Use dedicated temporary directory for all rsync pulls

[happz]

This should prevent rsync process from creating temporary files in the destination directory tree. In case multiple rsync commands operate on the same directory structure, they may run into race condition:

• rsync A is pull into the tree, creates a temporary file to hold partially transfered file;
• rsync B is pushing the tree elsewhere, notices the file, and intends to push it;
• rsync A completes the transfer, removes the temporary file;
• rsync B sees the file is gone, and reports an error.

Isolating temporary files between rsync processes and from the manipulated tree should prevent this conflict.

The patch comes with new "tmpdir" attribute to provide run-specific temporary directories in a way compatible with tempfile.TemporaryDirectory.

Pull Request Checklist

• implement the feature

[gemini-code-assist]

Code Review

This pull request adds the --temp-dir option to rsync for pulls and local syncs. To fully resolve potential race conditions, also apply this to the push method in tmt/guest/__init__.py. Use the self.mkdtemp() context manager to create a temporary directory on the guest.

_{Note: Security Review did not run due to the size of the PR.}

[LecrisUT]

Interesting. Would be nice if this could be handled more centrally at the TransferOptions.to_rsync, but should be good for now

[thrix]

The fix correctly targets all 3 rsync invocations in the codebase (verified via ag). push() is correctly NOT modified since --temp-dir applies to the receiving side which is remote for push.

One concern: tempfile.TemporaryDirectory() creates in /tmp (tmpfs) while tmt workdirs are under /var/tmp/tmt/ (disk) - different filesystems. This makes rsync fall back to copy-in-place instead of atomic rename, introducing a brief window where destination files could have truncated data. See inline comment for details.

For small files (typical in tmt) this window is negligible and the fix is a clear net improvement over the original race condition.

[thrix]

Cross-filesystem concern addressed in bce2f718. All 3 rsync invocations covered. Clean fix for the race condition.