exp show: unable to run in parallel with status and diff

[mattseddon]

Bug Report

Description

In the vs code extension we use file watchers to trigger commands that provide data for the UI. status, list and diff are run on any change in the workspace. exp show is run whenever there is a change to .git/refs/exps. If these four commands run at the same time then exp show will always fail.

Note: we have excluded .git/refs/exps from the base watcher to work around this issue.

Reproduce

Run status, list, diff and exp show at exactly the same time.

Expected

Read only commands should be able to run concurrently or wait for the repo to be released before running.

Environment information

Output of dvc doctor:

$ dvc doctor
DVC version: 2.3.0 (pip)
---------------------------------
Platform: Python 3.8.7 on macOS-11.3.1-x86_64-i386-64bit
Supports: http, https
Cache types: reflink, hardlink, symlink
Cache directory: apfs on /dev/disk1s5s1
Caches: local
Remotes: https
Workspace directory: apfs on /dev/disk1s5s1
Repo: dvc (subdir), git

Additional Information (if any):

This particular issue is not a show stopper for us as we can work around but the larger issue of not being able to run read only command concurrently is definitely a problem that we need addressed.