From 5689ac39146775a2983432498465216b0806b6f1 Mon Sep 17 00:00:00 2001
From: Hayodea Hekol <hayodea@protonmail.com>
Date: Tue, 9 Jun 2026 10:43:20 -0400
Subject: [PATCH] Nursery: Don't pass exceptions to main loop

---
 README.md                                  | 12 ++++----
 include/spinscale/co/nonViralTaskNursery.h | 32 ++++++++--------------
 2 files changed, 17 insertions(+), 27 deletions(-)

diff --git a/README.md b/README.md
index 31e46a3..c7fa402 100644
--- a/README.md
+++ b/README.md
@@ -190,11 +190,10 @@ nursery.openAdmission();
 
 auto lease = nursery.getNewSlotLease();
 lease.getSyncCanceler().startAcceptingWork();
-std::exception_ptr &exceptionStorage = lease.getExceptionStorage();
 lease.setOnSettledHook(
-	[&exceptionStorage]()
+	[](std::exception_ptr &exceptionPtr)
 	{
-		sscl::co::NonViralCompletion nvc(exceptionStorage);
+		sscl::co::NonViralCompletion nvc(exceptionPtr);
 		nvc.checkAndRethrowException();
 	});
 lease.fillSlot(
@@ -221,10 +220,9 @@ all slots have retired naturally and throw if admission is still open.
 `Slot::Lease` is commit-required: an uncommitted lease removes its reservation on
 destruction. `fillSlot()` takes an invoker factory (deferred construction) because
 non-viral coroutines may complete synchronously during invoker construction.
-The factory may capture `lease` by reference; `setOnSettledHook()` may not.
-The hook runs after the lease is destroyed, so capture slot-owned state such as
-`lease.getExceptionStorage()` (a reference to the slot's `exceptionPtr`), not
-`lease` itself.
+The factory may capture `lease` by reference; `setOnSettledHook()` may not
+capture `lease` itself. The nursery passes the slot's `exceptionPtr` into the
+hook at retirement.
 `Slot::Handle` is an opaque slot pointer valid only while the slot remains in the
 nursery.
 
diff --git a/include/spinscale/co/nonViralTaskNursery.h b/include/spinscale/co/nonViralTaskNursery.h
index 4c9de96..4501875 100644
--- a/include/spinscale/co/nonViralTaskNursery.h
+++ b/include/spinscale/co/nonViralTaskNursery.h
@@ -4,7 +4,6 @@
 #include <cstddef>
 #include <exception>
 #include <functional>
-#include <iostream>
 #include <list>
 #include <memory>
 #include <stdexcept>
@@ -116,7 +115,8 @@ public:
 			sscl::SyncCancelerForAsyncWork &getSyncCanceler()
 				{ return slot.syncCanceler; }
 
-			void setOnSettledHook(std::function<void()> hook)
+			void setOnSettledHook(
+				std::function<void(std::exception_ptr &exceptionPtr)> hook)
 			{
 				if (slot.leaseStatus != SlotLeaseStatus::RESERVED)
 				{
@@ -214,7 +214,7 @@ public:
 		std::exception_ptr exceptionPtr = nullptr;
 		sscl::SyncCancelerForAsyncWork syncCanceler;
 		std::unique_ptr<detail::MemberInvokerBase> memberInvoker;
-		std::function<void()> onSettledHook;
+		std::function<void(std::exception_ptr &exceptionPtr)> onSettledHook;
 	};
 
 	void openAdmission()
@@ -411,6 +411,8 @@ private:
 	void retireSlot(Slot &slot)
 	{
 		std::function<void()> waiterToInvoke;
+		std::function<void(std::exception_ptr &exceptionPtr)> onSettledHook;
+		std::exception_ptr settledExceptionPtr;
 
 		{
 			sscl::SpinLock::Guard guard(s.lock);
@@ -430,33 +432,23 @@ private:
 				return;
 			}
 
-			try {
-				if (slot.onSettledHook) { slot.onSettledHook(); }
-			}
-			catch (const std::exception &e)
-			{
-				std::cerr
-					<< "NonViralTaskNursery: onSettled hook exception: "
-					<< e.what() << std::endl;
-			}
-			catch (...)
-			{
-				std::cerr
-					<< "NonViralTaskNursery: onSettled hook unknown exception"
-					<< std::endl;
-			}
-
-			if (slot.exceptionPtr) {
+			settledExceptionPtr = slot.exceptionPtr;
+			if (settledExceptionPtr) {
 				slot.settlementStatus = SlotSettlementStatus::EXCEPTION_THROWN;
 			} else {
 				slot.settlementStatus = SlotSettlementStatus::COMPLETED;
 			}
 
+			onSettledHook = std::move(slot.onSettledHook);
 			slot.leaseStatus = SlotLeaseStatus::RETIRED;
 			slot.memberInvoker.reset();
 			waiterToInvoke = takeDrainWaiterIfDrainedUnlocked();
 		}
 
+		if (onSettledHook) {
+			onSettledHook(settledExceptionPtr);
+		}
+
 		if (waiterToInvoke) { waiterToInvoke(); }
 	}