Please describe the problem.

Originally detected while testing newer (7.20200204+git1-gb2c529b88-1~ndall+1) git-annex-standalone build against DataLad: one of our tests started to hang. Even using a different way of invoking external commands which was recently added to DataLad, it looks like git annex enableremote simply does not return:

[DEBUG  ] Async run ['git-annex', 'enableremote', '--debug', '-c', 'remote.target1.annex-ssh-options=-o ControlMaster=auto -S /home/travis/.cache/datalad/sockets/3d284eaf', 'target2'] 
[DEBUG  ] Launching process ['git-annex', 'enableremote', '--debug', '-c', 'remote.target1.annex-ssh-options=-o ControlMaster=auto -S /home/travis/.cache/datalad/sockets/3d284eaf', 'target2'] 
[DEBUG  ] Process 16621 started 
[DEBUG  ] Waiting for process 16621 to complete 
[Level 5] Read 46 bytes from 16621[stdout]: 
[Level 9] stdout| (merging target2/git-annex into git-annex...)
|   
[Level 5] Read 28 bytes from 16621[stdout]: 
[Level 9] stdout| (recording state in git...)
|   
No output has been received in the last 10m0s, this potentially indicates a stalled build or something wrong with the build itself.

Full log link.

Unfortunately ATM I do not have any further information, besides that we all failed to reproduce it locally and the issue is not happening with older build of git-annex-standalone (e.g. 7.20190819+git2-g908476a9b-1~ndall+1).