Some deferred observers get destroyed during updates

ql/patterns/observable.cpp

@@ -33,8 +33,12 @@ namespace QuantLib {
         if (!deferredObservers_.empty()) {
             bool successful = true;
             std::string errMsg;
+            runningDeferredUpdates_ = true;
+
+            for (const auto& [deferredObserver, isValid] : deferredObservers_) {
+                if (!isValid)
+                    continue;
 
-            for (auto* deferredObserver : deferredObservers_) {
                 try {
                     deferredObserver->update();
                 } catch (std::exception& e) {
@@ -46,10 +50,12 @@ namespace QuantLib {
             }
 
             deferredObservers_.clear();
+            runningDeferredUpdates_ = false;
 
             QL_ENSURE(successful,
                   "could not notify one or more observers: " << errMsg);
         }
+
     }
 
 

ql/patterns/observable.hpp

@@ -34,6 +34,7 @@ FOR A PARTICULAR PURPOSE.  See the license for more details.
 #include <ql/shared_ptr.hpp>
 #include <ql/types.hpp>
 #include <set>
+#include <map>
 
 #if !defined(QL_USE_STD_SHARED_PTR) && BOOST_VERSION < 107400
 
@@ -96,11 +97,12 @@ namespace QuantLib {
 
         bool updatesEnabled() const { return updatesEnabled_; }
         bool updatesDeferred() const { return updatesDeferred_; }
+        bool runningDeferredUpdates() const { return runningDeferredUpdates_; }
 
       private:
         ObservableSettings() = default;
 
-        typedef std::set<Observer*> set_type;
+        typedef std::map<Observer*, bool> set_type;
         typedef set_type::iterator iterator;
 
         void registerDeferredObservers(const Observable::set_type& observers);
@@ -109,6 +111,7 @@ namespace QuantLib {
         set_type deferredObservers_;
 
         bool updatesEnabled_ = true, updatesDeferred_ = false;
+        bool runningDeferredUpdates_ = false;
     };
 
     //! Object that gets notified when a given observable changes
@@ -160,12 +163,20 @@ namespace QuantLib {
 
     inline void ObservableSettings::registerDeferredObservers(const Observable::set_type& observers) {
         if (updatesDeferred()) {
-            deferredObservers_.insert(observers.begin(), observers.end());
+            for (Observer* obs : observers)
+                deferredObservers_.emplace(obs, true);
         }
     }
 
     inline void ObservableSettings::unregisterDeferredObserver(Observer* o) {
-        deferredObservers_.erase(o);
+        if (updatesDeferred())
+            deferredObservers_.erase(o);
+        else
+        {
+            auto it = deferredObservers_.find(o);
+            if (it != deferredObservers_.end())
+                it->second = false;
+        }
     }
 
     inline Observable::Observable(const Observable&) {
@@ -195,7 +206,8 @@ namespace QuantLib {
     }
 
     inline Size Observable::unregisterObserver(Observer* o) {
-        if (ObservableSettings::instance().updatesDeferred())
+        if (ObservableSettings::instance().updatesDeferred() ||
+            ObservableSettings::instance().runningDeferredUpdates())
             ObservableSettings::instance().unregisterDeferredObserver(o);
 
         return observers_.erase(o);

test-suite/observable.cpp

@@ -20,14 +20,19 @@
 #include "toplevelfixture.hpp"
 #include "utilities.hpp"
 #include <ql/indexes/ibor/euribor.hpp>
+#include <ql/indexes/inflation/euhicp.hpp>
 #include <ql/math/randomnumbers/mt19937uniformrng.hpp>
 #include <ql/patterns/observable.hpp>
 #include <ql/quotes/simplequote.hpp>
+#include <ql/termstructures/bootstraphelper.hpp>
+#include <ql/termstructures/inflation/inflationhelpers.hpp>
 #include <ql/termstructures/volatility/capfloor/capfloortermvolsurface.hpp>
 #include <ql/termstructures/volatility/optionlet/strippedoptionlet.hpp>
 #include <ql/termstructures/volatility/optionlet/strippedoptionletadapter.hpp>
 #include <ql/termstructures/yield/flatforward.hpp>
+#include <ql/time/daycounters/actualactual.hpp>
 #include <ql/time/calendars/nullcalendar.hpp>
+#include <ql/time/calendars/target.hpp>
 #include <chrono>
 #include <thread>
 
@@ -395,6 +400,20 @@ BOOST_AUTO_TEST_CASE(testAddAndDeleteObserverDuringNotifyObservers) {
     }
 }
 
+BOOST_AUTO_TEST_CASE(testDeferredObserverLifetime)
+{
+    Date today(24, Dec, 2025);
+    Settings::instance().evaluationDate() = today;
+    Handle<Quote> quote(ext::make_shared<SimpleQuote>(0.02));
+    auto zciHelper = ext::make_shared<ZeroCouponInflationSwapHelper>(
+        quote, 3 * Months, Date(29, Dec, 2026), TARGET(), ModifiedFollowing,
+        ActualActual(ActualActual::ISDA), ext::make_shared<EUHICPXT>(), CPI::Flat);
+
+    ObservableSettings::instance().disableUpdates(true);
+    Settings::instance().evaluationDate() = Date(29, Dec, 2025);
+    ObservableSettings::instance().enableUpdates();
+}
+
 BOOST_AUTO_TEST_SUITE_END()
 
 BOOST_AUTO_TEST_SUITE_END()