[NFC][LSAN] Limit the number of concurrent threads is the test

Test still fails with D88184 reverted.

The test was flaky on https://bugs.chromium.org/p/chromium/issues/detail?id=1206745 and
https://lab.llvm.org/buildbot/#/builders/sanitizer-x86_64-linux

Reviewed By: morehouse

Differential Revision: https://reviews.llvm.org/D102218

Author: vitalybuka

compiler-rt/test/lsan/TestCases/many_threads_detach.cpp

@@ -3,6 +3,8 @@
 // Use `-pthread` so that its driver will DTRT (ie., ignore it).
 // RUN: %clangxx_lsan %s -o %t -pthread && %run %t
 
+#include <assert.h>
+#include <dirent.h>
 #include <pthread.h>
 #include <stdlib.h>
 #include <unistd.h>
@@ -12,15 +14,33 @@
 // reused.
 static const size_t kTestThreads = 10000;
 
+// Limit the number of simultaneous threads to avoid reaching the limit.
+static const size_t kTestThreadsBatch = 100;
+
 void *null_func(void *args) {
   return NULL;
 }
 
+int count_threads() {
+  DIR *d = opendir("/proc/self/task");
+  assert(d);
+  int count = 0;
+  while (readdir(d))
+    ++count;
+  closedir(d);
+  assert(count);
+  return count;
+}
+
 int main(void) {
-  for (size_t i = 0; i < kTestThreads; i++) {
-    pthread_t thread;
-    if (pthread_create(&thread, NULL, null_func, NULL) == 0)
+  for (size_t i = 0; i < kTestThreads; i += kTestThreadsBatch) {
+    for (size_t j = 0; j < kTestThreadsBatch; ++j) {
+      pthread_t thread;
+      assert(pthread_create(&thread, NULL, null_func, NULL) == 0);
       pthread_detach(thread);
+    }
+    while (count_threads() > 10)
+      sched_yield();
   }
   return 0;
 }