdef SCIP_RETCODE PyNodeselInit (SCIP* scip, SCIP_NODESEL* nodesel) noexcept with gil:
diff --git a/src/pyscipopt/presol.pxi b/src/pyscipopt/presol.pxi
@@ -37,7 +37,6 @@ cdef SCIP_RETCODE PyPresolFree (SCIP* scip, SCIP_PRESOL* presol) noexcept with g
     presoldata = SCIPpresolGetData(presol)
     PyPresol = <Presol>presoldata
     PyPresol.presolfree()
-    Py_DECREF(PyPresol)
     return SCIP_OKAY
 
 cdef SCIP_RETCODE PyPresolInit (SCIP* scip, SCIP_PRESOL* presol) noexcept with gil:
diff --git a/src/pyscipopt/pricer.pxi b/src/pyscipopt/pricer.pxi
@@ -42,7 +42,6 @@ cdef SCIP_RETCODE PyPricerFree (SCIP* scip, SCIP_PRICER* pricer) noexcept with g
     pricerdata = SCIPpricerGetData(pricer)
     PyPricer = <Pricer>pricerdata
     PyPricer.pricerfree()
-    Py_DECREF(PyPricer)
     return SCIP_OKAY
 
 cdef SCIP_RETCODE PyPricerInit (SCIP* scip, SCIP_PRICER* pricer) noexcept with gil:
diff --git a/src/pyscipopt/propagator.pxi b/src/pyscipopt/propagator.pxi
@@ -53,7 +53,6 @@ cdef SCIP_RETCODE PyPropFree (SCIP* scip, SCIP_PROP* prop) noexcept with gil:
     propdata = SCIPpropGetData(prop)
     PyProp = <Prop>propdata
     PyProp.propfree()
-    Py_DECREF(PyProp)
     return SCIP_OKAY
 
 cdef SCIP_RETCODE PyPropInit (SCIP* scip, SCIP_PROP* prop) noexcept with gil:
diff --git a/src/pyscipopt/reader.pxi b/src/pyscipopt/reader.pxi
@@ -26,7 +26,6 @@ cdef SCIP_RETCODE PyReaderFree (SCIP* scip, SCIP_READER* reader) noexcept with g
     readerdata = SCIPreaderGetData(reader)
     PyReader = <Reader>readerdata
     PyReader.readerfree()
-    Py_DECREF(PyReader)
     return SCIP_OKAY
 
 cdef SCIP_RETCODE PyReaderRead (SCIP* scip, SCIP_READER* reader, const char* filename, SCIP_RESULT* result) noexcept with gil:
diff --git a/src/pyscipopt/relax.pxi b/src/pyscipopt/relax.pxi
@@ -36,7 +36,6 @@ cdef SCIP_RETCODE PyRelaxFree (SCIP* scip, SCIP_RELAX* relax) noexcept with gil:
     relaxdata = SCIPrelaxGetData(relax)
     PyRelax = <Relax>relaxdata
     PyRelax.relaxfree()
-    Py_DECREF(PyRelax)
     return SCIP_OKAY
 
 cdef SCIP_RETCODE PyRelaxInit (SCIP* scip, SCIP_RELAX* relax) noexcept with gil:
diff --git a/src/pyscipopt/scip.pxd b/src/pyscipopt/scip.pxd
@@ -2254,11 +2254,18 @@ cdef class Model:
     cdef int _generated_event_handlers_count
     # store references to Benders subproblem Models for proper cleanup
     cdef _benders_subproblems
+    # Strong references to every included plugin. Each plugin in turn holds a
+    # strong reference to the Model via `self.model`, so both stay alive until
+    # SCIP teardown callbacks (consfree, eventexit, ...) have finished running.
+    # The resulting cycle is broken by `Model.__del__` or by `Model.free()`.
+    # See `Model.free` for the full lifecycle policy.
+    cdef _plugins
     # store iis, if found
     cdef SCIP_IIS* _iis
     # helper methods for later var and cons cleanup
     cdef _getOrCreateCons(self, SCIP_CONS* scip_cons)
     cdef _getOrCreateVar(self, SCIP_VAR* scip_var)
+    cdef _free_scip_instance(self)
 
     @staticmethod
     cdef create(SCIP* scip)
diff --git a/src/pyscipopt/scip.pxi b/src/pyscipopt/scip.pxi
diff --git a/src/pyscipopt/scip.pyi b/src/pyscipopt/scip.pyi
diff --git a/src/pyscipopt/sepa.pxi b/src/pyscipopt/sepa.pxi
diff --git a/tests/test_conshdlr.py b/tests/test_conshdlr.py
diff --git a/tests/test_event.py b/tests/test_event.py
diff --git a/tests/test_pricer.py b/tests/test_pricer.py
