Kernel: Implement multi-watch InodeWatcher :^)

This patch modifies InodeWatcher to switch to a one watcher, multiple
watches architecture.  The following changes have been made:

- The watch_file syscall is removed, and in its place the
  create_iwatcher, iwatcher_add_watch and iwatcher_remove_watch calls
  have been added.
- InodeWatcher now holds multiple WatchDescriptions for each file that
  is being watched.
- The InodeWatcher file descriptor can be read from to receive events on
  all watched files.

Co-authored-by: Gunnar Beutner <gunnar@beutner.name>

Kernel/Syscalls/inode_watcher.cpp

@@ -0,0 +1,88 @@
+/*
+ * Copyright (c) 2018-2020, Andreas Kling <kling@serenityos.org>
+ * Copyright (c) 2021, sin-ack <sin-ack@protonmail.com>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <Kernel/API/InodeWatcherFlags.h>
+#include <Kernel/FileSystem/Custody.h>
+#include <Kernel/FileSystem/FileDescription.h>
+#include <Kernel/FileSystem/InodeWatcher.h>
+#include <Kernel/Process.h>
+
+namespace Kernel {
+
+KResultOr<int> Process::sys$create_inode_watcher(u32 flags)
+{
+    REQUIRE_PROMISE(rpath);
+
+    int fd = alloc_fd();
+    if (fd < 0)
+        return fd;
+
+    auto description_or_error = FileDescription::create(*InodeWatcher::create());
+    if (description_or_error.is_error())
+        return description_or_error.error();
+
+    m_fds[fd].set(description_or_error.release_value());
+    m_fds[fd].description()->set_readable(true);
+
+    if (flags & static_cast<unsigned>(InodeWatcherFlags::Nonblock))
+        m_fds[fd].description()->set_blocking(false);
+    if (flags & static_cast<unsigned>(InodeWatcherFlags::CloseOnExec))
+        m_fds[fd].set_flags(m_fds[fd].flags() | FD_CLOEXEC);
+
+    return fd;
+}
+
+KResultOr<int> Process::sys$inode_watcher_add_watch(Userspace<const Syscall::SC_inode_watcher_add_watch_params*> user_params)
+{
+    REQUIRE_PROMISE(rpath);
+
+    Syscall::SC_inode_watcher_add_watch_params params;
+    if (!copy_from_user(&params, user_params))
+        return EFAULT;
+
+    auto description = file_description(params.fd);
+    if (!description)
+        return EBADF;
+    if (!description->is_inode_watcher())
+        return EBADF;
+    auto inode_watcher = description->inode_watcher();
+
+    auto path = get_syscall_path_argument(params.user_path.characters, params.user_path.length);
+    if (path.is_error())
+        return path.error();
+
+    auto custody_or_error = VFS::the().resolve_path(path.value(), current_directory());
+    if (custody_or_error.is_error())
+        return custody_or_error.error();
+
+    auto& custody = custody_or_error.value();
+    if (!custody->inode().fs().supports_watchers())
+        return ENOTSUP;
+
+    auto wd_or_error = inode_watcher->register_inode(custody->inode(), params.event_mask);
+    if (wd_or_error.is_error())
+        return wd_or_error.error();
+    return wd_or_error.value();
+}
+
+KResultOr<int> Process::sys$inode_watcher_remove_watch(int fd, int wd)
+{
+    auto description = file_description(fd);
+    if (!description)
+        return EBADF;
+    if (!description->is_inode_watcher())
+        return EBADF;
+    auto inode_watcher = description->inode_watcher();
+
+    auto result = inode_watcher->unregister_by_wd(wd);
+    if (result.is_error())
+        return result;
+
+    return 0;
+}
+
+}