[RFC PATCH v3 07/21] apps: common: event_loop: Make it possible to exit with exception

[Barnabás Pőcze]

Instead of taking a single integer in `EventLoop:exit()`, change it
so that an `std::exception_ptr` can also be passed. And in `EventLoop::exec()`
rethrow the stored exception (if any).

Furthermore, catch exceptions from deferred calls, and stop the event
loop with the caught exception.

Signed-off-by: Barnabás Pőcze <pobrn at protonmail.com>
---
 src/apps/common/event_loop.cpp | 35 +++++++++++++++++++++++++++++-----
 src/apps/common/event_loop.h   |  6 ++++--
 2 files changed, 34 insertions(+), 7 deletions(-)

diff --git a/src/apps/common/event_loop.cpp b/src/apps/common/event_loop.cpp
index 0d7a4a024..13c2d6057 100644
--- a/src/apps/common/event_loop.cpp
+++ b/src/apps/common/event_loop.cpp
@@ -12,6 +12,7 @@
 #include <event2/thread.h>
 #include <algorithm>
 #include <iostream>
+#include <utility>
 
 EventLoop *EventLoop::instance_ = nullptr;
 
@@ -38,7 +39,14 @@ EventLoop::EventLoop()
 				self->calls_.pop_front();
 			}
 
-			call();
+			try {
+				call();
+			}
+			catch (...) {
+				::event_active(self->callsTrigger_, 0, 0);
+				self->exit(std::current_exception());
+				break;
+			}
 		}
 	}, this);
 	assert(callsTrigger_);
@@ -63,14 +71,31 @@ EventLoop *EventLoop::instance()
 
 int EventLoop::exec()
 {
-	exitCode_ = -1;
+	{
+		std::lock_guard locker(lock_);
+		result_ = 0;
+	}
+
 	event_base_loop(base_, EVLOOP_NO_EXIT_ON_EMPTY);
-	return exitCode_;
+
+	auto result = [&] {
+		std::lock_guard locker(lock_);
+		return std::exchange(result_, 0);
+	}();
+
+	if (auto *exc = std::get_if<std::exception_ptr>(&result))
+		std::rethrow_exception(std::move(*exc));
+
+	return std::get<int>(result);
 }
 
-void EventLoop::exit(int code)
+void EventLoop::exit(std::variant<int, std::exception_ptr> result)
 {
-	exitCode_ = code;
+	{
+		std::lock_guard locker(lock_);
+		result_ = std::move(result);
+	}
+
 	event_base_loopbreak(base_);
 }
 
diff --git a/src/apps/common/event_loop.h b/src/apps/common/event_loop.h
index 6d7d0497a..380c3483a 100644
--- a/src/apps/common/event_loop.h
+++ b/src/apps/common/event_loop.h
@@ -9,11 +9,13 @@
 
 #include <chrono>
 #include <deque>
+#include <exception>
 #include <functional>
 #include <list>
 #include <memory>
 #include <mutex>
 #include <optional>
+#include <variant>
 
 #include <libcamera/base/class.h>
 
@@ -35,7 +37,7 @@ public:
 	static EventLoop *instance();
 
 	int exec();
-	void exit(int code = 0);
+	void exit(std::variant<int, std::exception_ptr> result = 0);
 
 	void callLater(std::function<void()> &&func, std::optional<std::uintptr_t> cookie = {});
 	void cancelLater(std::optional<std::uintptr_t> cookie = {});
@@ -63,7 +65,7 @@ private:
 	static EventLoop *instance_;
 
 	struct event_base *base_;
-	int exitCode_;
+	std::variant<int, std::exception_ptr> result_ = 0;
 
 	std::deque<std::pair<std::function<void()>, std::optional<std::uintptr_t>>> calls_;
 	::event *callsTrigger_ = nullptr;
-- 
2.48.1