Lighttpd

[core] graceful restart with SIGUSR1 (fixes #2785)

more consistent cleanup of resources at shutdown
(e.g. upon error conditions)

Notes: graceful restart with SIGUSR1
- not available if chroot()ed, oneshot mode, or if idle timeout occurs
- preserve process id (pid)
- preserve existing listen sockets
- i.e. does not close old listen sockets from prior configs
(even if old listen sockets no longer in the new config)
(sockets may have been bound w/ root privileges no longer available)
- will fail to add listen sockets from new config if privileges
lighttpd configured to drop privileges to non-root user, and
new listen socket attempts to bind to low-numbered port requiring
root privileges.
- will fail if listen sockets in new config conflict with any previous
old listen sockets
- These failure modes will result in lighttpd shutting down instead of
graceful restart. These failure modes are not detectable with
preflight checks ('lighttpd tt -f lighttpd.conf') because the
new instance of lighttpd running the preflight check does not
known config state of n prior graceful restarts, or even the
config state of the currently running lighttpd server.
due to lighttpd feature of optionally managing backends
(e.g. fastcgi and scgi via "bin-path"), lighttpd must wait for
all child processes to exit prior to restarting. Restarting new
workers while old workers (and old backends) were still running would
result in failure of restarted lighttpd process to be able to bind to
sockets already in use by old backends (e.g. unix "socket" path)

x-ref:
"graceful restart with SIGUSR1"
https://redmine.lighttpd.net/issues/2785