From 3566c7ee5ccfab33434ce239fe15b3fef549ebf7 Mon Sep 17 00:00:00 2001
From: shockp <masque2204@gmail.com>
Date: Sun, 5 Apr 2026 12:26:10 +0200
Subject: [PATCH] GH-49433: [C++] Buffer ARROW_LOG output to prevent thread
 interleaving

When using the ARROW_LOG macros from multiple threads, messages were getting
mingled together in stderr because the operator<< was writing directly to
the global stream piece-by-piece.

This commit introduces an internal std::ostringstream buffer to CerrLog so
that messages are accumulated locally and flushed atomically upon destruction.
---
 cpp/src/arrow/util/logging.cc | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/cpp/src/arrow/util/logging.cc b/cpp/src/arrow/util/logging.cc
index 993c5306ca4a..15e3369dcdeb 100644
--- a/cpp/src/arrow/util/logging.cc
+++ b/cpp/src/arrow/util/logging.cc
@@ -24,6 +24,7 @@
 #endif
 #include <cstdlib>
 #include <iostream>
+#include <sstream>
 
 #ifdef ARROW_USE_GLOG
 
@@ -67,7 +68,7 @@ class CerrLog {
 
   virtual ~CerrLog() {
     if (has_logged_) {
-      std::cerr << std::endl;
+      std::cerr << buffer_.str() << std::endl;
     }
     if (severity_ == ArrowLogLevel::ARROW_FATAL) {
       PrintBackTrace();
@@ -77,14 +78,14 @@ class CerrLog {
 
   std::ostream& Stream() {
     has_logged_ = true;
-    return std::cerr;
+    return buffer_;
   }
 
   template <class T>
   CerrLog& operator<<(const T& t) {
     if (severity_ != ArrowLogLevel::ARROW_DEBUG) {
       has_logged_ = true;
-      std::cerr << t;
+      buffer_ << t;
     }
     return *this;
   }
@@ -92,6 +93,7 @@ class CerrLog {
  protected:
   const ArrowLogLevel severity_;
   bool has_logged_;
+  std::ostringstream buffer_;
 
   void PrintBackTrace() {
 #ifdef ARROW_WITH_BACKTRACE