LibThreading: Add ConditionVariable wrapper

ConditionVariable is a thin wrapper over the pthread_cond_* APIs, just
as Mutex is a wrapper over pthread_mutex.

Because ConditionVariable might want to wait on a high-level Mutex, it
needs to be friends with it.

Userland/Libraries/LibThreading/ConditionVariable.h

@@ -0,0 +1,64 @@
+/*
+ * Copyright (c) 2021, kleines Filmröllchen <malu.bertsch@gmail.com>.
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Function.h>
+#include <LibThreading/Mutex.h>
+#include <pthread.h>
+#include <sys/cdefs.h>
+#include <sys/types.h>
+
+namespace Threading {
+
+// A signaling condition variable that wraps over the pthread_cond_* APIs.
+class ConditionVariable {
+    friend class Mutex;
+
+public:
+    ConditionVariable(Mutex& to_wait_on)
+        : m_to_wait_on(to_wait_on)
+    {
+        auto result = pthread_cond_init(&m_condition, nullptr);
+        VERIFY(result == 0);
+    }
+
+    ALWAYS_INLINE ~ConditionVariable()
+    {
+        auto result = pthread_cond_destroy(&m_condition);
+        VERIFY(result == 0);
+    }
+
+    // As with pthread APIs, the mutex must be locked or undefined behavior ensues.
+    ALWAYS_INLINE void wait()
+    {
+        auto result = pthread_cond_wait(&m_condition, &m_to_wait_on.m_mutex);
+        VERIFY(result == 0);
+    }
+    ALWAYS_INLINE void wait_while(Function<bool()> condition)
+    {
+        while (condition())
+            wait();
+    }
+    // Release at least one of the threads waiting on this variable.
+    ALWAYS_INLINE void signal()
+    {
+        auto result = pthread_cond_signal(&m_condition);
+        VERIFY(result == 0);
+    }
+    // Release all of the threads waiting on this variable.
+    ALWAYS_INLINE void broadcast()
+    {
+        auto result = pthread_cond_broadcast(&m_condition);
+        VERIFY(result == 0);
+    }
+
+private:
+    pthread_cond_t m_condition;
+    Mutex& m_to_wait_on;
+};
+
+}

Userland/Libraries/LibThreading/Mutex.h

@@ -13,6 +13,8 @@
 namespace Threading {
 
 class Mutex {
+    friend class ConditionVariable;
+
 public:
     Mutex()
     {