Reset write-source only when necessary

When I/O port-counts do not change, plugin re-order happens
in sync in the process-thread. ::configure_io() is only called
to ensure that the current configuration is valid.

In case that the ChanCount does not change, the method must
be realtime-safe and not block.

DiskWriter::reset_write_sources() is not realtime-safe and
implicitly causes a session-save:

Write-sources are destroyed and re-created. This includes
a call to write_source->drop_references(), which triggers
ARDOUR::Session::remove_source(), which saves the session.

Furthermore adding/removing plugins will likewise call
::configure_io().

Previously any processor change on a track lead to
saving the session!

1 files changed, 15 insertions, 1 deletions

diff --git a/libs/ardour/disk_writer.cc b/libs/ardour/disk_writer.cc
index d93f8680bf..d9bf2fac1e 100644
--- a/libs/ardour/disk_writer.cc
+++ b/libs/ardour/disk_writer.cc
@@ -1497,11 +1497,25 @@ DiskWriter::steal_write_source_name ()
 bool
 DiskWriter::configure_io (ChanCount in, ChanCount out)
 {
+	bool changed = false;
+	{
+		boost::shared_ptr<ChannelList> c = channels.reader();
+		if (in.n_audio() != c->size()) {
+			changed = true;
+		}
+		if ((0 == in.n_midi ()) != (0 == _midi_buf)) {
+			changed = true;
+		}
+	}
+
+
 	if (!DiskIOProcessor::configure_io (in, out)) {
 		return false;
 	}
 
-	reset_write_sources (false, true);
+	if (record_enabled() || changed) {
+		reset_write_sources (false, true);
+	}
 
 	return true;
 }