From 432fda7bfa0167403b8dd30d3cc6b43c9b80f468 Mon Sep 17 00:00:00 2001
From: Victor Zverovich <victor.zverovich@gmail.com>
Date: Wed, 29 Apr 2026 16:55:06 -0700
Subject: [PATCH] Fix false positive in tsan (fixes #4755)

---
 include/fmt/format-inl.h | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/include/fmt/format-inl.h b/include/fmt/format-inl.h
index 539b1fd7..3e04ddf5 100644
--- a/include/fmt/format-inl.h
+++ b/include/fmt/format-inl.h
@@ -8,6 +8,11 @@
 #ifndef FMT_FORMAT_INL_H_
 #define FMT_FORMAT_INL_H_
 
+#ifdef __SANITIZE_THREAD__
+extern "C" void __tsan_acquire(void*);
+extern "C" void __tsan_release(void*);
+#endif
+
 #ifndef FMT_MODULE
 #  include <stddef.h>  // ptrdiff_t
 
@@ -1696,6 +1701,9 @@ class file_print_buffer<F, enable_if_t<has_flockfile<F>::value>>
  public:
   explicit file_print_buffer(F* f) : buffer(grow, size_t()), file_(f) {
     flockfile(f);
+#ifdef __SANITIZE_THREAD__
+    __tsan_acquire(f);
+#endif
     file_.init_buffer();
     auto buf = file_.get_write_buffer();
     set(buf.data, buf.size);
@@ -1703,7 +1711,10 @@ class file_print_buffer<F, enable_if_t<has_flockfile<F>::value>>
   ~file_print_buffer() {
     file_.advance_write_buffer(size());
     bool flush = file_.needs_flush();
-    F* f = file_;    // Make funlockfile depend on the template parameter F
+    F* f = file_;    // Make funlockfile depend on the template parameter F.
+#ifdef __SANITIZE_THREAD__
+    __tsan_release(f);
+#endif
     funlockfile(f);  // for the system API detection to work.
     if (flush) fflush(file_);
   }