Prevent recursion when invoking AI

This can result in stack overflow errors for missing commands.

https://fizzy.37signals.com/5986089/collections/7/cards/1135

Follow up to https://github.com/basecamp/fizzy/pull/766

app/models/command/ai/parser.rb

@@ -34,7 +34,9 @@ class Command::Ai::Parser
     end
 
     def commands_from_query(normalized_query, context)
-      parser = Command::Parser.new(context)
+      # The query should only contain supported /commands. If that's not the case,
+      # we don't want to fall back to AI again (potential stack overflow).
+      parser = Command::Parser.new(context, fall_back_to_ai: false)
       if command_lines = normalized_query[:commands].presence
         command_lines.collect { parser.parse(it) }
       end

app/models/command/parser.rb

@@ -3,8 +3,9 @@ class Command::Parser
 
   delegate :user, :cards, :filter, :script_name, to: :context
 
-  def initialize(context)
+  def initialize(context, fall_back_to_ai: true)
     @context = context
+    @fall_back_to_ai = fall_back_to_ai
   end
 
   def parse(string)
@@ -17,6 +18,10 @@ class Command::Parser
   end
 
   private
+    def fall_back_to_ai?
+      @fall_back_to_ai
+    end
+
     def as_plain_text(string)
       ActionText::Content.new(string).to_plain_text
     end
@@ -100,7 +105,7 @@ class Command::Parser
       elsif card = single_card_from(string)
         Command::GoToCard.new(card_id: card.id)
       else
-        Command::Ai::Parser.new(context).parse(string)
+        parse_with_fallback(string)
       end
     end
 
@@ -114,4 +119,12 @@ class Command::Parser
     def single_card_from(string)
       user.accessible_cards.find_by_id(string)
     end
+
+    def parse_with_fallback(string)
+      if fall_back_to_ai?
+        Command::Ai::Parser.new(context).parse(string)
+      else
+        Command::Search.new(terms: string)
+      end
+    end
 end

test/models/command/ai/parser_test.rb

@@ -25,4 +25,15 @@ class Command::Ai::ParserTest < ActionDispatch::IntegrationTest
     assert_equal [ collections(:writebook).id.to_s ], params[:collection_ids]
     assert_equal [ tags(:web).id.to_s ], params[:tag_ids]
   end
+
+  test "if the AI translator emitted an unknown command, this will result in a regular search (not on AI recursion)" do
+    Command::Ai::Translator.any_instance.stubs(:translate).returns(commands: [ "/missing" ])
+
+    command = parse_command "assign @kevin and close"
+    assert command.commands.one?
+
+    search_command = command.commands.first
+    assert search_command.is_a?(Command::Search)
+    assert_equal "/missing", search_command.terms
+  end
 end