In the datalad test suite, a test involving one of our special remotes hangs after 7245a9ed5 (Improve shutdown process for external special remotes and external backends, 2020-11-02). The hang depends on the remote program creating an SSH socket but not cleaning it up on failure. That's being addressed on our end (

While that's clearly bad handling on the part of datalad's special remote, I'm opening this bug report in case the above stall indicates that 7245a9ed5 brings an increased susceptibility to hanging that's worth worrying about. Based on 7245a9ed5's commit message, it sounds like that resolved some hanging on Windows (and generally sounds like a good change), so it probably makes sense keep that at the expense of hanging that is restricted to buggy special remotes. But I don't have a good enough understanding of the issue to know if it's limited to misbehaving special remotes, or if perhaps there's a way to avoid both hangs.

In case it's helpful, here's a demo that tweaks the example special remote to trigger a hang:

set -eu

ga_srcdir="$(realpath "${1?requires git-annex source directory as first argument}")"
host="${2?requires ssh host as second argument}"
cd "$(mktemp -d "${TMPDIR:-/tmp}"/gx-testremote-XXXXXXX)"

mkdir bin
git -C "$ga_srcdir" cat-file -p cfae74ae25a791a8dbf8b476fe17d5c6feacd8b0 \

# Open a socket in setupcreds().
cmd="ssh -fN -o ControlMaster=auto -o ControlPersist=15m -o ControlPath=$top/socket $host"
sed -i "s|setupcreds () {|&\\n\\t$cmd|" bin/git-annex-remote-foo

chmod +x bin/git-annex-remote-foo
export PATH="$top/bin:$PATH"

git annex version
git init a
    cd a
    git annex init main
    # This will trigger an INITREMOTE-FAILURE failure because
    # MYPASSWORD and MYLOGIN are not set.  This hangs after 7245a9ed5.
    git annex initremote d --debug \
        type=external externaltype=foo directory="$top/d" encryption=none
$ sh /tmp/ ~/src/haskell/git-annex smaug
git-annex version: 8.20201103
build flags: Assistant Pairing Inotify MagicMime Feeds Testsuite S3 WebDAV
dependency versions: aws-0.20 bloomfilter- cryptonite-0.25 DAV-1.3.4 feed- ghc-8.6.5 http-client-0.6.4 persistent-sqlite- uuid-1.3.13
[... 40 lines ...]
[2020-11-12 11:40:01.886013862] /tmp/gx-testremote-3MudMuQ/bin/git-annex-remote-foo[1] --> SETCONFIG directory /tmp/gx-testremote-3MudMuQ/d
[2020-11-12 11:40:02.451619481] /tmp/gx-testremote-3MudMuQ/bin/git-annex-remote-foo[1] --> INITREMOTE-FAILURE You need to set MYPASSWORD and MYLOGIN environment variables when running initremote.