Fixed runtime handling of systems without EPOLLRDHUP support. In 7583:efd71d49bde0 (nginx 1.17.5) along with introduction of the ioctl(FIONREAD) support proper handling of systems without EPOLLRDHUP support in the kernel (but with EPOLLRDHUP in headers) was broken. Before the change, rev->available was never set to 0 unless ngx_use_epoll_rdhup was also set (that is, runtime test for EPOLLRDHUP introduced in 6536:f7849bfb6d21 succeeded). After the change, rev->available might reach 0 on systems without runtime EPOLLRDHUP support, stopping further reading in ngx_readv_chain() and ngx_unix_recv(). And, if EOF happened to be already reported along with the last event, it is not reported again by epoll_wait(), leading to connection hangs and timeouts on such systems. This affects Linux kernels before 2.6.17 if nginx was compiled with newer headers, and, more importantly, emulation layers, such as DigitalOcean's App Platform's / gVisor's epoll emulation layer. Fix is to explicitly check ngx_use_epoll_rdhup before the corresponding rev->pending_eof tests in ngx_readv_chain() and ngx_unix_recv().

--- a/src/os/unix/ngx_readv_chain.c
+++ b/src/os/unix/ngx_readv_chain.c
@@ -55,7 +55,9 @@ ngx_readv_chain(ngx_connection_t *c, ngx
 
 #if (NGX_HAVE_EPOLLRDHUP)
 
-    if (ngx_event_flags & NGX_USE_EPOLL_EVENT) {
+    if ((ngx_event_flags & NGX_USE_EPOLL_EVENT)
+        && ngx_use_epoll_rdhup)
+    {
         ngx_log_debug2(NGX_LOG_DEBUG_EVENT, c->log, 0,
                        "readv: eof:%d, avail:%d",
                        rev->pending_eof, rev->available);

--- a/src/os/unix/ngx_recv.c
+++ b/src/os/unix/ngx_recv.c
@@ -52,7 +52,9 @@ ngx_unix_recv(ngx_connection_t *c, u_cha
 
 #if (NGX_HAVE_EPOLLRDHUP)
 
-    if (ngx_event_flags & NGX_USE_EPOLL_EVENT) {
+    if ((ngx_event_flags & NGX_USE_EPOLL_EVENT)
+        && ngx_use_epoll_rdhup)
+    {
         ngx_log_debug2(NGX_LOG_DEBUG_EVENT, c->log, 0,
                        "recv: eof:%d, avail:%d",
                        rev->pending_eof, rev->available);