Set realtime priorities if available

This significantly reduces the latency with no real drawbacks from what
I've noticed. Wineserver is still run using the normal scheduling
policies because from my testing running that with realtime priority
that can actually increase latencies, although doing so will greatly
reduce the variance in processing time.

src/common/utils.cpp

@@ -0,0 +1,24 @@
+// yabridge: a Wine VST bridge
+// Copyright (C) 2020  Robbert van der Helm
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+#include "utils.h"
+
+#include <sched.h>
+
+bool set_realtime_priority() {
+    sched_param params{.sched_priority = 10};
+    return sched_setscheduler(0, SCHED_FIFO, &params) == 0;
+}

src/common/utils.h

@@ -0,0 +1,27 @@
+// yabridge: a Wine VST bridge
+// Copyright (C) 2020  Robbert van der Helm
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+#pragma once
+
+/**
+ * Set the scheduling policy to `SCHED_FIFO` with priority 10 for this process.
+ * We explicitly don't do this for wineserver itself since from my testing that
+ * can actually increase latencies.
+ *
+ * @return Whether the operation was successful or not. This will fail if the
+ *   user does not have the privileges to set realtime priorities.
+ */
+bool set_realtime_priority();