Prevent Cylc 7 from running suite.rc workflow that was previously run with Cylc 8

[MetRonnie]

These changes close #4835.

Fix bug where Cylc 7 would happily run a suite.rc workflow that was previously run with Cylc 8. (This bug came after we stopped doing the flow.cylc -> suite.rc symlink)

Do not run if:

• flow.cylc detected in run dir
• _cylc-install dir detected in run dir or 1 above
• Cylc 8 workflow DB detected

This PR does not do anything about:

• cylc register or the auto-register that happens on start. All this does is create a .service/source symlink, which is relatively harmless
• Creation of auth files on start. This is also relatively harmless.

Testing

Pytest

Functional tests I have run (cylc test-battery):

• tests/cylc8
• tests/database
• tests/registration
• tests/restart
• tests/startup

Manual tests to run (from #4320):

First create a suite.rc workflow that is compatible with both Cylc 7 and 8

Run in Cylc 8:

# cylc8 conda env
$ cylc play myflow -n --pause

Then open a new terminal for Cylc 7:

1. cylc hold myflow
2. cylc scan --debug
3. cylc run myflow
4. cylc restart myflow
5. cylc stop myflow
6. cylc get-suite-contact myflow
7. cylc get-suite-version

Now stop the workflow:

# cylc8 conda env
$ cylc stop myflow

Then run those same Cylc 7 commands again.

Requirements check-list

• I have read CONTRIBUTING.md and added my name as a Code Contributor.
• Contains logically grouped changes (else tidy your branch by rebase).
• Does not contain off-topic changes (use other PRs for other changes).
• Applied any dependency changes to both setup.cfg and conda-environment.yml.
• Appropriate tests are included
• Appropriate change log entry included.
• No docs update needed

[wxtim]

• read.
• Manually tested.

[oliver-sanders]

LGTM.

One thing I noticed is that Cylc 7 let me run a workflow which had been "installed" by Cylc 8.

Perhaps this should also be blocked, could/should we add a check for the existence of _cylc-install, ../_cylc-install?

[MetRonnie]

What if a user is trying to make use of the back-compat interoperability even though they have installed from a newfangled source dir?

[oliver-sanders]

The back compat is with the workflow definition not with the install process.

The cylc register and rose suite-run commands both worked in very different ways to cylc install. Running a workflow that was installed by Cylc 8 under Cylc 7 could be dangerous, especially for Rose workflows as the Rose config will be completely ignored by Rose 2019 (it was only loaded during installation) and remote tasks will fail or pick up an older installation (Rose performed remote installation during workflow installation).

[hjoliver]

Looks like this is good to go now 🌮