#10883 adding const error string DismissPostError to be returned by MessageWillBePosted (#10921)

Adds an exported const string that can be used by plugins to signal a client that the incoming post should be dismissed, instead of the current behavior which just leaves it pending.

plugin/hooks.go

@@ -36,6 +36,11 @@ const (
 	TotalHooksId            = iota
 )
 
+const (
+	// DismissPostError dismisses a pending post when the error is returned from MessageWillBePosted.
+	DismissPostError = "plugin.message_will_be_posted.dismiss_post"
+)
+
 // Hooks describes the methods a plugin may implement to automatically receive the corresponding
 // event.
 //
@@ -90,6 +95,7 @@ type Hooks interface {
 	// To reject a post, return an non-empty string describing why the post was rejected.
 	// To modify the post, return the replacement, non-nil *model.Post and an empty string.
 	// To allow the post without modification, return a nil *model.Post and an empty string.
+	// To dismiss the post, return a nil *model.Post and the const DismissPostError string.
 	//
 	// If you don't need to modify or reject posts, use MessageHasBeenPosted instead.
 	//