From 1efccf34b12ceaf3565bd70b8c3b3465e65d4a18 Mon Sep 17 00:00:00 2001
From: Eelco Dolstra <edolstra@gmail.com>
Date: Thu, 13 Mar 2025 13:58:35 +0100
Subject: [PATCH] JSONLogger: Acquire a lock to prevent log messages from
 clobbering each other

---
 src/libutil/logging.cc | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/src/libutil/logging.cc b/src/libutil/logging.cc
index fcbc61d5e..c3ccfba42 100644
--- a/src/libutil/logging.cc
+++ b/src/libutil/logging.cc
@@ -6,6 +6,7 @@
 #include "config-global.hh"
 #include "source-path.hh"
 #include "position.hh"
+#include "sync.hh"
 
 #include <atomic>
 #include <sstream>
@@ -192,11 +193,22 @@ struct JSONLogger : Logger {
                 unreachable();
     }
 
+    struct State
+    {
+    };
+
+    Sync<State> _state;
+
     void write(const nlohmann::json & json)
     {
-        writeLine(fd,
+        auto line =
             (includeNixPrefix ? "@nix " : "") +
-            json.dump(-1, ' ', false, nlohmann::json::error_handler_t::replace));
+            json.dump(-1, ' ', false, nlohmann::json::error_handler_t::replace);
+
+        /* Acquire a lock to prevent log messages from clobbering each
+           other. */
+        auto state(_state.lock());
+        writeLine(fd, line);
     }
 
     void log(Verbosity lvl, std::string_view s) override