This fixed a false-positive "ambiguous latency" warning for
MIDI busses:
MIDI track -midi-> MIDI Bus w/instrument -audio|midi-> master
The master-bus only has audio inputs, so the MIDI Bus'
MIDI out is left unconnected. Its latency does not matter,
it can float freely.
Previously it was assumed to be zero. So the MIDI Bus' input
playback latency range was [0, master-bus playback-latency].
jack2 can process in parallel with calling a graph-order or
latency-callback, so Ardour takes a lock to update those in sync.
Yet jack2 can also block while port-registrations happen,
and concurrently emit latency-callback from another thread..
Port and connection changes always imply a latency_callback from
the engine. Worst I/O latency is updated directly in
Session::update_latency() which is called from
AudioEngine::latency_callback.
Explicit subscriptions to route->output()->changed() is not needed
to update the worst I/O latency.
Only set_block_size() needs to to update the I/O latency when
the buffer-size changes.
The backend may call update_latency() while at the same time
the auto-connect-port calls set_worst_io_latencies().
The latter already holds the process-lock, so update_latency()
first needs to acquire it, as well. If one already holds the
_update_latency_lock, one must not ask for the process-lock.
---
Previously Ardour's connection manager first took the process_lock
and then waited to the _update_latency_lock.
Meanwhile jack calls latency_callback(), takes the
_update_latency_lock and waits for the process_lock.
Classic deadlock.
LV2 plugins may add a MIDNAM file when they're loaded.
This can be happen concurrently with the MidiPatchManager,
loading the system-wide patch files.
Session load should not be interrupted by the initial
background scan of patch files, which can take a very long time.
We need to allow update_custom_midnam() to grab the _lock.
Various backends have different strategies for updating latencies,
and different thread semantics for latency update callbacks.
With jack2 it is possible that processing happens concurrently
while port latencies are being changed.
Ardour internal backends emit the latency callback from the
main process thread, serializing latency changes with processing.
Various user actions can also trigger latency changes. e.g.
with a stopped engine it is possible to re-order plugins.
The semantics are that if the user configures things to allow transport commands while
using a transport master, issuing such commands first stops using the master
Auditioner seek calls are explicitly handled by the butler,
and DR:overwrite_existing_buffers() is called explicitly
to read MIDI data for audition.
Session::non_realtime_overwrite is no longer required.
This reverts commit 6fbcf83779
and parts of 2d11667ce3.
The Auditioner is not part of the session route-list and the
auditioner route's I/O latency is never updated.
Session::process_audition() does not handle pre-roll either,
so it need to be zeroed, otherwise Route::roll skips samples.
This has lead to initial samples being skipped, IFF the
auditioner's output-port had non-zero latency.
Since private port-latencies are usually only set for routes
in the route-list, and _remaining_latency_preroll is reset
at transport-stop, this *usually* worked...
Last but not least, MIDI notes need to be resolved when seeking.
IO::latency iterates over the port-set retrieving the
private_latency_range of each port. Since it can only change
when connections and latency changes, we can instead cache the value.
This is also in preparation to allow the auditioner to override it.
DiskReader::seek() no longer fills MIDI buffers.
MIDI is now read into memory via DR:overwrite_existing_buffers()
There is still some edge-case remaining to be fixed. For
some reasons the synth does not receive initial patch/program
changes when starting auditioning.
update_custom_midnam() is usually called from the GUI or butler
thread when a LV2 plugin is instantiated, or the plugin changes
its MIDNAM. It must be exclusive with load_midnams_in_thread()
and calls from MTAV to ::maybe_use().
