How did I fix the «Could not get socket :6081: Address already in use» / «Syntax check failed, not restarting» Varnish 6.4 bug in Debian 10?

Unix Debian
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service varnish stop and service varnish start works correctly, but service varnish restart is not:

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Fixed by github.com/vegandthecity/server/commit/0cd5c0e6
github.com/varnishcache/pkg-varnish-cache/issues/75#issuecomment-316727624