Is the default `initDataTransform` identity function configuration intended?

[mseeley]

👋 there. :)

I'm working on converting our bespoke FairPlay implementation to work within 2.5.5.

While stepping through Shaka's implementation I arrived at this code in drm_engine:

https://github.com/google/shaka-player/blob/c0e7a9b62a57f73ef26d511e3d40d2dd0f014cb6/lib/media/drm_engine.js#L1020-L1029

I read this as, if a client has not specified a playerConfiguration.drm.initDataTransform callback then the default implementation should be used.

But, it turns out Shaka has a default identity function setup in configuration:

https://github.com/google/shaka-player/blob/c0e7a9b62a57f73ef26d511e3d40d2dd0f014cb6/lib/util/player_configuration.js#L85

So, as a client, if I omit playerConfiguration.drm.initDataTransform than the default handling in drm_engine is ignored.

I want to believe the default impl in player_configuration is unintended based on the function | undefined documentation: https://shaka-player-demo.appspot.com/docs/api/shaka.extern.html#.PlayerConfiguration

Thanks for your feedback.

[mseeley]

Providing drm.initDataTransform a value of false or null still results in the default identify function taking precedent.

I've gone ahead and copied the implementation from drm_engine into my client implementation. The identity function default isn't appropriate for FairPlay and it's transform requirement.

[joeyparrish]

The way our configuration system works, a value of undefined means "reset to the default value". A value of false or null fails our type checking system (which is looking for function), so that call would have no effect.

For example:

// Show the original value
player.getConfiguration().drm.initDataTransform
  (initData) => initData

// Change the value
player.configure('drm.initDataTransform', (initData) => 5)
  true

// Show the changed value
player.getConfiguration().drm.initDataTransform
  (initData) => 5

// Configure an invalid type, which is ignored
player.configure('drm.initDataTransform', null)
  Invalid config, wrong type for .drm.initDataTransform
  false

// This retains the previous value
player.getConfiguration().drm.initDataTransform
  (initData) => 5

// Reset to default with undefined
player.configure('drm.initDataTransform', undefined)
  true

// Show the default value has taken effect again
player.getConfiguration().drm.initDataTransform
  (initData) => initData

As far as I can tell, the use of the identity function is an intentional default. If you want to transform it in some other way, just supply the transformation function you need.

I don't understand what you mean when you say you're copying the implementation into your client. The implementation of what, exactly? The identity transform? Something else?

[TheModMaker]

@joeyparrish He is talking about the first code snippet he linked to. Since the default function exists, it is "truthy" so the first branch is always hit, even with a default. This means the second branch never happens and the default FairPlay transform isn't applied. We'll need some way of detecting that the function is a default without checking for "truthy" values since the function always exists under our current model.

[mseeley]

Hi @joeyparrish + @TheModMaker. Yep this was it:

This means the second branch never happens and the default FairPlay transform isn't applied

Thanks a lot for breaking down configure behavior @joeyparrish. That was really thoughtful. 👍

When reusing a player object between playbacks would either approach below reset all drm options to their defaults? Or, would we perhaps have to provide configure an undefined value for each drm key previously configured?

player.configure('drm', undefined)

player:configure({ drm: undefined });

[TheModMaker]

Yes, either of those options should recursively reset all the DRM configurations. There is also a player.resetConfiguration() method you can use to reset all configurations to the default.

[mseeley]

Thank you very much @TheModMaker.