LibCore: Add DirectoryWatcher

DirectoryWatcher is a wrapper around watch_file, and can be used to
watch for changes in directories.

Libraries/LibCore/DirectoryWatcher.h

@@ -0,0 +1,61 @@
+/*
+ * Copyright (c) 2020, Itamar S. <itamar8910@gmail.com>
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice, this
+ *    list of conditions and the following disclaimer.
+ *
+ * 2. Redistributions in binary form must reproduce the above copyright notice,
+ *    this list of conditions and the following disclaimer in the documentation
+ *    and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#pragma once
+
+#include <AK/Function.h>
+#include <AK/Noncopyable.h>
+#include <AK/String.h>
+#include <Kernel/API/InodeWatcherEvent.h>
+
+namespace Core {
+
+class DirectoryWatcher {
+    AK_MAKE_NONCOPYABLE(DirectoryWatcher);
+
+public:
+    explicit DirectoryWatcher(const String& path);
+    ~DirectoryWatcher();
+
+    struct Event {
+        enum class Type {
+            ChildAdded,
+            ChildRemoved,
+        };
+        Type type;
+        String child_path;
+    };
+
+    Optional<Event> wait_for_event();
+
+private:
+    String get_child_with_inode_index(unsigned) const;
+
+    String m_path;
+    int m_watcher_fd { -1 };
+};
+
+}