Ruby: Fix rack response tracking

Use type tracking instead of getReturningNode, which seems to be faster
and works correctly for the cases I've tried.

ruby/ql/lib/codeql/ruby/frameworks/Rack.qll

@@ -9,6 +9,8 @@ private import codeql.ruby.DataFlow
  * Provides modeling for the Rack library.
  */
 module Rack {
+  import codeql.ruby.typetracking.TypeTracker
+
   /**
    * A class that may be a rack application.
    * This is a class that has a `call` method that takes a single argument
@@ -20,7 +22,7 @@ module Rack {
     AppCandidate() {
       call = this.getInstanceMethod("call") and
       call.getNumberOfParameters() = 1 and
-      exists(DataFlow::LocalSourceNode resp | isRackResponse(resp) | resp.flowsTo(call.getReturn()))
+      call.getReturn() = trackRackResponse()
     }
 
     /**
@@ -37,4 +39,15 @@ module Rack {
       exists(DataFlow::LocalSourceNode n | n.asExpr() = arr | n.flowsTo(r))
     )
   }
+
+  private DataFlow::LocalSourceNode trackRackResponse(TypeTracker t) {
+    t.start() and
+    isRackResponse(result)
+    or
+    exists(TypeTracker t2 | result = trackRackResponse(t2).track(t2, t))
+  }
+
+  private DataFlow::Node trackRackResponse() {
+    trackRackResponse(TypeTracker::end()).flowsTo(result)
+  }
 }

ruby/ql/test/library-tests/frameworks/rack/Rack.expected

@@ -1,3 +1,4 @@
 | rack.rb:1:1:5:3 | HelloWorld | rack.rb:2:12:2:14 | env |
 | rack.rb:7:1:16:3 | Proxy | rack.rb:12:12:12:18 | the_env |
 | rack.rb:18:1:31:3 | Logger | rack.rb:24:12:24:14 | env |
+| rack.rb:45:1:61:3 | Baz | rack.rb:46:12:46:14 | env |

ruby/ql/test/library-tests/frameworks/rack/rack.rb

@@ -41,3 +41,21 @@ class Bar
     nil
   end
 end
+
+class Baz
+  def call(env)
+    run(env)
+  end
+
+  def run(env)
+    if env[:foo] == "foo"
+      [200, {}, "foo"]
+    else
+      error
+    end
+  end
+
+  def error
+    [400, {}, "nope"]
+  end
+end