fix lockless io-stealing

The uninitialized continuation pointer may be returned without being written resulting in dispatching an invalid fiber pointer.

If a CQE is stolen but no fiber is blocked on the corresponding future IoContext::getContinuationsFromCompletions will call future->completeAndGetContinuation which will set the future's completion but will not return a continuation.

If no continuation is returned from Future::completeAndGetContinuation the continuation buffer is not advanced by IoContext::getContinuationsFromCompletions (emper/io/IoContext.hpp:259).

IoContext::reapSingleCompletion does not check if a continuation was created and assumed that continuation contains a valid Fiber* if a CQE was successfully stolen.

Initialize continuation with nullptr, which is a valid Fiber*.

emper/io/IoContext.cpp

@@ -233,7 +233,7 @@ static constexpr auto LL_WRITE_MEM_ORDER =
 				: std::memory_order_seq_cst;
 
 template <CallerEnvironment callerEnvironment>
-auto IoContext::tryReapCompletionWaitFree(Fiber *continuation) -> emper::StealingResult {
+auto IoContext::tryReapCompletionWaitFree(Fiber **continuation) -> emper::StealingResult {
 	Completion completion;
 
 	struct io_uring_cq *cq = &ring.cq;
@@ -270,7 +270,7 @@ auto IoContext::tryReapCompletionWaitFree(Fiber *continuation) -> emper::Stealin
 	completion.first = cqe->res;
 	completion.second = cqe_data;
 
-	if (!ahead->compare_exchange_weak(head, head + 1, LL_WRITE_MEM_ORDER, LL_READ_MEM_ORDER))
+	if (!ahead->compare_exchange_strong(head, head + 1, LL_WRITE_MEM_ORDER, LL_READ_MEM_ORDER))
 		return emper::StealingResult::LostRace;
 
 	trackReqsInUring(-1);
@@ -283,6 +283,14 @@ auto IoContext::tryReapCompletionWaitFree(Fiber *continuation) -> emper::Stealin
 	return emper::StealingResult::Stolen;
 }
 
+// show the compiler our template incarnations
+template auto IoContext::tryReapCompletionWaitFree<CallerEnvironment::OWNER>(Fiber **continuation)
+		-> emper::StealingResult;
+template auto IoContext::tryReapCompletionWaitFree<CallerEnvironment::EMPER>(Fiber **continuation)
+		-> emper::StealingResult;
+template auto IoContext::tryReapCompletionWaitFree<CallerEnvironment::ANYWHERE>(
+		Fiber **continuation) -> emper::StealingResult;
+
 template <CallerEnvironment callerEnvironment, unsigned toReap>
 auto IoContext::reapCompletionsLockless(Fiber **continuations) -> unsigned {
 	Completion reapedCompletions[toReap];	 // NOLINT(modernize-avoid-c-arrays)