Read the derivation from PATH (`/gnu/store/xxx.drv'), and return the store path of its output OUTPUT.

Return #t if DRV is a fixed-output derivation, such as the result of a download with a fixed hash (aka. `fetchurl').

Return true if DRV can be offloaded, false otherwise.

Read the derivation in FILE, a '.drv' file, and return the corresponding <derivation> object.

Return a one-argument procedure that, when passed a store file name, returns a 'substitutable?' if it's substitutable and #f otherwise.

The returned procedure knows about all substitutes for all the derivation inputs or derivations listed in INPUTS-OR-DRV, *except* those that are already valid (that is, it won't bother checking whether an item is substitutable if it's already on disk); it also knows about their prerequisites, unless they are themselves substitutable.

Creating a single oracle (thus making a single 'substitutable-path-info' call) and reusing it is much more efficient than calling 'has-substitutes?' or similar repeatedly, because it avoids the costs associated with launching the substituter many times.

Return the hash of DRV, modulo its fixed-output inputs, as a bytevector.