From 34c76f527d12d7f0fba739e41dcdb06d82d4910d Mon Sep 17 00:00:00 2001
From: Yunjia Wang <yunjiaw@google.com>
Date: Mon, 15 Jul 2019 17:58:51 -0700
Subject: [PATCH] Add default size 1 for thread pool

---
 src/core/lib/iomgr/executor/threadpool.cc |  3 ++
 src/core/lib/iomgr/executor/threadpool.h  |  3 +-
 test/core/iomgr/threadpool_test.cc        | 45 +++++++++++++----------
 3 files changed, 30 insertions(+), 21 deletions(-)

diff --git a/src/core/lib/iomgr/executor/threadpool.cc b/src/core/lib/iomgr/executor/threadpool.cc
index 166bf0a34a0..9bb1fd1d1cd 100644
--- a/src/core/lib/iomgr/executor/threadpool.cc
+++ b/src/core/lib/iomgr/executor/threadpool.cc
@@ -51,6 +51,9 @@ void ThreadPool::SharedThreadPoolConstructor() {
   // All worker threads in thread pool must be joinable.
   thread_options_.set_joinable(true);
 
+  // Create at least 1 worker threads.
+  if (num_threads_ <= 0) num_threads_ = 1;
+
   queue_ = New<InfLenFIFOQueue>();
   threads_ = static_cast<ThreadPoolWorker**>(
       gpr_zalloc(num_threads_ * sizeof(ThreadPoolWorker*)));
diff --git a/src/core/lib/iomgr/executor/threadpool.h b/src/core/lib/iomgr/executor/threadpool.h
index 3f79bbe9d08..e37f5727ced 100644
--- a/src/core/lib/iomgr/executor/threadpool.h
+++ b/src/core/lib/iomgr/executor/threadpool.h
@@ -101,7 +101,8 @@ class ThreadPoolWorker {
 class ThreadPool : public ThreadPoolInterface {
  public:
   // Creates a thread pool with size of "num_threads", with default thread name
-  // "ThreadPoolWorker" and all thread options set to default.
+  // "ThreadPoolWorker" and all thread options set to default. If the given size
+  // is 0 or less, there will be 1 worker threads created inside pool.
   ThreadPool(int num_threads);
 
   // Same as ThreadPool(int num_threads) constructor, except
diff --git a/test/core/iomgr/threadpool_test.cc b/test/core/iomgr/threadpool_test.cc
index 9c678fa4304..1219d25f3b7 100644
--- a/test/core/iomgr/threadpool_test.cc
+++ b/test/core/iomgr/threadpool_test.cc
@@ -20,10 +20,29 @@
 
 #include "test/core/util/test_config.h"
 
-const int kSmallThreadPoolSize = 20;
-const int kLargeThreadPoolSize = 100;
-const int kThreadSmallIter = 100;
-const int kThreadLargeIter = 10000;
+static const int kSmallThreadPoolSize = 20;
+static const int kLargeThreadPoolSize = 100;
+static const int kThreadSmallIter = 100;
+static const int kThreadLargeIter = 10000;
+
+static void test_size_zero(void) {
+  gpr_log(GPR_INFO, "test_size_zero");
+  grpc_core::ThreadPool* pool_size_zero =
+      grpc_core::New<grpc_core::ThreadPool>(0);
+  GPR_ASSERT(pool_size_zero->pool_capacity() == 1);
+  Delete(pool_size_zero);
+}
+
+static void test_constructor_option(void) {
+  gpr_log(GPR_INFO, "test_constructor_option");
+  // Tests options
+  grpc_core::Thread::Options options;
+  options.set_stack_size(192 * 1024);  // Random non-default value
+  grpc_core::ThreadPool* pool = grpc_core::New<grpc_core::ThreadPool>(
+      0, "test_constructor_option", options);
+  GPR_ASSERT(pool->thread_options().stack_size() == options.stack_size());
+  Delete(pool);
+}
 
 // Simple functor for testing. It will count how many times being called.
 class SimpleFunctorForAdd : public grpc_experimental_completion_queue_functor {
@@ -109,21 +128,6 @@ class WorkThread {
   grpc_core::Thread thd_;
 };
 
-static void test_constructor(void) {
-  // Size is 0 case
-  grpc_core::ThreadPool* pool_size_zero =
-      grpc_core::New<grpc_core::ThreadPool>(0);
-  GPR_ASSERT(pool_size_zero->pool_capacity() == 0);
-  Delete(pool_size_zero);
-  // Tests options
-  grpc_core::Thread::Options options;
-  options.set_stack_size(192 * 1024);  // Random non-default value
-  grpc_core::ThreadPool* pool =
-      grpc_core::New<grpc_core::ThreadPool>(0, "test_constructor", options);
-  GPR_ASSERT(pool->thread_options().stack_size() == options.stack_size());
-  Delete(pool);
-}
-
 static void test_multi_add(void) {
   gpr_log(GPR_INFO, "test_multi_add");
   const int num_work_thds = 10;
@@ -178,7 +182,8 @@ static void test_one_thread_FIFO(void) {
 int main(int argc, char** argv) {
   grpc::testing::TestEnvironment env(argc, argv);
   grpc_init();
-  test_constructor();
+  test_size_zero();
+  test_constructor_option();
   test_add();
   test_multi_add();
   test_one_thread_FIFO();