Made thread_pool and parallel_for propagate exceptions from task threads to

calling code.

dlib/test/thread_pool.cpp

@@ -369,6 +369,39 @@ namespace
                     DLIB_TEST(d == 4);
                 }
 
+
+                tp.wait_for_all_tasks();
+
+                // make sure exception propagation from tasks works correctly.
+                auto f_throws = []() { throw dlib::error("test exception");};
+                bool got_exception = false;
+                try
+                {
+                    tp.add_task_by_value(f_throws);
+                    tp.wait_for_all_tasks();
+                }
+                catch(dlib::error& e)
+                {
+                    DLIB_TEST(e.info == "test exception");
+                    got_exception = true;
+                }
+                DLIB_TEST(got_exception);
+
+                dlib::future<int> aa;
+                auto f_throws2 = [](int& a) { a = 1; throw dlib::error("test exception");};
+                got_exception = false;
+                try
+                {
+                    tp.add_task(f_throws2, aa);
+                    aa.get();
+                }
+                catch(dlib::error& e)
+                {
+                    DLIB_TEST(e.info == "test exception");
+                    got_exception = true;
+                }
+                DLIB_TEST(got_exception);
+
             }
         }
 

dlib/threads/parallel_for_extension_abstract.h

@@ -24,7 +24,6 @@ namespace dlib
         requires
             - begin <= end
             - chunks_per_thread > 0
-            - funct does not throw any exceptions
         ensures
             - This is a convenience function for submitting a block of jobs to a thread_pool.  
               In particular, given the half open range [begin, end), this function will
@@ -61,7 +60,6 @@ namespace dlib
         requires
             - begin <= end
             - chunks_per_thread > 0
-            - funct does not throw any exceptions
         ensures
             - This function is equivalent to the following block of code:
                 thread_pool tp(num_threads);
@@ -82,7 +80,6 @@ namespace dlib
         requires
             - chunks_per_thread > 0
             - begin <= end
-            - funct does not throw any exceptions
         ensures
             - This is a convenience function for submitting a block of jobs to a
               thread_pool.  In particular, given the range [begin, end), this function will
@@ -117,7 +114,6 @@ namespace dlib
         requires
             - begin <= end
             - chunks_per_thread > 0
-            - funct does not throw any exceptions
         ensures
             - This function is equivalent to the following block of code:
                 thread_pool tp(num_threads);
@@ -137,7 +133,6 @@ namespace dlib
         requires
             - begin <= end
             - chunks_per_thread > 0
-            - funct does not throw any exceptions
         ensures
             - This function is equivalent to the following block of code:
                 parallel_for_blocked(default_thread_pool(), begin, end, funct, chunks_per_thread);
@@ -159,7 +154,6 @@ namespace dlib
         requires
             - begin <= end
             - chunks_per_thread > 0
-            - funct does not throw any exceptions
         ensures
             - This function is equivalent to the following function call:
               parallel_for_blocked(tp, begin, end, [&](long begin_sub, long end_sub) 
@@ -189,7 +183,6 @@ namespace dlib
         requires
             - begin <= end
             - chunks_per_thread > 0
-            - funct does not throw any exceptions
         ensures
             - This function is equivalent to the following block of code:
                 thread_pool tp(num_threads);
@@ -210,7 +203,6 @@ namespace dlib
         requires
             - begin <= end
             - chunks_per_thread > 0
-            - funct does not throw any exceptions
         ensures
             - This function is equivalent to the following function call:
               parallel_for_blocked(tp, begin, end, [&](long begin_sub, long end_sub) 
@@ -238,7 +230,6 @@ namespace dlib
         requires
             - begin <= end
             - chunks_per_thread > 0
-            - funct does not throw any exceptions
         ensures
             - This function is equivalent to the following block of code:
                 thread_pool tp(num_threads);
@@ -258,7 +249,6 @@ namespace dlib
         requires
             - begin <= end
             - chunks_per_thread > 0
-            - funct does not throw any exceptions
         ensures
             - This function is equivalent to the following block of code:
                 parallel_for(default_thread_pool(), begin, end, funct, chunks_per_thread);
@@ -280,7 +270,6 @@ namespace dlib
         requires
             - begin <= end
             - chunks_per_thread > 0
-            - funct does not throw any exceptions
         ensures
             - This function is identical to the parallel_for() routine defined above except
               that it will print messages to cout showing the progress in executing the
@@ -302,7 +291,6 @@ namespace dlib
         requires
             - begin <= end
             - chunks_per_thread > 0
-            - funct does not throw any exceptions
         ensures
             - This function is identical to the parallel_for() routine defined above except
               that it will print messages to cout showing the progress in executing the
@@ -323,7 +311,6 @@ namespace dlib
         requires
             - begin <= end
             - chunks_per_thread > 0
-            - funct does not throw any exceptions
         ensures
             - This function is identical to the parallel_for() routine defined above except
               that it will print messages to cout showing the progress in executing the
@@ -344,7 +331,6 @@ namespace dlib
         requires
             - begin <= end
             - chunks_per_thread > 0
-            - funct does not throw any exceptions
         ensures
             - This function is identical to the parallel_for() routine defined above except
               that it will print messages to cout showing the progress in executing the
@@ -364,7 +350,6 @@ namespace dlib
         requires
             - begin <= end
             - chunks_per_thread > 0
-            - funct does not throw any exceptions
         ensures
             - This function is identical to the parallel_for() routine defined above except
               that it will print messages to cout showing the progress in executing the
@@ -388,7 +373,6 @@ namespace dlib
         requires
             - begin <= end
             - chunks_per_thread > 0
-            - funct does not throw any exceptions
         ensures
             - This function is identical to the parallel_for_blocked() routine defined
               above except that it will print messages to cout showing the progress in
@@ -410,7 +394,6 @@ namespace dlib
         requires
             - begin <= end
             - chunks_per_thread > 0
-            - funct does not throw any exceptions
         ensures
             - This function is identical to the parallel_for_blocked() routine defined
               above except that it will print messages to cout showing the progress in
@@ -431,7 +414,6 @@ namespace dlib
         requires
             - begin <= end
             - chunks_per_thread > 0
-            - funct does not throw any exceptions
         ensures
             - This function is identical to the parallel_for_blocked() routine defined
               above except that it will print messages to cout showing the progress in
@@ -452,7 +434,6 @@ namespace dlib
         requires
             - begin <= end
             - chunks_per_thread > 0
-            - funct does not throw any exceptions
         ensures
             - This function is identical to the parallel_for_blocked() routine defined
               above except that it will print messages to cout showing the progress in
@@ -472,7 +453,6 @@ namespace dlib
         requires
             - begin <= end
             - chunks_per_thread > 0
-            - funct does not throw any exceptions
         ensures
             - This function is identical to the parallel_for_blocked() routine defined
               above except that it will print messages to cout showing the progress in

dlib/threads/thread_pool_extension.cpp

@@ -61,6 +61,11 @@ namespace dlib
         }
 
         wait();
+
+        // Throw any unhandled exceptions.  Since shutdown_pool() is only called in the
+        // destructor this will kill the program.
+        for (auto&& task : tasks)
+            task.propagate_exception();
     }
 
 // ----------------------------------------------------------------------------------------
@@ -94,6 +99,9 @@ namespace dlib
             const unsigned long idx = task_id_to_index(task_id);
             while (tasks[idx].task_id == task_id)
                 task_done_signaler.wait();
+
+            for (auto&& task : tasks)
+                task.propagate_exception();
         }
     }
 
@@ -124,6 +132,10 @@ namespace dlib
             if (found_task)
                 task_done_signaler.wait();
         }
+
+        // throw any exceptions generated by the tasks
+        for (auto&& task : tasks)
+            task.propagate_exception();
     }
 
 // ----------------------------------------------------------------------------------------
@@ -177,6 +189,9 @@ namespace dlib
                 task = tasks[idx];
             }
 
+            std::exception_ptr eptr;
+            try
+            {
                 // now do the task
                 if (task.bfp)
                     task.bfp();
@@ -186,6 +201,11 @@ namespace dlib
                     task.mfp1(task.arg1);
                 else if (task.mfp2)
                     task.mfp2(task.arg1, task.arg2);
+            }
+            catch(...)
+            {
+                eptr = std::current_exception();
+            }
 
             // Now let others know that we finished the task.  We do this
             // by clearing out the state of this task
@@ -198,6 +218,7 @@ namespace dlib
                 tasks[idx].mfp2.clear();
                 tasks[idx].arg1 = 0;
                 tasks[idx].arg2 = 0;
+                tasks[idx].eptr = eptr;
                 task_done_signaler.broadcast();
             }
 
@@ -210,6 +231,9 @@ namespace dlib
     find_empty_task_slot (
     ) const
     {
+        for (auto&& task : tasks)
+            task.propagate_exception();
+
         for (unsigned long i = 0; i < tasks.size(); ++i)
         {
             if (tasks[i].is_empty())

dlib/threads/thread_pool_extension.h

@@ -13,6 +13,7 @@
 #include "../array.h"
 #include "../smart_pointers_thread_safe.h"
 #include "../smart_pointers.h"
+#include <exception>
 
 namespace dlib
 {
@@ -451,6 +452,17 @@ namespace dlib
             bfp_type bfp;
 
             shared_ptr<function_object_copy> function_copy;
+            mutable std::exception_ptr eptr; // non-null if the task threw an exception
+
+            void propagate_exception() const
+            {
+                if (eptr)
+                {
+                    auto tmp = eptr;
+                    eptr = nullptr;
+                    std::rethrow_exception(tmp);
+                }
+            }
 
         };
 

dlib/threads/thread_pool_extension_abstract.h

@@ -225,9 +225,11 @@ namespace dlib
                 such as mutex objects. 
 
             EXCEPTIONS
-                Note that if an exception is thrown inside a task thread and 
-                is not caught then the normal rule for uncaught exceptions in
-                threads applies. That is, the application will be terminated.
+                Note that if an exception is thrown inside a task thread and is not caught
+                then the exception will be trapped inside the thread pool and rethrown at a
+                later time when someone calls one of the add task or wait member functions
+                of the thread pool.  This allows exceptions to propagate out of task threads
+                and into the calling code where they can be handled.
         !*/
 
     public: