This issue with GN appeared while working on Android-related changes to the Chromium build rules. It can be reproduced with the following simple CL: https://chromium-review.googlesource.com/c/chromium/src/+/1131740

Essentially, the 'testonly' variable doesn't seem to be propagated correctly through template calls that wraps an action. As an example, first consider the following:

  template("my_template") {
    forward_variables_from(invoker, ["testonly"])
    ...
    action(first_target) {
       # This inherits the 'testonly' from the upper scope
       ...
    }
    action(second_target) {
       # This inherits 'testonly' from the upper scope too.
       ...
    }
  }

Here's things occur as designed, the value of 'testonly' that was forwarded into the |my_template| scope is propagated to the actions.

Now, let's write a template that wraps an action without doing anything else:

  template("action_wrapper") {
    action(target_name) {
      forward_variables_from(invoker, "*")
    }
  }

My understanding of the GN language and rules is that calling action() or action_wrapper() should be equivalent for 'testonly'. But that's not the case:

  template("my_template") {
    forward_variables_from(invoker, ["testonly"])
    ...
    action(first_target) {
       # This inherits the 'testonly' from the upper scope
       ...
    }
    action_wrapper(second_target) {
       # This DOES NOT inherit 'testonly' from the upper scope.
       ...
    }
  }

In the code above, |second_target| will never have the testonly flag set to true, and any target that depends on it, that is testonly, will actually generate an error at GN time.

This doesn't happen with a simple action() call.

I've tried to look at the GN sources, the FillTestonly() method in tools/gen/target_generator.cc sets the 'testonly' flag associated with each target by calling Scope::GetValue(), which performs a recursive lookup (i.e. it is supposed to find the 'testonly' value defined in the |my_template| scope).

We can work around by forwarding testonly explicitly, either with:

  action_wrapper(second_target) {
    testonly = defined(testonly) && testonly
    ...
  }

Or:

  action_wrapper(second_target) {
    if (defined(testonly)) {
      testonly = testonly
    }
    ...
  }

Also, note that the problem only arises when testonly is first forwarded in the outer template's scope, i.e. the following works correctly:

  template("my_template") {
    ...
    action(first_target) {
      # Inherit 'testonly' from the calling scope.
      forward_variables_from(invoker, ["testonly"])
       ...
    }
    action_wrapper(second_target) {
       # This DOES inherit 'testonly' from the calling scope.
      forward_variables_from(invoker, ["testonly"])
       ...
    }
  }