Do not crash when lookup yields empty group in indirect table

Instead just log an error and treat it as an error.

include/bm/bm_sim/match_tables.h

@@ -434,7 +434,7 @@ class MatchTableIndirect : public MatchTableAbstract {
   std::unique_ptr<MatchUnitAbstract<IndirectIndex> > match_unit;
   ActionProfile *action_profile{nullptr};
   bool default_set{false};
-  ActionEntry empty_action{};
+  ActionEntry empty_action{};  // for lookups yielding empty groups
 };
 
 class MatchTableIndirectWS : public MatchTableIndirect {

src/bm_sim/match_tables.cpp

@@ -663,6 +663,12 @@ MatchTableIndirect::lookup(const Packet &pkt,
   if (!(*hit) && !default_set) return empty_action;
 
   const IndirectIndex &index = (*hit) ? *res.value : default_index;
+  if (index.is_grp() && action_profile->group_is_empty(index.get_grp())) {
+    BMLOG_ERROR_PKT(
+        pkt, "Lookup in table '{}' yielded empty group", get_name());
+    return empty_action;
+  }
+
   auto &entry = action_profile->lookup(pkt, index);
   // TODO(antonin): unfortunately this has to be done at this stage and cannot
   // be done when inserting a member because for 2 match tables sharing the same

tests/test_tables.cpp

@@ -1828,6 +1828,27 @@ TEST_F(TableIndirectWS, DefaultGroup) {
   EXPECT_EQ(rc, MatchErrorCode::SUCCESS);
 }
 
+// It can be quite complicated to prevent the control-plane from referencing
+// empty groups (is it even desirable?). We must at least make sure that we do
+// not crash. In the current implementation, I believe the table apply is a
+// no-op.
+TEST_F(TableIndirectWS, EmptyGroup) {
+  MatchErrorCode rc;
+  grp_hdl_t grp;
+  entry_handle_t lookup_handle;
+  bool hit;
+
+  auto pkt = get_pkt(64);
+
+  rc = action_profile.create_group(&grp);
+  EXPECT_EQ(MatchErrorCode::SUCCESS, rc);
+
+  rc = table->set_default_group(grp);
+  EXPECT_EQ(rc, MatchErrorCode::SUCCESS);
+
+  table->lookup(pkt, &hit, &lookup_handle);
+}
+
 TEST_F(TableIndirectWS, CustomGroupSelection) {
   using GroupSelectionIface = ActionProfile::GroupSelectionIface;
   using hash_t = ActionProfile::hash_t;