(FACT-1373) Prevent fact calls from external facts

Prior to this commit, it was possible to make a facter call from
within an external fact. This would result in a fork bomb where
facter would be constantly calling into itself. In order to prevent
this, set an environment variable that allows us to ensure facter
is not already running when we make the call to evaluate external
facts.

exe/facter.cc

@@ -2,6 +2,8 @@
 #include <facter/logging/logging.hpp>
 #include <facter/facts/collection.hpp>
 #include <facter/ruby/ruby.hpp>
+#include <leatherman/logging/logging.hpp>
+#include <leatherman/util/environment.hpp>
 #include <leatherman/util/scope_exit.hpp>
 #include <boost/algorithm/string.hpp>
 // Note the caveats in nowide::cout/cerr; they're not synchronized with stdio.
@@ -25,8 +27,10 @@
 using namespace std;
 using namespace facter::facts;
 using namespace facter::logging;
+using leatherman::util::environment;
 namespace po = boost::program_options;
 
+
 void help(po::options_description& desc)
 {
     boost::nowide::cout <<
@@ -251,7 +255,15 @@ int main(int argc, char **argv)
         facts.add_default_facts(ruby);
 
         if (!vm.count("no-external-facts")) {
+          string inside_facter;
+          environment::get("INSIDE_FACTER", inside_facter);
+
+          if (inside_facter == "true") {
+            LOG_WARNING("Facter was called recursively, skipping external facts. Add '--no-external-facts' to silence this warning");
+          } else {
+            environment::set("INSIDE_FACTER", "true");
             facts.add_external_facts(external_directories);
+          }
         }
 
         // Add the environment facts