@tonistiigi this LGTY?

So, that crossed my mind, but "it's complicated"

The current situation is:

• Layer and RWLayer are distinct interfaces
• They do share some properties:
  • both have a TarStreamer interface (embedded)
  • both have Parent()
  • both have Size()
  • both have Metadata()
• So one possible option would be to extract the common properties to a common interface
• However... Layer.Size() and RWLayer.Size() are not equivalents; Layer.Size() is the size of the whole layer chain (Layer + Parents), but RWLayer.Size() is only the size of the RWLayer itself
• So the closest "equivalent" of RWLayer.Size() would be Layer.DiffSize()

Based on the above, at least in the current state of things, I think there's enough differences between the two to change the signature independently

Alternative

So, an alternative could be to;

• Add a Layer.ChainSize() function, which would be the equivalent to the current Layer.Size()
• Change Layer.Size() to do what Layer.DiffSize() does currently (return the (RW)Layer's own size); this function would never return an error in both cases.
• Move Size() and Parent() into a common interface
• TBD: move Metadata() into that interface (but TBD: Metadata() originates from storage-drivers, so perhaps it shoud be an interface there that we can embed)

this function would never return an error in both cases.

Arf, so that's not correct; RWLayer.Size() can return an error, but Layer.Size() won't

Do we gain anything from having an explicitly shared interface?