tsan: add another fork deadlock test

[dvyukov]

Summary

The test tries to provoke internal allocator to be locked during fork
and then force the child process to use the internal allocator.
This test sometimes deadlocks with the new tsan runtime.

Depends on D114514.

compiler-rt/test/tsan/Linux/fork_multithreaded4.cpp

+// RUN: %clangxx_tsan -O1 %s -o %t && %run %t 2>&1 | FileCheck %s
+
+// The test tries to provoke internal allocator to be locked during fork
+// and then force the child process to use the internal allocator.
+
+#include "../test.h"
+#include <errno.h>
+#include <sys/types.h>
+#include <sys/wait.h>
+
+static void *forker(void *arg) {
+  void *p = calloc(1, 16);
+  static_cast<volatile int *>(p)[0]++;
+  __atomic_fetch_add(static_cast<int *>(p), 1, __ATOMIC_SEQ_CST);
+  int pid = fork();
+  if (pid < 0) {
+    fprintf(stderr, "failed to fork (%d)\n", errno);
+    exit(1);
+  }
+  if (pid == 0) {
+    __atomic_fetch_add(&static_cast<int *>(p)[1], 1, __ATOMIC_SEQ_CST);
+    exit(0);
+  }
+  int status = 0;
+  while (waitpid(pid, &status, 0) != pid) {
+  }
+  if (!WIFEXITED(status) || WEXITSTATUS(status) != 0) {
+    fprintf(stderr, "subprocess failed (%d)\n", status);
+    exit(1);
+  }
+  free(p);
+  return 0;
+}
+
+int main() {
+  for (int i = 0; i < 10; i++) {
+    pthread_t threads[100];
+    for (auto &th : threads)
+      pthread_create(&th, 0, forker, 0);
+    for (auto th : threads)
+      pthread_join(th, 0);
+  }
+  fprintf(stderr, "DONE\n");
+}
+
+// CHECK: DONE