nghttpx: Send SIGQUIT to the original master process

Previously, after sending SIGUSR2 to the original master process, and
the new master process gets ready, user has to send SIGQUIT to the
original master process to shut it down gracefully.  With this commit,
the new master process sends SIGQUIT to the original master process
when it is ready to serve requests, eliminating for user to send
SIGQUIT manually.

This works nicely with systemd, because now you can replace nghttpx
binary with new one by "systemctl kill -s USR2 --kill-who=main
nghttpx".

doc/nghttpx.h2r

@@ -83,14 +83,18 @@ SIGUSR1
   Reopen log files.
 
 SIGUSR2
+
   Fork and execute nghttpx.  It will execute the binary in the same
-  path with same command-line arguments and environment variables.
-  After new process comes up, sending SIGQUIT to the original process
-  to perform hot swapping.  The difference between SIGUSR2 + SIGQUIT
-  and SIGHUP is that former is usually used to execute new binary, and
-  the master process is newly spawned.  On the other hand, the latter
-  just reloads configuration file, and the same master process
-  continues to exist.
+  path with same command-line arguments and environment variables.  As
+  of nghttpx version 1.20.0, the new master process sends SIGQUIT to
+  the original master process when it is ready to serve requests.  For
+  the earlier versions of nghttpx, user has to send SIGQUIT to the
+  original master process.
+
+  The difference between SIGUSR2 (+ SIGQUIT) and SIGHUP is that former
+  is usually used to execute new binary, and the master process is
+  newly spawned.  On the other hand, the latter just reloads
+  configuration file, and the same master process continues to exist.
 
 .. note::
 

doc/sources/nghttpx-howto.rst

@@ -229,12 +229,18 @@ Hot swapping
 nghttpx supports hot swapping using signals.  The hot swapping in
 nghttpx is multi step process.  First send USR2 signal to nghttpx
 process.  It will do fork and execute new executable, using same
-command-line arguments and environment variables.  At this point, both
-current and new processes can accept requests.  To gracefully shutdown
-current process, send QUIT signal to current nghttpx process.  When
-all existing frontend connections are done, the current process will
-exit.  At this point, only new nghttpx process exists and serves
-incoming requests.
+command-line arguments and environment variables.
+
+As of nghttpx version 1.20.0, that is all you have to do.  The new
+master process sends QUIT signal to the original process, when it is
+ready to serve requests, to shut it down gracefully.
+
+For earlier versions of nghttpx, you have to do one more thing.  At
+this point, both current and new processes can accept requests.  To
+gracefully shutdown current process, send QUIT signal to current
+nghttpx process.  When all existing frontend connections are done, the
+current process will exit.  At this point, only new nghttpx process
+exists and serves incoming requests.
 
 If you want to just reload configuration file without executing new
 binary, send SIGHUP to nghttpx master process.

src/shrpx.cc

@@ -123,6 +123,12 @@ constexpr auto ENV_UNIX_PATH = StringRef::from_lit("NGHTTP2_UNIX_PATH");
 // descriptor.  <PATH> is a path to UNIX domain socket.
 constexpr auto ENV_ACCEPT_PREFIX = StringRef::from_lit("NGHTTPX_ACCEPT_");
 
+// This environment variable contains PID of the original master
+// process, assuming that it created this master process as a result
+// of SIGUSR2.  The new master process is expected to send QUIT signal
+// to the original master process to shut it down gracefully.
+constexpr auto ENV_ORIG_PID = StringRef::from_lit("NGHTTPX_ORIG_PID");
+
 #ifndef _KERNEL_FASTOPEN
 #define _KERNEL_FASTOPEN
 // conditional define for TCP_FASTOPEN mostly on ubuntu
@@ -456,7 +462,8 @@ void exec_binary() {
 
   auto &listenerconf = get_config()->conn.listener;
 
-  auto envp = make_unique<char *[]>(envlen + listenerconf.addrs.size() + 1);
+  // 2 for ENV_ORIG_PID and terminal nullptr.
+  auto envp = make_unique<char *[]>(envlen + listenerconf.addrs.size() + 2);
   size_t envidx = 0;
 
   std::vector<ImmutableString> fd_envs;
@@ -479,6 +486,11 @@ void exec_binary() {
     envp[envidx++] = const_cast<char *>(fd_envs.back().c_str());
   }
 
+  auto ipc_fd_str = ENV_ORIG_PID.str();
+  ipc_fd_str += '=';
+  ipc_fd_str += util::utos(get_config()->pid);
+  envp[envidx++] = const_cast<char *>(ipc_fd_str.c_str());
+
   for (size_t i = 0; i < envlen; ++i) {
     auto env = StringRef{environ[i]};
     if (util::starts_with(env, ENV_ACCEPT_PREFIX) ||
@@ -486,7 +498,8 @@ void exec_binary() {
         util::starts_with(env, ENV_LISTENER6_FD) ||
         util::starts_with(env, ENV_PORT) ||
         util::starts_with(env, ENV_UNIX_FD) ||
-        util::starts_with(env, ENV_UNIX_PATH)) {
+        util::starts_with(env, ENV_UNIX_PATH) ||
+        util::starts_with(env, ENV_ORIG_PID)) {
       continue;
     }
 
@@ -1073,6 +1086,18 @@ void close_unused_inherited_addr(const std::vector<InheritedAddr> &iaddrs) {
 }
 } // namespace
 
+namespace {
+// Returns the PID of the original master process from environment
+// variable ENV_ORIG_PID.
+pid_t get_orig_pid_from_env() {
+  auto s = getenv(ENV_ORIG_PID.c_str());
+  if (s == nullptr) {
+    return -1;
+  }
+  return util::parse_uint(s);
+}
+} // namespace
+
 namespace {
 int create_acceptor_socket(Config *config, std::vector<InheritedAddr> &iaddrs) {
   std::array<char, STRERROR_BUFSIZE> errbuf;
@@ -1288,6 +1313,8 @@ int event_loop() {
     close_unused_inherited_addr(iaddrs);
   }
 
+  auto orig_pid = get_orig_pid_from_env();
+
   auto loop = ev_default_loop(config->ev_loop_flags);
 
   int ipc_fd;
@@ -1311,6 +1338,12 @@ int event_loop() {
   // ready to serve requests
   shrpx_sd_notifyf(0, "READY=1");
 
+  if (orig_pid != -1) {
+    LOG(NOTICE) << "Send QUIT signal to the original master process to tell "
+                   "that we are ready to serve requests.";
+    kill(orig_pid, SIGQUIT);
+  }
+
   ev_run(loop, 0);
 
   return 0;