From 29b73c5d603389bf26860ab94fe855651b8929e9 Mon Sep 17 00:00:00 2001
From: "copilot-swe-agent[bot]" <198982749+Copilot@users.noreply.github.com>
Date: Fri, 29 May 2026 18:15:03 +0000
Subject: [PATCH 1/2] Initial plan


From cb47a4a74b3e9dee494421cabb0cb5d6cdc53b79 Mon Sep 17 00:00:00 2001
From: "copilot-swe-agent[bot]" <198982749+Copilot@users.noreply.github.com>
Date: Fri, 29 May 2026 18:21:04 +0000
Subject: [PATCH 2/2] fix: resolve deadlock in beginJob when compiled debug

The deadlock was caused by two issues:
1. Lock ordering: beginJob() held m_ while creating XThreadTimer (which
   acquires gSystemMutex internally), while the app thread could hold
   gSystemMutex (inside ProcessEvents) and try to acquire m_ via a timer
   callback - classic ABBA deadlock.
2. Bare cv_.wait() without predicate: susceptible to missed notifications
   if the signal was sent before the wait began.

Fix:
- Add appStarted_ and eveSetupDone_ boolean state flags
- Use predicate-based cv_.wait() to handle missed notifications
- Don't hold m_ during XThreadTimer creation to avoid lock ordering issue
---
 src/Mu2eEventDisplay_module.cc | 19 ++++++++++++++-----
 1 file changed, 14 insertions(+), 5 deletions(-)

diff --git a/src/Mu2eEventDisplay_module.cc b/src/Mu2eEventDisplay_module.cc
index be59bd8..b9d3b5e 100644
--- a/src/Mu2eEventDisplay_module.cc
+++ b/src/Mu2eEventDisplay_module.cc
@@ -164,6 +164,8 @@ namespace mu2e
         // Control between the main thread and event-display thread
         std::condition_variable cv_{};
         std::mutex m_{};
+        bool appStarted_{false};
+        bool eveSetupDone_{false};
 
         int  diagLevel_;
         bool showCrv_;
@@ -255,22 +257,28 @@ namespace mu2e
   void Mu2eEventDisplay::signalAppStart()
   { 
       std::unique_lock lock{m_};
+      appStarted_ = true;
       cv_.notify_all();
   }
 
   void Mu2eEventDisplay::beginJob(){
       if(diagLevel_ == 1) std::cout<<"[Mu2eEventDisplay : beginJob()] -- starting ..."<<std::endl;
       {      
-        std::unique_lock lock{m_};
         appThread_ = std::thread{[this] { run_application(); }};
         // Wait for app init to finish ... this will process pending timer events.
         XThreadTimer sut([this]{ signalAppStart(); });
-        if(diagLevel_ == 1) std::cout<<"[Mu2eEventDisplay : beginJob()] -- starting wait on app start"<<std::endl;
-        cv_.wait(lock);
+        {
+          std::unique_lock lock{m_};
+          if(diagLevel_ == 1) std::cout<<"[Mu2eEventDisplay : beginJob()] -- starting wait on app start"<<std::endl;
+          cv_.wait(lock, [this]{ return appStarted_; });
+        }
         if(diagLevel_ == 1) std::cout<<"[Mu2eEventDisplay : beginJob()] -- app start signal received, starting eve init"<<std::endl;
         XThreadTimer suet([this]{ setup_eve(); });
-        if(diagLevel_ == 1) std::cout<<"[Mu2eEventDisplay : beginJob()] -- starting wait on eve setup"<<std::endl;
-        cv_.wait(lock);
+        {
+          std::unique_lock lock{m_};
+          if(diagLevel_ == 1) std::cout<<"[Mu2eEventDisplay : beginJob()] -- starting wait on eve setup"<<std::endl;
+          cv_.wait(lock, [this]{ return eveSetupDone_; });
+        }
         if(diagLevel_ == 1) std::cout<<"[Mu2eEventDisplay : beginJob()] -- eve setup apparently complete"<<std::endl;
       }
   }
@@ -643,6 +651,7 @@ void Mu2eEventDisplay::FillAnyCollection(const art::Event& evt, std::vector<std:
     std::unique_lock lock{m_}; 
     // Signal the waiting Art thread (in analyze() or beginRun()) to continue processing 
     // now that the REve display is fully initialized.
+    eveSetupDone_ = true;
     cv_.notify_all();
     //std::cout << "[DEBUG] TextSelect object ID assigned: " << fText->GetElementId() << std::endl; 
 }