Remove incorrect Py_INCREF/DECREF on Model in catchEvent/dropEvent

CHANGELOG.md

@@ -2,7 +2,9 @@
 
 ## Unreleased
 ### Added
+- Added `getMemUsed()`, `getMemTotal()`, and `getMemExternEstim()` methods
 ### Fixed
+- Removed incorrect `Py_INCREF`/`Py_DECREF` on `Model` in `catchEvent`/`dropEvent` that caused reference count imbalance
 ### Changed
 ### Removed
 

src/pyscipopt/scip.pxi

@@ -316,7 +316,7 @@
     if rc == SCIP_OKAY:
         pass
     elif rc == SCIP_ERROR:
         raise Exception('SCIP: unspecified error!')
     elif rc == SCIP_NOMEMORY:
         raise MemoryError('SCIP: insufficient memory error!')
     elif rc == SCIP_READERROR:
@@ -335,7 +335,7 @@
         raise Exception('SCIP: method cannot be called at this time'
                             + ' in solution process!')
     elif rc == SCIP_INVALIDDATA:
         raise Exception('SCIP: error in input data!')
     elif rc == SCIP_INVALIDRESULT:
         raise Exception('SCIP: method returned an invalid result code!')
     elif rc == SCIP_PLUGINNOTFOUND:
@@ -11268,7 +11268,6 @@
         else:
             raise Warning("event handler not found")
 
-        Py_INCREF(self)
         PY_SCIP_CALL(SCIPcatchEvent(self._scip, eventtype, _eventhdlr, NULL, NULL))
 
     def dropEvent(self, eventtype, Eventhdlr eventhdlr):
@@ -11288,7 +11287,6 @@
         else:
             raise Warning("event handler not found")
 
-        Py_DECREF(self)
         PY_SCIP_CALL(SCIPdropEvent(self._scip, eventtype, _eventhdlr, NULL, -1))
 
     def catchVarEvent(self, Variable var, eventtype, Eventhdlr eventhdlr):

tests/test_event.py

@@ -1,4 +1,4 @@
-import pytest, random
+import pytest, weakref, gc, random
 
 from pyscipopt import Model, Eventhdlr, SCIP_RESULT, SCIP_EVENTTYPE, SCIP_PARAMSETTING, quicksum
 
@@ -189,4 +189,40 @@ def eventexec(self, event):
     m.includeEventhdlr(ev, "var_event", "event handler for var events")
 
     with pytest.raises(Exception):
-        m.optimize()
+        m.optimize()
+
+def test_catchEvent_does_not_leak_model():
+    """catchEvent should not artificially increment the Model's reference count.
+
+    Previously, catchEvent called Py_INCREF(self) on the Model, and dropEvent
+    called Py_DECREF(self). Since many event handlers skip dropEvent (e.g. when
+    self.model is already dead), this caused the Model to leak — its refcount
+    never returned to zero, preventing garbage collection.
+    """
+
+    class SimpleEvent(Eventhdlr):
+        def eventinit(self):
+            self.model.catchEvent(SCIP_EVENTTYPE.NODEFOCUSED, self)
+
+        def eventexit(self):
+            pass  # intentionally no dropEvent, which is bad practice
+
+        def eventexec(self, event):
+            pass
+
+    m = Model()
+    m.hideOutput()
+    ev = SimpleEvent()
+    m.includeEventhdlr(ev, "simple", "test event handler")
+    m.addVar("x", obj=1, vtype="I")
+    m.optimize()
+
+    ref = weakref.ref(m)
+
+    del ev
+    gc.collect()
+    assert ref() is not None, "Model was garbage collected — event handler absorbed a reference"
+
+    del m
+    gc.collect()
+    assert ref() is None, "Model was not garbage collected — catchEvent likely leaked a reference"