Add a flag to force tsan's background thread

Reviewed By: dvyukov, vitalybuka

Differential Revision: https://reviews.llvm.org/D115759

compiler-rt/lib/tsan/rtl/tsan_flags.inc

@@ -43,6 +43,9 @@ TSAN_FLAG(
     bool, force_seq_cst_atomics, false,
     "If set, all atomics are effectively sequentially consistent (seq_cst), "
     "regardless of what user actually specified.")
+TSAN_FLAG(bool, force_background_thread, false,
+          "If set, eagerly launch a background thread for memory reclamation "
+          "instead of waiting for a user call to pthread_create.")
 TSAN_FLAG(bool, halt_on_error, false, "Exit after first reported error.")
 TSAN_FLAG(int, atexit_sleep_ms, 1000,
           "Sleep in main thread before exiting for that many ms "

compiler-rt/lib/tsan/rtl/tsan_rtl.cpp

@@ -698,6 +698,18 @@ void Initialize(ThreadState *thr) {
   OnInitialize();
 }
 
+#if !SANITIZER_GO
+#  pragma clang diagnostic push
+// We intentionally use a global constructor to delay the pthread call.
+#  pragma clang diagnostic ignored "-Wglobal-constructors"
+static bool UNUSED __local_tsan_dyninit = [] {
+  if (flags()->force_background_thread)
+    MaybeSpawnBackgroundThread();
+  return false;
+}();
+#  pragma clang diagnostic pop
+#endif
+
 void MaybeSpawnBackgroundThread() {
   // On MIPS, TSan initialization is run before
   // __pthread_initialize_minimal_internal() is finished, so we can not spawn

compiler-rt/test/tsan/force_background_thread.cpp

@@ -0,0 +1,21 @@
+// RUN: %clangxx_tsan -O1 %s -o %t
+// RUN: %deflake %env_tsan_opts=force_background_thread=0:verbosity=1:memory_limit_mb=1000 %run %t 2>&1 | FileCheck %s --implicit-check-not "memory flush check"
+// RUN: %deflake %env_tsan_opts=force_background_thread=1:verbosity=1:memory_limit_mb=1000 %run %t 2>&1 | FileCheck %s --check-prefixes=CHECK,THREAD
+// RUN: %deflake %env_tsan_opts=force_background_thread=0:verbosity=1:memory_limit_mb=1000 %run %t 1 2>&1 | FileCheck %s --check-prefixes=CHECK,THREAD
+#include "test.h"
+
+void *Thread(void *a) { return nullptr; }
+
+int main(int argc, char *argv[]) {
+  if (argc > 1) {
+    pthread_t t;
+    pthread_create(&t, nullptr, Thread, nullptr);
+    void *p;
+    pthread_join(t, &p);
+  }
+  sleep(3);
+  return 1;
+}
+
+// CHECK: Running under ThreadSanitizer
+// THREAD: ThreadSanitizer: memory flush check