more information in TEpollPoller::SetImpl error messages

- don't hide epoll_ctl add errors;
 - print "fd" and "what" args.

2 files changed, 17 insertions, 2 deletions

diff --git a/util/network/poller_ut.cpp b/util/network/poller_ut.cpp
index 6df0dda8ec..e5914b7df3 100644
--- a/util/network/poller_ut.cpp
+++ b/util/network/poller_ut.cpp
@@ -232,5 +232,12 @@ Y_UNIT_TEST_SUITE(TSocketPollerTest) {
         UNIT_ASSERT_EQUAL(TPoller::ExtractFilter(&e), CONT_POLL_RDHUP);
         UNIT_ASSERT_EQUAL(TPoller::ExtractEvent(&e), (void*)17);
     }
+
+    Y_UNIT_TEST(TestSetSocketErrors) {
+        TGenericPoller<TEpollPoller<TWithoutLocking>> poller;
+
+        UNIT_ASSERT_EXCEPTION_CONTAINS(poller.Set(nullptr, Max<int>(), CONT_POLL_READ), TSystemError, "epoll add failed");
+        UNIT_ASSERT_EXCEPTION_CONTAINS(poller.Set(nullptr, Max<int>(), CONT_POLL_READ | CONT_POLL_MODIFY), TSystemError, "epoll modify failed");
+    }
 #endif
 }
diff --git a/util/network/pollerimpl.h b/util/network/pollerimpl.h
index 21534967fa..0ec45e24f2 100644
--- a/util/network/pollerimpl.h
+++ b/util/network/pollerimpl.h
@@ -251,9 +251,17 @@ public:
 
         e.data.ptr = data;
 
-        if ((what & CONT_POLL_MODIFY) || epoll_ctl(Fd_, EPOLL_CTL_ADD, fd, &e) == -1) {
+        if (what & CONT_POLL_MODIFY) {
             if (epoll_ctl(Fd_, EPOLL_CTL_MOD, fd, &e) == -1) {
-                ythrow TSystemError() << "epoll add failed";
+                ythrow TSystemError() << "epoll modify failed (fd=" << fd << ", what=" << what << ")";
+            }
+        } else if (epoll_ctl(Fd_, EPOLL_CTL_ADD, fd, &e) == -1) {
+            if (LastSystemError() != EEXIST) {
+                ythrow TSystemError() << "epoll add failed (fd=" << fd << ", what=" << what << ")";
+            }
+
+            if (epoll_ctl(Fd_, EPOLL_CTL_MOD, fd, &e) == -1) {
+                ythrow TSystemError() << "epoll modify failed (fd=" << fd << ", what=" << what << ")";
             }
         }
     }