make status rule handling fall back on sensible defaults if no match

Author: yasunariw

documentation/config_docs.md

@@ -136,7 +136,11 @@ Monorobot supports additional behavior for GitHub status notifications, which ar
 
 The following takes place when a status notification is received.
 
-1. Check whether a status notification should be *allowed* for further processing or *ignored*, according to a list of **status rules**. The bot will check the list *in order*, and use the policy defined by the first rule that the notification satisfies. If no rule matches, the default behavior is to allow the notification.
+1. Check whether a status notification should be *allowed* for further processing or *ignored*, according to a list of **status rules**. The bot will check the list *in order*, and use the policy defined by the first rule that the notification satisfies. If no rule matches, the default behavior of the status state is used:
+   - `pending`: `ignore`
+   - `failure`: `allow`
+   - `error`: `allow`
+   - `success`: `allow_once`
 1. For those payloads allowed by step 1, if it isn't a main branch build notification, route to the default channel to reduce spam in topic channels. Otherwise, check the notification commit's files according to the prefix rules.
 
 Internally, the bot keeps track of the status of the last allowed payload, for a given pipeline and branch. This information is used to evaluate the status rules (see below).

lib/rule.atd

@@ -30,7 +30,14 @@ type status_policy = [
 type build_status <ocaml from="Github" t="status_state"> = abstract
 
 (* A status matches a status rule with the policy with the build status is in
-   the list, and the condition is true *)
+   the list, and the condition is true.
+
+   The default behavior for each status state is:
+   - pending: ignore
+   - failure: allow
+   - error: allow
+   - success: allow_once
+*)
 type status_rule = {
   trigger <json name="on"> : build_status list;
   ?condition <json name="when"> : status_condition nullable;

lib/rule.ml

@@ -2,9 +2,17 @@ open Base
 open Rule_t
 
 module Status = struct
+  let default_rules =
+    [
+      { trigger = [ Pending ]; condition = None; policy = Ignore };
+      { trigger = [ Failure; Error ]; condition = None; policy = Allow };
+      { trigger = [ Success ]; condition = None; policy = Allow_once };
+    ]
+
   (** `match_rules n rs` returns the policy declared by the first rule in `rs`
-      to match status notification `n`, if one exists. A rule `r` matches `n`
-      if `n.state` is in `r.trigger` and `n` meets `r.condition`. *)
+      to match status notification `n` if one exists, falling back to
+      the default rules otherwise. A rule `r` matches `n` if `n.state` is in
+      `r.trigger` and `n` meets `r.condition`. *)
   let match_rules (notification : Github_t.status_notification) ~rules =
     let match_rule rule =
       let value_of_field = function
@@ -27,7 +35,7 @@ module Status = struct
       then Some rule.policy
       else None
     in
-    List.find_map rules ~f:match_rule
+    List.find_map (List.append rules default_rules) ~f:match_rule
 end
 
 module Prefix = struct

test/notabot.json

@@ -15,10 +15,7 @@
                     }
                 },
                 "policy": "ignore"
-            },
-            { "on": ["pending"], "policy": "ignore"},
-            { "on": ["failure", "error"], "policy": "allow"},
-            { "on": ["success"], "policy": "allow_once"}
+            }
         ]
     },
     "prefix_rules": {