rule: use main_branch_name for global filtering

Author: yasunariw

documentation/config_docs.md

@@ -29,7 +29,7 @@ Refer [here](https://docs.github.com/en/free-pro-team@latest/developers/webhooks
 
 | value | description | optional | default |
 |-|-|-|-|
-| `main_branch_name` | main branch used for the repo; filtering notifications about merges of main into other branches | Yes | - |
+| `main_branch_name` | main branch used for the repo; filtering notifications about merges of main into other branches, and constraining prefix rule application | Yes | - |
 | `label_rules` | label rules config object | No | - |
 | `prefix_rules` | prefix rules config object | No | - |
 | `status_rules` | status rules config object | No | - |
@@ -97,7 +97,7 @@ A **label rule** specifies whether or not a Slack channel should be notified, ba
 ```json
 "prefix_rules": {
     "default_channel": "default",
-    "default_branch_filters": ["develop"],
+    "filter_main_branch": true,
     "rules": [
         {
             "match": [
@@ -122,7 +122,7 @@ A **label rule** specifies whether or not a Slack channel should be notified, ba
 | value | description | if absent |
 |-|-|-|
 | `default_channel` | same behavior as label rule `default_channel` |  |
-| `default_branch_filters` | fallback branch filters (see below) if none are declared locally for a rule | don't apply branch filtering and show `distinct` commits only |
+| `filter_main_branch` | if true and `main_branch_name` is declared, use main branch to filter rules that have no local filter; otherwise, don't apply branch filtering and show `distinct` commits only | false |
 | `rules` | list of `prefix_rule` objects | required field |
 
 ### Prefix Rule
@@ -131,14 +131,14 @@ A **prefix rule** specifies whether or not a Slack channel should be notified, b
 
 Default behavior is to apply each rule regardless of what branch is pushed, and when a rule is matched, show its `distinct` commits only.
 Branch filters limit rule application to selected branches, and shows _all_ commits on match.
-The filters can be declared globally with `default_branch_filters` (see above), or locally per rule with `branch_filters`, where the latter takes precedence.
+The filters can be declared globally with `filter_main_branch` (see above), or locally per rule with `branch_filters`, where the latter takes precedence.
 To ignore a globally declared filter for a single rule, declare one locally with an empty list, as shown in the example above.
 
 | value | description | if absent |
 |-|-|-|
 | `match` | if commit files have any prefix in this list, they should be routed to the channel | all prefixes matched |
 | `ignore` | if commit files have any prefix in this list, they shouldn't be routed to the channel (even if they have any `match` prefixes) | fall back on `match` field behavior |
-| `branch_filters` | consider commits only if pushed ref branch is in this list | fall back on `default_branch_filters` field behavior (see above) |
+| `branch_filters` | consider commits only if pushed ref branch is in this list | fall back on `filter_main_branch` field behavior (see above) |
 | `channel` | channel to notify if the rule is matched | required field |
 
 ## Status Options

lib/action.ml

@@ -16,8 +16,8 @@ module Action (Github_api : Api.Github) (Slack_api : Api.Slack) = struct
     let default = Option.to_list cfg.prefix_rules.default_channel in
     let rules = cfg.prefix_rules.rules in
     let branch = Github.commits_branch_of_ref n.ref in
-    let default_branch_filters = cfg.prefix_rules.default_branch_filters in
-    let filter_by_branch = Rule.Prefix.filter_by_branch ~branch ~default_branch_filters in
+    let main_branch = if cfg.prefix_rules.filter_main_branch then cfg.main_branch_name else None in
+    let filter_by_branch = Rule.Prefix.filter_by_branch ~branch ~main_branch in
     n.commits
     |> List.filter ~f:(fun c ->
          let skip = Github.is_main_merge_message ~msg:c.message ~branch cfg in

lib/config.atd

@@ -12,7 +12,7 @@ type status_rules = {
   files they are related to. *)
 type prefix_rules = {
   ?default_channel: string nullable; (* if none of the rules is matching *)
-  ~default_branch_filters <ocaml default="[]">: string list;
+  ~filter_main_branch <ocaml default="false">: bool;
   rules: prefix_rule list;
 }
 

lib/rule.ml

@@ -40,16 +40,16 @@ end
 
 module Prefix = struct
   (** Filters prefix rules based on branch filtering config and current commit.
-      Prioritizes local filters over default ones. Only allows distinct commits
+      Prioritizes local filters over main branch one. Only allows distinct commits
       if no filter is matched. *)
-  let filter_by_branch ~branch ~default_branch_filters ~distinct rule =
+  let filter_by_branch ~branch ~main_branch ~distinct rule =
     match rule.branch_filters with
     | Some (_ :: _ as filters) -> List.mem filters branch ~equal:String.equal
     | Some [] -> distinct
     | None ->
-    match default_branch_filters with
-    | _ :: _ as filters -> List.mem filters branch ~equal:String.equal
-    | [] -> distinct
+    match main_branch with
+    | Some main_branch -> String.equal main_branch branch
+    | None -> distinct
 
   (** `match_rules f rs` returns the channel name of a rule in `rs` that matches
       file name `f` with the longest prefix, if one exists. A rule `r` matches