Ludovic Courtès <ludo@gnu.org> writes:

>> The attached patch fixes the issue for me.  However, since I'm not sure
>> how %derivation-cache is or was supposed to be used, I would appreciate
>> a second opinion.
>
> You forgot to attach the patch, but I think it’s enough to remove the
> ‘hash-clear!’ call from ‘call-with-external-store’.

Sorry - but yes, that's all it did.  I removed the hash-clear! call.
I've gone ahead and committed this as
7f0af119a1e3ea9d0ae53811b619437b3e942702 on core-updates.

-- 
Chris