fmf discover copies the whole source directory

[imphil]

The fmf discover implementation copies the whole source directory (fmf_root) to the working directory in (in my case) /var/tmp/tmt/run-009/packages/my-package/plans/232-0-0/discover/default-0/. In my case, the source directory contains many gigabytes of build outputs. Copying all this data takes a many minutes on our CI runners (yes, network-attached slow disk). The whole tree is copied, .gitignore is not honored (unlike in other copy steps that are done with rsync).

This is the code in question:

tmt/tmt/steps/discover/fmf.py

Lines 445 to 449 in 2af39b3

	self.info('directory', directory, 'green')
	if not dist_git_source or dist_git_merge:
	self.debug(f"Copy '{directory}' to '{self.testdir}'.")
	if not self.is_dry_run:
	shutil.copytree(directory, self.testdir, symlinks=True)

I tried, but there are too many file copies and conditional code paths going on for me to really see what a good solution could be. I observed that the shell discovery plugin uses symlinks (#953). Changing the code to use symlinks makes things work for me with for local execution, but not when executing inside a container. Do you have a suggestion how to fix this issue? I'm happy to do the implementation.

Steps to reproduce

1. Have a directory with a working tmt setup and many GB of (unrelated) files in it.

2. Run the discover step, e.g. (in my case)

   $ tmt run discover -vvv plan --name /packages/my-package/plans/232.0.0 provision --how local --feeling-safe
   /var/tmp/tmt/run-001
    Found 22 plans.
    /packages/my-package/plans/230.0.0
    summary: Smoke test for my-package
        discover
            how: fmf
            order: 50
            directory: /my/workdir
   
    		LONG WAIT HERE
   
            hash: e7c193a
            filter: tag:targets-my-package
            summary: 1 test selected
                /packages/my-package/tests/smoke
        provision
            queued provision.provision task #1: default-0
   

3. Observe the output to hang after the line starting with directory: until the file copy has completed.