From f8bf8083af8e3c80b8957ce7fda6cceabb077468 Mon Sep 17 00:00:00 2001
From: Hayodea Hakol <hayodea@protonmail.com>
Date: Sat, 27 Sep 2025 20:51:20 -0400
Subject: [PATCH] Locking: Add contin tracing to detect deadlocks

We added the code to trace all the contins linked to a particular
Lockvoker, into SerializedAsyncContinuation. This basically
ensures that we'll almost never deal with a deadlock. So cool.
---
 include/asynchronousContinuation.h            |  6 +-
 include/asynchronousContinuationChainLink.h   |  3 +
 include/serializedAsynchronousContinuation.h  | 14 ++++-
 .../serializedAsynchronousContinuation.cpp    | 58 +++++++++++++++++--
 4 files changed, 73 insertions(+), 8 deletions(-)

diff --git a/include/asynchronousContinuation.h b/include/asynchronousContinuation.h
index cd76783..10e725f 100644
--- a/include/asynchronousContinuation.h
+++ b/include/asynchronousContinuation.h
@@ -38,7 +38,6 @@ public:
 		std::shared_ptr<AsynchronousContinuation<OriginalCbFnT>>
 			lifetimePreservingConveyance);
 
-
 	/**	EXPLANATION:
 	 * When an exception is thrown in a an async callee, which pertains to an
 	 * error in the data given by the caller, we ought not to throw the
@@ -67,6 +66,11 @@ public:
 			{ std::rethrow_exception(exception); }
 	}
 
+	// Implement the virtual method from AsynchronousContinuationChainLink
+	virtual std::shared_ptr<AsynchronousContinuationChainLink>
+	getCallersContinuation() override
+		{ return originalCallback.callerContinuation; }
+
 public:
 	Callback<OriginalCbFnT> originalCallback;
 	std::exception_ptr exception;
diff --git a/include/asynchronousContinuationChainLink.h b/include/asynchronousContinuationChainLink.h
index 5b5eb0c..63fb1b9 100644
--- a/include/asynchronousContinuationChainLink.h
+++ b/include/asynchronousContinuationChainLink.h
@@ -18,6 +18,9 @@ class AsynchronousContinuationChainLink
 {
 public:
     virtual ~AsynchronousContinuationChainLink() = default;
+
+	virtual std::shared_ptr<AsynchronousContinuationChainLink>
+	getCallersContinuation() = 0;
 };
 
 } // namespace smo
diff --git a/include/serializedAsynchronousContinuation.h b/include/serializedAsynchronousContinuation.h
index b019e59..48d634d 100644
--- a/include/serializedAsynchronousContinuation.h
+++ b/include/serializedAsynchronousContinuation.h
@@ -100,7 +100,14 @@ public:
 					{ return; }
 
 #ifdef CONFIG_ENABLE_DEBUG_LOCKS
-				handleLikelyDeadlock(*firstFailedQutexPtr);
+				Qutex	&firstFailedQutex = *firstFailedQutexPtr;
+				bool isDeadlock = traceContinuationHistoryForDeadlockOn(
+					firstFailedQutex);
+
+				if (!isDeadlock)
+					{ return; }
+
+				handleDeadlock(firstFailedQutex);
 #endif
 				return;
 			}
@@ -186,12 +193,13 @@ public:
 #endif
 		}
 
+#ifdef CONFIG_ENABLE_DEBUG_LOCKS
+		bool traceContinuationHistoryForDeadlockOn(Qutex &firstFailedQutex);
 		/**
 		 * @brief Handle a likely deadlock situation by logging debug information
 		 * @param firstFailedQutex The first qutex that failed acquisition
 		 */
-#ifdef CONFIG_ENABLE_DEBUG_LOCKS
-		void handleLikelyDeadlock(Qutex& firstFailedQutex);
+		void handleDeadlock(Qutex& firstFailedQutex);
 #endif
 
 	private:
diff --git a/smocore/serializedAsynchronousContinuation.cpp b/smocore/serializedAsynchronousContinuation.cpp
index a5e4f56..19ffdea 100644
--- a/smocore/serializedAsynchronousContinuation.cpp
+++ b/smocore/serializedAsynchronousContinuation.cpp
@@ -5,22 +5,72 @@
 namespace smo {
 
 #ifdef CONFIG_ENABLE_DEBUG_LOCKS
+
+template <class OriginalCbFnT>
+template <class InvocationTargetT>
+bool
+SerializedAsynchronousContinuation<OriginalCbFnT>
+::LockerAndInvoker<InvocationTargetT>
+::traceContinuationHistoryForDeadlockOn(Qutex& firstFailedQutex)
+{
+	/**	EXPLANATION:
+	 * In this function we will trace through the chain of continuations that
+	 * led up to this Lockvoker's continuation. For each continuation which is
+	 * a SerializedAsynchronousContinuation, we check through its LockSet to see
+	 * if it contains the lock that failed acquisition. If it does, we have a
+	 * deadlock.
+	 */
+
+	/* We can't start with the continuation directly referenced by this starting
+	 * Lockvoker as it would contain the all locks we're currently trying to
+	 * acquire...and rightly so because it's the continuation for this current
+	 * lockvoker.
+	 */
+	for (std::shared_ptr<AsynchronousContinuationChainLink> currContin =
+			this->serializedContinuation.getCallersContinuation();
+		 currContin != nullptr;
+		 currContin = currContin->getCallersContinuation())
+	{
+		auto serializedCont = std::dynamic_pointer_cast<
+			SerializedAsynchronousContinuation<OriginalCbFnT>>(currContin);
+
+		if (serializedCont == nullptr) { continue; }
+
+		// Check if the firstFailedQutex is in this continuation's LockSet
+		try {
+			const auto& lockUsageDesc = serializedCont->requiredLocks
+				.getLockUsageDesc(firstFailedQutex);
+		} catch (const std::runtime_error& e) {
+			std::cerr << __func__ << ": " << e.what() << std::endl;
+			continue;
+		}
+
+		std::cout << __func__ << ":Deadlock detected: Found "
+			<< "firstFailedQutex @" << &firstFailedQutex
+			<< " (" << firstFailedQutex.name << ") in LockSet of "
+			<< "SerializedAsynchronousContinuation @"
+			<< serializedCont.get() << std::endl;
+
+		return true;
+	}
+
+	return false;
+}
+
 template <class OriginalCbFnT>
 template <class InvocationTargetT>
 void
 SerializedAsynchronousContinuation<OriginalCbFnT>
 ::LockerAndInvoker<InvocationTargetT>
-::handleLikelyDeadlock(Qutex& firstFailedQutex)
+::handleDeadlock(Qutex& firstFailedQutex)
 {
-	std::cerr << __func__ << ": Deadlock likely: "
+	std::cerr << __func__ << ": Deadlock: "
 		<< "Lockvoker has been waiting for "
 		<< std::chrono::duration_cast<std::chrono::milliseconds>(
 			std::chrono::steady_clock::now() - this->creationTimestamp)
 			.count()
 		<< "ms, failed on qutex @" << &firstFailedQutex
 		<< " (" << firstFailedQutex.name << ")" << std::endl;
-
-	
 }
 #endif