From f08d42fcb84a3969a47ef4b0714177ac9245c4ce Mon Sep 17 00:00:00 2001
From: Dennis Klein <d.klein@gsi.de>
Date: Wed, 10 Jun 2026 16:12:35 +0200
Subject: [PATCH] fix: serialize console output in fair::mq::tools::execute

- concurrent execute() calls print captured subprocess lines to
  std::cout from multiple threads; the standard allows that, but
  libstdc++ maintains the formatted-output state (ios_base::width)
  with plain reads and writes -- a data race ThreadSanitizer reports
  once libstdc++ itself is instrumented
- a mutex around the insertion also keeps whole lines from
  interleaving
---
 fairmq/tools/Process.cxx | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/fairmq/tools/Process.cxx b/fairmq/tools/Process.cxx
index 046d1dae..05be0aae 100644
--- a/fairmq/tools/Process.cxx
+++ b/fairmq/tools/Process.cxx
@@ -18,6 +18,7 @@
 #include <chrono>
 #include <csignal>   // kill, signals
 #include <iostream>
+#include <mutex>
 #include <sstream>
 #include <stdexcept>
 #include <thread>
@@ -40,10 +41,18 @@ class LinePrinter
         , fPrefix(std::move(prefix))
     {}
 
-    // prints line with prefix on both cout (thread-safe) and output stream
+    // prints line with prefix on both cout and output stream
     void Print(const string& line)
     {
-        cout << fair::mq::tools::ToString(fPrefix, line, "\n") << flush;
+        // Serialize: the standard allows concurrent insertion on std::cout,
+        // but libstdc++ maintains the formatted-output state (e.g.
+        // ios_base::width) with plain reads and writes that constitute data
+        // races; the lock also keeps whole lines from interleaving.
+        static mutex sCoutMutex;
+        {
+            lock_guard<mutex> lock(sCoutMutex);
+            cout << fair::mq::tools::ToString(fPrefix, line, "\n") << flush;
+        }
         fOut << fPrefix << line << endl;
     }