Improve from search prefix error handling

app/controllers/api/v2/search_controller.rb

@@ -11,6 +11,16 @@ class Api::V2::SearchController < Api::BaseController
   def index
     @search = Search.new(search_results)
     render json: @search, serializer: REST::SearchSerializer
+
+  # TODO: in the front end, these will show a toast that is only barely helpful
+  # TODO: semantics?
+
+  # user searched with a prefix that does exist
+  rescue Mastodon::SyntaxError
+    unprocessable_entity
+  # user searched for posts from an account the instance is not aware of
+  rescue Mastodon::NotFound
+    not_found
   end
 
   private

app/lib/search_query_transformer.rb

@@ -88,14 +88,16 @@ def initialize(prefix, term)
       case prefix
       when 'from'
         @filter = :account_id
-        username, domain = term.split('@')
-        account = Account.find_remote(username, domain)
 
-        raise "Account not found: #{term}" unless account
+        username, domain = Account.validate_account_string(term)
+        raise Mastodon::SyntaxError, "Account string invalid: #{term}" if username.nil?
+
+        account = Account.find_local_or_remote(username, domain)
+        raise Mastodon::NotFound, "Account not found: #{term}" if account.nil?
 
         @term = account.id
       else
-        raise "Unknown prefix: #{prefix}"
+        raise Mastodon::SyntaxError, "Unknown prefix: #{prefix}"
       end
     end
   end

app/models/concerns/account_finder_concern.rb

@@ -25,8 +25,21 @@ def find_local(username)
     def find_remote(username, domain)
       AccountFinder.new(username, domain).account
     end
+
+    # sigh
+    def find_local_or_remote(username, domain)
+      TagManager.instance.local_domain?(domain) ? find_local(username) : find_remote(username, domain)
+    end
+
+    def validate_account_string(account_string)
+      match = ACCOUNT_STRING_RE.match(account_string)
+
+      [match[:username], match[:domain]] if match
+    end
   end
 
+  ACCOUNT_STRING_RE = /^@?(?<username>#{Account::USERNAME_RE})(?:@(?<domain>[[:word:]\.\-]+))?$/i
+
   class AccountFinder
     attr_reader :username, :domain
 

lib/exceptions.rb

@@ -10,6 +10,8 @@ class DimensionsValidationError < ValidationError; end
   class StreamValidationError < ValidationError; end
   class RaceConditionError < Error; end
   class RateLimitExceededError < Error; end
+  class NotFound < Error; end
+  class SyntaxError < Error; end
 
   class UnexpectedResponseError < Error
     attr_reader :response