[hwasan] Stress test for thread creation.

This test has two modes - testing reused threads with multiple loops of
batch create/join, and testing new threads with a single loop of
create/join per fork.

The non-reuse variant catches the problem that was fixed in D101881 with
a high probability.

Differential Revision: https://reviews.llvm.org/D101936

compiler-rt/test/hwasan/TestCases/Linux/create-thread-stress.cpp

@@ -0,0 +1,61 @@
+// Stress test for https://reviews.llvm.org/D101881
+// RUN: %clangxx_hwasan -DREUSE=0 %s -pthread -O2 -o %t && %run %t 2>&1
+// RUN: %clangxx_hwasan -DREUSE=1 %s -pthread -O2 -o %t_reuse && %run %t_reuse 2>&1
+
+#include <thread>
+#include <vector>
+
+#include <sys/types.h>
+#include <sys/wait.h>
+#include <unistd.h>
+
+#include <stdio.h>
+
+constexpr int kTopThreads = 20;
+constexpr int kChildThreads = 30;
+constexpr int kChildIterations = REUSE ? 8 : 1;
+
+constexpr int kProcessIterations = 20;
+
+void Thread() {
+  for (int i = 0; i < kChildIterations; ++i) {
+    std::vector<std::thread> threads;
+    for (int i = 0; i < kChildThreads; ++i)
+      threads.emplace_back([]() {});
+    for (auto &t : threads)
+      t.join();
+  }
+}
+
+void run() {
+  std::vector<std::thread> threads;
+  for (int i = 0; i < kTopThreads; ++i)
+    threads.emplace_back(Thread);
+  for (auto &t : threads)
+    t.join();
+}
+
+int main() {
+#if REUSE
+  // Test thread reuse with multiple iterations of thread create / join in a single process.
+  run();
+#else
+  // Test new, non-reused thread creation by running a single iteration of create / join in a freshly started process.
+  for (int i = 0; i < kProcessIterations; ++i) {
+    int pid = fork();
+    if (pid) {
+      int wstatus;
+      do {
+        waitpid(pid, &wstatus, 0);
+      } while (!WIFEXITED(wstatus) && !WIFSIGNALED(wstatus));
+      if (!WIFEXITED(wstatus) || WEXITSTATUS(wstatus)) {
+        fprintf(stderr, "failed at iteration %d / %d\n", i, kProcessIterations);
+        return 1;
+      }
+    } else {
+      run();
+      return 0;
+    }
+  }
+#endif
+}