probe

Opens path, reads what it says about itself, and closes it again.

Blocking, exactly like MediaSource.Companion.open, which it is: a probe is an open, a read of already-parsed fields, and a close. Call it off the UI thread, and expect a network URL to cost what opening one costs.

An extension on the companion rather than a member of it: MediaSource is an expect class, so a member would owe an actual in four platform source sets for a body that is the same everywhere and calls only what each of them already has.

Throws

The same over caller-supplied bytes. The source owns io and closes it, so io is spent when this returns, exactly as it would be after an MediaSource.Companion.open and a close.