Check initgroups with AC_CHECK_DECLS for cygwin

configure.ac

@@ -604,7 +604,6 @@ AC_CHECK_FUNCS([ \
   dup2 \
   getcwd \
   getpwnam \
-  initgroups \
   localtime_r \
   memchr \
   memmove \
@@ -626,6 +625,10 @@ AC_CHECK_FUNCS([ \
 AC_CHECK_FUNC([timerfd_create],
               [have_timerfd_create=yes], [have_timerfd_create=no])
 
+# For cygwin: we can link initgroups, so AC_CHECK_FUNCS succeeds, but
+# cygwin disables initgroups due to feature test macro magic with our
+# configuration.
+AC_CHECK_DECLS([initgroups], [], [], [[#include <grp.h>]])
 
 # Checks for epoll availability, primarily for examples/tiny-nghttpd
 AX_HAVE_EPOLL([have_epoll=yes], [have_epoll=no])

src/shrpx.h

@@ -44,4 +44,8 @@
 
 #define DIE() _Exit(EXIT_FAILURE)
 
+#if defined(HAVE_DECL_INITGROUPS) && !HAVE_DECL_INITGROUPS
+inline int initgroups(const char *user, gid_t group) { return 0; }
+#endif // defined(HAVE_DECL_INITGROUPS) && !HAVE_DECL_INITGROUPS
+
 #endif // SHRPX_H

src/shrpx_worker_process.cc

@@ -64,14 +64,12 @@ void drop_privileges(
 #endif // HAVE_NEVERBLEED
     ) {
   if (getuid() == 0 && get_config()->uid != 0) {
-#ifdef HAVE_INITGROUPS
     if (initgroups(get_config()->user.get(), get_config()->gid) != 0) {
       auto error = errno;
       LOG(FATAL) << "Could not change supplementary groups: "
                  << strerror(error);
       exit(EXIT_FAILURE);
     }
-#endif // HAVE_INITGROUPS
     if (setgid(get_config()->gid) != 0) {
       auto error = errno;
       LOG(FATAL) << "Could not change gid: " << strerror(error);