BUG/MEDIUM: mux-fcgi: put BE conn back in idle list after deferred shut

BUG/MEDIUM: mux-fcgi: put BE conn back in idle list after deferred shut

When a stream cannot send its FCGI_ABORT record or its end of stream at
detach time because the mux buffer is busy or full, fcgi_detach() leaves
early to keep the stream as an orphan. This skips the idle/avail list
management performed at the end of the function. The orphan is later
destroyed by fcgi_deferred_shut() once the record can be sent, but nothing
makes the connection idle or available again at this point. The timeout is
not refreshed either.

If the orphan was the last stream of the connection, the connection is then
in no idle list, it is not released and it remains accounted in
srv->curr_used_conns. Its timeout is the one armed when the stream was
orphaned, so the connection is closed after "timeout server" instead of
being reused, and it is wasted in the mean time.

To fix the issue, the idle/avail list management is extracted from
fcgi_detach() into a new fcgi_may_idle_conn() function, which is now also
called from fcgi_deferred_shut() after the orphan was destroyed and the
timeout was refreshed, as fcgi_detach() does.

In fcgi_deferred_shut(), the stream's session cannot be used, since it may
have been released in the mean time. The connection owner is used instead,
but only if the connection is still attached to its session, because
session_free() resets conn->owner only in this case. For a private
connection, if there is no session anymore when the last stream leaves, the
connection can no longer be reused and it is destroyed.

The orphans destroyed from fcgi_strm_wake_one_stream() are not covered here.
This mostly happens on connection errors or shutdowns, in which case the
connection is released anyway.

This is the same fix as the one applied on mux-h2 by the commit
"BUG/MEDIUM: mux-h2: put BE conn back in idle list after deferred shut".

This could be backported to all supported versions, after the recent
fcgi_detach() fixes: a167d39a5 ("BUG/MEDIUM: mux-fcgi: arm the connection
timeout only when data remains"), 170a6469c ("BUG/MEDIUM: mux-fcgi: ignore
conn->owner when deciding if a connection is dead"), b77b044d3 ("MINOR:
mux-fcgi: check for a dead connection before refreshing the timeout") and
2acf116bf ("MINOR: mux-fcgi: don't add a closed connection to an idle list
on detach").