Add support for SCGI systemd socket activation

Add a new command, `network.scgi.open_systemd`, that binds to a file
descriptor passed in via systemd socket activation.

configure.ac

@@ -56,6 +56,7 @@ AC_LANG_POP(C++)
 
 TORRENT_WITH_LUA
 TORRENT_WITH_TINYXML2
+TORRENT_WITH_SYSTEMD
 
 if test ${with_xmlrpc_c+y} && test ${with_xmlrpc_tinyxml2+y}; then
   AC_MSG_ERROR([--with-xmlrpc-c and --with-xmlrpc-tinyxml2 cannot be used together. Please choose only one])

scripts/checks.m4

@@ -358,3 +358,20 @@ AC_DEFUN([TORRENT_DISABLE_PTHREAD_SETNAME_NP], [
     ]
   )
 ])
+
+
+AC_DEFUN([TORRENT_WITH_SYSTEMD], [
+  AC_ARG_WITH(systemd,
+    AS_HELP_STRING([--with-systemd],[enable systemd socket activation support [[default=no]]]),
+    [
+      if test "$withval" = "yes"; then
+        PKG_CHECK_MODULES([SYSTEMD], [libsystemd],
+          [
+            CXXFLAGS="$CXXFLAGS $SYSTEMD_CFLAGS"
+            LIBS="$LIBS $SYSTEMD_LIBS"
+            AC_DEFINE(HAVE_SYSTEMD, 1, [Support for systemd socket activation.])
+          ],
+          [AC_MSG_ERROR([libsystemd not found. Install libsystemd-dev (or the equivalent for your distribution).])])
+      fi
+    ])
+])

src/command_network.cc

@@ -3,6 +3,10 @@
 #include <functional>
 #include <cstdio>
 #include <unistd.h>
+#ifdef HAVE_SYSTEMD
+#include <sys/socket.h>
+#include <systemd/sd-daemon.h>
+#endif
 #include <rak/address_info.h>
 #include <torrent/torrent.h>
 #include <torrent/rate.h>
@@ -143,6 +147,41 @@ apply_scgi(const std::string& arg, int type) {
   return torrent::Object();
 }
 
+#ifdef HAVE_SYSTEMD
+torrent::Object
+apply_scgi_systemd() {
+  if (scgi_thread::scgi() != nullptr)
+    throw torrent::input_error("SCGI already enabled.");
+
+  int n = sd_listen_fds(0);
+  if (n < 1)
+    throw torrent::input_error("No systemd socket(s) provided (sd_listen_fds returned " +
+                               std::to_string(n) + ").");
+
+  // Iterate over all provided fds. Use the first listening stream socket;
+  // close the rest. The systemd docs say unused fds should be closed.
+  int selected_fd = -1;
+  for (int i = 0; i < n; i++) {
+    int fd = SD_LISTEN_FDS_START + i;
+    if (selected_fd == -1 && sd_is_socket(fd, AF_UNSPEC, SOCK_STREAM, 1) > 0)
+      selected_fd = fd;
+    else
+      ::close(fd);
+  }
+
+  if (selected_fd == -1)
+    throw torrent::input_error("No listening stream socket found among systemd-provided fds.");
+
+  initialize_rpc_handlers();
+
+  rpc::SCgi* scgi = new rpc::SCgi;
+  scgi->open_fd(selected_fd);
+
+  scgi_thread::set_scgi(scgi);
+  return torrent::Object();
+}
+#endif
+
 torrent::Object
 apply_xmlrpc_dialect(const std::string& arg) {
   int value;
@@ -243,6 +282,9 @@ initialize_command_network() {
   CMD2_ANY_STRING  ("network.scgi.open_port",        std::bind(&apply_scgi, std::placeholders::_2, 1));
   CMD2_ANY_STRING  ("network.scgi.open_local",       std::bind(&apply_scgi, std::placeholders::_2, 2));
   CMD2_VAR_BOOL    ("network.scgi.dont_route",       false);
+#ifdef HAVE_SYSTEMD
+  CMD2_ANY_VALUE_V ("network.scgi.open_systemd",     [](auto, auto& value) { if (value != 0) apply_scgi_systemd(); });
+#endif
 
   CMD2_ANY_STRING  ("network.xmlrpc.dialect.set",    [](const auto&, const auto& arg) { return apply_xmlrpc_dialect(arg); })
   CMD2_ANY         ("network.xmlrpc.size_limit",     [](const auto&, const auto&)     { return rpc::rpc.size_limit(); });

src/rpc/scgi.cc

@@ -2,6 +2,7 @@
 
 #include <algorithm>
 #include <cassert>
+#include <fcntl.h>
 #include <unistd.h>
 #include <sys/un.h>
 #include <torrent/connection_manager.h>
@@ -80,6 +81,20 @@ SCgi::open_named(const std::string& filename) {
   m_path = filename;
 }
 
+void
+SCgi::open_fd(int fd) {
+  torrent::runtime::socket_manager()->open_event_or_throw(this, [&]() {
+      int flags = ::fcntl(fd, F_GETFL, 0);
+      if (flags == -1 || ::fcntl(fd, F_SETFL, flags | O_NONBLOCK) == -1)
+        throw torrent::resource_error("Could not set non-blocking on systemd fd: " +
+                                      std::string(std::strerror(errno)));
+      set_file_descriptor(fd);
+      // fd is already bound and listening; no bind()/listen() needed.
+    });
+
+  torrent::connection_manager()->inc_socket_count();
+}
+
 void
 SCgi::open(sockaddr* sa, unsigned int length) {
   try {

src/rpc/scgi.h

@@ -21,6 +21,7 @@ public:
 
   void                open_port(sockaddr* sa, unsigned int length, bool dont_route);
   void                open_named(const std::string& filename);
+  void                open_fd(int fd);
 
   void                activate();