Add configurable exception handler (#637)

* Added exception_handler()

* Fixed worker crash if exception thrown in catch-all handler

include/crow/app.h

@@ -252,6 +252,24 @@ namespace crow
             return *this;
         }
 
+        /// Set the function to call to handle uncaught exceptions generated in routes (Default generates error 500).
+
+        ///
+        /// The function must have the following signature: void(crow::response&).
+        /// It must set the response passed in argument to the function, which will be sent back to the client.
+        /// See Router::default_exception_handler() for the default implementation.
+        template<typename Func>
+        self_t& exception_handler(Func&& f)
+        {
+            router_.exception_handler() = std::forward<Func>(f);
+            return *this;
+        }
+
+        std::function<void(crow::response&)>& exception_handler()
+        {
+            return router_.exception_handler();
+        }
+
         /// Set a custom duration and function to run on every tick
         template<typename Duration, typename Func>
         self_t& tick(Duration d, Func f)

include/crow/routing.h

@@ -1461,22 +1461,13 @@ namespace crow
 
             CROW_LOG_DEBUG << "Matched rule (upgrade) '" << rules[rule_index]->rule_ << "' " << static_cast<uint32_t>(req.method) << " / " << rules[rule_index]->get_methods();
 
-            // any uncaught exceptions become 500s
             try
             {
                 rules[rule_index]->handle_upgrade(req, res, std::move(adaptor));
             }
-            catch (std::exception& e)
-            {
-                CROW_LOG_ERROR << "An uncaught exception occurred: " << e.what();
-                res = response(500);
-                res.end();
-                return;
-            }
             catch (...)
             {
-                CROW_LOG_ERROR << "An uncaught exception occurred. The type was unknown so no information was available.";
-                res = response(500);
+                exception_handler_(res);
                 res.end();
                 return;
             }
@@ -1534,7 +1525,14 @@ namespace crow
                 std::vector<uint16_t> bpi = found.blueprint_indices;
                 if (bps_found[i]->catchall_rule().has_handler())
                 {
-                    bps_found[i]->catchall_rule().handler_(req, res);
+                    try
+                    {
+                        bps_found[i]->catchall_rule().handler_(req, res);
+                    }
+                    catch (...)
+                    {
+                        exception_handler_(res);
+                    }
 #ifdef CROW_ENABLE_DEBUG
                     return std::string("Redirected to Blueprint \"" + bps_found[i]->prefix() + "\" Catchall rule");
 #else
@@ -1544,7 +1542,14 @@ namespace crow
             }
             if (catchall_rule_.has_handler())
             {
-                catchall_rule_.handler_(req, res);
+                try
+                {
+                    catchall_rule_.handler_(req, res);
+                }
+                catch (...)
+                {
+                    exception_handler_(res);
+                }
 #ifdef CROW_ENABLE_DEBUG
                 return std::string("Redirected to global Catchall rule");
 #else
@@ -1704,23 +1709,14 @@ namespace crow
 
             CROW_LOG_DEBUG << "Matched rule '" << rules[rule_index]->rule_ << "' " << static_cast<uint32_t>(req.method) << " / " << rules[rule_index]->get_methods();
 
-            // any uncaught exceptions become 500s
             try
             {
                 auto& rule = rules[rule_index];
                 handle_rule<App>(rule, req, res, found.r_params);
             }
-            catch (std::exception& e)
-            {
-                CROW_LOG_ERROR << "An uncaught exception occurred: " << e.what();
-                res = response(500);
-                res.end();
-                return;
-            }
             catch (...)
             {
-                CROW_LOG_ERROR << "An uncaught exception occurred. The type was unknown so no information was available.";
-                res = response(500);
+                exception_handler_(res);
                 res.end();
                 return;
             }
@@ -1787,6 +1783,30 @@ namespace crow
             return blueprints_;
         }
 
+        std::function<void(crow::response&)>& exception_handler()
+        {
+            return exception_handler_;
+        }
+
+        static void default_exception_handler(response& res)
+        {
+            // any uncaught exceptions become 500s
+            res = response(500);
+
+            try
+            {
+                throw;
+            }
+            catch (const std::exception& e)
+            {
+                CROW_LOG_ERROR << "An uncaught exception occurred: " << e.what();
+            }
+            catch (...)
+            {
+                CROW_LOG_ERROR << "An uncaught exception occurred. The type was unknown so no information was available.";
+            }
+        }
+
     private:
         CatchallRule catchall_rule_;
 
@@ -1802,5 +1822,6 @@ namespace crow
         std::array<PerMethod, static_cast<int>(HTTPMethod::InternalMethodCount)> per_methods_;
         std::vector<std::unique_ptr<BaseRule>> all_rules_;
         std::vector<Blueprint*> blueprints_;
+        std::function<void(crow::response&)> exception_handler_ = &default_exception_handler;
     };
 } // namespace crow

tests/unittest.cpp

@@ -3261,6 +3261,79 @@ TEST_CASE("blueprint")
     }
 } // blueprint
 
+TEST_CASE("exception_handler")
+{
+    SimpleApp app;
+
+    CROW_ROUTE(app, "/get_error")
+    ([&]() -> std::string {
+        throw std::runtime_error("some error occurred");
+    });
+
+    CROW_ROUTE(app, "/get_no_error")
+    ([&]() {
+        return "Hello world";
+    });
+
+    app.validate();
+
+    {
+        request req;
+        response res;
+
+        req.url = "/get_error";
+        app.handle_full(req, res);
+
+        CHECK(500 == res.code);
+        CHECK(res.body.empty());
+    }
+
+    {
+        request req;
+        response res;
+
+        req.url = "/get_no_error";
+        app.handle_full(req, res);
+
+        CHECK(200 == res.code);
+        CHECK(res.body.find("Hello world") != std::string::npos);
+    }
+
+    app.exception_handler([](crow::response& res) {
+        try
+        {
+            throw;
+        }
+        catch (const std::exception& e)
+        {
+            res = response(501, e.what());
+        }
+    });
+
+    {
+        request req;
+        response res;
+
+        req.url = "/get_error";
+        app.handle_full(req, res);
+
+        CHECK(501 == res.code);
+        CHECK(res.body.find("some error occurred") != std::string::npos);
+    }
+
+    {
+        request req;
+        response res;
+
+        req.url = "/get_no_error";
+        app.handle_full(req, res);
+
+        CHECK(200 == res.code);
+        CHECK(res.body.find("some error occurred") == std::string::npos);
+        CHECK(res.body.find("Hello world") != std::string::npos);
+    }
+} // exception_handler
+
 TEST_CASE("base64")
 {
     unsigned char sample_bin[] = {0x14, 0xfb, 0x9c, 0x03, 0xd9, 0x7e};