show the group we failed to drop via setgroups

This also deals with the unlikely case of groups being mentioned
multiple times or if the effective group isn't mentioned at all.
In practice, it is a debugging aid through like for #806475.

Git-Dch: Ignore

apt-pkg/contrib/fileutl.cc

@@ -2322,12 +2322,17 @@ bool DropPrivileges()							/*{{{*/
       return _error->Errno("seteuid", "Failed to seteuid");
 #endif
 
-   // Verify that the user has only a single group, and the correct one
-   gid_t groups[1];
-   if (getgroups(1, groups) != 1)
-      return _error->Errno("getgroups", "Could not get new groups");
-   if (groups[0] != pw->pw_gid)
-      return _error->Error("Could not switch group");
+   // Verify that the user isn't still in any supplementary groups
+   long const ngroups_max = sysconf(_SC_NGROUPS_MAX);
+   std::unique_ptr<gid_t[]> gidlist(new gid_t[ngroups_max]);
+   if (unlikely(gidlist == NULL))
+      return _error->Error("Allocation of a list of size %lu for getgroups failed", ngroups_max);
+   ssize_t gidlist_nr;
+   if ((gidlist_nr = getgroups(ngroups_max, gidlist.get())) < 0)
+      return _error->Errno("getgroups", "Could not get new groups (%lu)", ngroups_max);
+   for (ssize_t i = 0; i < gidlist_nr; ++i)
+      if (gidlist[i] != pw->pw_gid)
+	 return _error->Error("Could not switch group, user %s is still in group %d", toUser.c_str(), gidlist[i]);
 
    // Verify that gid, egid, uid, and euid changed
    if (getgid() != pw->pw_gid)