Merge GH #2723 Allow disabling thread pool at runtime via env var

2 files changed, 56 insertions, 10 deletions

diff --git a/src/lib/utils/thread_utils/thread_pool.cpp b/src/lib/utils/thread_utils/thread_pool.cpp
index 405f79585..ce326fdc0 100644
--- a/src/lib/utils/thread_utils/thread_pool.cpp
+++ b/src/lib/utils/thread_utils/thread_pool.cpp
@@ -1,5 +1,5 @@
 /*
-* (C) 2019 Jack Lloyd
+* (C) 2019,2021 Jack Lloyd
 *
 * Botan is released under the Simplified BSD License (see license.txt)
 */
@@ -11,19 +11,53 @@
 
 namespace Botan {
 
+namespace {
+
+std::optional<size_t> global_thread_pool_size()
+   {
+   std::string var;
+   if(OS::read_env_variable(var, "BOTAN_THREAD_POOL_SIZE"))
+      {
+      try
+         {
+         return std::optional<size_t>(std::stoul(var, nullptr));
+         }
+      catch(std::exception&) { /* ignore it */ }
+
+      if(var == "none")
+         return std::nullopt;
+      }
+
+   // If it was neither a number nor a special value, then ignore it.
+   return std::optional<size_t>(0);
+   }
+
+}
+
 //static
 Thread_Pool& Thread_Pool::global_instance()
    {
-   static Thread_Pool g_thread_pool(OS::read_env_variable_sz("BOTAN_THREAD_POOL_SIZE"));
+   static Thread_Pool g_thread_pool(global_thread_pool_size());
    return g_thread_pool;
    }
 
-Thread_Pool::Thread_Pool(size_t pool_size)
+Thread_Pool::Thread_Pool(std::optional<size_t> opt_pool_size)
    {
+   m_shutdown = false;
+
+   if(!opt_pool_size.has_value())
+      return;
+
+   size_t pool_size = opt_pool_size.value();
+
    if(pool_size == 0)
       {
       pool_size = OS::get_cpu_available();
 
+      // Unclear if this can happen, but be defensive
+      if(pool_size == 0)
+         pool_size = 2;
+
       /*
       * For large machines don't create too many threads, unless
       * explicitly asked to by the caller.
@@ -32,11 +66,6 @@ Thread_Pool::Thread_Pool(size_t pool_size)
          pool_size = 16;
       }
 
-   if(pool_size <= 1)
-      pool_size = 2;
-
-   m_shutdown = false;
-
    for(size_t i = 0; i != pool_size; ++i)
       {
       m_workers.push_back(std::thread(&Thread_Pool::worker_thread, this));
@@ -70,6 +99,11 @@ void Thread_Pool::queue_thunk(std::function<void ()> fn)
    if(m_shutdown)
       throw Invalid_State("Cannot add work after thread pool has shut down");
 
+   if(m_workers.empty())
+      {
+      return fn();
+      }
+
    m_tasks.push_back(fn);
    m_more_tasks.notify_one();
    }
diff --git a/src/lib/utils/thread_utils/thread_pool.h b/src/lib/utils/thread_utils/thread_pool.h
index 8f818d1b7..d94a0e75f 100644
--- a/src/lib/utils/thread_utils/thread_pool.h
+++ b/src/lib/utils/thread_utils/thread_pool.h
@@ -18,6 +18,7 @@
 #include <thread>
 #include <future>
 #include <condition_variable>
+#include <optional>
 
 namespace Botan {
 
@@ -32,9 +33,20 @@ class BOTAN_TEST_API Thread_Pool
       /**
       * Initialize a thread pool with some number of threads
       * @param pool_size number of threads in the pool, if 0
-      *        then some default value is chosen
+      *        then some default value is chosen. If the optional
+      *        is nullopt then the thread pool is disabled; all
+      *        work is executed immediately when queued.
       */
-      Thread_Pool(size_t pool_size = 0);
+      Thread_Pool(std::optional<size_t> pool_size);
+
+      /**
+      * Initialize a thread pool with some number of threads
+      * @param pool_size number of threads in the pool, if 0
+      *        then some default value is chosen.
+      */
+      Thread_Pool(size_t pool_size = 0) :
+         Thread_Pool(std::optional<size_t>(pool_size))
+         {}
 
       ~Thread_Pool() { shutdown(); }