Make our own RW engine mutex, so we can do priority inversion

Signed-off-by: falkTX <falktx@falktx.com>

include/mutex.hpp

@@ -0,0 +1,84 @@
+/*
+ * DISTRHO Cardinal Plugin
+ * Copyright (C) 2021-2022 Filipe Coelho <falktx@falktx.com>
+ *
+ * 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 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.
+ *
+ * For a full copy of the GNU General Public License see the LICENSE file.
+ */
+
+#pragma once
+
+#include <pthread.h>
+
+/* replace Rack's mutex with our own custom one, which can do priority inversion. */
+
+namespace rack {
+
+
+struct SharedMutex {
+    pthread_mutex_t readLock, writeLock;
+
+    SharedMutex() noexcept {
+        pthread_mutexattr_t attr;
+
+        pthread_mutexattr_init(&attr);
+        pthread_mutexattr_setprotocol(&attr, PTHREAD_PRIO_INHERIT);
+        pthread_mutexattr_settype(&attr, PTHREAD_MUTEX_RECURSIVE);
+        pthread_mutex_init(&readLock, &attr);
+        pthread_mutexattr_destroy(&attr);
+
+        pthread_mutexattr_init(&attr);
+        pthread_mutexattr_setprotocol(&attr, PTHREAD_PRIO_INHERIT);
+        pthread_mutexattr_settype(&attr, PTHREAD_MUTEX_NORMAL);
+        pthread_mutex_init(&writeLock, &attr);
+        pthread_mutexattr_destroy(&attr);
+    }
+
+    ~SharedMutex() noexcept {
+        pthread_mutex_destroy(&readLock);
+        pthread_mutex_destroy(&writeLock);
+    }
+
+    // for std::lock_guard usage, writers lock
+    void lock() noexcept {
+        pthread_mutex_lock(&writeLock);
+        pthread_mutex_lock(&readLock);
+    }
+    void unlock() noexcept {
+        pthread_mutex_unlock(&readLock);
+        pthread_mutex_unlock(&writeLock);
+    }
+
+    // for SharedLock usage, readers lock
+    void lock_shared() noexcept {
+        pthread_mutex_lock(&readLock);
+    }
+    void unlock_shared() noexcept {
+        pthread_mutex_unlock(&readLock);
+    }
+};
+
+
+template <class Mutex>
+struct SharedLock {
+    Mutex& mutex;
+
+    SharedLock(Mutex& m) noexcept : mutex(m) {
+        mutex.lock_shared();
+    }
+    ~SharedLock() noexcept {
+        mutex.unlock_shared();
+    }
+};
+
+
+} // namespace rack