Bug#866002: apt-get update fails with missing InRelease file on local mirror

[Andreas Heinlein]

Package: apt

For testing and development purposes, I have created an air-gapped 
machine and use a local mirror on hard disk (created with debmirror) for 
installation and updates. The mirror is a full mirror of the stretch, 
stretch-updates and stretch-security archives.
That method used to work fine with jessie, but fails since I upgraded to 
stretch. When I run apt-get update on the air-gapped machine, it always 
complains about a missing InRelease file in the main stretch archive. 
Indeed there is no InRelease on the official mirrors, only Release and 
Release.gpg. The Debian FTP masters told me that this was "on purpose", 
whatever that might be.
I thought that apt was supposed to try Release.gpg instead if InRelease 
is missing, but it doesn't.
Turning on "Debug::Acquire::FTP=true" on the internet connected machine 
using the official mirrors shows that it works there; but the air-gapped 
machine doesn't even try to download Release.gpg but immediately fails.

I don't know if that has anything to do with the local mirror at all, or 
if something got screwed during the update, however I have no idea where 
to look.

Thanks,
Andreas