LibCore: Added FileWatcher, a binding for the watch_file syscall

This wrapper abstracts the watch_file setup and file handling, and
allows using the watch_file events as part of the event loop via the
Core::Notifier class.

Also renames the existing DirectoryWatcher class to BlockingFileWatcher,
and adds support for the Modified mode in this class.

Userland/Libraries/LibCore/FileWatcher.h

@@ -0,0 +1,80 @@
+/*
+ * Copyright (c) 2020, Itamar S. <itamar8910@gmail.com>
+ * Copyright (c) 2021, the SerenityOS developers.
+ * 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/NonnullRefPtr.h>
+#include <AK/RefCounted.h>
+#include <AK/Result.h>
+#include <AK/String.h>
+#include <LibCore/Notifier.h>
+
+namespace Core {
+
+struct FileWatcherEvent {
+    enum class Type {
+        Modified,
+        ChildAdded,
+        ChildRemoved,
+    };
+    Type type;
+    String child_path;
+};
+
+class BlockingFileWatcher {
+    AK_MAKE_NONCOPYABLE(BlockingFileWatcher);
+
+public:
+    explicit BlockingFileWatcher(const String& path);
+    ~BlockingFileWatcher();
+
+    Optional<FileWatcherEvent> wait_for_event();
+
+private:
+    String m_path;
+    int m_watcher_fd { -1 };
+};
+
+class FileWatcher : public RefCounted<FileWatcher> {
+    AK_MAKE_NONCOPYABLE(FileWatcher);
+
+public:
+    static Result<NonnullRefPtr<FileWatcher>, String> watch(const String& path);
+    ~FileWatcher();
+
+    Function<void(FileWatcherEvent)> on_change;
+
+private:
+    FileWatcher(NonnullRefPtr<Notifier>, const String& path);
+
+    NonnullRefPtr<Notifier> m_notifier;
+    String m_path;
+};
+
+}