Kernel: Move TTY-related code to a new subdirectory under Devices

The TTY subsystem is represented with unix devices, so it should be
under the Devices directory like the Audio, Storage, GPU and HID
subsystems.

Kernel/Devices/TTY/PTYMultiplexer.cpp

@@ -0,0 +1,65 @@
+/*
+ * Copyright (c) 2018-2020, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <AK/Singleton.h>
+#include <Kernel/API/POSIX/errno.h>
+#include <Kernel/Debug.h>
+#include <Kernel/Devices/TTY/MasterPTY.h>
+#include <Kernel/Devices/TTY/PTYMultiplexer.h>
+#include <Kernel/FileSystem/OpenFileDescription.h>
+#include <Kernel/Sections.h>
+
+namespace Kernel {
+
+static Singleton<PTYMultiplexer> s_the;
+
+PTYMultiplexer& PTYMultiplexer::the()
+{
+    return *s_the;
+}
+
+UNMAP_AFTER_INIT PTYMultiplexer::PTYMultiplexer()
+    : CharacterDevice(5, 2)
+{
+    m_freelist.with([&](auto& freelist) {
+        freelist.ensure_capacity(max_pty_pairs);
+        for (int i = max_pty_pairs; i > 0; --i)
+            freelist.unchecked_append(i - 1);
+    });
+}
+
+UNMAP_AFTER_INIT PTYMultiplexer::~PTYMultiplexer() = default;
+
+UNMAP_AFTER_INIT void PTYMultiplexer::initialize()
+{
+    MUST(the().after_inserting());
+}
+
+ErrorOr<NonnullRefPtr<OpenFileDescription>> PTYMultiplexer::open(int options)
+{
+    return m_freelist.with([&](auto& freelist) -> ErrorOr<NonnullRefPtr<OpenFileDescription>> {
+        if (freelist.is_empty())
+            return EBUSY;
+
+        auto master_index = freelist.take_last();
+        auto master = TRY(MasterPTY::try_create(master_index));
+        dbgln_if(PTMX_DEBUG, "PTYMultiplexer::open: Vending master {}", master->index());
+        auto description = TRY(OpenFileDescription::try_create(*master));
+        description->set_rw_mode(options);
+        description->set_file_flags(options);
+        return description;
+    });
+}
+
+void PTYMultiplexer::notify_master_destroyed(Badge<MasterPTY>, unsigned index)
+{
+    m_freelist.with([&](auto& freelist) {
+        freelist.append(index);
+        dbgln_if(PTMX_DEBUG, "PTYMultiplexer: {} added to freelist", index);
+    });
+}
+
+}