[compiler-rt] [TSan] Add interceptors for flockfile, ftrylockfile, funlockfile (PR #203470)

[James Lim via llvm-commits]

https://github.com/lim-james created https://github.com/llvm/llvm-project/pull/203470

This patch adds TSan interceptors for `flockfile`, `ftrylockfile`, and `funlockfile`.

`flockfile`/`funlockfile` are POSIX stdio locking primitives for mutual exclusion on FILE streams. However, TSan does not have interceptors to account for them, so happens-before relations are not tracked, thus causing false positive data race reports.

This fixes false positives for `std::println` on Glibc platforms as libstdc++'s _File_sink acquires `flockfile`, which was invisible to TSan.

Verified by:
- Confirming TSan reports a race on flockfile-protected code without the fix
- Confirming silence with the fix applied
- Confirming real unprotected races are still caught

Resolves #203451


>From 3fe88eaf48005dd03acd583d1d4f287fa45963f0 Mon Sep 17 00:00:00 2001
From: james <jameslimbj at gmail.com>
Date: Fri, 12 Jun 2026 14:47:39 +0800
Subject: [PATCH] [tsan] Add interceptors for flockfile, ftrylockfile,
 funlockfile

flockfile/funlockfile are POSIX stdio locking primitives for mutual exclusion on FILE
streams. However, TSan does not have interceptors to account for them, so
happens-before relations are not tracked, thus causing false positive data race
reports.

Signed-off-by: james <jameslimbj at gmail.com>
---
 .../lib/tsan/rtl/tsan_interceptors_posix.cpp  | 27 +++++++++++++++++++
 1 file changed, 27 insertions(+)

diff --git a/compiler-rt/lib/tsan/rtl/tsan_interceptors_posix.cpp b/compiler-rt/lib/tsan/rtl/tsan_interceptors_posix.cpp
index e00853bd1b426..ed123cae3b8a9 100644
--- a/compiler-rt/lib/tsan/rtl/tsan_interceptors_posix.cpp
+++ b/compiler-rt/lib/tsan/rtl/tsan_interceptors_posix.cpp
@@ -1762,6 +1762,30 @@ TSAN_INTERCEPTOR(int, fstat64, int fd, void *buf) {
 #  define TSAN_MAYBE_INTERCEPT_FSTAT64
 #endif
 
+TSAN_INTERCEPTOR(void, flockfile, void *file) {
+  SCOPED_TSAN_INTERCEPTOR(flockfile, file);
+  MutexPreLock(thr, pc, (uptr)file,
+               MutexFlagWriteReentrant | MutexFlagNotStatic);
+  BLOCK_REAL(flockfile)(file);
+  MutexPostLock(thr, pc, (uptr)file,
+                MutexFlagWriteReentrant | MutexFlagNotStatic);
+}
+
+TSAN_INTERCEPTOR(int, ftrylockfile, void *file) {
+  SCOPED_TSAN_INTERCEPTOR(ftrylockfile, file);
+  int res = REAL(ftrylockfile)(file);
+  if (res == 0)
+    MutexPostLock(thr, pc, (uptr)file,
+                  MutexFlagWriteReentrant | MutexFlagNotStatic | MutexFlagTryLock);
+  return res;
+}
+
+TSAN_INTERCEPTOR(void, funlockfile, void *file) {
+  SCOPED_TSAN_INTERCEPTOR(funlockfile, file);
+  MutexUnlock(thr, pc, (uptr)file);
+  REAL(funlockfile)(file);
+}
+
 TSAN_INTERCEPTOR(int, open, const char *name, int oflag, ...) {
   mode_t mode = 0;
   if (OpenReadsVaArgs(oflag)) {
@@ -3142,6 +3166,9 @@ void InitializeInterceptors() {
   TSAN_MAYBE_INTERCEPT___FXSTAT;
   TSAN_MAYBE_INTERCEPT_FSTAT;
   TSAN_MAYBE_INTERCEPT_FSTAT64;
+  TSAN_INTERCEPT(flockfile);
+  TSAN_INTERCEPT(ftrylockfile);
+  TSAN_INTERCEPT(funlockfile);
   TSAN_INTERCEPT(open);
   TSAN_MAYBE_INTERCEPT_OPEN64;
   TSAN_INTERCEPT(creat);