Allow append skipped rules (#346)

* Allow appending to skipped rules

If a rule has an append attribute but the original rule was skipped (due
to having lower priority than the configured priority), silently skip
the appending rule instead of returning an error.

* Unit test for appending to skipped rules

Unit test verifies fix for appending to skipped rules. One rules file
defines a rule with priority WARNING, a second rules file appends to
that rules file, and the configured priority is ERROR.

Ensures that falco rules without errors.

test/falco_tests.yaml

@@ -642,6 +642,14 @@ trace_files: !mux
       - rules/rule_append_failure.yaml
     trace_file: trace_files/cat_write.scap
 
+  rule_append_skipped:
+    detect: False
+    priority: ERROR
+    rules_file:
+      - rules/single_rule.yaml
+      - rules/append_single_rule.yaml
+    trace_file: trace_files/cat_write.scap
+
   rule_append:
     detect: True
     detect_level: WARNING

test/rules/append_single_rule.yaml

@@ -0,0 +1,3 @@
+- rule: open_from_cat
+  append: true
+  condition: and fd.name=/tmp

userspace/engine/lua/rule_loader.lua

@@ -132,7 +132,8 @@ end
 -- object. The by_name index is used for things like describing rules,
 -- and the by_idx index is used to map the relational node index back
 -- to a rule.
-local state = {macros={}, lists={}, filter_ast=nil, rules_by_name={}, macros_by_name={}, lists_by_name={},
+local state = {macros={}, lists={}, filter_ast=nil, rules_by_name={},
+	       skipped_rules_by_name={}, macros_by_name={}, lists_by_name={},
 	       n_rules=0, rules_by_idx={}, ordered_rule_names={}, ordered_macro_names={}, ordered_list_names={}}
 
 local function reset_rules(rules_mgr)
@@ -291,10 +292,12 @@ function load_rules(rules_content, rules_mgr, verbose, all_events, extra, replac
 	    end
 
 	    if state.rules_by_name[v['rule']] == nil then
+	       if state.skipped_rules_by_name[v['rule']] == nil then
 		  error ("Rule " ..v['rule'].. " has 'append' key but no rule by that name already exists")
 	       end
-
+	    else
 	       state.rules_by_name[v['rule']]['condition'] = state.rules_by_name[v['rule']]['condition'] .. " " .. v['condition']
+	    end
 
 	 else
 
@@ -320,6 +323,8 @@ function load_rules(rules_content, rules_mgr, verbose, all_events, extra, replac
 	       v['output'] = compiler.trim(v['output'])
 
 	       state.rules_by_name[v['rule']] = v
+	    else
+	       state.skipped_rules_by_name[v['rule']] = v
 	    end
 	 end
       else