From 717d30337093917064b5bbdb6b456b3ff1ce424e Mon Sep 17 00:00:00 2001
From: Vitaliy Filippov <vitalif@yourcmc.ru>
Date: Mon, 31 Jan 2022 02:52:18 +0300
Subject: [PATCH] Handle get_sqe failures, don't die with "will fall out of
 sync" in epoll_manager

Problem is that in recent kernels io_uring may return completions BEFORE
clearing the submission queue. I.e. for example its capacity is 512, there
were 512 requests, one of them completed, so when the request completion is
processed the queue "should have" 1 free slot. But sometimes it doesn't because
io_uring doesn't always clear the submission queue before sending CQE :-/
---
 src/epoll_manager.cpp | 16 +++++++++++++++-
 src/epoll_manager.h   |  2 ++
 2 files changed, 17 insertions(+), 1 deletion(-)

diff --git a/src/epoll_manager.cpp b/src/epoll_manager.cpp
index bc37409c..2e4d5923 100644
--- a/src/epoll_manager.cpp
+++ b/src/epoll_manager.cpp
@@ -13,6 +13,7 @@
 epoll_manager_t::epoll_manager_t(ring_loop_t *ringloop)
 {
     this->ringloop = ringloop;
+    this->pending = false;
 
     epoll_fd = epoll_create(1);
     if (epoll_fd < 0)
@@ -22,11 +23,19 @@ epoll_manager_t::epoll_manager_t(ring_loop_t *ringloop)
 
     tfd = new timerfd_manager_t([this](int fd, bool wr, std::function<void(int, int)> handler) { set_fd_handler(fd, wr, handler); });
 
+    consumer.loop = [this]()
+    {
+        if (pending)
+            handle_epoll_events();
+    };
+    ringloop->register_consumer(&consumer);
+
     handle_epoll_events();
 }
 
 epoll_manager_t::~epoll_manager_t()
 {
+    ringloop->unregister_consumer(&consumer);
     if (tfd)
     {
         delete tfd;
@@ -64,8 +73,13 @@ void epoll_manager_t::handle_epoll_events()
     io_uring_sqe *sqe = ringloop->get_sqe();
     if (!sqe)
     {
-        throw std::runtime_error("can't get SQE, will fall out of sync with EPOLLET");
+        // Don't handle epoll events until we manage to post the next event handler
+        // otherwise we'll fall out of sync with EPOLLET
+        pending = true;
+        ringloop->wakeup();
+        return;
     }
+    pending = false;
     ring_data_t *data = ((ring_data_t*)sqe->user_data);
     my_uring_prep_poll_add(sqe, epoll_fd, POLLIN);
     data->callback = [this](ring_data_t *data)
diff --git a/src/epoll_manager.h b/src/epoll_manager.h
index bb9161f6..6cdee46e 100644
--- a/src/epoll_manager.h
+++ b/src/epoll_manager.h
@@ -11,6 +11,8 @@
 class epoll_manager_t
 {
     int epoll_fd;
+    bool pending;
+    ring_consumer_t consumer;
     ring_loop_t *ringloop;
     std::map<int, std::function<void(int, int)>> epoll_handlers;
 public: