iostream: shutdown socket fd before close

If a socket fd is shared by a child process, closing it in the parent
will not shut down the underlying connection. As a result, the server
may hang executing the graceful shutdown protocol. Fix this problem by
explicitly shutting down the connection socket fd before closing it.

This is a recommended way to terminate a Unix socket connection, see
http://www.faqs.org/faqs/unix-faq/socket/#:~:text=2.6.%20%20When%20should%20I%20use%20shutdown()%3F

Closes #7256

NO_DOC=bug fix

changelogs/unreleased/gh-7256-iostream-shutdown.md

+## bugfix/core
+
+* Fixed a bug because of which a net.box connection was not properly terminated
+  when the process had a child (for example, started with `popen`) sharing the
+  connection socket fd. The bug could lead to a server hanging at exit while
+  executing the graceful shutdown protocol (gh-7256).

src/lib/core/iostream.c

@@ -9,6 +9,7 @@
 #include <assert.h>
 #include <errno.h>
 #include <stddef.h>
+#include <sys/socket.h>
 #include <sys/types.h>
 #include <unistd.h>
 
@@ -33,6 +34,12 @@ iostream_close(struct iostream *io)
 {
 	int fd = io->fd;
 	iostream_destroy(io);
+	/*
+	 * Explicitly shut down the socket before closing its fd so that
+	 * the connection will be terminated even if the Tarantool process
+	 * forked and the child process did not close parent fds.
+	 */
+	shutdown(fd, SHUT_RDWR);
 	close(fd);
 }
 

test/box-luatest/graceful_shutdown_test.lua

 local fiber = require('fiber')
 local net = require('net.box')
+local popen = require('popen')
 local server = require('test.luatest_helpers.server')
 local t = require('luatest')
 local g = t.group()
@@ -524,3 +525,17 @@ g.test_net_box_callback_garbage_collected = function()
     g.server:stop()
     conn:close()
 end
+
+-- Checks that the client closes the connection socket fd even if there is
+-- a child processes that shares it (gh-6820).
+g.test_graceful_shutdown_and_fork = function()
+    g.server:exec(function()
+        box.ctl.set_on_shutdown_timeout(9000)
+    end)
+    local c = net.connect(g.server.net_box_uri)
+    local p = popen.new({'/usr/bin/sleep', '9000'}, {close_fds = false})
+    t.assert(p)
+    c:close()
+    g.server:stop()
+    p:close()
+end