add --no-tty option to gpg

[riedel]

Proposed Commit Message

Make sure the gpg works even if VM provides no /dev/tty (e.g. debian cloud init)

LP:#1813396

Additional Context

fixes https://bugs.launchpad.net/cloud-init/+bug/1813396

Test Steps

on a VM without a terminal mounted as /dev/tty define an external keyserver such used for docker

apt:
  sources:
   docker:
     source: 'deb [arch=amd64] https://download.docker.com/linux/debian stretch stable'
     keyserver: keyserver.ubuntu.com
     keyid: 0EBFCD88

Checklist:

• My code follows the process laid out in the documentation
• I have updated or added any unit tests accordingly
• I have updated or added any documentation accordingly

[OddBloke]

Hi @riedel, thanks for the submission! This is definitely a fix we want.

Do you know what messages go to /dev/tty by default? Are we losing anything on systems with /dev/tty by passing --no-tty?

[riedel]

The tty is normally used for the interactive parts (like prompts). However, here only recv-keys is needed and even only for custom apt repos. From what I can see there are no side effects at all. In https://github.com/gpg/gnupg/blob/6a4443c8425fd548020553b22d5a16ffad98371f/g10/keyserver.c#L1081 I see only logging stuff, which actually need no tty, which is only part of the interactive stuff here: https://github.com/gpg/gnupg/blob/6a4443c8425fd548020553b22d5a16ffad98371f/g10/keyserver.c#L733.

All that verbose logging of gpg still seems to work fine.

>gpg --no-tty --keyserver keyserver.ubuntu.com --recv-keys 0EBFCD88

gpg: requesting key 0EBFCD88 from hkp server keyserver.ubuntu.com
gpg: key 0EBFCD88: public key "Docker Release (CE deb) <docker@docker.com>" imported
gpg: no ultimately trusted keys found
gpg: Total number processed: 1
gpg:               imported: 1  (RSA: 1)

or an error case:

>gpg --no-tty --keyserver keyserver.ubuntu.com --recv-keys 0FBFCD88

gpg: requesting key 0FBFCD88 from hkp server keyserver.ubuntu.com
gpgkeys: key 0FBFCD88 not found on keyserver
gpg: no valid OpenPGP data found.
gpg: Total number processed: 0
gpg: keyserver communications error: key not found
gpg: keyserver communications error: bad public key
gpg: keyserver receive failed: bad public key

don't look any different.

[OddBloke]

Excellent, thank you for the detailed explanation! I'm going to push a commit to your branch to alphabetise .github-cla-signers, then this can land.