🧑🏼‍💻 [feat/0.8] Rules: Fallback rule implementation (aka rules that will run only if no other rule was executed)

harp_apps/rules/examples/teapot.yml

@@ -5,3 +5,9 @@ rules:
       on_request: |
         from harp.http import HttpResponse
         response = HttpResponse("I'm a teapot.", status=418)
+  "__default__":
+    "GET /*":
+      # language=python
+      on_request: |
+        from harp.http import HttpResponse
+        response = HttpResponse("I'm the fallback for acme.", status=418)

harp_apps/rules/models/rulesets.py

@@ -4,15 +4,37 @@
 
 from harp_apps.rules.constants import DEFAULT_LEVELS, DEFAULT_RULES_LEVELS
 from harp_apps.rules.models.compilers import BaseRuleSetCompiler
+from harp_apps.rules.models.patterns import Pattern
+
+
+DEFAULT_RULE_MATCHER = Pattern("__default__")
+
+
+class Itemize:
+    def __init__(self, seq) -> None:
+        self._seq = seq
+
+    def __iter__(self):
+        try:
+            return iter(self._seq.items())
+        except AttributeError:
+            return iter(self._seq)
 
 
 def _match_level(rules, against):
+    default = None
+    has_match = False
     for pattern, rules in rules:
+        if pattern == DEFAULT_RULE_MATCHER:
+            default = rules
+            continue
+
         if pattern.match(against):
-            if hasattr(rules, "items"):
-                yield from rules.items()
-            else:
-                yield from rules
+            has_match = True
+            return Itemize(rules)
+
+    if not has_match and default:
+        return Itemize(default)
 
 
 def _rules_as_human_dict(rules: dict, *, show_scripts=True):
@@ -62,6 +84,9 @@ def match(self, *args):
         """
         Match the given arguments against the rules. Each argument must match a "level" in this ruleset.
         """
+        if not self.rules:
+            return
+
         if len(args) != len(self._levels):
             raise ValueError(f"Expected {len(self._levels)} arguments, got {len(args)}")
 

harp_apps/rules/tests/examples/test_teapot.py

@@ -19,12 +19,24 @@ def get_rules(self):
         settings = RulesSettings(**self.source)
         return settings.ruleset
 
+    def test_on_proxy_no_match_gets_default(self):
+        rules = self.get_rules()
+        event = self.create_proxy_filter_event("proxy.filter.request", endpoint="xxx*", request=self.create_request())
+
+        scripts = list(rules.match(*event.criteria))
+        assert len(scripts) == 1
+        assert "I'm the fallback for acme." in scripts[0].source
+
+        event.execute_script(scripts[0])
+        assert event.response.status == 418
+
     def test_on_proxy_filter_request(self):
         rules = self.get_rules()
         event = self.create_proxy_filter_event("proxy.filter.request", endpoint="acme1", request=self.create_request())
 
         scripts = list(rules.match(*event.criteria))
         assert len(scripts) == 1
+        assert "I'm a teapot." in scripts[0].source
 
         event.execute_script(scripts[0])
         assert event.response.status == 418