From c5fb7e5b73ca587ea59c1657bd221af0743929d5 Mon Sep 17 00:00:00 2001
From: Craig Tiller <ctiller@google.com>
Date: Sun, 8 Oct 2017 03:25:31 +0000
Subject: [PATCH] Use a mutex for an exact test

---
 build.yaml                           |  1 +
 test/core/debug/stats_test.cc        | 14 +++++++++-----
 tools/run_tests/generated/tests.json |  2 +-
 3 files changed, 11 insertions(+), 6 deletions(-)

diff --git a/build.yaml b/build.yaml
index 19ce721be4e..211cf6913fd 100644
--- a/build.yaml
+++ b/build.yaml
@@ -4607,6 +4607,7 @@ targets:
   - grpc
   - gpr_test_util
   - gpr
+  uses_polling: false
 - name: status_test
   build: test
   language: c++
diff --git a/test/core/debug/stats_test.cc b/test/core/debug/stats_test.cc
index 35f11eae378..c652e446b8e 100644
--- a/test/core/debug/stats_test.cc
+++ b/test/core/debug/stats_test.cc
@@ -20,6 +20,7 @@ extern "C" {
 #include "src/core/lib/debug/stats.h"
 }
 
+#include <mutex>
 #include <thread>
 
 #include <grpc/grpc.h>
@@ -86,10 +87,12 @@ class HistogramTest : public ::testing::TestWithParam<int> {};
 
 TEST_P(HistogramTest, IncHistogram) {
   const int kHistogram = GetParam();
-  const int kThreads = std::max(1, (int)gpr_cpu_num_cores());
+  const int kBuckets = grpc_stats_histo_buckets[kHistogram];
+  const int kThreads = kBuckets;
   std::vector<std::thread> threads;
+  std::vector<std::mutex> mutexes(kBuckets);
   for (int thread = 0; thread < kThreads; thread++) {
-    threads.emplace_back([kHistogram, kThreads, thread]() {
+    threads.emplace_back([kHistogram, kThreads, thread, &mutexes]() {
       std::vector<int> test_values;
       for (int j = -1000 + thread;
            j < grpc_stats_histo_bucket_boundaries
@@ -100,9 +103,10 @@ TEST_P(HistogramTest, IncHistogram) {
       }
       std::random_shuffle(test_values.begin(), test_values.end());
       for (auto j : test_values) {
-        Snapshot snapshot;
-
         int expected_bucket = FindExpectedBucket(kHistogram, j);
+	std::lock_guard<std::mutex> lock(mutexes[expected_bucket]);
+
+        Snapshot snapshot;
 
         grpc_exec_ctx exec_ctx = GRPC_EXEC_CTX_INIT;
         grpc_stats_inc_histogram[kHistogram](&exec_ctx, j);
@@ -110,7 +114,7 @@ TEST_P(HistogramTest, IncHistogram) {
 
         auto delta = snapshot.delta();
 
-        EXPECT_GE(delta.histograms[grpc_stats_histo_start[kHistogram] +
+        EXPECT_EQ(delta.histograms[grpc_stats_histo_start[kHistogram] +
                                    expected_bucket],
                   1)
             << "\nhistogram:" << kHistogram
diff --git a/tools/run_tests/generated/tests.json b/tools/run_tests/generated/tests.json
index 6f9b3cab897..72e840e2f20 100644
--- a/tools/run_tests/generated/tests.json
+++ b/tools/run_tests/generated/tests.json
@@ -4195,7 +4195,7 @@
       "posix", 
       "windows"
     ], 
-    "uses_polling": true
+    "uses_polling": false
   }, 
   {
     "args": [],