From 92bcddb43784a372df2acae5d0acb3b3f7919fb1 Mon Sep 17 00:00:00 2001
From: Robin Gareus <robin@gareus.org>
Date: Sat, 16 Aug 2025 21:10:17 +0200
Subject: [PATCH] Emit Signal when MCSS Threads are exhausted

By default Windows has limit of 32 MMCSS threads.
Ardour uses MMCSS scheduling for for hardware I/O (audio and MIDI)
as well as for realtime process threads, and a user on a
recent machine may run into this limit.
---
 libs/pbd/pbd/windows_mmcss.h | 3 +++
 libs/pbd/windows_mmcss.cc    | 4 ++++
 2 files changed, 7 insertions(+)

diff --git a/libs/pbd/pbd/windows_mmcss.h b/libs/pbd/pbd/windows_mmcss.h
index 6c61edfbe6..7624bc5022 100644
--- a/libs/pbd/pbd/windows_mmcss.h
+++ b/libs/pbd/pbd/windows_mmcss.h
@@ -24,6 +24,7 @@
 
 #include <string>
 
+#include "pbd/signals.h"
 #include "pbd/libpbd_visibility.h"
 
 namespace PBD {
@@ -55,6 +56,8 @@ bool LIBPBD_API revert_thread_characteristics (HANDLE task_handle);
 
 bool LIBPBD_API set_thread_priority (HANDLE, AVRT_PRIORITY);
 
+extern Signal<void()> MMCSSError;
+
 } // namespace MMCSS
 
 } // namespace PBD
diff --git a/libs/pbd/windows_mmcss.cc b/libs/pbd/windows_mmcss.cc
index 302d7ed805..396f9e5f2b 100644
--- a/libs/pbd/windows_mmcss.cc
+++ b/libs/pbd/windows_mmcss.cc
@@ -41,6 +41,8 @@ namespace PBD {
 
 namespace MMCSS {
 
+Signal<void()> MMCSSError;
+
 bool
 initialize ()
 {
@@ -133,6 +135,8 @@ set_thread_characteristics (const std::string& task_name, HANDLE* task_handle)
 			DEBUG_THREADS("MMCSS: Privilege not held\n");
 			break;
 		default:
+			/* this is likely because the maximum number of MMCSS threads has been reached */
+			MMCSSError (); /* EMIT SIGNAL */
 			DEBUG_THREADS("MMCSS: Unknown error setting thread characteristics\n");
 			break;
 		}