From 1e86ca5834b94cae7d5e6d219056c0fc895cf95d Mon Sep 17 00:00:00 2001
From: AJ Heller <hork@google.com>
Date: Wed, 12 Jul 2023 18:42:09 -0700
Subject: [PATCH] [backport][iomgr][EventEngine] Improve server handling of
 file descriptor exhaustion (#33672)

Backport of #33656
---
 src/core/lib/iomgr/tcp_server_posix.cc        | 46 ++++++++++++++-----
 src/core/lib/iomgr/tcp_server_utils_posix.h   | 13 +++++
 .../iomgr/tcp_server_utils_posix_common.cc    | 21 ++++++++
 3 files changed, 67 insertions(+), 13 deletions(-)

diff --git a/src/core/lib/iomgr/tcp_server_posix.cc b/src/core/lib/iomgr/tcp_server_posix.cc
index a1db16d916..6804928fe3 100644
--- a/src/core/lib/iomgr/tcp_server_posix.cc
+++ b/src/core/lib/iomgr/tcp_server_posix.cc
@@ -16,13 +16,17 @@
  *
  */
 
+#include <grpc/support/port_platform.h>
+
+#include <utility>
+
+#include <grpc/support/atm.h>
+
 /* FIXME: "posix" files shouldn't be depending on _GNU_SOURCE */
 #ifndef _GNU_SOURCE
 #define _GNU_SOURCE
 #endif
 
-#include <grpc/support/port_platform.h>
-
 #include "src/core/lib/iomgr/port.h"
 
 #ifdef GRPC_POSIX_SOCKET_TCP_SERVER
@@ -45,6 +49,7 @@
 #include "absl/strings/str_cat.h"
 #include "absl/strings/str_format.h"
 
+#include <grpc/event_engine/event_engine.h>
 #include <grpc/support/alloc.h>
 #include <grpc/support/log.h>
 #include <grpc/support/sync.h>
@@ -350,21 +357,35 @@ static void on_read(void* arg, grpc_error_handle err) {
     if (fd < 0) {
       if (errno == EINTR) {
         continue;
-      } else if (errno == EAGAIN || errno == ECONNABORTED ||
-                 errno == EWOULDBLOCK) {
+      }
+      // When the process runs out of fds, accept4() returns EMFILE. When this
+      // happens, the connection is left in the accept queue until either a
+      // read event triggers the on_read callback, or time has passed and the
+      // accept should be re-tried regardless. This callback is not cancelled,
+      // so a spurious wakeup may occur even when there's nothing to accept.
+      // This is not a performant code path, but if an fd limit has been
+      // reached, the system is likely in an unhappy state regardless.
+      if (errno == EMFILE) {
+        grpc_fd_notify_on_read(sp->emfd, &sp->read_closure);
+        if (gpr_atm_full_xchg(&sp->retry_timer_armed, true)) return;
+        grpc_timer_init(&sp->retry_timer,
+                        grpc_core::ExecCtx::Get()->Now() + 1 * GPR_MS_PER_SEC,
+                        &sp->retry_closure);
+        return;
+      }
+      if (errno == EAGAIN || errno == ECONNABORTED || errno == EWOULDBLOCK) {
         grpc_fd_notify_on_read(sp->emfd, &sp->read_closure);
         return;
+      }
+      gpr_mu_lock(&sp->server->mu);
+      if (!sp->server->shutdown_listeners) {
+        gpr_log(GPR_ERROR, "Failed accept4: %s", strerror(errno));
       } else {
-        gpr_mu_lock(&sp->server->mu);
-        if (!sp->server->shutdown_listeners) {
-          gpr_log(GPR_ERROR, "Failed accept4: %s", strerror(errno));
-        } else {
-          /* if we have shutdown listeners, accept4 could fail, and we
-             needn't notify users */
-        }
-        gpr_mu_unlock(&sp->server->mu);
-        goto error;
+        // if we have shutdown listeners, accept4 could fail, and we
+        // needn't notify users
       }
+      gpr_mu_unlock(&sp->server->mu);
+      goto error;
     }
 
     /* For UNIX sockets, the accept call might not fill up the member sun_path
@@ -558,6 +581,7 @@ static grpc_error_handle clone_port(grpc_tcp_listener* listener,
     sp->port_index = listener->port_index;
     sp->fd_index = listener->fd_index + count - i;
     GPR_ASSERT(sp->emfd);
+    grpc_tcp_server_listener_initialize_retry_timer(sp);
     while (listener->server->tail->next != nullptr) {
       listener->server->tail = listener->server->tail->next;
     }
@@ -791,6 +815,7 @@ static void tcp_server_shutdown_listeners(grpc_tcp_server* s) {
   if (s->active_ports) {
     grpc_tcp_listener* sp;
     for (sp = s->head; sp; sp = sp->next) {
+      grpc_timer_cancel(&sp->retry_timer);
       grpc_fd_shutdown(sp->emfd,
                        GRPC_ERROR_CREATE_FROM_STATIC_STRING("Server shutdown"));
     }
diff --git a/src/core/lib/iomgr/tcp_server_utils_posix.h b/src/core/lib/iomgr/tcp_server_utils_posix.h
index 26cef0209f..de5a888cff 100644
--- a/src/core/lib/iomgr/tcp_server_utils_posix.h
+++ b/src/core/lib/iomgr/tcp_server_utils_posix.h
@@ -30,6 +30,7 @@
 #include "src/core/lib/iomgr/resolve_address.h"
 #include "src/core/lib/iomgr/socket_utils_posix.h"
 #include "src/core/lib/iomgr/tcp_server.h"
+#include "src/core/lib/iomgr/timer.h"
 
 /* one listening port */
 typedef struct grpc_tcp_listener {
@@ -52,6 +53,11 @@ typedef struct grpc_tcp_listener {
      identified while iterating through 'next'. */
   struct grpc_tcp_listener* sibling;
   int is_sibling;
+  // If an accept4() call fails, a timer is started to drain the accept queue in
+  // case no further connection attempts reach the gRPC server.
+  grpc_closure retry_closure;
+  grpc_timer retry_timer;
+  gpr_atm retry_timer_armed;
 } grpc_tcp_listener;
 
 /* the overall server */
@@ -139,4 +145,10 @@ grpc_error_handle grpc_tcp_server_prepare_socket(
 /* Ruturn true if the platform supports ifaddrs */
 bool grpc_tcp_server_have_ifaddrs(void);
 
+// Initialize (but don't start) the timer and callback to retry accept4() on a
+// listening socket after file descriptors have been exhausted. This must be
+// called when creating a new listener.
+void grpc_tcp_server_listener_initialize_retry_timer(
+    grpc_tcp_listener* listener);
+
 #endif /* GRPC_CORE_LIB_IOMGR_TCP_SERVER_UTILS_POSIX_H */
diff --git a/src/core/lib/iomgr/tcp_server_utils_posix_common.cc b/src/core/lib/iomgr/tcp_server_utils_posix_common.cc
index 574fd02d0d..a32f542c4a 100644
--- a/src/core/lib/iomgr/tcp_server_utils_posix_common.cc
+++ b/src/core/lib/iomgr/tcp_server_utils_posix_common.cc
@@ -18,6 +18,8 @@
 
 #include <grpc/support/port_platform.h>
 
+#include <grpc/support/atm.h>
+
 #include "src/core/lib/iomgr/port.h"
 
 #ifdef GRPC_POSIX_SOCKET_TCP_SERVER_UTILS_COMMON
@@ -81,6 +83,24 @@ static int get_max_accept_queue_size(void) {
   return s_max_accept_queue_size;
 }
 
+static void listener_retry_timer_cb(void* arg, grpc_error_handle err) {
+  // Do nothing if cancelled.
+  if (err != GRPC_ERROR_NONE) return;
+  grpc_tcp_listener* listener = static_cast<grpc_tcp_listener*>(arg);
+  gpr_atm_no_barrier_store(&listener->retry_timer_armed, false);
+  if (!grpc_fd_is_shutdown(listener->emfd)) {
+    grpc_fd_set_readable(listener->emfd);
+  }
+}
+
+void grpc_tcp_server_listener_initialize_retry_timer(
+    grpc_tcp_listener* listener) {
+  gpr_atm_no_barrier_store(&listener->retry_timer_armed, false);
+  grpc_timer_init_unset(&listener->retry_timer);
+  GRPC_CLOSURE_INIT(&listener->retry_closure, listener_retry_timer_cb, listener,
+                    grpc_schedule_on_exec_ctx);
+}
+
 static grpc_error_handle add_socket_to_server(grpc_tcp_server* s, int fd,
                                               const grpc_resolved_address* addr,
                                               unsigned port_index,
@@ -112,6 +132,7 @@ static grpc_error_handle add_socket_to_server(grpc_tcp_server* s, int fd,
     sp->server = s;
     sp->fd = fd;
     sp->emfd = grpc_fd_create(fd, name.c_str(), true);
+    grpc_tcp_server_listener_initialize_retry_timer(sp);
     memcpy(&sp->addr, addr, sizeof(grpc_resolved_address));
     sp->port = port;
     sp->port_index = port_index;
-- 
2.33.0