Add support for socket activation via systemd

This prevents the need for starting fcgiwrap explicitly, or using a
tool such as spawn-fcgi. The type of socket does not matter, we merely
accept a single FD passed from pid 1 and listen on it.

Makefile.in

@@ -11,7 +11,7 @@ install: all
 	install -m 644 fcgiwrap.8 $(man8dir)
 
 fcgiwrap: fcgiwrap.c
-	@CC@ @AM_CFLAGS@ fcgiwrap.c -o fcgiwrap -lfcgi @LDFLAGS@
+	@CC@ @AM_CFLAGS@ fcgiwrap.c -o fcgiwrap -lfcgi @systemd_LIBS@ @LDFLAGS@
 
 #>+ 21
 clean:

configure.ac

@@ -14,13 +14,30 @@ AC_SUBST([AM_CFLAGS])
 
 # Checks for programs.
 AC_PROG_CC
+PKG_PROG_PKG_CONFIG
 
 # Create the config.h.
 AC_CONFIG_HEADERS([config.h])
 
 # Checks for libraries.
 AC_CHECK_LIB([fcgi], [FCGX_Init],, [AC_MSG_ERROR([FastCGI library is missing])])
 
+# systemd support.
+AC_ARG_WITH([systemd],
+  AS_HELP_STRING([--with-systemd], [support systemd socket activation]),
+  [], [with_systemd=check])
+have_systemd=no
+if test "x$with_systemd" != "xno"; then
+  PKG_CHECK_MODULES(systemd, [libsystemd-daemon],
+    [AC_DEFINE(HAVE_SYSTEMD, 1, [Define if systemd is available])
+    have_systemd=yes],
+  have_systemd=no)
+  if test "x$have_systemd" = xno -a "x$with_systemd" = xyes; then
+    AC_MSG_ERROR([systemd support requested but libraries not found])
+  fi
+fi
+AM_CONDITIONAL(HAVE_LIBSSL, [test "x$have_systemd" = "xyes"])
+
 # Checks for header files.
 AC_CHECK_HEADERS([fcntl.h],, [AC_MSG_ERROR([fcntl.h header missing])])
 AC_CHECK_HEADERS([limits.h stdlib.h string.h unistd.h],, [AC_MSG_ERROR([at least one important system header file is missing])])

fcgiwrap.c

@@ -47,6 +47,10 @@
 #include <sys/un.h>
 #include <netinet/in.h>
 
+#ifdef HAVE_SYSTEMD
+#include <systemd/sd-daemon.h>
+#endif
+
 /* glibc doesn't seem to export it */
 #ifndef UNIX_PATH_MAX
 #define UNIX_PATH_MAX 108
@@ -788,6 +792,14 @@ int main(int argc, char **argv)
 		}
 	}
 
+#ifdef HAVE_SYSTEMD
+	if (sd_listen_fds(true) > 0) {
+		/* systemd woke us up. we should never see more than one FD passed to us. */
+		if (listen_on_fd(SD_LISTEN_FDS_START) < 0) {
+			return 1;
+		}
+	} else
+#endif
 	if (socket_url) {
 		if (setup_socket(socket_url) < 0) {
 			return 1;